Construction method for flat pilot tunnel in ultra-large section tunnel chamber of karst landform

Abstract

The invention discloses a construction method of a flat pilot tunnel in a karst landform super-large section tunnel chamber. The construction method comprises the following steps: 1) geological investigation before construction; 2) advanced geological forecast; (3) a flat pilot tunnel is excavated; (4) primary supporting is conducted; (5) treating a karst cave and a karst cavity; (6) water prevention and drainage measures are taken; (7) monitoring and measuring; and 8) ecological restoration. The method has the following technical advantages that 1, disasters such as collapse, water bursting and mud bursting in the construction process are effectively avoided through detailed geological survey, advanced geological forecast and a reasonable construction scheme, and the safety of constructors and equipment is guaranteed; 2, a proper excavation method and supporting measures are adopted, the construction process is optimized, the construction period is shortened, and the construction efficiency is improved; loss caused by geological disasters and improper construction is reduced, and engineering investment is saved; ecological restoration is emphasized, damage of construction to the surrounding environment is reduced, and coordinated development of engineering construction and environmental protection is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnel engineering construction, and in particular to a flat pilot tunnel construction method suitable for use in a super-large cross-section chamber in karst landforms. Background Art

[0002] The construction of ultra-large cross-section tunnel chambers in karst regions presents numerous complex technical challenges. For example, the complex geological conditions, widespread distribution of unfavorable geological formations such as caves, cavities, and underground rivers, and poor surrounding rock stability make construction prone to disasters such as collapse, water inrush, and mud inrush. The excavation and support of ultra-large cross-section chambers are challenging, and limited conditions make it difficult to meet the construction deadline with a single face construction method, and traditional construction methods are difficult to meet project requirements. Furthermore, environmental protection and ecological restoration during construction are also key considerations. Summary of the Invention

[0003] In order to solve the deficiencies in the above-mentioned prior art, the present invention provides a method for constructing a horizontal pilot tunnel in a karst landform super-large cross-section tunnel chamber, which improves the safety and efficiency of existing construction, reduces construction costs and takes into account environmental protection.

[0004] The object of the present invention is achieved like this:

[0005] A method for constructing a horizontal pilot tunnel in a karst landform tunnel with a large cross section comprises the following steps:

[0006] Step 1) Geological survey before construction;

[0007] Step 2) Advanced geological forecast;

[0008] Step 3) excavation of the horizontal pilot hole;

[0009] Step 4), initial support;

[0010] Step 5), cave and cavity treatment;

[0011] Step 6), anti-drainage measures;

[0012] Step 7) Monitoring and measurement;

[0013] Step 8) Ecological restoration.

[0014] The step 1) geological survey before construction: detailed survey of the geological conditions of the karst landform area where the chamber is located, including the distribution, scale and filling of caves, cavities and underground rivers, to provide an accurate geological basis for the formulation of the construction plan;

[0015] The step 2) of advanced geological prediction is to use a variety of advanced geological prediction methods, including geological radar, TSP seismic wave method and advanced drilling, to predict the geological conditions ahead, discover unfavorable geological bodies in advance, and adjust the construction plan in time;

[0016] The step 3) of excavating the horizontal pilot tunnel is as follows: according to the geological conditions and the size of the cross section, a suitable excavation method is selected, including the bench method, CD method and CRD method, and the excavation footage and blasting parameters are strictly controlled to reduce the disturbance to the surrounding rock;

[0017] The step 4) of initial support is to promptly implement initial support, including anchor rods, shotcrete and steel arches, to improve the stability of the surrounding rock.

[0018] The step 5) of treating caves and cavities is as follows: for caves and cavities encountered during the construction process, different treatment measures are taken according to their size and filling conditions, including backfilling, spanning and reinforcement.

[0019] The anti-drainage measures in step 6) are: setting up a complete anti-drainage system, including a waterproof layer, drainage blind pipes and drainage ditches, to prevent groundwater from affecting construction and operation.

[0020] The step 7) monitoring and measuring is to monitor and measure the surrounding rock deformation, support structure stress, etc. in real time during the construction process, and adjust the construction parameters in time according to the monitoring data to ensure construction safety.

[0021] The step 8) ecological restoration is: after the construction is completed, the construction area is promptly restored to restore the vegetation and landforms.

[0022] The step 2) advanced geological prediction includes: 1) comprehensive advanced geological prediction: based on geological survey data, strengthening advanced geological prediction and construction geological work in karst-developed sections, accurately grasping the karst development morphology, cavity filling conditions, and groundwater development conditions in front of the tunnel face, and preventing karst disasters including sudden water inrush, mud inrush, and sudden collapse; in karst-developed sections, comprehensive advanced geological prediction should be conducted based on geological survey methods, mainly using advanced drilling methods, combined with various geophysical exploration methods, and adopting the method of macro-forecast guiding micro-forecast and long-distance forecast guiding medium- and short-distance forecast;

[0023] 1.2) Geological radar scanning: Use the elastic wave reflection method (TSP203) to make a relatively detailed forecast of the location, scale, nature, and groundwater conditions of unfavorable geological bodies within 30 to 100 meters in front of the tunnel face. The scan is carried out every 100 meters and is appropriately increased when abnormal conditions are found.

[0024] 1.2) Medium- and short-range advance prediction: Using electromagnetic wave reflection or infrared detection, a more detailed forecast of geological conditions within 30 m in front of the tunnel face, including karst morphology, filling properties, and the presence of water in karst cavities, is made.

[0025] 1.3) Advance horizontal drilling: Verify the predictions of TSP203 and infrared detection using advance horizontal drilling and face-deepening blastholes. One to five advance horizontal drill holes are arranged in each section, with one hole used for core sampling. A 25-meter cycle is used, with a single hole length of 30 meters and a 5-meter overlap between adjacent detection holes. When anomalies are detected, interpretation is conducted based on the TSP203 and infrared detection prediction results, and appropriate drilling increases or lengthens are implemented.

[0026] Comprehensively analyze and interpret the data obtained from various prediction and forecasting methods, verify each other, and make predictions and judgments based on the geological conditions, development laws, trends and precursors revealed by the tunnel face; optimize and adjust support measures accordingly based on the results of comprehensive advanced geological prediction and forecasting.

[0027] 2) Karst treatment: Tunnel excavation and support in karst-developed areas should be comprehensively studied based on the karst morphology and size, filling conditions, and relative position of the tunnel determined by comprehensive advanced geological forecasts;

[0028] 2.1) In principle, the measures of pre-support with pipe sheds and subsequent excavation should be adopted for filling karst caves;

[0029] 2.2) For small karst cavities, after excavation, they will be treated based on the size and morphology of the karst caves. Karst caves at the tunnel bottom will be backfilled if possible based on the hydrological conditions; otherwise, beams and slabs will be used to span the tunnel. Karst caves in the side walls and arches will generally be backfilled with concrete.

[0030] 2.3) For karst areas with the risk of water and mud inrush, pre-treatment measures such as advanced curtain grouting and advanced peripheral grouting should be adopted for water-rich areas of geological structures based on geological survey data;

[0031] 2.4) Special safety measures should be formulated for the cleaning of cave fillings and the treatment of cave surrounding rocks in accordance with the actual conditions on site;

[0032] 2.5) During the construction of the interval karst tunnel, the drainage design should be carried out according to the predicted water inflow.

[0033] 3) Surface settlement control measures: Design and construction should be carried out according to the principle of pre-excavation reinforcement of surrounding rock and roof protection. The tunnel should be pre-grouted with small pipes and medium or large pipe sheds to reinforce the surrounding rock in front.

[0034] Mining tunnels adopt information technology for design and construction. During construction, attention should be paid to and strengthened monitoring and measurement work, which should be carried out throughout the construction process. Information should be fed back in a timely manner to guide design and construction.

[0035] During construction, disturbance to the surrounding rock should be minimized and short advances should be adopted. The length of the steps should be strictly controlled. When the steps are long, a temporary arch should be used to seal the bottom if necessary. Anchor rods or anchor pipes should be used to reinforce the arch feet of the steel frame.

[0036] After excavation, initial support or temporary support should be carried out in a timely manner, with close connection between the working procedures, early arch suppression and early closure into a ring; when the stability of the tunnel face is poor, concrete should be sprayed to seal the working face at any time;

[0037] During construction, drainage should be strengthened and, if necessary, pre-drilled holes should be installed under the tunnel face for local drainage. This includes conducting necessary monitoring and measurement of the surrounding rock, supporting structures, and ground buildings during the initial construction phase after completion, so as to obtain timely information and take timely measures to control ground subsidence.

[0038] 4) Monitoring and measurement of tunnel construction using the mining method: On-site monitoring and measurement are divided into mandatory and optional items. Mandatory items include observations inside and outside the tunnel, geological description of the tunnel face, measurement of vault subsidence, measurement of peripheral displacement convergence, and measurement of surface subsidence. Optional items include measurement of the internal forces of the primary support and secondary lining, the axial force of the anchor bolts, the pressure between the surrounding rock and the support, and the internal displacement of the surrounding rock.

[0039] To ensure the safety of the tunnel's surrounding rock, adjacent buildings, and caverns, during construction, monitoring and measurement work should be strengthened. A dedicated monitoring and measurement team should be established, and this work should be entrusted to an experienced and qualified unit. The team should track the deformation of the surrounding rock, foundation pit, and existing buildings throughout the entire process, analyze, judge, and predict possible situations that may arise during construction, and adjust support parameters and excavation methods in a timely manner based on the measurement results, thus achieving dynamic design and information-based construction to ensure construction safety.

[0040] 5) Intersection opening treatment measures:

[0041] 5.1) Based on the designed intersection angle between the pilot tunnel and the main tunnel, the initial support at the intersection was reinforced to provide safe support for the subsequent main tunnel crossing the C2-type large section. This meant that reinforcement began several steel frames before the pilot tunnel and the main tunnel intersected. Because the angle between the pilot tunnel and the main tunnel was 90°, the last few steel frames were adjusted to be perpendicular to the longitudinal direction of the main tunnel.

[0042] 5.2) Considering that the last steel frame of the initial support of the horizontal pilot tunnel needs to bear the load transmitted by the main tunnel steel frame and the surrounding rock, it must be reinforced for safety. That is, two I20b steel frames are welded together to form a reinforcement ring at the end of the horizontal pilot tunnel, at the inner edge of the maximum span of the main tunnel, to improve the load-bearing conditions;

[0043] 5.3) To ensure the footing and firmness of the arch frame on the main tunnel section of the intersection, after the initial support reinforcement ring steel frame of the horizontal pilot tunnel is erected, the upper step is continued to excavate forward 0.8 meters and the section is expanded; two arch frames are erected along the east position of the corresponding positions of the side walls on both sides of the horizontal pilot tunnel and welded together; the elevation of the steel frame top is controlled to be consistent with the initial support steel frame of the horizontal pilot tunnel, and a longitudinal horizontal joist is set on the top of the steel frame along the forward direction of the main tunnel. The joist is made of two I20b steel sections welded together; the top surface of the horizontal joist is welded with steel plates at intervals of 100 cm to facilitate the overlap of the arch steel frame and the joist of the main tunnel of the intersection section;

[0044] 5.4) The joists and supports are reinforced with additional locking anchors and sprayed with anchors for protection; the triangular area formed between the main tunnel unit steel frame and the horizontal pilot tunnel reinforcement ring steel frame is sprayed and compacted;

[0045] 5.5) Lower the steel frame of the upper steps of the horizontal pilot tunnel and the steel frame of the main tunnel unit in sequence. To prevent the support from sinking due to insufficient bearing capacity of the base, first tamp the base, weld steel plates to the bottom of the steel frame, and install bottom channel steel.

[0046] The construction process of the horizontal pilot tunnel excavation in step 3) is as follows: 3.1 setting up the tunnel portal operating platform; 3.2 surveying and setting out; 3.3 drilling; 3.4 blasting; 3.5 ventilation; 3.6 troubleshooting; 3.7 slag removal; 3.8 support;

[0047] The construction method of the steps at the intersection of the main tunnel and the horizontal pilot tunnel in the construction process of the horizontal pilot tunnel excavation is as follows: the first ten steel frames at the intersection are densely arranged with a spacing of 0.5m / frame;

[0048] 1) Construction preparation: Before the construction of the tunnel portal, assemble the steel frame first, check its clearance size and connection quality, and notify all parties for acceptance after passing the inspection. Only after passing the inspection can the next process be carried out; first, lay out the position of the outer contour line of the horizontal pilot tunnel excavation on the right line, and mark the dividing lines of the upper and lower steps respectively; 2) Demolish the tunnel portal in sections, erect the steel frame, hang the net, and spray concrete.

[0049] The construction method for the intersection of the horizontal pilot tunnel and the main tunnel in the construction process of the horizontal pilot tunnel excavation is as follows: 1) according to the designed intersection angle of the horizontal pilot tunnel and the main tunnel, the initial support at the intersection is strengthened to provide a safe support guarantee for the main tunnel to cross the C2 type large section; that is, the steel frames are strengthened before the intersection of the horizontal pilot tunnel and the main tunnel. Since the horizontal pilot tunnel is parallel to the main tunnel, the last few steel frames are adjusted to be perpendicular to the longitudinal direction of the main tunnel;

[0050] 2) Considering that the last steel frame of the initial support of the horizontal pilot tunnel needs to bear the load transmitted by the main tunnel steel frame and the surrounding rock, it must be strengthened for safety. That is, six I20b steel frames are welded together as a reinforcement ring at the end of the horizontal pilot tunnel, at the inner edge of the maximum span of the main tunnel, to improve the load-bearing conditions;

[0051] 3) To ensure the footing and firmness of the arch frame on the upper section of the main tunnel at the intersection, after the initial support reinforcement ring steel frame of the horizontal pilot tunnel is erected, the step is stepped and excavated to the right line section. Two arch frames are erected along the main tunnel at the corresponding positions on the side walls of the horizontal pilot tunnel and welded together. The top elevation of the steel frame is controlled to be consistent with the initial support steel frame of the horizontal pilot tunnel. A longitudinal horizontal joist is installed on the top of the steel frame along the direction of the pilot tunnel. The joist is made of two superimposed welded I20b steel sections.

[0052] 4) The joists and supports are reinforced with locking anchor rods and anchor spraying is carried out for protection; the triangular area formed between the main tunnel unit steel frame and the horizontal pilot tunnel reinforcement ring steel frame is fully sprayed and compacted;

[0053] 5) Lower the stepped steel frames on the horizontal guide tunnel one by one; to prevent the support from sinking due to insufficient bearing capacity of the base, first compact the base, weld steel plates to the bottom of the steel frame, and install bottom channel steel.

[0054] In the construction process of the primary support for the main line, the horizontal pilot tunnel adopts the "step method" and the principle of "exploration before excavation" should be adhered to during the tunnel construction process. The main operations include 1) earthwork excavation: according to the design drawings and construction site conditions, the horizontal pilot tunnel adopts conventional blasting construction; the excavation plan adopts two-step blasting construction according to the cross-section design;

[0055] 2) Steel frame production: The steel frame is made of I18, I20b and I-beams. The steel frame is cut according to the design size of the steel frame. The length of each section of the steel frame is determined by the design drawing and on-site construction methods. The deformation of the steel frame should be controlled during bending. After the steel frame is manufactured, a trial assembly should be carried out to check whether the size and outline of the steel frame are qualified.

[0056] 3) Steel frame installation process: Before installing the steel frame, the excavation clearance of the face should be checked, and the loose slag at the bottom of the steel frame should be cleared. It is not allowed to fill the bottom of the over-excavated arch foot with loose slag. Square wood or steel should be padded for height adjustment. The excavation size allows an error of ±5cm in the horizontal direction and ±5cm in the height difference. In areas with weak surrounding rock, channel steel should be padded at the bottom of the steel frame to prevent the steel frame from sinking. The segmented steel frame should be manually assembled into a whole steel frame on the face, and the connecting bolts should be tightened. After the steel frame is installed, the center line error is allowed to be ±3cm, the elevation error is allowed to be ±3cm, and the verticality error of the steel frame is allowed to be ±2°. After the steel frame is calibrated, prefabricated concrete wedges should be used to wedge the steel frame and the rock layer every 2.0m. The steel frame bottom extension is carried out in a staggered manner, with 1 to 3 rows of steel frames extended on one side each time. In soft strata, the bottom extension and invert arch can be connected at the same time and concrete sprayed in time. The connection between the extended steel frame and the original steel frame must be firm and accurate. The two rows of steel frames are connected with φ22 connecting bars every 1.0m along the steel frame circumference to form a whole, improve the longitudinal stress state of the steel frame and prevent overturning. After the steel frame is longitudinally connected, the mesh spraying anchor support is quickly carried out according to the design, and the back of the steel frame is sprayed and filled with concrete to make it close to the surrounding rock.

[0057] 4) Fabrication and installation of steel mesh: The steel mesh is prefabricated into mesh sheets outside the hole and then transported to the working surface for hanging. If a double-layer mesh is used, the two layers can be staggered. The steel mesh, sprayed concrete, and steel frame form a joint support and are laid continuously. When laying, please note that the steel mesh must be firmly welded to the anchor rod, steel frame or other anchoring device, and the overlap length of the sheet steel mesh must be no less than 250mm. The steel bars must be covered with sprayed concrete, with at least a 20mm protective layer.

[0058] 5) Small conduit construction: The small conduit adopts Φ42 seamless steel pipe. First, cut the steel pipe into the small conduit length designed in the drawing, make a 10cm long cone at one end of the steel pipe, and weld a φ8 steel hoop 10cm away from the other end; no hole is opened 50cm away from one end of the steel hoop, and φ6-8mm grouting holes are opened symmetrically every 20cm in the remaining part. The grouting holes are arranged in a plum blossom shape according to the principle of precipitation funnel. Start from the top and pressurize from top to bottom. First pressurize the holes without water and then the holes with water. In case of slurry leakage or slurry leakage, pressurize one or several holes at intervals;

[0059] 6) Mortar anchor construction: The anchor is made of mortar anchor. After the pneumatic drill is driven into the rock formation to form a hole, the hole is cleaned with high-pressure air, and then grouting is done with a grouting machine, and finally the anchor is inserted;

[0060] (1) The anchor rods are cut according to the design requirements, and are straightened, derusted and degreased. The anchor rods without pads at the outer ends are first bent into elbows with a length of 0.25 meters.

[0061] ⑵Use ordinary silicate cement, the sand particle size should not be greater than 3mm, and should be sieved;

[0062] ⑶ The mortar grade shall not be lower than 20MPa, and the mix ratio shall be cement: sand: water = 100:35:45. A construction mix ratio test must be carried out during construction.

[0063] (4) The drilling should meet the following requirements: the hole diameter should match the pipe diameter; the hole diameter should be 15mm larger than the pipe diameter; the hole position tolerance should be ±15mm to 50mm; the hole depth tolerance should be ±50mm; the anchor length should not be less than the designed length; after drilling, flush the hole with high-pressure air;

[0064] ⑸ The mortar should be mixed evenly and its workability adjusted. Use it as soon as it is mixed. Slowly and evenly pull out the grouting pipe, and always keep the grouting pipe mouth buried in the mortar to avoid slurry voids. The grouting volume should be approximately larger than the required volume; after the grouting pipe is completely pulled out, the anchor rod should be forcibly inserted immediately. The length of the rod inserted into the hole should not be shorter than the designed length. The grouting is judged to be full based on whether there is slurry squeezed out of the hole mouth.

[0065] (6) Immediately weld the rod to the steel grid after it is in place to prevent the anchor rod from shifting. Do not collide with it until the slurry reaches 70% of the design strength. Do not hang heavy objects on it within three days.

[0066] 7. Spray concrete promptly after excavation and close the initial support as soon as possible. During the excavation process, pay close attention to the deformation of the anchor rods, and the cracking and bulging of the sprayed concrete layer, so as to understand the dynamics of the surrounding rock and adjust the excavation and support parameters in time;

[0067] 7) Hollow anchor construction: (1) Construction preparation: Prepare a concrete pump, a 7655 type pneumatic rock machine, a large diameter anchor and a skilled machine operator before construction;

[0068] ⑵ Drilling: Use a trolley or handheld rock drill to drill the anchor rod with the drill bit installed to the designed depth or slightly deeper than the designed depth;

[0069] ⑶ Drilling: Remove the drilling rig and pull out the anchor rod, then manually drive the hollow anchor rod into the eye hole (the drill hole is slightly larger than the anchor rod eye hole) and install the grouting plug, which should be installed 25 cm away from the hole mouth. In special cases such as when the grouting pressure or surrounding rock is large, the hole can also be sealed with an anchor runner agent;

[0070] (4) Connection: Connect the grouting joint and the tail end of the anchor rod through the connecting sleeve to ensure close contact and prevent leakage during the grouting process;

[0071] (5) Grouting: Start the grouting machine and stop when the grouting is full and the pressure reaches the design value. The grouting pressure is determined according to the design parameters and the performance of the grouting machine;

[0072] 8) Sprayed concrete construction includes the following procedures: ⑴ Construction preparation: remove loose rocks and rock chips, and flush the sprayed surface with wind and water. If there is water leakage, it should be properly handled; check the excavation section size, and set signs to control the thickness of the sprayed concrete; check machinery and equipment and air, water, electricity and other pipelines, and test run them to pass the test; ⑵ Operation process control procedure; ⑶ Net spraying procedure; ⑷ Wet spraying procedure.

[0073] The operation process control includes: the spraying concrete operation is carried out immediately after the support frame is completed; in order to ensure the quality of the sprayed concrete, reduce the rebound rate and reduce dust, the wet spraying method is used for the spraying concrete construction; the working air pressure of the sprayer must meet the pressure of 0.1MPa at the nozzle;

[0074] The spraying of concrete is carried out in sections and pieces in sequence; the spraying operation is from bottom to top, first spraying the concrete between the steel frame support and the arch wall, and then spraying the concrete between the two arch frames;

[0075] If the rock surface is uneven, the concave part should be sprayed first, and then sprayed upwards. When spraying, the nozzle beam moves in a rotating trajectory, one circle at a time, and moves in a serpentine manner in the longitudinal direction; the rotation radius is about 15 cm, and the amplitude of each serpentine movement is 1.5 to 2.0 m; when spraying the second row in the longitudinal direction, start from the starting point of the first row in sequence, and the rows must overlap by 2 to 3 cm. The beam rotation speed should be uniform; the shotcrete operation should maintain continuous and uniform material supply; the shotcrete nozzle should be perpendicular to the sprayed surface, and the distance between the nozzle and the sprayed surface should be 0.6 to 1.0 m;

[0076] Concrete is sprayed in layers, and the initial spraying is to seal the working surface to prevent weathering; the thickness of the spraying layer is 3 to 5 cm, and it is carried out immediately after excavation; the next layer of spraying should be carried out after the previous layer of concrete has finally set. If spraying is carried out 1 hour after final setting, the surface of the spraying layer must be cleaned with water first; under normal circumstances, the rebound of the sprayed concrete is not more than 15% for the side walls and not more than 25% for the top; when the spraying operation is followed by blasting excavation of the excavation face, the time from final setting of the concrete to the next cycle of blasting should not be less than 3 hours; curing begins 2 hours after the final setting of the sprayed concrete.

[0077] The wet spraying process is as follows: after the spraying machine is installed, water is first injected, ventilation is performed, and debris in the pipe is cleaned; at the same time, high-pressure air is used to blow the sprayed surface to remove dust on the sprayed surface;

[0078] When conditions permit, the nozzle should be fixed on a manipulator for spraying operations; when conditions do not permit and the nozzle must be manually controlled, two people should operate the nozzle together; the mixture of shotcrete should be mixed with a forced concrete mixer, the mixing time should be no less than 2 minutes, the slump of the discharge should be controlled at 10-18 cm, and continuous feeding should be ensured; the spraying route of shotcrete should be from bottom to top, in an "S" shape; during spraying, the nozzle should make continuous circular motion and form a spiral forward, with the last circle pressing one-third of the previous circle;

[0079] The pressure at the spraying machine nozzle is required to be 0.1Mpa, and the nozzle should be kept perpendicular to the sprayed surface. If the sprayed surface is covered by steel mesh or grid steel frame, the nozzle can be slightly tilted by 10° to 20°. When spraying concrete, observe the following points:

[0080] The initial spraying of concrete follows the tunnel face closely. Before the re-spraying, the installation of the advance small duct, steel mesh and grid steel frame shall be completed according to the design requirements.

[0081] Before spraying construction, a trial spraying should be carried out. After the trial spraying is qualified, the spraying construction can be started. Large panels should be sprayed according to regulations and inspection specimens should be made.

[0082] After each concrete spraying, check the thickness immediately. If the thickness is not enough, additional spraying is required to reach the designed thickness. It is strictly forbidden to use rebound material as spraying material.

[0083] Positive beneficial effects:

[0084] Compared with the existing technology, the present invention has the following technical advantages:

[0085] 1. Improve construction safety: Through detailed geological surveys, advanced geological forecasts and reasonable construction plans, we can effectively avoid disasters such as collapse, water inrush, and mud inrush during the construction process, and ensure the safety of construction personnel and equipment.

[0086] 2. Improve construction efficiency: adopt appropriate excavation methods and support measures, optimize construction technology, shorten construction period and improve construction efficiency.

[0087] 3. Reduce construction costs: reduce losses caused by geological disasters and improper construction, and save project investment.

[0088] 4. Protect the environment: focus on ecological restoration, reduce the damage of construction to the surrounding environment, and achieve coordinated development of engineering construction and environmental protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Figure 1 This is a schematic diagram of the initial support construction structure for the upper steps;

[0090] Figure 2 This is a schematic diagram of the initial support construction structure for the lower steps;

[0091] Figure 3 This is a schematic diagram of the strengthening of the flat guide tunnel door;

[0092] Figure 4 This is the process flow chart for initial support construction;

[0093] Figure 5 Provide a process flow chart for steel frame installation;

[0094] Figure 6 This is the process flow chart of small conduit grouting;

[0095] Figure 7 This is a schematic diagram of small catheter processing;

[0096] Figure 8 The mortar anchor construction process;

[0097] Figure 9 The invention relates to a flat pilot tunnel construction process. DETAILED DESCRIPTION

[0098] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0099] Example

[0100] 1. Overview of implementation case projects:

[0101] 1.1.1 Project Introduction

[0102] Topography of the second phase of a city's Rail Transit Line 2:

[0103] The landform type along this section is an alternating topography of high and medium mountain karst hills and karst depressions. The highest point of the ground elevation is located at the low mileage end of the line, with a mountain top elevation of approximately 1296.0m; the lowest point is located at the high mileage end of the section line, with an elevation of approximately 1270.0m; the terrain between the starting point of the section DK0+000 and DK0+106.91 is relatively flat, with a gentle slope with low altitude in the west and high altitude in the east, and an average slope of less than 2°.

[0104] According to the construction design drawings and geological survey reports, the flat guide tunnel is made entirely of rock, requiring blasting. According to the Ministry of Construction's safety management regulations, tunnel excavation and blasting are considered sub-projects exceeding a certain scale and are considered dangerous, requiring experts to conduct a plan review. 3 The actual volume is determined based on on-site measurements or calculations based on drawings.

[0105] 1.1.2 Hydrogeology

[0106] (1) Groundwater types and occurrence

[0107] The proposed site's groundwater is primarily phreatic, specifically divided into Quaternary loose rock pore water and karst water. Quaternary loose rock pore water, primarily found within the Quaternary loose overburden, has a relatively small overall volume and significant seasonal fluctuations. It is primarily recharged by atmospheric precipitation and has low permeability, with some areas showing good permeability but lacking water, resulting in a weak water-rich level. Carbonate karst water, primarily found within carbonate karst fissures and pores, is abundant and drains centrally in the form of pools or underground river outlets. Its water content is relatively uniform, with minimal dynamic fluctuations. The groundwater runoff modulus in this layer is 3 to 8 L / s·km², resulting in a weak to moderate water-rich level.

[0108] The underlying bedrock of the underground excavation area of ​​the proposed site is all soluble rock, with weak karst development, mainly manifested as dissolved pores and dissolved gaps. Caves are developed in some local areas. Exploration reveals that the caves are fully or semi-filled. The occurrence of groundwater is affected by the degree of karst development, and the runoff is controlled by the topography, karst morphology and filling state. The distribution of groundwater is complex and differentiated.

[0109] (2) Groundwater recharge, runoff, discharge and dynamic characteristics

[0110] Atmospheric precipitation is the primary source of groundwater recharge at the proposed site, particularly rainfall of low intensity and long duration, which is most beneficial for groundwater recharge; the second is year-round lateral recharge from the surface water network. The infiltration rate of atmospheric precipitation varies depending on the structure of the aquifer. The shallow miscellaneous fill at the proposed site is relatively loose and porous, and the underlying bedrock has well-developed karst pores and crevices, resulting in a high infiltration rate. Atmospheric precipitation infiltrates directly vertically, and after the recharge water source infiltrates into the water table, shallow groundwater primarily flows along shallow drainage channels, relying on the terrain. Deep karst water primarily flows along karst fissures, karst pipes, structural fracture zones, fissures, and layered runoff. Its runoff direction is significantly affected by factors such as rock surface undulations, rock formation dip, fissure development, and structural development direction.

[0111] (3) Hydrochemical characteristics and corrosiveness evaluation

[0112] Based on the engineering experience of Line 1 in this city, the environmental type of this market is considered as Class II, which is slightly corrosive to concrete structures;

[0113] (4) Water content and permeability of rock and soil layers

[0114] Based on regional hydrogeological data, on-site investigations, and analysis of drilling data, the site's hydrogeological conditions are considered average. Groundwater is divided into two main types: pore water in the overburden and karst fissure water in the bedrock. Pore water has moderate water yield, while bedrock fissure water has low water yield. The permeability coefficient of the rock mass within the cavern is generally low, with significant variation, typically ranging from 0.10 to 0.5 m / d. This variation in permeability is influenced by the degree of karst development, rock mass integrity, and the presence of karst water.

[0115] (5) Estimation of tunnel water inflow

[0116] The calculated average daily water inflow of the tunnel is 75m(3) / d from the starting point of the section (DK39+000) to the mileage DK39+577. The above tunnel water inflow predictions do not take into account the karst pipeline water. Based on the relevant engineering experience in karst areas, it is recommended that the water inflow in the karst pipeline be considered at 70-80m(3) / h.

[0117] 1.1.3 Unfavorable geology and special rock and soil

[0118] (1) Karst

[0119] The tunnel area is located in a karst trough valley with a wide distribution of carbonate rocks. Due to urban development altering the original landform, no obvious karst depressions, sinkholes, or funnels are visible. Drilling and geological surveys reveal that the underlying bedrock is primarily soluble rocks such as dolomite and limestone. Karst gullies, karst troughs, and rock buds are common near the interface between the bedrock and the non-soluble rock. Karst caves, karst fracture zones, and karst fissures are also common within the rock mass, particularly at the boundary between soluble and non-soluble rocks.

[0120] The karst in the tunnel area is located beneath a 1-5 meter thick overburden, representing a covered karst type. The karst development intensity in the area is classified as moderate, with higher intensity near the short-distance intermediate loop road. The main impacts of karst on the project include a reduction or loss of foundation bearing capacity, a reduction or loss of the tunnel surrounding rock grade, decreased stability of the tunnel walls and pit walls, increased risk of collapse during tunnel construction, significant water inrush when karst pipes are filled with water, and ground subsidence caused by large-scale groundwater extraction.

[0121] (2) Ground wall collapse

[0122] The thickness of the Quaternary cover layer within the interval tunnel is about 1.0 to 6.0 m. The cover layer is relatively thin and is mainly composed of loose miscellaneous fill, which is conducive to the formation of karst collapse; the lower bedrock is mainly dolomite and limestone, which are soluble rocks and provide the necessary lithological conditions for karst ground collapse; drilling reveals that there are hidden karst forms such as crevices and pores developed underground, providing a good channel for groundwater migration.

[0123] (3) Bedding

[0124] The intersection angle α between the rock strata direction of the line and the center line of the line is generally greater than 30°, and the apparent inclination angle of the rock stratum layer on the cross section of the tunnel body (β apparent) is 10°~24°. However, it should be noted that the intersection of the open and dark excavation of the inclined shaft at the small mileage end, the west side wall of the construction shaft, and the open and dark excavation of the drainage tunnel need to consider the problem of bedding. At the same time, there is a combination of bedding and unfavorable joint surfaces during construction excavation, which is prone to block falling and collapse.

[0125] (4) Fault

[0126] According to geological surveys, a fault, the Tuyunguan Fault (f1), develops near this section. This reverse fault strikes nearly north-south, dipping eastward at an angle of 35° to 45°. Its hanging wall is composed of limestone and dolomite of the Lower Carboniferous Baizuo Formation (C1b+h), while its footwall is composed of thin limestone of the Lower Triassic Daye Formation (T1d). It intersects the proposed track at a 60° angle near YDK39+320. Ground surveys and magnetotelluric geophysical exploration revealed a significant negative topography at the fault site, with a throw exceeding the Permian stratum thickness, conservatively estimated at over 2 km. Mechanically, it is a compressional fault with a width of approximately 200 m and low resistivity near the fault zone. This indicates karst development on both sides of the fault zone, enriched groundwater, and widespread, thick, and moderately water-rich aquifers. Tectonic rocks within the fault fracture zone are primarily fault breccias, which are irregularly angular and vary in size, cemented by calcareous rock. During tunnel excavation, support measures should be strengthened within the fault-affected area.

[0127] (5) Dangerous rocks and collapse

[0128] Located in the area affected by the footwall of the Tuyunguan Fault, it has obvious negative terrain and is close to the steep cliff of the fault hanging wall on the east side, which poses a risk of dangerous rock collapse. In addition, the west side of this section is the Central Ring Road, and there are densely distributed residential houses along the hillside. The surrounding environmental conditions are sensitive and complex, and attention should be paid during construction. Non-blasting methods should be used during excavation. At the same time, the inspection and investigation of surrounding residential houses and dangerous rocks should be strengthened. Reinforcement measures can be taken when necessary to ensure construction safety.

[0129] (6) Artificial fill

[0130] Artificial fill is widely distributed on the surface of the tunnel section, with layer thicknesses ranging from 0.5 to 4.8 meters, with an average thickness of 2.09 meters. This fill is characterized by its mixed composition, looseness, low bearing capacity, and high compressibility, which has a certain impact on the entrance and exit excavation slopes.

[0131] (7) Red clay

[0132] It is mainly distributed at the small mileage end of the interval, with a layer thickness of 1.7 to 9.0 m and an average thickness of 4.64 m. It has weak expansion and high liquid limit. It softens when exposed to water, shrinks strongly when losing water, has developed cracks, and is easy to peel off. It has a certain impact on the excavation slopes of the entrance and exit.

[0133] (8) Weathered rock

[0134] There are strongly weathered rocks (dolomite limestone and mudstone) distributed in some parts of the surface along the section tunnel. Joints and fissures are well developed and the rock mass is broken. Most of them are located above the tunnel, which has a certain impact on the tunnel excavation of this project.

[0135] (9) Water

[0136] Because the groundwater level along some of the proposed subway lines is shallow and karst is well developed, particularly within the pure, medium-thick layered limestone and dolomite, where karst is relatively extensive, karst conduits have formed locally. Once these conduits are exposed through excavation, large amounts of water may gush along them. The project area also includes tunnel sections crossing rivers, which are shallow, have fractured rock masses, are highly permeable, and traverse soluble rock formations with well-developed karst. This could result in large amounts of water gush along these conduits and fissures during construction, creating significant challenges for construction. Furthermore, lowering the groundwater level during construction could cause karst collapse in the surrounding area. Lowering the groundwater level within the overburden layer could increase effective stress, leading to additional surface subsidence, potentially posing a significant threat to surrounding buildings and structures.

[0137] 1.1.4 Engineering Environment

[0138] 1. This section passes under the Forest Park Mountains, and a fault has developed nearby - the Tuyunguan Fault (f1). Geophysical exploration shows that the rock mass in this section is relatively fragmented, and water gushing and mud bursting are expected to occur in the nearby areas.

[0139] 2. The tunnel passes through moderately thick layers of massive powder crystal dolomite and dolomitic limestone in the Baizuo and Huanglong Formations, with a dip of 115° to 145°∠25°. The roof and sidewalls are moderately weathered dolomite, and the overlying rock mass is 160 to 225 meters thick. The rock mass is well-integrity and the surrounding rock is relatively stable. The arch and sidewalls are largely self-stabilizing. The tunnel is located below the groundwater level, with localized karst caves. Water seepage and linear water inrush are possible along karst fissures. This project is 12 meters parallel to the C2 major section, meeting construction safety requirements.

[0140] 1.1.6 Structure Introduction

[0141] The primary branch of the flat pilot tunnel is divided into three types of cross-sections and lining types: type A lining, type B lining, and type B lining of the flat pilot tunnel gradient section.

[0142] 1.2 Analysis of key points and difficulties in the project

[0143] 1. The focus of construction is to reinforce the entrances of the main tunnel and the horizontal pilot tunnel to ensure the safety of the entrance structure.

[0144] 2. The horizontal pilot tunnel passes through the area affected by the Tuyunguan reverse fault. Dealing with water gushing and mud bursting is the key and difficult point of this project.

[0145] 1.3. Construction requirements and technical guarantee conditions

[0146] The total length of the additional horizontal pilot tunnel in the Fu-Sen section is 106.91 meters. The upper and lower step blasting method is used for construction. Anchor rods, steel mesh, sprayed concrete and steel frames are used as initial support, supplemented by small grouting pipes as initial support. During the construction process, the lining construction and clearance dimensions must be strictly controlled to ensure structural safety and subsequent performance.

[0147] Technically, take the following measures:

[0148] ⑴ Comprehensive advanced geological forecast

[0149] Based on geological survey data, advance geological prediction and construction geology work should be strengthened in karst-rich areas. Accurately understand the karst development morphology, cavity filling conditions, and groundwater development ahead of the tunnel face, so that targeted engineering measures can be taken to prevent karst disasters such as water and mud inrush, and sudden landslides. In karst-rich areas, comprehensive advance geological prediction should be conducted based on geological survey methods, primarily using advance drilling, combined with various geophysical methods. Macroscopic predictions should guide microscopic predictions, and long-range predictions should guide medium- and short-range predictions.

[0150] 1) Geological radar scanning: The elastic wave reflection method (TSP203) is used to make a relatively detailed forecast of the location, scale, nature, and groundwater conditions of the adverse geological bodies within 30 to 100 meters in front of the tunnel face. The forecast is carried out every 100 meters and is appropriately increased when there are abnormal conditions.

[0151] 2) Medium- and short-range advance prediction: The electromagnetic wave reflection method (infrared detection) is used to make a more detailed forecast of geological conditions such as karst morphology, filling properties, and whether there is water in the karst cavity within a range of about 30 meters in front of the tunnel face.

[0152] 3) Advance Horizontal Drilling: Verify the predictions from TSP203 and infrared detection using advance horizontal drilling and face-deepening blastholes. One to five advance horizontal drill holes (one for core sampling) are drilled in each section, with a 25-meter cycle and a single hole length of approximately 30 meters. The overlap between adjacent detection holes is 5 meters. When anomalies are detected, interpretation is conducted based on the TSP203 and infrared detection prediction results, and appropriate drilling increases or increases in length are made.

[0153] Comprehensively analyze and interpret the data obtained from various forecasting methods, verifying each other and combining them with the geological conditions, development patterns, trends, and precursors revealed by the tunnel face to make predictions and judgments. Based on the results of comprehensive advanced geological forecasting, support measures are optimized and adjusted accordingly to ensure construction safety and structural security, ensuring smooth construction.

[0154] ⑵ Karst treatment

[0155] Tunnel excavation and support in karst-developed areas should be comprehensively studied based on the specific conditions such as the karst morphology and size, filling conditions and relative position of the tunnel determined by comprehensive advanced geological forecasts.

[0156] 1) In principle, the measures of pre-support with pipe sheds and subsequent excavation should be adopted for filling karst caves;

[0157] 2) For small karst cavities, they can be excavated and treated according to the size and morphology of the karst caves. Generally, karst caves at the bottom of the tunnel should be backfilled if possible according to the hydrological conditions, otherwise they should be spanned with beams and slabs; karst caves in the side walls and arches should generally be backfilled with concrete;

[0158] 3) For karst treatment with the risk of water and mud inrush, advance curtain grouting and advance peripheral grouting measures should be adopted for pre-treatment in water-rich areas of geological structures based on geological survey data;

[0159] 4) Special safety measures should be formulated for the cleaning of cave fillings and the treatment of cave surrounding rocks in accordance with the actual situation on site;

[0160] 3) During the construction of the interval karst tunnel, the drainage design should be carried out according to the predicted water inflow.

[0161] ⑶Surface subsidence control measures

[0162] The design and construction are based on the principle of pre-reinforcement of surrounding rock and roof protection before excavation. The tunnel in this section uses pre-grouting of small pipes + medium or large pipe sheds to reinforce the surrounding rock in front.

[0163] Mining-method tunnels adopt information-based design and construction. During construction, attention should be paid to and the monitoring and measurement work should be strengthened. The monitoring and measurement work should be carried out throughout the construction process. Information should be fed back in a timely manner to guide the design and construction and ensure the safety of the tunnel structure construction.

[0164] During construction, disturbance of the surrounding rock should be minimized, and short advances should be adopted. The length of the steps should be strictly controlled. If the steps are long, a temporary inverted arch should be used to seal the bottom when necessary. Anchor rods (tubes) should be used to reinforce the arch feet of the steel frame.

[0165] After excavation, initial or temporary support should be carried out promptly, with closely linked processes and close connections. Arch suppression should be implemented as soon as possible, and ring closure should be achieved as soon as possible. If the tunnel face is unstable, concrete should be sprayed to seal the working surface at any time.

[0166] During construction, construction drainage should be strengthened. If necessary, pre-drilled holes should be set up under the face for local drainage to ensure that the excavation face is in a water-free state and improve the self-stabilizing ability of the surrounding rock.

[0167] During the construction process (including the initial stage of completion), necessary monitoring and measurement of the surrounding rock, supporting structures, and ground buildings (structures) should be carried out in order to obtain information in a timely manner and take timely measures to control surface subsidence.

[0168] ⑷Monitoring and measurement of tunnel construction using the mining method

[0169] On-site monitoring and measurement are divided into mandatory items and optional items. The mandatory items include observations inside and outside the tunnel and geological description of the tunnel face, arch subsidence measurement, peripheral displacement convergence measurement, and surface subsidence measurement. The optional items include the internal forces of the initial support and secondary lining, the axial force of the anchor rod, the pressure between the surrounding rock and the support, and the measurement of the internal displacement of the surrounding rock.

[0170] To ensure the safety of the tunnel's surrounding rock, adjacent buildings, and caverns, construction monitoring and measurement efforts should be strengthened during construction. A dedicated monitoring and measurement team should be established, and this work should be entrusted to experienced and qualified testing organizations. Deformation of the surrounding rock, foundation pit, and existing buildings should be tracked throughout the entire process. Potential construction scenarios should be analyzed, assessed, and predicted. Based on these measurement results, support parameters and excavation methods should be adjusted promptly, enabling dynamic design and information-based construction to ensure construction safety. Emergency plans should be established for risky areas, and if observations reveal abnormalities exceeding warning thresholds, emergency measures must be implemented immediately.

[0171] 1) Geological description of the tunnel face

[0172] After each tunnel excavation is completed, personnel from a dedicated construction safety monitoring and measurement unit will enter the site to describe and record the geological conditions exposed at the tunnel face and determine whether the surrounding rock grade matches the design. If not, appropriate changes will be made to support parameters and construction methods. This ensures construction safety and that the lining structure's loads meet design requirements.

[0173] 2) Surface subsidence monitoring

[0174] High-precision levels are used to measure the settlement of the ground surface and surrounding buildings, which is carried out by geological professionals. The layout of the monitoring points and the measurement frequency can be found in the corresponding design drawings.

[0175] The maximum surface subsidence is 40 mm, and the maximum differential settlement near the building foundation and pipelines is determined according to the requirements of the relevant underground pipeline department.

[0176] 3) Measurement of arch subsidence and perimeter convergence displacement

[0177] Tunnel crown subsidence and perimeter convergence displacement measurements were performed using a dedicated tunnel displacement convergence meter and a high-precision level. This was carried out by personnel from a dedicated construction safety monitoring and measurement unit. The layout of monitoring points and measurement frequency are shown in the corresponding design drawings.

[0178] Based on the values ​​of tunnel vault subsidence and convergence displacement around the tunnel, as well as the deformation trend and the stability of the surrounding rock, the safety of the excavation method, the rationality of the support parameters and the construction time of the secondary lining are determined. At the same time, the value of the reserved deformation is studied and determined.

[0179] 4) Measurement of selected items such as lining internal force, anchor axial force and surrounding rock pressure

[0180] Rebar gauges, concrete strain gauges, pressure cells and multi-point displacement gauges are used to measure the internal forces of the initial support steel frame, the internal forces of the secondary lining, the axial forces of the anchor rods, the interaction between the surrounding rock and the lining, and the internal displacement of the stratum caused by tunnel excavation, targeting different tunnel cross-sectional structures, different geological conditions and different tunnel burial depths.

[0181] By analyzing the measured results of anchor axial forces and the internal forces of the initial support caused by internal displacement of the stratum, and conducting numerical simulation analysis of the tunnel excavation process, we can evaluate the safety and feasibility of the excavation method used and whether the initial support parameters can meet the requirements for stabilizing the surrounding rock. This allows us to modify the design parameters of the initial support and select a reasonable excavation method.

[0182] ⑸Intersection opening

[0183] 1. Based on the designed intersection angle between the horizontal pilot tunnel and the main tunnel, the initial support at the intersection was reinforced to provide safe support for the subsequent main tunnel crossing the C2-type large section. This meant that reinforcement began several steel frames before the horizontal pilot tunnel and the main tunnel intersected. Because the angle between the horizontal pilot tunnel and the main tunnel was 90°, the last few steel frames were adjusted to be perpendicular to the longitudinal direction of the main tunnel.

[0184] 2. Considering that the last steel frame of the initial support of the horizontal pilot tunnel needs to bear the load transmitted by the main tunnel steel frame and the surrounding rock, it must be strengthened for safety. That is, two I20b steel frames are used and welded together as a reinforcement ring at the end of the horizontal pilot tunnel away from the inner edge of the maximum span of the main tunnel to improve the stress conditions.

[0185] 3. To ensure the footing and securement of the arch frame in the intersection section, after the initial support ring steel frame for the horizontal pilot tunnel was erected, the upper steps were excavated 0.8 meters forward and the section was expanded. Two arch frames were erected along the east side walls of the horizontal pilot tunnel at corresponding locations and welded together. The top elevation of the steel frame was controlled to match that of the initial support steel frame for the horizontal pilot tunnel. A longitudinal horizontal joist was installed at the top of the steel frame, running along the direction of the main tunnel. The joist was constructed from two superimposed welded I20b steel beams. Steel plates were welded to the top of the horizontal joists at 100 cm intervals to facilitate the overlap of the steel frame and joist in the main tunnel section of the intersection.

[0186] 4. The joists and supports were reinforced with additional locking anchors and sprayed with anchors. The triangular area formed between the main tunnel unit steel frame and the horizontal pilot tunnel reinforcement ring steel frame was sprayed and compacted.

[0187] 5. Lower the steel frame of the upper steps of the horizontal pilot tunnel and the steel frame of the main tunnel unit in sequence. To prevent the support from sinking due to insufficient bearing capacity of the base, first tamp the base, weld steel plates to the bottom of the steel frame, and install bottom channel steel.

[0188] ⑹ Personnel training and briefing

[0189] Before underground excavation construction, the plan preparer or technical person in charge shall provide written safety and technical briefings to on-site management personnel, operators, etc., and the briefing person and the person being briefed shall sign the briefing documents.

[0190] 2. Construction process:

[0191] 2.1. Construction technical parameters

[0192]

[0193] 2.2. Flat pilot tunnel construction technology

[0194] 2.2.1 Construction process flow chart, such as Figure 9 As shown:

[0195] 2.2.2 Construction of portal joists

[0196] 1. After the construction of the small duct and grouting of the arch is completed, the joist construction can be carried out. The surveying team will lay out the joist outline according to the design drawing;

[0197] 2. Use a jackhammer to break up the concrete within the joist range and reinforce the exposed right arch frame. Install two C22 locking anchor rods (3m long) on ​​the right arch frame above the joist, and weld the locking anchor rods to the arch frame firmly.

[0198] 3. Install HW250×250×14 steel joists: Before installing the joists, first install the lower positioning steel bars of the joists, then install the joists, weld the joists to the right arch frame firmly, and finally install the upper positioning steel bars of the joists. The joists are positioned using C22 system anchor rods with a length of 1.0m and a spacing of 2.0m.

[0199] 4. After the construction of the supporting beam is completed, a circle of φ42 small advance guide tubes (3.5m long, with a spacing of circumferential × longitudinal = 0.35 × 2.0m) is installed within 120° of the arch.

[0200] 2.2.3 Construction technology for steps at the intersection of the main tunnel and the pilot tunnel

[0201] The first ten steel frames at each intersection are arranged more densely, with a spacing of 0.5m per frame.

[0202] ⑴Construction preparation

[0203] Before the portal is constructed, the steel frame is assembled and its clearance dimensions and connection quality are inspected. Once qualified, the acceptance is notified to all parties. Only after the acceptance is qualified can the next process be carried out. First, the position of the outer contour line of the horizontal pilot tunnel excavation is laid out on the right line (and the dividing lines of the upper and lower steps are marked respectively).

[0204] ⑵Partially demolish the tunnel door, erect steel frame, hang nets, spray concrete

[0205] Step 1: Surveying and setting out. The surveying team lays out the tunnel excavation outline based on the design drawings (and the reserved deformation);

[0206] Step 2: Use a jackhammer to break the concrete on the right side according to the contour line position. After the overall concrete of the upper step is broken, cut off the right side steel frame (retain a connecting bar or steel frame every 0.4m during the cutting process, with a length of not less than 30D to ensure connection with the section main line steel frame on all sides);

[0207] Step 3: Spray 4cm thick C25 concrete, and promptly install small advance conduits, system anchors, and steel mesh. The arch uses A42 small advance conduits and A25 hollow grouting anchors, while the side walls use C22 mortar anchors. The small advance grouting conduits are 3.5m long, 0.35m x 2.0m (circular x longitudinal), the hollow anchors are 3.0m long (the mortar anchors are 3.0m long), and are arranged in a plum blossom pattern of 1.0m x 1.0m (circular x longitudinal). The steel mesh uses A8@200 x 200 steel mesh.

[0208] Step 4: Erect the first and second steel frames of the upper steps with a spacing of 0.5m. After the steel frames are erected, use "7-shaped reinforcement" to weld them to the reserved steel bars or steel sections on the right line (single-sided welding 22D, double-sided welding 11D), and reserve the longitudinal connecting reinforcement and connecting steel plates for the third steel frame (a total of 3 longitudinal connecting steel plates are set at each node).

[0209] Step 5: Install locking anchors and connecting reinforcement (plates): This work is carried out during or after the steel frame is erected. The locking anchors are two C22 threaded steel bars, L = 3.0m, with a downward inclination of 35° / 55°, and are welded firmly to the steel frame. The connecting reinforcement between the arches is a single layer of C22 threaded steel arranged at 1.0m intervals. Finally, a longitudinal connecting steel plate (h = 16mm, 50mm wide) is installed.

[0210] Step 6: Normal section construction: After the construction of the first ten arch frames is completed, the normal section construction will begin. Figure 1 shown.

[0211] 2.2.4 Construction technology of steps under the tunnel gate

[0212] After the tunnel is excavated to 3-5m, the concrete at the side wall is broken. The construction method is the same as the construction of the upper step. Figure 2 As shown;

[0213] 2.2.5 Tunnel Gate Reinforcement

[0214] Use I20b I-beam to weld with the I-beam at the flat guide tunnel portal, and use h=16mm steel plate for reinforcement welding at the interface. The reinforcement range is 10 arch frames on one side of the flat guide tunnel centerline, a total of 20 frames (the rest of the parts are reinforced with 7-shaped reinforcement welding). Use steel plates or steel bars to plug the gap between the right-line I-beam and the reinforced I-beam to weld tightly and weld firmly. Before spray mixing, the excess I-beams can be cut off and then spray mixed. In the process of cutting off the right-line steel frame, it must be cut off in sections. After the first steel arch frame of the flat guide tunnel portal is erected and welded firmly, the right-line steel frame of the next unit can be cut off. Figure 3 As shown;

[0215] 2.3 Construction technology of intersection between horizontal pilot tunnel and main tunnel

[0216] 1. Based on the designed intersection angle between the horizontal pilot tunnel and the main tunnel, the initial support at the intersection was reinforced to provide safe support for the main tunnel across the C2-type large section. This meant that reinforcement began several steel frames before the horizontal pilot tunnel and the main tunnel intersected. Because the horizontal pilot tunnel and the main tunnel are parallel, the last few steel frames were adjusted to be perpendicular to the longitudinal direction of the main tunnel.

[0217] 2. Considering that the last steel frame of the initial support of the horizontal pilot tunnel needs to bear the load transmitted by the main tunnel steel frame and the surrounding rock, it must be strengthened for safety. That is, six I20b steel frames are used and welded together as a reinforcement ring at the end of the horizontal pilot tunnel away from the inner edge of the maximum span of the main tunnel to improve the stress conditions.

[0218] 3. To ensure the location and stability of the arch support in the upper section of the main tunnel at the intersection, after the initial support reinforcement ring steel frame for the horizontal pilot tunnel was erected, the steps were stepped and excavated to the right-line section. Two arch frames were erected along the main tunnel at corresponding locations on the side walls of the horizontal pilot tunnel and welded together. The top elevation of the steel frame was controlled to be consistent with the initial support steel frame for the horizontal pilot tunnel. A longitudinal horizontal joist was installed at the top of the steel frame along the direction of the pilot tunnel. The joist was constructed from two superimposed welded I20b steel sections.

[0219] 4. The joists and supports were reinforced with additional locking anchors and sprayed with anchors. The triangular area formed between the main tunnel unit steel frame and the horizontal pilot tunnel reinforcement ring steel frame was sprayed and compacted.

[0220] 5. Lower the steel frame of the steps on the horizontal guide tunnel to the bottom one by one. To prevent the support from sinking due to insufficient bearing capacity of the base, first tamp the base, weld steel plates to the bottom of the steel frame, and install bottom channel steel.

[0221] 2.4. Initial support construction of main line

[0222] 2.4.1 Construction technology of primary support for mainline

[0223] The horizontal pilot tunnel adopts the "step method" construction, and the principle of "exploration before excavation" should be adhered to during the tunnel construction process; the initial support construction technology, specifically Figure 4 shown.

[0224] 2.4.2 Main process measures

[0225] 2.4.2.1 Earthwork Excavation

[0226] According to the design drawings and construction site conditions, the horizontal pilot tunnel was constructed using conventional blasting; the excavation plan adopted two-step blasting according to the cross-section design;

[0227] 2.4.2.2 Steel frame production

[0228] The steel frame is constructed from I18, I20b, and I-beams. Cut according to the designed dimensions of the steel frame. The length of each section is determined based on the design drawings and on-site construction methods. Deformation must be controlled during bending. After fabrication, the steel frame must be trial-assembled to check for acceptable dimensions and contours.

[0229] 2.4.2.3 Steel frame installation process

[0230] Before installing the steel frame, check the excavation clearance of the face and remove any loose ballast from the steel frame footing. Never allow loose ballast to fill the bottom of the over-excavated arch footing. Pad with timber or steel sections to adjust for height differences. The allowable error for excavation dimensions is ±5cm horizontally and ±5cm vertically. In areas with weak surrounding rock, place channel steel under the steel frame footing to prevent the frame from sinking.

[0231] The segmented steel frames are manually assembled into a whole steel frame at the face of the tunnel, and the connecting bolts must be tightened.

[0232] After the steel frame is installed, the center line error is allowed to be ±3cm, the elevation error is allowed to be ±3cm, and the verticality error is allowed to be ±2°.

[0233] After the steel frame is aligned, precast concrete wedges are used to tighten the steel frame against the rock formation every 2.0m. The steel frame is extended in a staggered manner, with one to three rows of steel frames extended on each side. In weak strata, the steel frame can be extended and connected to the invert at the same time, and concrete can be sprayed immediately. The connection between the extended steel frame and the original steel frame must be firm and accurate.

[0234] The two rows of steel frames were connected with φ22 connecting bars every 1.0m along the circumference of the steel frames to form a whole, improving the longitudinal stress state of the steel frames and preventing overturning. After the steel frames were longitudinally connected, mesh spraying and anchoring support were quickly implemented according to the design. The back of the steel frames was sprayed and filled with concrete to ensure a close contact with the surrounding rock.

[0235] 2.4.2.4. Fabrication and installation of steel mesh

[0236] Steel frame installation process flow chart Figure 5 As shown, the steel mesh is prefabricated outside the tunnel and then transported to the work surface for hanging. If a double-layer mesh is used, the two layers can be staggered. The steel mesh, sprayed concrete, and steel frame form a combined support and are laid continuously. During laying, please note: the steel mesh must be securely welded to the anchor rods, steel frame, or other anchoring devices. The overlap length of the sheet steel mesh must be no less than 250mm. The steel bars must be covered with sprayed concrete, with a protective layer of at least 20mm.

[0237] 2.4.2.5 Small duct construction

[0238] ⑴ The small conduit is made of Φ42 seamless steel pipe. First, cut the steel pipe into the length of the small conduit designed in the drawing. Make a 10cm long cone at one end of the steel pipe and weld a φ8 steel hoop 10cm away from the other end. No holes are opened 50cm away from one end of the steel hoop. The remaining part is symmetrically opened with φ6~8mm grouting holes every 20cm. The grouting holes are arranged in a plum blossom shape. The schematic diagram of the small conduit processing is as follows Figure 7 shown.

[0239] The small conduit is installed using the hole-in-the-hole method. The installation steps are as follows:

[0240] 1) Use a YT-28 pneumatic drill to drill holes. The hole depth depends on the length of the conduit.

[0241] 2) Use a blower to blow out the gravel in the pipe or use a digging hook to dig out the sand and gravel;

[0242] 3) Insert the catheter. If insertion is difficult, use a pneumatic drill to push it in.

[0243] 4) Seal the gaps around the catheter with plasticine and plug the opening with cotton yarn.

[0244] (2) According to the principle of precipitation funnel, start from the top and inject from top to bottom, first inject into the holes without water, then inject into the holes with water. If slurry is leaking or leaking, inject one or several holes at intervals.

[0245] Grouting process see: small tube grouting process flow chart as follows Figure 6 As shown:

[0246] 1) Drilling and driving the small conduit: Before driving the small conduit, position the small conduit according to the design requirements. Using a pneumatic drill as the power source, use a dedicated jack to push the small conduit in. The tail of the small conduit is placed under the steel frame to increase the supporting capacity. After the small conduit is installed, seal the outer opening of the conduit with plastic cement.

[0247] 2) Cover: Before grouting, spray 4cm thick concrete to seal the working surface to prevent leakage.

[0248] 3) Grouting: Use the YSH-3 grouting machine produced in Zhengzhou for grouting, and adopt the dual control principle of grouting volume and grouting pressure to control the grouting time.

[0249] Precautions for small tube grouting

[0250] ① When preparing slurry, operators must wear rubber gloves, goggles, protective hats, and long rubber shoes. Workers are not allowed to wear short-sleeved shirts or shorts to work.

[0251] ② During grouting, workers are not allowed to stand near the grouting port.

[0252] ③ If any abnormality is found in the pressure gauge, stop grouting and find the fault.

[0253] ④ Prepare the slurry immediately before use, discard the remaining slurry, and clean the slurry storage barrel.

[0254] ⑤ Before each grouting, a simple bonding test is performed on site to determine the bonding time and early strength.

[0255] ⑶ Grouting final pressure: 0.5MPa

[0256] Grouting materials and slurry ratio

[0257] Pure cement slurry: The raw material is ordinary cement mixed with 10% micro-expansion agent, with a water-cement ratio of 1:0.6.

[0258] Grouting volume control: The grouting volume of small pipes can be calculated according to the following formula:

[0259] Q=π·R2·L·n·K

[0260] In the formula: R is the slurry diffusion radius, which can be considered as 0.3m; L is the length of the small pipe; n is the porosity of the rock mass; K is the filling coefficient, which is determined according to different geological conditions; generally, the grouting volume at a pressure of 0.35MPa is taken; construction equipment: pneumatic drill, dual-liquid grouting pump.

[0261] 2.4.2.6 Mortar anchor construction

[0262] Mortar anchor construction process Figure 8 As shown, the anchor rod adopts mortar anchor rod. After the air drill is driven into the rock formation to form a hole, the hole is cleaned with high-pressure air, and then grouting is done with a grouting machine, and finally the anchor rod is inserted.

[0263] ⑴ The anchor rods are cut according to the design requirements, and are straightened, derusted and degreased. The anchor rods without pads at the outer ends are first bent into elbows with a length of 0.25 meters.

[0264] ⑵ Ordinary silicate cement should be used, and the sand particle size should not exceed 3mm and should be sieved.

[0265] ⑶ The mortar grade shall not be lower than 20MPa, and the mix ratio shall be cement: sand: water = 100:35:45. A construction mix ratio test must be carried out during construction.

[0266] (4) Drilling should meet the following requirements: The hole diameter should be well matched to the pipe diameter. The hole diameter should be 15mm larger than the pipe diameter. The hole position tolerance should be ±15mm to 50mm. The hole depth tolerance should be ±50mm. The anchor length should be no less than the designed length. After drilling, flush the hole with high-pressure air.

[0267] 5. The mortar should be mixed evenly and adjusted for workability. Use as you mix. Slowly and evenly withdraw the grouting tube, always keeping the grouting tube mouth buried in the mortar to prevent cavities in the slurry. The grouting volume should be approximately larger than the required volume. After the grouting tube is completely withdrawn, immediately force the anchor rod into the hole. The length of the rod inserted into the hole must not be less than the designed length. The grouting is fully filled by determining whether slurry is squeezed out of the hole.

[0268] ⑹ After the rod body is in place, it should be welded to the steel grid immediately to prevent the anchor rod from shifting. Do not collide it randomly until the slurry reaches 70% of the design strength. No heavy objects should be hung within three days.

[0269] ⑺ Spray concrete in time after excavation and close the initial support as soon as possible. During the excavation process, pay close attention to the deformation of the anchor rods, and the cracking and bulging of the sprayed concrete layer to understand the dynamics of the surrounding rock and adjust the excavation and support parameters in time.

[0270] 2.4.2.7 Hollow Anchor Construction

[0271] ⑴ Construction preparation: Before construction, prepare a concrete pump, a 7655 type pneumatic rock machine, a large diameter anchor rod and a skilled machine operator.

[0272] ⑵ Drilling: Use a trolley or handheld rock drill to drill the anchor rod with the drill bit installed to the designed depth or slightly deeper than the designed depth.

[0273] ⑶ Drilling: Remove the drilling rig and pull out the anchor rod. Then manually drive the hollow anchor rod into the eye hole (the drill hole is slightly larger than the anchor rod eye hole) and install the grouting plug. Install it in the anchor hole 25cm away from the hole mouth. In special cases such as high grouting pressure or surrounding rock fragmentation, you can also use anchor runner agent to seal the hole.

[0274] (4) Connection: Connect the grouting joint and the tail end of the anchor rod through the connecting sleeve to ensure close contact and prevent leakage during the grouting process.

[0275] ⑸ Grouting: Start the grouting machine and stop when the grouting is full and the pressure reaches the design value. The grouting pressure is determined according to the design parameters and the performance of the grouting machine.

[0276] 2.4.2.8. Sprayed concrete construction

[0277] ⑴Construction preparation

[0278] Remove loose rocks and rock debris, and flush the sprayed surface with air and water. If there is water leakage, it should be properly handled; check the excavation section size and set signs to control the thickness of the sprayed concrete; check machinery and equipment and air, water, electricity and other pipelines, and test run them to ensure they are qualified.

[0279] ⑵Operation process control

[0280] The spraying concrete operation is carried out immediately after the support is completed. In order to ensure the quality of the sprayed concrete, reduce the rebound rate and reduce dust, the wet spraying method is used for the sprayed concrete construction.

[0281] The working air pressure of the jet machine must meet the pressure at the nozzle, which is about 0.1MPa.

[0282] The spraying of concrete is carried out in sections and pieces in sequence; the spraying operation is from bottom to top, first spraying the concrete between the steel frame support and the arch wall, and then spraying the concrete between the two arch frames.

[0283] If the rock surface is uneven, spray the concave areas first, then spray upward. During spraying, the nozzle beam moves in a rotating trajectory, one circle at a time, in a serpentine motion. The rotation radius is generally around 15 cm, with each serpentine swing of 1.5 to 2.0 m. When spraying the second vertical row, start sequentially from the starting point of the first row, with a 2 to 3 cm overlap between rows. The beam rotation speed should be uniform, not too slow or too fast.

[0284] The spraying concrete operation should maintain a continuous and uniform supply of materials. The spraying concrete nozzle should be perpendicular to the sprayed surface, and the distance between the nozzle and the sprayed surface should be 0.6 to 1.0m.

[0285] Concrete is sprayed in layers. The initial spraying serves to seal the working surface and prevent weathering. The thickness of the spraying layer is generally 3-5 cm and should be applied immediately after excavation. The subsequent spraying layer should be applied after the previous layer has completely set. If spraying is performed more than 1 hour after final setting, the sprayed layer surface must be cleaned with water first.

[0286] Under normal circumstances, the rebound of shotcrete should not exceed 15% on the side walls and 25% on the top. When shotcrete is sprayed immediately following blasting excavation, the time from final setting of the concrete to the next blasting cycle should be no less than 3 hours. Curing should begin 2 hours after the shotcrete has finally set.

[0287] ⑶ Screen spraying process:

[0288] ⑷Wet spraying process:

[0289] After the sprayer is installed, first fill it with water, ventilate it, and clean the debris in the pipe. At the same time, use high-pressure air to blow the sprayed surface to remove dust.

[0290] When conditions permit, the nozzle should be fixed on a manipulator for spraying operations; when conditions do not permit and the nozzle needs to be manually controlled, two people should operate the nozzle together.

[0291] The mixture of shotcrete is mixed with a forced concrete mixer. The mixing time is not less than 2 minutes. The slump of the discharge should be controlled at 10 to 18 cm. Continuous feeding should be ensured.

[0292] The spraying route of shotcrete should be from bottom to top, in an "S" shape; when spraying, the nozzle makes continuous circular motion and moves forward in a spiral, with the latter circle pressing one-third of the former circle.

[0293] The pressure at the sprayer nozzle is required to be around 0.1 MPa. The nozzle should be kept perpendicular to the sprayed surface. If the sprayed surface is covered by steel mesh or grid steel frame, the nozzle can be slightly tilted by 10° to 20°. Spraying concrete operations must comply with the following points on the basis of meeting the relevant provisions of the "Technical Specification for Rock and Soil Anchors and Sprayed Concrete Support Engineering" (GB 50086-2015):

[0294] The initial spraying of concrete follows the palm face. Before the re-spraying, the installation of the advance small duct, steel mesh and grid steel frame shall be completed according to the design requirements.

[0295] Before spraying construction, a trial spraying should be carried out. After the trial spraying is qualified, the spraying construction can be started. Large panels should be sprayed according to regulations and inspection specimens should be made.

[0296] After each shotcrete spraying, check the thickness immediately. If the thickness is not enough, re-spray to reach the designed thickness. It is strictly forbidden to use rebound material as shotcrete.

[0297] Firmly implement the "Four No's" system, i.e., do not proceed with the face until the concrete spraying process is complete; do not proceed until the thickness of the sprayed concrete is insufficient; do not proceed until problems are found and resolved after spraying; and do not proceed until monitoring results indicate unsafe conditions. Responsibilities for the above system are assigned to individuals, and records are kept in the construction log for inspection. The project management department is responsible for supervision. Wet spraying concrete is made by adding water to the raw materials for spraying concrete according to the mix ratio to make concrete. The concrete is then fed into the sprayer hopper. The sprayer piston sends the concrete into the mixing chamber, where it is mixed with compressed air and then enters the spraying pipe. Liquid accelerator is added to the nozzle, and the mixed material beam is ejected from the nozzle to the sprayed surface.

[0298] Effect of implementation case: In the implementation case, the construction method of the present invention was used to add a tunnel working surface to the C2 super-large section in the Fu-Sen section. The two ends of the C2 super-large section were excavated relative to each other, so that the tunnel of this section was completed 2 months ahead of schedule, saving construction time and enabling the entire Fu-Sen section to be completed ahead of schedule.

Claims

1. A method for constructing a horizontal pilot tunnel in a karst landform tunnel chamber with a large cross-section, characterized in that: The following steps are involved: Step 1), geological survey before construction; Step 2), advanced geological forecast; Step 3), excavation of horizontal pilot tunnel; Step 4), initial support; Step 5), cave and cavity treatment; Step 6), drainage measures; Step 7), monitoring and measurement; Step 8), ecological restoration.

2. The method for constructing a horizontal pilot tunnel in a karst landform super-large cross-section tunnel chamber according to claim 1, characterized in that: Step 1) Pre-construction geological survey: Detailed survey of the geological conditions of the karst landform area where the chamber is located, including the distribution, scale and filling of caves, cavities and underground rivers, to provide an accurate geological basis for the formulation of the construction plan; Step 2) of the advanced geological forecast is to use a variety of advanced geological forecasting methods, including geological radar, TSP seismic wave method and advanced drilling, to predict the geological conditions ahead, identify unfavorable geological bodies in advance, and adjust the construction plan in a timely manner; Step 3) of horizontal pilot tunnel excavation is to select an appropriate excavation method, including the bench method, CD method, and CRD method, based on geological conditions and cross-section size, and strictly control the excavation footage and blasting parameters to reduce disturbance to the surrounding rock; The step 4) initial support is to promptly implement initial support, including anchor rods, shotcrete and steel arches, to improve the stability of the surrounding rock; Step 5) of treating caves and cavities is as follows: for caves and cavities encountered during construction, different treatment measures are taken according to their size and filling conditions, including backfilling, spanning and reinforcement; Step 6) of the drainage prevention and control measures is to set up a complete drainage prevention and control system, including a waterproof layer, drainage blind pipes and drainage ditches, to prevent groundwater from affecting construction and operation; Step 7) monitoring and measuring is to monitor and measure the surrounding rock deformation and support structure stress during the construction process in real time, and adjust the construction parameters in a timely manner according to the monitoring data to ensure construction safety; The aforementioned step 8) ecological restoration is: after the construction is completed, timely ecological restoration of the construction area is carried out to restore vegetation and landforms.

3. The method for constructing a horizontal pilot tunnel in a karst landform super-large cross-section tunnel chamber according to claim 1 or 2, characterized in that: The step 2) advanced geological prediction includes: 1) comprehensive advanced geological prediction: based on geological survey data, strengthening advanced geological prediction and construction geological work in karst-developed sections, accurately understanding the karst development morphology, cavity filling conditions, and groundwater development conditions in front of the tunnel face, and preventing karst disasters including sudden water inrush, mud inrush, and sudden collapse; in karst-developed sections, comprehensive advanced geological prediction should be conducted based on geological survey methods, mainly using advanced drilling methods, combined with various geophysical exploration methods, and adopting the method of macro-forecast guiding micro-forecast and long-distance forecast guiding medium- and short-distance forecast; 1.2) Geological radar scanning: Using the elastic wave reflection method (TSP203), a relatively detailed forecast of the location, size, nature, and groundwater conditions of unfavorable geological bodies within 30 to 100 meters in front of the tunnel face is conducted. This is conducted every 100 meters and more frequently when abnormal conditions are detected. 1.2) Medium- and short-range advance prediction: Using electromagnetic wave reflection or infrared detection, a more detailed forecast of geological conditions within 30 meters in front of the tunnel face, including karst morphology, filling properties, and the presence of water in karst cavities, is provided. 1.3) Advance horizontal drilling: Verify the predictions of TSP203 and infrared detection using advance horizontal drilling and face-deepening blastholes. One to five advance horizontal drill holes are arranged in each section, with one hole used for core extraction. A 25-meter cycle is used, with a single hole length of 30 meters and a 5-meter overlap between adjacent detection holes. In the event of anomalies, interpretation is conducted based on the TSP203 and infrared detection prediction results, and appropriate drilling increases or lengthens are implemented. Comprehensively analyze and interpret the data obtained from various forecasting methods, verify each other, and make predictions and judgments based on the geological conditions, development laws, trends and precursors revealed by the tunnel face; optimize and adjust support measures accordingly based on the results of comprehensive advanced geological forecasting; 2) Karst treatment: Tunnel excavation and support in karst-developed areas should be comprehensively studied based on the karst morphology and size, filling conditions, and relative position of the tunnel determined by comprehensive advanced geological forecasts; 2.1) In principle, the measures of pre-support with pipe sheds and subsequent excavation should be adopted for filling karst caves; 2.2) For small karst cavities, treatment should be conducted based on the size and morphology of the karst caves after excavation. Caves at the tunnel bottom should be backfilled if possible, depending on the hydrological conditions; otherwise, beams and slabs should be used to span the caves. Karst caves in the side walls and arches should generally be backfilled with concrete. 2.3) For karst areas with the risk of water and mud inrush, pre-treatment measures such as advanced curtain grouting and advanced peripheral grouting should be adopted for water-rich areas of the geological structure based on geological survey data; 2.4) Special safety measures should be formulated for the cleaning of cave fillings and the treatment of cave surrounding rocks in accordance with the actual conditions on site; 2.5) During the construction of inter-regional karst tunnels, drainage and pumping design should be carried out based on the predicted water inflow; 3) Surface settlement control measures: Design and construction should be based on the principle of pre-excavation followed by reinforcement of the surrounding rock and roof protection. Pre-excavation grouting of small guide tubes plus medium or large pipe sheds will be used to reinforce the surrounding rock ahead. Mining tunnels adopt information technology for design and construction. During construction, attention should be paid to and strengthened monitoring and measurement work, which should be carried out throughout the construction process. Information should be fed back in a timely manner to guide design and construction. Minimize disturbance to the surrounding rock during construction and adopt short advances; strictly control the length of steps; if the steps are long, a temporary inverted arch should be used to seal the bottom when necessary; anchor rods or anchor pipes should be used to reinforce the arch feet of the steel frame; After excavation, initial support or temporary support should be carried out in a timely manner, with close connection between the working procedures, early arch suppression and early closure into a ring; when the stability of the tunnel face is poor, concrete should be sprayed to seal the working face at any time; During construction, drainage should be strengthened and, if necessary, pre-drilled holes should be installed under the tunnel face for local drainage. This includes conducting necessary monitoring and measurement of the surrounding rock, supporting structures, and ground buildings during the initial construction phase after completion, so as to obtain timely information and take timely measures to control ground subsidence. 4) Monitoring and measurement of tunnel construction using the mining method: On-site monitoring and measurement are divided into mandatory and optional items. Mandatory items include observations inside and outside the tunnel, geological description of the tunnel face, measurement of vault subsidence, measurement of peripheral displacement convergence, and measurement of surface subsidence. Optional items include measurement of the internal forces of the primary support and secondary lining, the axial force of the anchor bolts, the pressure between the surrounding rock and the support, and the internal displacement of the surrounding rock. To ensure the safety of the tunnel's surrounding rock, adjacent buildings, and caverns, during construction, monitoring and measurement work should be strengthened. A dedicated monitoring and measurement team should be established, and this work should be entrusted to an experienced and qualified unit. The team should track the deformation of the surrounding rock, foundation pit, and existing buildings throughout the entire process, analyze, judge, and predict possible situations that may arise during construction, and adjust support parameters and excavation methods in a timely manner based on the measurement results, thus achieving dynamic design and information-based construction to ensure construction safety. 5) Intersection opening treatment measures: 5.1) Based on the designed intersection angle between the pilot tunnel and the main tunnel, reinforce the initial support at the intersection to ensure safe support for the subsequent main tunnel crossing the C2-type large section. This means that reinforcement should begin several steel frames before the pilot tunnel and the main tunnel intersect. Because the angle between the pilot tunnel and the main tunnel is 90°, the last few steel frames should be adjusted to be perpendicular to the longitudinal direction of the main tunnel. 5.2) Considering that the last steel frame of the initial support of the horizontal pilot tunnel needs to bear the load transmitted by the main tunnel steel frame and the surrounding rock, it must be reinforced for safety. Specifically, two I20b steel frames are welded together to form a reinforcement ring at the end of the horizontal pilot tunnel, at the inner edge of the maximum road span of the main tunnel, to improve the load-bearing conditions. 5.3) To ensure the footing and firmness of the arch frame on the main tunnel section of the intersection, after the initial support reinforcement ring steel frame of the horizontal pilot tunnel was erected, the upper step was continued forward for 0.8 meters and the section was expanded. Two arch frames were erected along the east direction of the corresponding positions of the side walls on both sides of the horizontal pilot tunnel and welded together. The elevation of the top of the steel frame was controlled to be consistent with the initial support steel frame of the horizontal pilot tunnel. A longitudinal horizontal joist was set on the top of the steel frame along the forward direction of the main tunnel. The joist was made of two superimposed welded I20b steels. Steel plates were welded on the top of the horizontal joists at intervals of 100 cm to facilitate the overlap of the arch steel frame and the joist of the main tunnel of the intersection section. 5.4) The joists and supports are reinforced with additional locking anchors and sprayed with anchors for protection. The triangular area formed between the main tunnel unit steel frame and the horizontal pilot tunnel reinforcement ring steel frame is sprayed and compacted. 5.5) Lower the steel frame of the steps on the horizontal pilot tunnel and the main tunnel unit steel frame to the bottom in sequence; to prevent the support from sinking due to insufficient bearing capacity of the base, first compact the base, weld steel plates to the bottom of the steel frame, and install bottom channel steel.

4. The method for constructing a horizontal pilot tunnel in a karst landform super-large cross-section tunnel chamber according to claim 1, characterized in that: The construction method for excavating the horizontal pilot tunnel in step 3) is as follows: 3.1 setting up the tunnel portal operating platform; 3.2 measuring and setting out; 3.3 drilling; 3.4 blasting; 3.5 ventilation; 3.6 troubleshooting; 3.7 slag removal; and 3.8 support.

5. The method for constructing a horizontal pilot tunnel in a karst landform super-large cross-section tunnel chamber according to claim 4, characterized in that: The construction method of the steps at the intersection of the main tunnel and the horizontal pilot tunnel in the construction process of the horizontal pilot tunnel excavation is as follows: the first ten steel frames at the intersection are densely arranged with a spacing of 0.5m / frame; 1) Construction preparation: Before the construction of the tunnel portal, assemble the steel frame first, check its clearance size and connection quality, and notify all parties for acceptance after passing the inspection. Only after passing the inspection can the next process be carried out; first, lay out the position of the outer contour line of the horizontal pilot tunnel excavation on the right line, and mark the dividing lines of the upper and lower steps respectively; 2) Demolish the tunnel portal in sections, erect the steel frame, hang the net, and spray concrete.

6. The method for constructing a horizontal pilot tunnel in a karst landform super-large cross-section tunnel chamber according to claim 4, characterized in that: The construction method for the intersection of the horizontal pilot tunnel and the main tunnel in the construction process of the horizontal pilot tunnel excavation is as follows: 1) according to the designed intersection angle of the horizontal pilot tunnel and the main tunnel, the initial support at the intersection is strengthened to provide a safe support guarantee for the main tunnel to cross the C2 type large section; that is, the reinforcement of the first few steel frames begins before the intersection of the horizontal pilot tunnel and the main tunnel. Since the horizontal pilot tunnel is parallel to the main tunnel, the last few steel frames are adjusted to be perpendicular to the longitudinal direction of the main tunnel; 2) Considering that the last steel frame of the initial support of the horizontal pilot tunnel needs to bear the load transmitted by the main tunnel steel frame and the surrounding rock, it must be reinforced for safety. That is, six I20b steel frames are welded together as a reinforcement ring at the end of the horizontal pilot tunnel, at the inner edge of the maximum road span of the main tunnel, to improve the load-bearing conditions; 3) To ensure the footing and firmness of the arch frame on the upper section of the main tunnel at the intersection, after the initial support reinforcement ring steel frame of the horizontal pilot tunnel is erected, the step is stepped and excavated to the right line section. Two arch frames are erected along the main tunnel at the corresponding positions on the side walls of the horizontal pilot tunnel and welded together. The top elevation of the steel frame is controlled to be consistent with the initial support steel frame of the horizontal pilot tunnel. A longitudinal horizontal joist is installed on the top of the steel frame along the direction of the pilot tunnel. The joist is made of two superimposed welded I20b steel sections. 4) The joists and supports are reinforced with locking anchor rods and anchor spraying is carried out for protection; the triangular area formed between the main tunnel unit steel frame and the horizontal pilot tunnel reinforcement ring steel frame is fully sprayed and compacted; 5) Lower the steel frames on the steps of the horizontal guide tunnel one by one; to prevent the support from sinking due to insufficient bearing capacity of the base, first compact the base, weld steel plates to the bottom of the steel frame, and install bottom channel steel.

7. The method for constructing a horizontal pilot tunnel in a karst landform super-large cross-section tunnel chamber according to claim 1 or 2, characterized in that: In the construction process of the primary support for the mainline, the horizontal pilot tunnel is constructed using the "step method". During the tunnel construction process, the principle of "exploration before excavation" should be adhered to. The main operations include 1) earthwork excavation: according to the design drawings and construction site conditions, the horizontal pilot tunnel is constructed using conventional blasting. The excavation plan adopts two-step blasting construction according to the cross-section design; 2) Steel frame fabrication: The steel frame is made of I18, I20b, and I-beams. Cutting is done according to the designed dimensions of the steel frame. The length of each section of the steel frame is determined by the design drawing and on-site construction methods. Deformation of the steel frame must be controlled during bending. After the steel frame is manufactured, a trial assembly must be carried out to check whether the size and outline of the steel frame are qualified. 3) Steel frame installation process: Before installing the steel frame, the excavation clearance of the face should be checked and the ballast at the foot of the steel frame should be removed. Ballast is not allowed to be filled at the bottom of the over-excavated arch foot. Square wood or steel should be used to adjust the height difference. The excavation size tolerance is ±5cm in the horizontal direction and ±5cm in the height difference. In areas with weak surrounding rock, channel steel should be used to pad the foot of the steel frame to prevent the steel frame from sinking. The segmented steel frame should be manually assembled into a whole steel frame at the face of the face, and the connecting bolts should be tightened. After the steel frame is installed, the centerline tolerance is ±3cm, the elevation tolerance is ±3cm, and the verticality tolerance is ±2°. After the steel frame is calibrated, the tolerance should be ±5° every 2.0 Use prefabricated concrete wedges to wed the steel frame tightly against the rock formation. Extend the steel frame by staggering on one side, extending 1 to 3 rows of steel frames at a time. In soft strata, the steel frame can be extended and connected to the invert at the same time, and concrete can be sprayed in time. The connection between the extended steel frame and the original steel frame must be firm and accurate. Use φ22 connecting bars every 1.0m along the circumference of the steel frame to connect them as a whole, improve the longitudinal stress state of the steel frame, and prevent it from overturning. After the steel frame is connected longitudinally, quickly carry out the mesh spraying and anchoring support according to the design, and spray the back of the steel frame with concrete to make it close to the surrounding rock. 4) Fabrication and installation of steel mesh: The steel mesh is prefabricated into mesh sheets outside the tunnel and then transported to the work surface for hanging. If a double-layer mesh is used, the two layers can be staggered. The steel mesh, sprayed concrete, and steel frame form a combined support and are laid continuously. When laying, please note that the steel mesh must be firmly welded to the anchor rods, steel frame, or other anchoring devices, and the overlap length of the sheet steel mesh must be no less than 250mm. The steel bars must be covered with sprayed concrete, with a protective layer of at least 20mm. 5) Small conduit construction: The small conduit uses Φ42 seamless steel pipe. First, cut the steel pipe into the small conduit length designed in the drawing, make a 10cm long cone at one end of the steel pipe, and weld a φ8 steel hoop 10cm away from the other end; no holes are drilled 50cm away from one end of the steel hoop, and φ6~8mm grouting holes are symmetrically drilled every 20cm in the remaining part. The grouting holes are arranged in a plum blossom shape according to the principle of precipitation funnel. Start from the top and pressurize from top to bottom, first pressurize the holes without water, and then pressurize the holes with water; in case of slurry leakage or slurry leakage, pressurize one or several holes at intervals; 6) Mortar anchor construction: The anchor is made of mortar anchor. After the pneumatic drill is driven into the rock formation to form a hole, the hole is cleaned with high-pressure air, and then grouting is done with a grouting machine before the anchor is inserted. (1) The anchor rods are cut according to the design requirements, and are straightened, derusted and degreased. The anchor rods without pads at the outer ends are first bent into elbows with a length of 0.25 meters. ⑵Use ordinary silicate cement, the sand particle size should not be greater than 3mm, and should be sieved; (3) The mortar grade shall not be less than 20MPa, and the mix ratio shall be cement: sand: water = 100:35:

45. A construction mix ratio test must be carried out during construction; (4) The drilling should meet the following requirements: the hole diameter should match the pipe diameter; the hole diameter should be 15mm larger than the pipe diameter; the hole position tolerance should be ±15mm to 50mm; the hole depth tolerance should be ±50mm; the anchor length should not be less than the designed length; after the hole is drilled, flush the hole with high-pressure air; ⑸ The mortar should be mixed evenly and its workability should be adjusted. It should be used immediately after mixing. The grouting pipe should be pulled out slowly and evenly, and the grouting pipe mouth should always be buried in the mortar to avoid cavities in the slurry. The grouting volume should be approximately larger than the required volume. After the grouting pipe is completely pulled out, the anchor rod should be forcibly inserted into the hole immediately. The length of the rod inserted into the hole should not be shorter than the designed length. Whether the grouting is full should be judged by whether there is slurry squeezed out of the hole mouth. (6) Immediately weld the rod to the steel grid after it is in place to prevent the anchor rod from shifting. Do not collide with it until the slurry reaches 70% of the design strength. Do not hang heavy objects on it within three days.

7. Spray concrete promptly after excavation and close the initial support as soon as possible. During the excavation process, pay close attention to the deformation of the anchor rods, and the cracking and bulging of the sprayed concrete layer, so as to understand the dynamics of the surrounding rock and adjust the excavation and support parameters in time; 7) Hollow anchor construction: (1) Construction preparation: Before construction, prepare a concrete pump, a 7655 type pneumatic rock machine, a large diameter anchor, and a skilled machine operator; ⑵ Drilling: Use a trolley or handheld rock drill to drill the anchor rod with the drill bit installed to the designed depth or slightly deeper than the designed depth; ⑶ Drilling: Remove the drilling rig and pull out the anchor rod. Then manually drive the hollow anchor rod into the eye hole (the drill hole is slightly larger than the anchor rod eye hole) and install the grouting plug. Install it in the anchor hole 25 cm away from the hole mouth. In special cases such as when the grouting pressure is large or the surrounding rock is broken, you can also use an anchor runner to seal the hole. (4) Connection: Connect the grouting joint and the tail end of the anchor rod through the connecting sleeve to ensure close contact and prevent leakage during the grouting process; ⑸ Grouting: Start the grouting machine and stop when the grouting is full and the pressure reaches the designed value. The grouting pressure is determined according to the design parameters and the performance of the grouting machine; Spray concrete construction includes the following procedures: ⑴ Construction preparation: remove loose rocks and rock chips, and flush the sprayed surface with wind and water. If there is water leakage, it should be properly handled; check the excavation section size, and set signs to control the thickness of the sprayed concrete; check machinery and equipment and air, water, electricity and other pipelines, and test run them to pass the test; ⑵ Operation process control procedure; ⑶ Net spraying procedure; ⑷ Wet spraying procedure.

8. The method for constructing a horizontal pilot tunnel in a karst landform super-large cross-section tunnel chamber according to claim 7, characterized in that: The operation process control includes: the spraying concrete operation is carried out immediately after the support frame is completed; in order to ensure the quality of the sprayed concrete, reduce the rebound rate and reduce dust, the wet spraying method is used for the spraying concrete construction; the working air pressure of the sprayer must meet the pressure of 0.1MPa at the nozzle; The spraying of concrete is carried out in sections and pieces in sequence; the spraying operation is from bottom to top, first spraying the concrete between the steel frame support and the arch wall, and then spraying the concrete between the two arch frames; If the rock surface is uneven, spray the concave areas first and then spray upwards. During spraying, the nozzle beam moves in a rotating trajectory, one circle at a time, and in a serpentine motion vertically. The rotation radius is about 15 cm, and the amplitude of each serpentine motion is 1.5 to 2.0 m. When spraying the second vertical row, start sequentially from the starting point of the first row, with an overlap of 2 to 3 cm between rows. The beam rotation speed should be uniform. The shotcrete operation should maintain a continuous and uniform feed. The shotcrete nozzle should be perpendicular to the sprayed surface, and the distance between the nozzle and the sprayed surface should be 0.6 to 1.0 m. Concrete is sprayed in layers, and the initial spraying is to seal the working surface to prevent weathering; the thickness of the spraying layer is 3 to 5 cm, and it is carried out immediately after excavation; the next layer of spraying should be carried out after the previous layer of concrete has finally set. If spraying is carried out 1 hour after final setting, the surface of the spraying layer must be cleaned with water first; under normal circumstances, the rebound of the sprayed concrete is not more than 15% for the side walls and not more than 25% for the top; when the spraying operation is followed by blasting excavation of the excavation face, the time from final setting of the concrete to the next cycle of blasting should not be less than 3 hours; curing begins 2 hours after the final setting of the sprayed concrete.

9. The method for constructing a horizontal pilot tunnel in a karst landform super-large cross-section tunnel chamber according to claim 7, characterized in that: The wet spraying process is as follows: after the spraying machine is installed, first fill it with water, ventilate it, and clean the debris in the pipe; at the same time, use high-pressure air to blow the sprayed surface to remove the dust on the sprayed surface; When conditions permit, the nozzle should be fixed on a manipulator for spraying operations. When conditions do not permit and the nozzle must be manually controlled, two people should operate the nozzle together. The mixture of shotcrete should be mixed with a forced concrete mixer for at least 2 minutes. The slump of the discharge should be controlled at 10-18 cm, and continuous feeding should be ensured. The spraying route of shotcrete should be from bottom to top, in an "S" shape. During spraying, the nozzle should make continuous circular motion and form a spiral forward, with the last circle pressing one-third of the previous circle. The pressure at the spray nozzle is required to be 0.1Mpa, and the nozzle should be kept perpendicular to the sprayed surface. If the sprayed surface is covered by steel mesh or grid steel frame, the nozzle can be slightly tilted by 10° to 20°. When spraying concrete, observe the following points: The initial spraying of concrete follows the tunnel face closely. Before the re-spraying, the installation of the advance small duct, steel mesh and grid steel frame shall be completed according to the design requirements. Before spraying construction, a test spraying shall be carried out. After the test spraying is qualified, the spraying construction shall be started. The large plate shall be sprayed according to the regulations and the test specimens shall be made; After each concrete spraying, check the thickness immediately. If the thickness is not enough, re-spray to reach the designed thickness. It is strictly prohibited to use rebound material as injection material.

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