Construction method for diversion tunnel adverse geological disasters

Abstract

The application discloses a construction method for adverse geological disasters of a diversion tunnel, and comprises the following steps: stabilizing the upstream and downstream surrounding rocks of the adverse geological disaster area; supporting and constructing the upstream and downstream surrounding rocks of the adverse geological disaster area; processing the adverse geological disaster area: closing the adverse geological disaster area, reinforcing the support of a composite arch support stress body, deep support and shallow support; establishing a whole support arch stress body formed by a steel arch frame, model spraying concrete and secondary backfilling; processing the layered blasting excavation of the rock plug section of the tail water main tunnel intersecting with the diversion tunnel; and establishing a safety detection and early warning system to timely monitor the surrounding rocks of the adverse geological disaster area of the diversion tunnel to guide the on-site construction. The construction method guarantees the stability of the adverse geological disaster area, the stability of the surrounding rocks during excavation, and the safety and stability of the large-scale collapse area of the partition wall in the diversion tunnel, especially in the super-large section diversion tunnel.

Description

Technical Field

[0001] This invention relates to the field of construction in the context of adverse geological hazards. More specifically, this invention relates to a construction method for diversion tunnels in the event of minor geological hazards. Background Technology

[0002] In the construction of power stations in high mountains and deep valleys, tunnel diversion is generally used. With the continuous expansion of hydropower station construction, the size and complexity of underground diversion tunnels are increasing, as are the geological conditions. Furthermore, due to energy shortages, the demand for early commissioning of hydropower projects is becoming increasingly strong, leading to a trend towards shorter construction cycles. The diversion tunnel is a crucial part of the river closure process, typically requiring extremely tight deadlines. How to adapt to extremely complex geological conditions within a very short construction period has become an unavoidable problem in the construction of underground tunnels today.

[0003] Based on nearly a decade of experience in underground tunnel construction, including the Xiluodu left bank diversion tunnel, Jinping diversion tunnel, and Guandi underground powerhouse, technological innovations were carried out during the construction of the Wudongde hydropower station's right bank diversion tunnel. Addressing the unique thin- to extremely thin-layered limestone geological conditions of Wudongde, the rock mass unloading and stabilization issues after excavation were extremely prominent. The surrounding rock had a short self-stabilization time, making it prone to collapses, bedding spalling, and other damage. During construction, issues such as disturbance and deformation of large-section rocks, rock mass reinforcement, prevention of large collapses, and construction safety were all urgent problems to be solved. Summary of the Invention

[0004] One object of the present invention is to solve at least the above-mentioned problems and to provide at least the advantages that will be described later.

[0005] Another objective of this invention is to provide a construction method for diversion tunnels with adverse geological hazards. This method ensures the stability of the adverse geological hazard area in the diversion tunnel, the stability of the surrounding rock during excavation, and strictly controls the excavation progress to ensure excavation quality. For a series of special adverse geological conditions encountered during construction, targeted reinforcement treatment processes are adopted, and a "monitoring, measurement, analysis, and feedback system" is established to verify the rationality of the construction process and design parameters, thereby ensuring the safety and stability of the adverse geological hazard area in the diversion tunnel.

[0006] To achieve these objectives and other advantages according to the present invention, a construction method for diversion tunnels in adverse geological conditions is provided, comprising the following steps:

[0007] Step 1: Stabilization of surrounding rock upstream and downstream of the adverse geological hazard area: Construction of support for the surrounding rock upstream and downstream of the adverse geological hazard area;

[0008] Step 2, Construction Treatment of Adverse Geological Hazard Areas: The adverse geological hazard areas are sealed off, reinforced with composite arch support, and subjected to deep and shallow support.

[0009] Step 3: Construct the integral support arch load-bearing body formed by steel arch frame - shotcrete - secondary backfilling;

[0010] Step 4: Layered blasting excavation of the rock-blocked section of the tailrace main tunnel intersecting with the diversion tunnel;

[0011] Step 5: Establish a safety monitoring and early warning system to monitor the surrounding rock in the adverse geological hazard zone of the diversion tunnel in a timely manner to guide on-site construction.

[0012] Preferably, step one specifically includes: setting multiple rows of through anchor cables on the partition wall of the diversion tunnel, setting multiple rows of anchor piles on the left side wall, and setting multiple rows of anchor piles on the right side wall.

[0013] Preferably, step two specifically includes the following steps:

[0014] S1, Sealing treatment: The cracked parts of the shotcrete layer on the upstream and downstream sidewalls of the adverse geological disaster area are initially sealed by spraying 15cm thick C25 coarse fiber concrete; the sidewalls of the adverse geological disaster area are protected by slag removal outside the hole, and steel gabions are added to the side of the access road in the adverse geological disaster area to protect the foot of the slag pile.

[0015] S2, Composite arch support for reinforced support: Steel supports are erected inside the diversion tunnel, and a system of shotcrete is applied.

[0016] S3, Deep and shallow support: For sections with adverse geological hazards, reinforced support treatment is carried out by combining deep and shallow anchor piles and anchor cables. At the same time, anchor piles and anchor cable holes are used to carry out systematic consolidation grouting construction on the entire slope.

[0017] Preferably, step three specifically includes the following steps:

[0018] 1) Based on the structural characteristics of the diversion tunnel cross-section, the middle layer of the diversion tunnel is excavated in a thin layer from top to bottom, and the rear two sides are excavated in an expanded manner; and the surrounding rock and working face of the excavation area are sealed in a timely manner.

[0019] 2) After sealing, anchor bolts are used to support the surrounding rock, and steel arch frames are installed, steel mesh is hung, and formwork is installed without dismantling.

[0020] 3) Apply shotcrete between the installed steel arch frame and the surrounding rock surface, and then fill the gap with secondary backfill grouting or pumped concrete, so that the steel arch frame, shotcrete and backfill concrete form an integral supporting arch load-bearing body.

[0021] Preferably, the anchor cables are arranged in 4 rows with a spacing of 4×4m between rows; the anchor piles on the left and right side walls are arranged in 6 rows with a spacing of 2.5×2.5m between rows.

[0022] Preferably, the steel gabion has dimensions of 3×1×1m, a spacing of 20cm×20cm between steel bars, a main bar diameter of 22mm, and a web bar diameter of 14mm.

[0023] Preferably, the step S2, which involves erecting steel supports within the diversion tunnel, specifically includes: installing 120b I-beams on the sidewalls of the diversion tunnel, which are welded to anchor rods installed in the sidewalls; longitudinally connecting channel steels are installed between the I-beams, with a circumferential spacing of 1.5m.

[0024] Preferably, in step 1), timely sealing of the surrounding rock and working face of the excavation area includes: spraying a 5-8cm layer of steel fiber reinforced concrete onto the surrounding rock and working face for sealing.

[0025] Preferably, step four specifically includes: first, shallow hole trenching is carried out in the middle section of 6-7m, and a 4-5m protective layer is reserved on both sides. The protective layer is excavated by vertical light blasting. All vertical light blasting holes are set up with movable standard templates, drilled with a hand drill, and vertical light blasting is carried out. The amount of explosive per blast is controlled within 10kg.

[0026] The present invention has at least the following beneficial effects:

[0027] First, the construction method for diversion tunnels in this invention for dealing with adverse geological hazards first stabilizes the surrounding rock upstream and downstream of the collapse area before treating the adverse geological area, and then treats the adverse geological area after the stress in the collapse area has stabilized.

[0028] Secondly, the construction method for diversion tunnels in this invention for adverse geological disasters adopts the technology of "rapid closure, composite arch support for the load-bearing body, and simultaneous deep and shallow support" for adverse geological areas, which ensures the stability of adverse geological areas.

[0029] Third, the construction method for diversion tunnels in the present invention, especially for the treatment of wall collapse in diversion tunnels with extra-large cross-sections, adopts steel support inside the tunnel and systematic shotcrete treatment, which effectively strengthens the rock mass and provides good support for the sidewalls.

[0030] Fourth, in the construction method for diversion tunnels in the present invention, deep and shallow reinforcement support measures such as anchor piles and anchor cables are used to strengthen the collapsed body in the adverse geological area. At the same time, the anchor piles and anchor cable holes are used to carry out systematic consolidation grouting measures for the entire slope, which has achieved good results and played a good role in reinforcing the rock mass around the collapsed body in the adverse geological disaster.

[0031] Fifth, in the construction method for diversion tunnels in the present invention, the rock blockage section of the tailwater main tunnel intersecting with the diversion tunnel is treated by layered blasting excavation to reduce the disturbance and deformation of large-section rocks caused by blasting, and effectively prevent the occurrence of large collapses.

[0032] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0033] Figure 1 This is a cross-sectional view of the steel support in the construction process of reinforcing the composite arch support load-bearing body of the present invention;

[0034] Figure 2 This is a cross-sectional view of the anchor cable arrangement in the deep support and shallow support construction process of this invention.

[0035] Figure 3 This is a cross-sectional view of the self-advancing anchor bolt / anchor pile support layout in the collapse area during the deep and shallow support construction process of this invention.

[0036] Figure 4 This is a flowchart of the construction method for diversion tunnels in the event of adverse geological hazards according to the present invention;

[0037] Figure 5 This is a structural diagram of the layout of diversion tunnels #3 and #4, and tailrace main tunnels #5 and #6. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to embodiments, so that those skilled in the art can implement it based on the description.

[0039] It should be noted that, unless otherwise specified, the experimental methods described in the following implementation plan are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified.

[0040] Example

[0041] The Wudongde Hydropower Station is the upstream cascade of the four hydropower cascades in the lower reaches of the Jinsha River (from Panzhihua City to Yibin City) – Wudongde, Baihetan, Xiluodu, and Xiangjiaba. The right bank of the river section where the dam site is located belongs to Luquan County, Kunming City, Yunnan Province, and the left bank belongs to Huidong County, Sichuan Province. This project is a Class I large (1) project. The main structures of the key project consist of water-retaining structures, water-discharging structures, and water diversion and power generation structures. The power plant is located in the mountains on both banks, both close to the riverbed. Each plant is equipped with 6 mixed-flow turbine generator units with a single unit capacity of 850MW, with a total installed capacity of 10200MW, an average annual power generation of 40.11 billion kWh, a normal water level of 975m, and a corresponding reservoir capacity of 5.863 billion m³. 3 Taking the construction of the water diversion system (which uses a one-machine-one-tunnel layout, and the tailrace system which uses a two-machine-one-chamber-one-tunnel layout, with two tailrace tunnels combined with diversion tunnels on each bank) as an example,

[0042] Three diversion tunnels are arranged on the right bank of the Wudongde Hydropower Station. The tunnel sections are parallel, with an axial spacing of 38m to 50m. The net cross-section of tunnels #3 and #4 after lining is 16.5 × 24.0m (width × height), with excavation dimensions of 19.9 × 27.2m. The tunnel outlet section is integrated with the power station's tailrace tunnel. The surrounding rock mass of the right bank diversion tunnels consists of a folded basement. From the inlet to the outlet, the strata traversed are: Yinmin Formation Pt2yl, Pt2y2, Luoxue Formation Pt211, and Pt2l8. Among them, Pt2y1 is a reddish-pink interbedded thin and medium-thick layers of quartz marble-altered dolomite, while Pt213-2-4 and Pt213-4-2 are approximately 5m thick quartzite. The rock is hard. The strata dip at 160°–180°, with a dip angle generally between 65° and 85°, dipping downstream.

[0043] The collapsed section of the diversion tunnel was primarily characterized by slightly weathered rock, with localized fissured dissolution and weathering. The bedding planes were yellowish, poorly cemented, and the tunnel walls showed signs of localized wrinkling. Medium- to gently dipping structural planes and micro-fractures were well-developed, the rock mass was fractured, and localized seepage was observed, classifying it as Class IV surrounding rock. Additionally, this section contained a set of long fissures, 2-3 mm wide, trending at 350° and dipping gently upstream at 25°. These fissures were filled with mud and calcareous material, which, together with the bedding planes, cut into easily unstable blocks. The excavation height of the sidewalls of diversion tunnels #3 and #4 at the Wudongde Hydropower Station was significant, and the central partition wall between the two tunnels was thin, only 30.5 m thick. High sidewalls and thin partition walls are prone to deformation and instability. After the collapse, only about 17 m of the central partition wall remained, with the remaining rock mass primarily in the plastic deformation zone, exhibiting extremely poor stability. During the handling of the landslide, further collapse is very likely to occur, resulting in the complete collapse of the central partition wall. This is extremely detrimental to the handling of the landslide area and construction safety. In addition, the landslide area is located at the tail section of the diversion tunnel. The central partition wall in this part will need to be excavated and modified by blasting in the later stage. In addition, the construction period of the diversion tunnel is already extremely tight. If it cannot be handled in the shortest possible time, it will seriously affect the timely flow and the closure of the main river.

[0044] A large-scale collapse occurred on the right-side wall below the arch corner of the No. 3 diversion tunnel, between chainages 1+220 and 1+270. The collapse height was approximately 20-22m, with elevations ranging from EL799 to EL820m, and the collapse depth was approximately 8-13m. The total volume of the collapse was approximately 4000m³. After the collapse, only about 17m of the central partition wall remained, and the remaining rock mass was primarily in the plastic deformation zone, exhibiting extremely poor stability.

[0045] To address the technical challenges of the construction project, the construction method for diversion tunnels in the face of adverse geological hazards includes the following steps (flowchart shown). Figure 4 As shown):

[0046] Step 1: Stabilization of the surrounding rock upstream and downstream of the adverse geological hazard area: Support construction is carried out on the surrounding rock upstream and downstream of the adverse geological hazard area; reinforced support measures are implemented at the downstream tail section of the collapse area. Four rows of through-cable cables are added to the partition wall of the No. 4 diversion tunnel from 1+530.02 to 1+598.02m (corresponding to the No. 3 diversion tunnel at chainages 1+402.373 to 1+470.373m). The anchor cables are installed from the No. 4 diversion tunnel towards the No. 3 diversion tunnel, using 1500KN anchors with a length of L = 30.5m and a spacing of 4×4 rows. m; Six rows of anchor piles with a length L = 18m and a spacing of 2.5×2.5m were added to the left sidewall of the No. 3 diversion tunnel from 1+361.373m to 1+388.873m; Six rows of anchor piles with a length L = 18m and a spacing of 2.5×2.5m were added to the right sidewall of the No. 4 diversion tunnel from 1+477.3m to 1+607.3m; Six rows of anchor piles with a length L = 18m and a spacing of 2.5×2.5m were added to the left sidewall of the No. 4 diversion tunnel from 1+600.5m to 1+610.5m;

[0047] Step Two: Construction Treatment of Areas with Adverse Geological Hazards: This involves sealing off the areas with adverse geological hazards, reinforcing the load-bearing structure with composite arch supports, and implementing both deep and shallow support methods.

[0048] S1, Sealing Treatment: For the cracked areas of the shotcrete layer on the upstream and downstream sidewalls of the adverse geological disaster area, a 15cm thick C25 coarse fiber concrete was sprayed for initial sealing; the pits and depressions in the collapsed area were filled to form an arc-shaped arch to strengthen the support; the right wall on the upstream side and the downstream side of the collapsed area of ​​the No. 3 diversion tunnel were protected by external slag removal; steel gabions were added to the right side of the No. 5 branch tunnel and the access road leading to the collapsed area to protect the foot of the slag pile. The steel gabion size was 3×1×1m, the spacing between the steel bars was 20cm×20cm, the main reinforcement was Φ22mm, and the web reinforcement was Φ14mm.

[0049] S2, Reinforced Support for Composite Arch Support: Steel supports are erected inside the diversion tunnel, and a system of shotcrete is applied simultaneously. I20b I-beams are added below the arch corners of the right sidewall of the No. 3 diversion tunnel (1+160~1+320m range) and the left sidewall of the No. 4 diversion tunnel (1+330~1+450m range), with a spacing of 1.25m, the same as the system anchors. These I-beams are welded to the system anchors on the sidewalls. [12 connecting channel steel is installed longitudinally between the I-beams, with a circumferential spacing of 1.5m. The I-beams are welded firmly to the system anchors as much as possible. Where welding to the system anchors is not possible, four Φ25, L=3m anchor bolts are installed on each I-beam. C25 steel fiber reinforced concrete is used for full shotcreting of the steel support areas; if... Figure 1 The image shown is a cross-sectional view of the steel support.

[0050] S3, Deep support and shallow support: Based on the geological conditions of the collapsed section and the actual construction conditions on site, the following treatment measures were mainly adopted for the collapsed body.

[0051] (1) Three rows of through anchor cables are added below the right wall arch angle of the No. 3 diversion tunnel from 1+160 to 1+183.15m (corresponding to the No. 4 diversion tunnel from 1+287.8 to 1+311.02m). The spacing between the anchor cables is 4×3m. The holes are drilled in the direction of the No. 4 diversion tunnel. The length of the anchor cables is about 30.5m, and 1000KN bonded type is used. Below this section of anchor cables, four rows of 3Φ28 anchor piles are driven from the No. 3 and No. 4 diversion tunnels towards each other. The spacing between the rows is 2.5×2.5m, and the length of the anchor piles is L=18m.

[0052] (2) Five rows of through-cable cables will be added below the right wall arch angle from 1+183.15 to 1+230m in Diversion Tunnel #3 (corresponding to 1+311.02 to 1+357.87m in Diversion Tunnel #4); five rows of through-cable cables will also be added below the right wall arch angle from 1+270 to 1+320m in Diversion Tunnel #3 (corresponding to 1+397.87 to 1+447.87m in Diversion Tunnel #4). The spacing between the rows of cables is 4×4m. The length of the cables is approximately 30.5m, and they are 1000KN bonded type. Figure 2 Cross-sectional view for anchor cable installation;

[0053] (3) For the No. 4 diversion tunnel, the corresponding chainage is 1+357.87~1+397.87. Eight rows of 3Φ28, L=18m anchor piles are added below the left wall arch angle, with a row spacing of 2.5×2.5m. The anchor pile holes are used to grout around the hole wall for consolidation.

[0054] (4) Add 3Φ28 anchor piles to the side walls of the 1+230~1+311 section of the No.4 diversion tunnel, with a spacing of 2.5×2.5m and a length of 18m.

[0055] (5) The original design support parameters for the 4# guide section from 1+350 to 1+398 were Φ28, L=6m / 9 mortar anchor rods with a spacing of 1.25×1.25. The anchor rods of the middle and lower side walls were adjusted to Φ28, L=6m / 9m self-advancing anchor rods with a spacing of 1.25×1.25m.

[0056] The original design support parameters for the anchor bolts on the lower sidewalls of the No. 4 diversion tunnel (sections 1+398 to 1+450) were Φ25, L=4.5m, and the spacing between rows was 1.5×1.5m. They were adjusted to Φ28, L=6 / 9m, and the spacing between rows was 1.25×1.25m for self-advancing anchor bolts.

[0057] The lower-level sidewall anchor bolts in the No. 5 diversion tunnel, from chainage 1+285 to 1+335, have been adjusted to self-drilling anchor bolts with parameters of Φ28, L=9m, and a spacing of 1.25×1.25m. Figure 3 Cross-sectional view of the self-advancing anchor bolt / anchor pile support layout in the collapsed area;

[0058] (6) Two rows of anchor piles are added to the left sidewall of the No.5 diversion tunnel from 1+240 to 1+340, and four rows of anchor piles are added to the right sidewall and the arch corner. The parameters are 3Φ28, L=18m, and the spacing between rows is 2×3m.

[0059] (7) Four rows of 3Φ28 anchor piles were added above the 808m elevation on the left sidewall of the K1+220~K1+320 section of the No. 3 diversion tunnel, and two rows of 3Φ28 anchor piles were added below the 808m elevation. The spacing between the rows was 2.5×2.5m and the length was 18m.

[0060] (8) Five rows of 3Φ28 anchor piles were added to the right sidewall of the section from K1+330 to K1+360 of the No. 4 diversion tunnel, with a spacing of 2.5×2.5m between rows and a length of 18m. Two rows were above the 815.5m elevation and three rows were below the 811.0m elevation.

[0061] (9) For the right side of the collapse area from 1+190 to 1+270m of the No. 3 diversion tunnel, add self-advancing anchor bolts and embed a Φ20PE pipe inside the self-advancing anchor bolt as a grouting pipe. Install a grout stop plug and a pad at the rock surface. Use the self-advancing anchor bolts to inject pure cement grout for simple consolidation grouting of the rock mass. The parameters of the self-advancing anchor bolts are: Φ38, L=12m, and the spacing between rows is 1m×1m.

[0062] (10) The dry spraying method for the right wall of the No. 3 diversion tunnel (1+160~1+180m) and the left wall of the No. 4 reverse flow tunnel (1+310~1+450m) below the arch corner was changed to wet spraying method. The bottom backfill stone slag was used as the construction platform, and the slag filling height was about 3~4m.

[0063] (11) At the top of the unexcavated rock platform (approximately 10.6m high) in sections 1+285 to 1+310 of the No. 4 diversion tunnel, an additional row of downward-inclined anchor piles is added to anchor the unexcavated rock mass. The anchor pile parameters are 3Φ28, L=12m, spacing 2m, and downward inclination 30°.

[0064] (12) Three rows of anchor piles were added to the right sidewall of the No. 4 diversion tunnel from 1+311 to 1+330. The anchor pile parameters are: 3Φ28, L=18m, and spacing 2.5m.

[0065] (14) Six rows of 18m long anchor piles were added to the left wall of the No. 3 diversion tunnel from 1+389 to 1+472m, with a spacing of 2.5×2.5m between rows.

[0066] (15) Two multi-point displacement meters with a range of 25m were added to the left sidewall of the No. 4 diversion tunnel at chainages K1+290 and K1+320 to strengthen the deep monitoring of the collapsed body.

[0067] (16) In order to reinforce the rock mass, simple consolidation grouting was carried out on the surrounding rock at the locations where anchor piles and anchor cables were installed, using the anchor pile holes and anchor cable holes as consolidation grouting holes. The consolidation grouting pressure was 0.3 to 0.5 MPa, and thick grout was first injected, with a water-cement ratio of 0.35 to 0.5:1.

[0068] (17) After the debris in the collapsed area is removed and the self-propelled anchor bolts are installed, the collapsed body will be backfilled with concrete in two stages. The first stage will begin at elevation 803.7, with a maximum depth of approximately 9.5m. Backfilling will be carried out in layers from bottom to top, with each layer being 3 to 3.5m high. C25 grade II concrete will be pumped into the formwork. The second stage will involve backfilling the remaining 3m thick concrete of the collapsed body and the 1.2m thick concrete of the side arch lining using a steel formwork trolley. The grade of the second backfill concrete will be the same as that of the side arch concrete, namely C9030. In addition, a row of through anchor cables will be installed at elevation EL808 in the 1+400 to 1+450m area of ​​the No. 4 diversion tunnel. The anchor cable spacing will be 4m, and the ends of the anchor cables will be led to the concrete lining surface using pre-embedded Φ150 steel pipes. Tensioning will be carried out after the concrete lining is completed.

[0069] To enhance the connection between the backfill concrete and the rock mass, gusset bars are arranged inside the backfill concrete. The gusset bars are welded to the rock surface system anchors. One gusset bar is arranged every other system anchor. The specifications are Φ25, L=1.5m (where the gusset length is 30cm and the straight length is 120cm), and the spacing between rows is 2×2m.

[0070] Since the backfill concrete is constructed in two stages, connecting reinforcing bars are arranged at the concrete construction joint surface. The reinforcing bars are Φ25, L=3.0m, and spaced 1.5×1.5m apart; 1m is exposed and 2m is embedded in the concrete. The exposed reinforcing bars are effectively connected to the later edge arch lining reinforcement with Φ25 steel bars to form a whole.

[0071] Step 3: Construct the integral support arch load-bearing body formed by steel arch frame - shotcrete - secondary backfilling;

[0072] 1) Based on the structural characteristics of the diversion tunnel cross-section, the middle layer of the diversion tunnel is excavated in a thin layer from top to bottom, and the rear two sides are excavated in an expanded manner; and the surrounding rock and working face of the excavation area are sealed in a timely manner.

[0073] Based on the structural characteristics of the extra-large cross-section cavern, a top-down thin-layer excavation method was adopted. The large cross-section cavern should be divided into 4-5 layers, with the upper layer height being 9.0m-10m. The excavation method involves "preliminary exploration of the central pilot tunnel, followed by expansion excavation on both sides after support is completed, or full-section excavation." On-site construction strictly followed a procedure of one cycle of excavation followed by one cycle of support. The middle and lower layers adopted an excavation method with a central trench and pre-reserved protective layers on both sides, with each layer height being 4-6m. By adopting thin-layer excavation and one-cycle support, the rock mass was promptly reinforced, reducing the disturbance and deformation of the large cross-section rock caused by blasting, effectively preventing large-scale landslides.

[0074] After excavation, a 5-8cm layer of steel fiber reinforced concrete should be sprayed onto the surrounding rock and the tunnel face to seal them, preventing secondary damage to the surrounding rock caused by weathering and forming a simple initial flexible support ring to increase the self-stabilization time of the surrounding rock. The plain shotcrete sealing measures at the tunnel face should be used to keep the excavation face roughly perpendicular to the tunnel axis to facilitate the operation of large machinery and accelerate the progress of subsequent safety support.

[0075] 2) After sealing, anchor bolts are used to support the surrounding rock, and steel arch frames are installed, steel mesh is hung, and formwork is installed without dismantling.

[0076] After spraying concrete, a certain number of system anchors should be installed in a timely manner to give full play to their suspension function as soon as possible and further enhance the stability of the surrounding rock.

[0077] System anchor bolts should preferably be installed using a multi-arm drill (compared to manual pneumatic drills, which are only suitable for Class II and III surrounding rock, multi-arm drills demonstrate greater adaptability in Class IV and V surrounding rock and fractured, weakly weathered surrounding rock, enabling faster, higher-quality, and safer construction). After drilling, the bolts should be inserted promptly to avoid prolonged placement that could lead to hole collapse. Self-drilling anchor bolts should be used in areas where drilling is impossible. The layout of system anchor bolt holes should consider the location of steel supports to facilitate welding extensions of the anchor bolts to reinforce the steel supports.

[0078] After the initial spraying, the steel arch frames are quickly installed using a drilling and blasting rig in conjunction with manual labor. The steel arch frames are generally made of I20 I-beams, with a spacing of 0.75–1m between frames. Each steel arch frame is only allowed to be used on-site after passing factory inspection. Before installation, measurements are taken and the layout is completed to ensure the installation position meets design requirements. During installation, key control measures are implemented for the bolt connections of the base plates, the welding of the arch frames to the connecting channel steel, the installation of the anchor bolts, and the welding quality of the system anchor bolts to the steel arch frames. Each process must pass inspection before proceeding to the next, and the installation quality of the steel arch frames must meet design and specification requirements.

[0079] The steel supports are installed outside the designed cross-section of the lining, leaving sufficient space for shotcrete thickness. The shotcrete must not encroach on the lining cross-section during subsequent installation. Due to the spalling structure and severe bedding spalling after excavation of the thin rock layer, the gap between the steel arch and the rock surface is relatively large. To effectively form a supporting arch, an auxiliary arch is added between the steel arch and the hollow rock surface. Channel steel is added between the steel support frames to form a steel truss structure, increasing the rigidity of the steel support frames without the need for formwork removal.

[0080] Double-layer steel mesh is hung on the side of the steel support facing the rock surface, and a quick-closing formwork that can be removed is installed. The steel mesh and the formwork mesh must be tightly connected to the system anchors and steel supports by welding or other means to prevent damage to the formwork during shotcreting.

[0081] 3) Apply shotcrete between the installed steel arch frame and the surrounding rock surface, and then fill the gaps using secondary backfill grouting or pumped concrete, so that the steel arch frame, shotcrete, and backfill concrete form an integral supporting arch load-bearing body. For cavities larger than 1m, pumped concrete is used to fill the gaps; for cavities smaller than 1m, backfill grouting is used to fill the gaps.

[0082] Step 4: Layered blasting excavation of the rock-blocked section of the tailrace main tunnel intersecting with the diversion tunnel;

[0083] like Figure 5 As shown, the tailrace main tunnels No. 5 and No. 6, which intersect with the No. 3 and No. 4 diversion tunnels at a small angle, are excavated in layers because the rock blockage in the middle of their intersection section is the area of ​​the collapsed partition wall in the No. 3 and No. 4 diversion tunnels, and both belong to Class IV surrounding rock. The excavation is carried out in four layers. The layer height of the first layer is 9.85m, and the layer height of the first layer of the H(K) type standard cross section is 9.35m; the excavation height of the second and third layers is 6.0m; and the layer height of the bottom plate of the lower layer of the fourth layer is 4.5m to 6.0m.

[0084] The design parameters for the first layer of blasting are shown in Table 1 below.

[0085] Table 1 Technical Parameters for Drilling and Blasting Construction in Layer I

[0086]

[0087] For layers II and III, horizontal holes were drilled using a YT-28 hand-held pneumatic drill with a diameter of Φ42mm. The hole depth per cycle was 4m, and the advance per cycle was approximately 3.5m. The spacing between the main horizontal blasting holes was 1.2m × 1.0m. The main blasting holes used Φ32mm emulsion explosive with continuous charging, a hole-clogging length of 1m, and a single-hole charge of 2.8–3.0kg. The maximum single-blast charge was controlled to be within 50kg. The blasting design parameters for layers II and III are shown in Table 2 below.

[0088] Table 2 Technical Parameters for Drilling and Blasting Construction in Layer II and Layer III

[0089]

[0090] Excavation of Layer IV commenced after the completion of Layer III, employing a full-face excavation method. All excavation of the lower layers utilized YT-28 hand-held pneumatic drills in conjunction with a drill-and-blast rig to drill horizontal holes, with smooth blasting employed for the designed outline. Each drilling cycle resulted in a hole depth of 4m and an advance of approximately 3.5m per cycle. The blasting design parameters for Layer IV (lower layer) excavation are shown in Table 3 below.

[0091] Table 3 Technical Parameters for Drilling and Blasting Construction in Layer IV

[0092]

[0093]

[0094] To ensure the excavation quality of the high sidewalls of the large underground cavern and to reduce the impact of blasting vibrations on the rock mass, a comprehensive approach was adopted: shallow-hole excavation in a 6-7m stepped section in the middle, with 4-5m protective layers reserved on both sides. The protective layers were excavated using vertical smooth blasting, strictly adhering to the principle of "thin-layer excavation, layer-by-layer support." The excavation plan specifically employed movable standard templates for all vertical smooth blasting holes to control the excavation quality. Small holes were drilled using pneumatic drills, and vertical smooth blasting was performed, with the charge per blast controlled to within 10kg. Simultaneously, the protective layers utilized strict and precise controlled blasting techniques to minimize the impact of blasting vibrations.

[0095] Step 5: Establish a safety monitoring and early warning system to monitor the surrounding rock in the adverse geological hazard zone of the diversion tunnel in a timely manner to guide on-site construction.

[0096] To promptly understand and monitor the deformation, stress, and strain of the surrounding rock during construction, it is essential to actively conduct temporary and permanent safety monitoring during the construction period, based on on-site engineering geological and hydrogeological data, design documents, and the actual conditions of the excavation work. For areas with unfavorable geological conditions such as Class III-V surrounding rock in diversion tunnels, areas with abundant groundwater, fault fracture zones, tunnel entrances and junctions, shallower sections significantly affected by adjacent excavation, and high-stress zones, monitoring sections should be established every 30-50 meters. During monitoring, more frequent observations should be conducted at locations with abrupt displacement changes or significant variations. Permanent monitoring instruments such as multi-point displacement gauges and anchor stress gauges should be deployed at key locations. This will allow for timely understanding of the surrounding rock deformation and provide timely feedback to guide on-site construction. Simultaneously, a dynamic design and construction technique combining numerical calculations and the construction process should be adopted. Construction parameters should be calculated, analyzed, determined, and optimized in a timely manner to guide construction and verify the correctness of the construction methods. This approach can effectively address construction safety and ensure construction progress.

[0097] The construction method of this invention has been successfully applied to the No. 3 and No. 4 diversion tunnels on the right bank of the Wudongde Hydropower Station, achieving excellent results. With the development of the national economy, the excavation of underground caverns under complex geological conditions is increasing, especially with the commencement of key national projects such as the West-East Gas Pipeline and the West-East Electricity Transmission Project in recent years. This has led to numerous challenging engineering problems under complex geological conditions, with serious collapse issues. Traditional geological exploration and experimental methods often cannot fully provide the necessary technology to address these complex geological conditions. This project, based on the case study of a large-scale collapse of the diversion tunnel wall in the Wudongde Hydropower Station, summarizes and refines the construction methods and techniques. These methods can provide a reference for other similar projects and have broad application prospects.

[0098] The construction method for diversion tunnels in this invention, addressing adverse geological conditions, employs a "rapid closure, composite arch support for reinforced load-bearing structure, and simultaneous deep and shallow support" approach in the collapsed area. First, the surrounding rock upstream and downstream of the collapsed area is stabilized. After the stress in the collapsed area stabilizes, further treatment is carried out. Later excavation utilizes a "first seal, then excavate" method to stabilize the rock mass at small angles of intersection. The sidewalls employ "thin-layer excavation, layer-by-layer support, vertical blasting, and precise blasting control," and a "safety monitoring and early warning system" is established. Through safety monitoring and guidance of excavation control measures, the safety and stability of large-scale collapse areas in the partition walls of extra-large cross-section diversion tunnels are ensured. This effectively solves the technical challenges of constructing diversion tunnels, especially those with extra-large chambers, to handle partition wall collapses. It avoids the safety hazards and treatment costs of large collapses, resulting in significant indirect economic and social benefits. The construction technology, quality, and efficiency have been consistently affirmed by quality expert groups.

[0099] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and embodiments shown and described herein.

Claims

1. A construction method for diversion tunnels in the event of adverse geological hazards, characterized in that, Includes the following steps: Step 1: Stabilization of the surrounding rock upstream and downstream of the adverse geological hazard area: Support construction is carried out on the surrounding rock upstream and downstream of the adverse geological hazard area; specifically, this includes: installing multiple rows of through-cable anchors on the partition wall of the diversion tunnel, installing multiple rows of anchor piles on the left side wall, and installing multiple rows of anchor piles on the right side wall; among them, the anchor cables are set in 4 rows with a spacing of 4×4m; the anchor piles on both the left and right side walls are set in 6 rows with a spacing of 2.5×2.5m. Step Two, Construction Treatment of Adverse Geological Hazard Areas: This involves sealing off the adverse geological hazard area, reinforcing the composite arch support structure, and implementing both deep and shallow support. Specifically, this includes: S1, Sealing Treatment: Initial sealing of cracked areas in the shotcrete layer of the upstream and downstream sidewalls of the adverse geological hazard area by spraying 15cm thick C25 coarse fiber concrete; setting up external slag protection at the toe of the sidewalls of the adverse geological hazard area; and adding reinforced gabions to the side of the access road in the adverse geological hazard area to protect the slag heap. The reinforced gabions are 3×1×1 m in size, with a spacing of 20 cm×20 cm between reinforcing bars, a main bar diameter of 22 mm, and a web bar diameter of 14 mm. S2, Reinforcing Support of the Composite Arch Support Structure: Steel supports are erected inside the diversion tunnel. I20b I-beams are installed on the sidewalls of the diversion tunnel, welded to anchor bolts installed in the sidewalls. Connecting channel steels are longitudinally installed between the I-beams, with a circumferential spacing of 1.5m. m; Simultaneously, systematic shotcrete treatment is carried out; S3, deep support and shallow support: for sections with adverse geological hazards, deep and shallow support treatment is carried out by combining anchor piles and anchor cables, and at the same time, systematic consolidation grouting construction is carried out on the entire slope using anchor piles and anchor cable holes; Step 3: Establishing an integral supporting arch structure formed by steel arch frame, shotcrete, and secondary backfill: Based on the structural characteristics of the diversion tunnel cross-section, the middle layer of the diversion tunnel is excavated in thin layers from top to bottom, while the sides are excavated by widening. The surrounding rock and tunnel face of the excavation area are sealed in a timely manner, including: spraying a 5-8cm layer of steel fiber reinforced concrete onto the surrounding rock and tunnel face; anchoring the sealed surrounding rock; installing the steel arch frame, hanging steel mesh, and installing formwork without dismantling; spraying shotcrete between the installed steel arch frame and the surrounding rock surface, and then filling the gaps with secondary backfill grouting or pumped concrete, so that the steel arch frame, shotcrete, and backfill concrete form an integral supporting arch structure. Step 4: Layered blasting excavation of the tailrace main tunnel rock block section intersecting with the diversion tunnel: shallow hole blasting is carried out first in the middle 6-7 m stepped section, and a 4-5 m protective layer is reserved on both sides. The protective layer is excavated by vertical smooth blasting. All vertical smooth blasting holes are set up with movable standard templates, drilled with a hand drill, and vertical smooth blasting is carried out. The amount of explosive per blast is controlled within 10 kg. Step 5: Establish a safety monitoring and early warning system to monitor the surrounding rock in the adverse geological hazard zone of the diversion tunnel in a timely manner to guide on-site construction.

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