High-altitude shallow-buried silty-fine sand layer tunnel portal section collaborative supporting structure and construction method

Abstract

The invention relates to the technical field of tunnel engineering construction, in particular to a high-altitude shallow-buried silty-fine sand layer tunnel portal section collaborative supporting structure and a construction method. The construction method comprises the following steps that S1, based on engineering geological survey data, an advanced pipe shed and a horizontal high-pressure jet grouting pile are integrally designed; and calculating and determining the design distance, the lap joint length and the rigidity matching parameter of the advanced pipe shed and the horizontal high-pressure jet grouting pile, and constructing a composite structure model taking the advanced pipe shed as a rigid framework and cooperatively bearing with the horizontal high-pressure jet grouting pile. According to the method, a collaborative supporting system with rigid-flexible coupling of the advanced pipe sheds and the horizontal jet grouting piles is constructed, jet grouting construction parameters are dynamically regulated and controlled on the basis of an environment parameter mapping library, and the problems that the self-stability of a high-altitude silty-fine sand layer is poor, and the influence of the air pressure temperature difference on the pile forming quality is large are effectively solved; and the water stopping effect and the overall bearing stability of the hole section supporting structure under the complex geology are ensured.

Description

Technical Field

[0001] This invention relates to the field of tunnel engineering construction technology, and in particular to the collaborative support structure and construction method for the portal section of a tunnel in a high-altitude shallow buried fine sand layer. Background Technology

[0002] As transportation infrastructure construction extends to high-altitude and complex terrain areas, tunnel entrance sections often encounter extremely soft strata such as water-rich fine sand. These strata are characterized by poor self-stability, looseness, and easy flow. During construction, pre-support technologies such as advanced pipe roofs or horizontal high-pressure jet grouting piles are usually used to pre-reinforce the surrounding rock, aiming to form a protective shell outside the excavation outline to maintain the stability of the tunnel face.

[0003] However, existing construction parameters mostly follow the experience of plains, ignoring the negative impact of high altitude, low air pressure and large temperature difference on jet diffusion and grout solidification, resulting in inconsistent pile quality and incomplete closure of the water-stop curtain, which can easily induce sand inrush and collapse accidents. Summary of the Invention

[0004] To overcome the above deficiencies, this invention provides a collaborative support structure and construction method for the tunnel portal section in shallow buried fine sand layers at high altitudes. It aims to improve the existing technology, which often uses construction parameters based on plain experience and ignores the negative impact of high altitude, low air pressure, and large temperature difference on jet diffusion and slurry solidification.

[0005] In a first aspect, the present invention provides the following technical solution: a construction method for a collaborative support structure for the portal section of a tunnel in a high-altitude shallow-buried fine sand layer, comprising the following steps: S1. Based on engineering geological survey data, the advanced pipe roof and horizontal high-pressure jet grouting pile are designed in an integrated manner. The design spacing, overlap length and stiffness matching parameters of the two are calculated and determined. A composite structure model with the advanced pipe roof as a rigid skeleton and the horizontal high-pressure jet grouting pile as a co-bearing structure is constructed. S2. Horizontal high-pressure jet grouting piles are constructed at the tunnel entrance arch and surrounding area. Based on real-time collected ambient temperature and atmospheric pressure data, the grouting pressure, grout flow rate and drill rod lifting speed are adjusted during the jet grouting construction. The water-cement ratio and admixture dosage of the grout are adjusted online based on the feedback of grouting pressure and grout flow rate to form an interlocking horizontal jet grouting reinforcement layer. S3. Set the control threshold for surrounding rock deformation and support structure stress during tunnel excavation. Pre-embed sensors in the advanced pipe roof and horizontal high-pressure jet grouting piles to collect real-time data. Compare the monitoring data with the control threshold and adjust the excavation advance, support construction timing and support parameters for the next cycle based on the comparison results. S4. Carry out tunnel excavation and support structure construction, and complete the initial support closure and secondary lining.

[0006] Preferably, in step S1, constructing a composite structural model with an advanced pipe roof as a rigid frame and horizontal high-pressure jet grouting piles working together to bear load specifically includes the following steps: A mechanical role definition model was established, in which the advanced pipe shed was set as the main bending member and the horizontal high-pressure jet grouting pile was set as the water-stopping and supplementary load-bearing member. Numerical simulation calculations were performed based on engineering geological survey data to determine the design spacing, longitudinal overlap length, and stiffness matching coefficient of the advanced pipe shed and horizontal high-pressure jet grouting piles, and to establish the composite structure model.

[0007] Preferably, in step S2, adjusting the grouting pressure, grout flow rate, and drill rod lifting speed of the jet grouting operation based on real-time collected ambient temperature and atmospheric pressure data specifically includes the following steps: An environmental monitoring station was established at the construction site to collect real-time data on ambient temperature and atmospheric pressure in the tunnel entrance area. The collected ambient temperature and atmospheric pressure data are input into a preset construction parameter mapping library, which contains the set grouting pressure, grout flow rate and drill rod lifting speed values ​​corresponding to different environmental conditions. Based on the mapping and matching results, control commands are automatically generated to drive the high-pressure grouting pump and drilling rig actuators, adjusting the jet grouting construction parameters to the set values ​​corresponding to the current environmental conditions.

[0008] Preferably, the construction of the preset construction parameter mapping library specifically includes the following steps: Before formal construction, on-site process test piles were carried out to test the pile formation effect under different combinations of grouting pressure, grout flow rate and drill rod lifting speed under different atmospheric pressure and ambient temperature conditions. Optimal parameter combinations were selected to ensure that both the pile diameter and pile strength met the design requirements. The effective parameter combinations are correlated with the corresponding environmental conditions data and entered into the database to form the construction parameter mapping library.

[0009] Preferably, in step S2, the process of adjusting the water-cement ratio and admixture dosage of the grout online based on feedback from grouting pressure and grout flow rate to form an interlocking horizontal jet grouting reinforcement layer specifically includes the following steps: Sensors installed on the grouting pipeline are used to monitor the grouting pressure and grout flow rate in real time during the high-pressure jet grouting process. The monitored grouting pressure and grout flow rate are compared with the preset benchmark diffusion range value to determine the diffusion resistance and loss of grout in the fine sand layer. Based on the comparison results, the water-cement ratio of the slurry and the amount of early strength agent and antifreeze agent added are adjusted online through an automatic slurry mixing system.

[0010] Preferably, in step S3, the process of pre-embedding sensors within the advanced pipe roof and horizontal high-pressure jet grouting piles to collect real-time data, and comparing the monitoring data with control thresholds, specifically includes the following steps: Stress sensors and strain sensors are installed on the pipe body of the advanced pipe shed and on the stress-bearing parts of the horizontal high-pressure jet grouting pile. The system reads sensor values ​​in real time through an automated data acquisition system and calculates the current deformation of the surrounding rock and the stress value of the support structure. Retrieve the preset safety threshold corresponding to the current construction stage, and compare the calculated surrounding rock deformation and support structure stress value with the preset safety threshold.

[0011] Preferably, in step S3, adjusting the excavation advance, support application timing, and support parameters for the next cycle based on the comparison results specifically includes the following steps: When the comparison results show that the monitoring data reaches or exceeds the preset warning value, a warning signal is generated and the tunnel excavation advance length of the next cycle is reduced; Shorten the exposure time of the excavation face and the closure time of the initial support construction; Increase the stiffness parameters of the support structure, including increasing the installation spacing of the steel arch frames and increasing the thickness of the shotcrete.

[0012] Preferably, in step S4, the tunnel excavation and support structure construction specifically includes the following steps: The three-stage reserved core soil method was adopted to excavate the tunnel face in sections, retaining the core soil of the upper and middle stages to support the stability of the tunnel face; Following the excavation face, a steel frame is erected, and anchor pipes are installed at the arch foot of the steel frame. The ends of the anchor pipes are welded and fixed to the steel frame. Shotcrete seals the surface of the surrounding rock, making the initial support structure a closed loop; A cold-proof and heat-insulating layer is laid on the inner side of the initial support structure, and then secondary lining concrete is cast.

[0013] Secondly, the present invention also provides the following technical solution: a collaborative support structure for the tunnel portal section in a high-altitude shallow-buried fine sand layer, comprising: The steel frame is set at intervals along the longitudinal direction of the tunnel, the guide pipe is fixedly set on the outer edge of the arch of the steel frame, the horizontal high-pressure jet grouting pile is set through the guide pipe, the initial support concrete layer covering the steel frame and the surrounding rock surface, the cold protection and heat preservation layer laid on the inner side of the initial support concrete layer, and the secondary lining built on the inner side of the cold protection and heat preservation layer. The guide tubes are arranged at intervals along the arch outline of the steel frame, and each guide tube is welded and fixed to the outside of the steel frame. One end of the horizontal high-pressure jet grouting pile passes through the guide pipe and extends forward into the rock strata, forming an advanced reinforcement curtain along the tunnel arch. The initial support concrete layer encloses the steel frame and the root of the guide pipe; The cold-proof and heat-insulating layer is set in close contact with the inner surface of the initial support concrete layer, and the secondary lining is molded on the inner surface of the cold-proof and heat-insulating layer.

[0014] The present invention has the following beneficial effects: 1. In this invention, by constructing a synergistic support system that combines advanced pipe roof and horizontal jet grouting piles with rigid-flexible coupling, and dynamically adjusting the jet grouting construction parameters based on an environmental parameter mapping library, the problem of poor self-stability of high-altitude fine sand layers and the large influence of air pressure and temperature difference on pile quality is effectively overcome, ensuring the water-stopping effect and overall bearing stability of the support structure at the tunnel entrance under complex geology.

[0015] 2. In this invention, a construction active control mechanism based on a safety factor algorithm is established by using pre-embedded sensors. The excavation advance and support parameters for the next cycle are dynamically optimized according to the real-time response of the stratum. This effectively avoids the risk of sudden collapse caused by excessive disturbance in extremely unstable fine sand strata, and significantly improves the safety and scientific nature of the construction process.

[0016] 3. In this invention, a cold-proof insulation layer design method based on thermal calculation is introduced in the lining construction. By accurately calculating the thickness of the insulation material, the heat exchange between the inside and outside of the tunnel is effectively blocked, which solves the problem of lining frost heave and cracking caused by freeze-thaw cycles in high-altitude cold regions, greatly reduces operation and maintenance costs and extends the service life of the structure. Attached Figure Description

[0017] Figure 1 This is a flowchart of the construction method for the collaborative support structure of the tunnel portal section in a high-altitude shallow buried fine sand layer proposed in this invention. Figure 2 This is a three-dimensional view of the collaborative support structure for the tunnel portal section in a high-altitude shallow buried fine sand layer proposed in this invention. Figure 3 This is a partial structural diagram of the horizontal high-pressure jet grouting piles in the collaborative support structure for the tunnel portal section of a high-altitude shallow buried fine sand layer proposed in this invention. Figure 4 This is a partial structural diagram of the initial support concrete layer of the collaborative support structure for the tunnel entrance section of a high-altitude shallow buried fine sand layer proposed in this invention.

[0018] The components include: 1. Steel frame; 2. Guide pipe; 3. Horizontal high-pressure jet grouting pile; 4. Initial support concrete layer; 5. Cold protection and heat insulation layer; 6. Secondary lining. Detailed Implementation

[0019] When the comparison results show that the monitoring data reaches or exceeds the preset warning value, a warning signal is generated and the tunnel excavation advance length of the next cycle is reduced; Shorten the exposure time of the excavation face and the closure time of the initial support construction; Increase the stiffness parameters of the support structure, including increasing the installation spacing of the steel arch frames and increasing the thickness of the shotcrete.

[0020] Specifically, during the tunnel construction preparation stage and the advanced support construction process, a closed-loop monitoring system integrating hardware sensing and software analysis is constructed. In the processing and fabrication of the advanced pipe roof steel pipes, key stress sections corresponding to the arch crown, arch waist, and arch foot are selected. The mounting base of the vibrating wire surface strain gauge is fixed to the inner wall of the steel pipe by spot welding. After the sensor is installed, epoxy resin is applied and a semi-circular steel protective cover is covered to prevent the grouting pressure from damaging the sensor element. The signal transmission cable is led out along the inner wall of the steel pipe through a pre-set PVC protective pipe. During the window period when the horizontal high-pressure jet grouting pile is completed but the grout has not yet fully solidified, a monitoring unit containing a miniature earth pressure cell and an internal concrete strain gauge is pressed into the designed depth of the pile body using a special rigid implantation rod. A positioning steel cage is set around the monitoring unit to prevent it from shifting position or settling during the grout solidification process. All sensor cables are gathered at the data acquisition station at the tunnel entrance and connected to a multi-channel automatic acquisition module.

[0021] During construction, the automated data acquisition system reads the raw frequency and temperature signals from each sensor at a set frequency. It then uses a built-in physical conversion model to calculate the stress state of the structural entity. For vibrating wire strain gauges, the system converts the frequency signal into micro-strain values ​​based on the principles of physical acoustics. Let the currently measured frequency be... The sensor's initial factory frequency is The calibration coefficient is The temperature correction factor is The current temperature is The initial temperature is The microstrain of the structure at the current moment The calculation formula is: ; In the formula, The calculated microstrain values ​​are dimensionless. The frequency value measured at the current moment, in Hertz; This is the initial frequency value after installation, in Hertz; This is the currently measured temperature value, in degrees Celsius. This is the initial temperature value, in degrees Celsius. The calibration coefficient of the sensor is expressed in microstrain per square hertz. This is the temperature correction factor, expressed in microstrain per degree Celsius.

[0022] The system further incorporates the elastic modulus of the steel pipes used in the pipe roof. The real-time stress value of the pipe roof structure was calculated. The calculation relationship is as follows: equal Multiply Meanwhile, the system reads the earth pressure cell values ​​in real time to obtain the radial compressive stress of the surrounding rock on the support structure.

[0023] The system retrieves preset control thresholds to perform a safety status assessment and executes a dynamic adjustment algorithm, setting stress warning thresholds according to design specifications. and surrounding rock pressure early warning threshold The system calculates the current comprehensive construction safety index in real time. This index is used to quantify the safety reserve under current operating conditions, and the calculation formula is: ; In the formula, This is a comprehensive index for construction safety, dimensionless. This represents the maximum stress value among all currently monitored measuring points, expressed in megapascals (MPa). The set stress warning threshold, in megapascals; This represents the maximum surrounding rock pressure currently monitored, expressed in megapascals (MPA). The set early warning threshold for surrounding rock pressure, in megapascals (MPa); Based on calculations The system automatically calculates the recommended excavation advance for the next cycle using an exponential decay model. The calculation formula is: ; In the formula, The recommended excavation advance length for the next cycle, in meters; This refers to the standard excavation advance length in the original design, in meters. The value is 1.5 for the formation sensitivity coefficient, specifically for silty sand formations.

[0024] If calculated If the value is less than 1.0, the system will immediately trigger an audible and visual alarm and will then calculate the value. The data is sent to the construction team's handheld terminal, forcibly requiring a shorter advance distance. Simultaneously, the system... Numerical output support enhancement command: when When the value is in the range of 0.8 to 1.0, the instruction requires that the steel frame installation spacing be shortened to 0.8 times the original design value. When the value is less than 0.8, the instruction requires that the steel frame installation spacing be shortened to 0.6 times the original design and that the thickness of the sprayed concrete be increased by 5 centimeters.

[0025] Through the above steps, the tunnel construction has been transformed from passive monitoring to active control. This method can capture abnormal changes in mechanical signals through algorithms in the early stage of surrounding rock pressure manifestation, and automatically quantify and output the optimal construction parameter adjustment scheme. While ensuring construction efficiency, it effectively curbs the risk of rheology and collapse caused by excessive disturbance in high-altitude silty sand layers.

[0026] S4. Carry out tunnel excavation and support structure construction, and complete the initial support closure and secondary lining.

[0027] Furthermore, in S4, the tunnel excavation and support structure construction specifically include the following steps: The three-stage reserved core soil method was adopted to excavate the tunnel face in sections, retaining the core soil of the upper and middle stages to support the stability of the tunnel face; Following the excavation face, a steel frame is erected, and anchor pipes are installed at the arch foot of the steel frame. The ends of the anchor pipes are welded and fixed to the steel frame. Shotcrete seals the surface of the surrounding rock, making the initial support structure a closed loop; A cold-proof and heat-insulating layer is laid on the inner side of the initial support structure, and then secondary lining concrete is cast.

[0028] Specifically, the tunnel excavation operation is executed according to the control command output in step S3. The construction team first receives the suggested excavation advance calculated in the previous cycle. Using this length as the actual tunneling parameter for this cycle, measurement and layout were carried out. The three-stage pre-reserved core soil method was adopted for excavation, dividing the tunnel cross-section into three parts from top to bottom: upper stage, middle stage, and lower stage. The upper stage advanced the middle stage by 3 to 5 meters, and the middle stage advanced the lower stage by 3 to 5 meters. During the excavation of the upper and middle stages, a trapezoidal core soil body was retained in the center of the tunnel face. The length of the core soil body was controlled between 1.5 meters and 2.5 meters, the top width was not less than 1.0 meter, and the slope of the two sides was controlled between 1:0.5 and 1:0.75. This core soil body can provide counter-pressure support to the extremely unstable silty fine sand layer tunnel face, offsetting the surrounding rock extrusion deformation monitored in step S3.

[0029] After the excavation of each step is completed, the exposed fine sand surface is immediately sealed with a 4-5 cm thick layer of concrete. Then, a steel frame made of I-beams or H-beams is erected. The longitudinal spacing of the steel frame is strictly in accordance with the instructions from step S3. If the structural safety factor output in step S3 is... If the spacing is greater than or equal to 1.0, then install according to the original design spacing. If less than 0.8, then use the adjusted encryption spacing. After the steel frame is in place, anchor pipes are immediately installed at the arch foot. The anchor pipes are made of seamless steel pipes with an outer diameter of 42 mm to 50 mm. Two anchor pipes are installed at each arch foot, with an angle of 30 to 45 degrees downward from the horizontal plane. The length is not less than 4.0 meters. The tail end of the anchor pipe is connected to the web of the steel frame by double-sided full welding through L-shaped steel bars. The welding length is not less than 10 cm, so that the steel frame and the deep surrounding rock form a stable triangular force system to control the subsidence of the arch foot.

[0030] After the steel frame installation and anchor pipe construction are completed, steel mesh is hung and longitudinal connecting bars are connected. Wet spraying process is used to spray concrete in layers until the design thickness is reached, so that the initial support structure is closely attached to the surrounding rock. After the lower bench excavation is completed, the initial support of the invert arch is constructed in time, so that the entire initial support structure closes into a ring in the ring direction, forming a closed bearing ring.

[0031] After the initial support deformation has basically stabilized, the construction of the frost protection and insulation layer and secondary lining is carried out. First, geotextile and waterproof board are laid on the inner surface of the initial support, followed by the frost protection and insulation board. To ensure the insulation effect in the high-altitude and low-temperature environment, the thickness of the frost protection and insulation layer is [not specified]. The extreme minimum temperature monitored in step S2 is used for calculation, and the calculation formula is as follows: ; In the formula, The calculated thickness of the cold-proof insulation layer is in meters; The thermal conductivity of the selected insulation material is expressed in watts per meter Kelvin. The minimum air temperature designed to be maintained inside the tunnel is usually set at 0 degrees Celsius or 5 degrees Celsius. The extreme minimum ambient temperature of the tunnel entrance area obtained from the monitoring and statistics in step S2 is expressed in degrees Celsius. The maximum heat flux density allowed through the lining structure is specified in watts per square meter. The design thickness of the secondary lining concrete is in meters. The thermal conductivity of concrete is expressed in watts per meter (Kelvin), calculated from... The appropriate rigid polyurethane or extruded polystyrene board is selected for laying, and a secondary reinforced concrete lining is constructed on the inner side using a hydraulic trolley, thus completing the construction of the composite lining structure system.

[0032] The thickness of the frost-proof layer, determined through thermal calculations, effectively blocks the heat exchange between the cold air outside the tunnel and the surrounding rock, preventing frost heave damage unique to high-altitude areas and ensuring the long-term stability and operational safety of the tunnel entrance section structure. Example

[0033] Taking a shallow-buried silty sand tunnel project in a high-altitude, cold region at an altitude of 3650 meters as an example, the tunnel entrance section is 120 meters long and 14 meters deep, with a designed excavation span of 11.8 meters. During step S1, geological engineers were first organized to conduct a special supplementary investigation of the entrance section. Soil samples were drilled at the arch crown and both sides of the arch foot. Through indoor geotechnical tests, the physical and mechanical parameters of the silty sand layer were determined as follows: natural unit weight... The specific gravity is 19.5 kN / m³, the saturated specific gravity is 21.0 kN / m³, and the internal friction angle is... At 28 degrees, cohesion The pressure is 12 kPa, the deformation modulus is 18 MPa, and the Poisson's ratio is 0.35. At the same time, the groundwater level is measured to be 2.5 meters above the arch. The selected advanced support materials are: the pipe roof uses Q345B hot-rolled seamless steel pipe with an outer diameter of 108 mm and a wall thickness of 6 mm, the horizontal high-pressure jet grouting piles are designed with a pile diameter of 600 mm, and the grouting material is P.O42.5 ordinary Portland cement.

[0034] Based on the above survey data, a mechanical role definition model was established, and the physical parameters were transformed into constitutive relations and element properties in the numerical calculation model: the advanced pipe roof was set as the main bending member, and the Timoshenko beam element was used for simulation in the finite element analysis software, with an elastic modulus of 210 GPa and a Poisson's ratio of 0.3. The horizontal high-pressure jet grouting pile was set as a water-stopping and supplementary bearing member, and the solid element was used for simulation in the software, with a Mohr-Coulomb constitutive model. According to the cement-soil mix ratio test, its elastic modulus was set to 300 MPa, the internal friction angle to be 35 degrees, and the cohesion to be 0.5 MPa. The contact surface between the pipe roof and the jet grouting pile was set as a friction contact element, and the normal stiffness and tangential stiffness were set to simulate the synergistic effect between the two.

[0035] Based on the defined mechanical roles and survey data, the design spacing between the advanced pipe roof and the horizontal high-pressure jet grouting piles is calculated. First, based on Terzaghi's earth pressure theory and combined with the groundwater influence correction factor, the vertical uniformly distributed load acting on the top of the composite support structure is calculated. : ; In the formula, The calculated vertical uniformly distributed load is expressed in kilopascals (kPa). To account for the soil loosening caused by high-altitude freeze-thaw cycles, a correction factor of 1.1 is used. The natural density of the soil is 19.5 kN / m³. The tunnel is 14 meters deep. The cohesion is 12 kPa. The tunnel has an excavation span of 11.8 meters. The water density is 10 kN / m³. Given a groundwater level of 2.5 meters, the calculation yielded... It is approximately 165.4 kPa.

[0036] Next, the advanced pipe roof was simplified into a multi-span continuous beam model, and the maximum theoretical spacing that meets the bearing capacity requirements was calculated. : ; In the formula, The calculated maximum center-to-center spacing of the pipe sheds is in meters. The section bending modulus of a 108 mm steel pipe is 68.5 cubic centimeters, as found in the table. The allowable bending stress of the steel is taken as 215 MPa. The spacing between support points, i.e., the spacing between steel arch frames, is taken as 0.8 meters. Substituting this value into the numerical calculation yields... The final design spacing was determined to be 0.45 meters, taking into account safety reserves and the interlocking requirements of the jet grouting piles. It is 0.40 meters.

[0037] After determining the spacing, the stiffness matching parameters are further calculated to ensure coordinated load-bearing, and the stiffness matching coefficient is defined. The formula used to characterize the deformation compatibility between the rigid skeleton and the flexible reinforcement is as follows: ; In the formula, The stiffness matching coefficient is dimensionless. The elastic modulus of the pipe roof is 210 gigapascals. Let be the moment of inertia of the pipe roof section, calculated to be 370 cm⁴. The composite elastic modulus of the jet grouting pile is 300 MPa. The cross-sectional area of ​​a single jet grouting pile is 0.283 square meters. The design spacing is 0.40 meters. Substituting these values ​​into the calculation yields... The value is 0.57, which is within the optimal coordination range of 0.5 to 1.0. This indicates that the bending stiffness of the pipe roof and the compressive stiffness of the jet grouting pile can achieve excellent deformation coordination at the design spacing, without the need to adjust the cement content of the jet grouting pile.

[0038] Finally, a three-dimensional full-size numerical model was established using the finite difference software FLAC3D. The parameters determined by the above calculations—108 mm pipe roof diameter, 600 mm jet grouting pile diameter, design spacing of 0.40 m, and longitudinal overlap length of 3.0 m—were input into the model. The excavation process was simulated using the three-step reserved core soil method. Monitoring points were set to extract the arch settlement and pipe roof stress. The simulation results showed that the final arch settlement was 22 mm, which was less than the control standard of 30 mm. The maximum bending stress of the pipe roof was 165 MPa, which was less than the allowable stress of 215 MPa. Based on this, the integrated design scheme was finally determined: 108 mm pipe roof and 600 mm jet grouting piles were arranged alternately with a center spacing of 400 mm and a longitudinal overlap of 3 m.

[0039] Reference Figures 2-4 The present invention also provides a collaborative support structure for the tunnel portal section in shallow buried fine sand layers at high altitudes, comprising: The steel frame 1 is set at intervals along the longitudinal direction of the tunnel, the guide pipe 2 is fixedly set on the outer edge of the arch of the steel frame 1, the horizontal high-pressure jet grouting pile 3 is set through the guide pipe 2, the initial support concrete layer covering the steel frame 1 and the surface of the surrounding rock 4, the cold protection and heat preservation layer 5 is laid on the inner side of the initial support concrete layer 4, and the secondary lining 6 is built on the inner side of the cold protection and heat preservation layer 5. The guide tubes 2 are arranged at intervals along the arch outline of the steel frame 1, and each guide tube 2 is welded and fixed to the outside of the steel frame 1. One end of the horizontal high-pressure jet grouting pile 3 passes through the guide pipe 2 and extends forward into the rock strata, forming an advanced reinforcement curtain along the tunnel arch; The initial support concrete layer 4 wraps around the root of the steel frame 1 and the guide pipe 2; The cold-proof insulation layer 5 is set in close contact with the inner surface of the initial support concrete layer 4, and the secondary lining 6 is cast on the inner surface of the cold-proof insulation layer 5.

[0040] Specifically, at the tunnel excavation face, steel frames 1 are erected at designed intervals, and guide pipes 2 are welded and fixed to the outer edge of the steel frame 1 along the outline of the steel frame arch at predetermined external angles and intervals. This serves as a rigid skeleton and positioning guide for subsequent construction. Then, drilling equipment is used to drill through the guide pipes 2 into the surrounding rock ahead. Through high-pressure jet grouting, horizontal high-pressure jet grouting piles 3 are formed deep into the rock mass. The interlocking between the piles forms an advanced reinforcement curtain. Tunnel excavation is then carried out, and shotcrete is promptly applied to wrap the steel frame 1 and the surface of the surrounding rock to construct the initial support concrete layer 4 to seal the tunnel face and limit the deformation of the surrounding rock. Finally, after the initial support deformation has stabilized, a cold-proof insulation layer 5 is laid across the entire cross-section of the inner surface of the initial support concrete layer 4, and secondary lining 6 is constructed by casting concrete on its inner side, thus completing the construction of a collaborative support structure system from the outside to the inside.

[0041] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A construction method for a collaborative support structure at the tunnel portal section in a high-altitude, shallowly buried silty fine sand layer, characterized in that: Includes the following steps: S1. Based on engineering geological survey data, the advanced pipe roof and horizontal high-pressure jet grouting pile are designed in an integrated manner. The design spacing, overlap length and stiffness matching parameters of the two are calculated and determined. A composite structure model with the advanced pipe roof as a rigid skeleton and the horizontal high-pressure jet grouting pile as a co-bearing structure is constructed. S2. Horizontal high-pressure jet grouting piles are constructed at the tunnel entrance arch and surrounding area. Based on real-time collected ambient temperature and atmospheric pressure data, the grouting pressure, grout flow rate and drill rod lifting speed are adjusted during the jet grouting construction. The water-cement ratio and admixture dosage of the grout are adjusted online based on the feedback of grouting pressure and grout flow rate to form an interlocking horizontal jet grouting reinforcement layer. S3. Set the control threshold for surrounding rock deformation and support structure stress during tunnel excavation. Pre-embed sensors in the advanced pipe roof and horizontal high-pressure jet grouting piles to collect real-time data. Compare the monitoring data with the control threshold and adjust the excavation advance, support construction timing and support parameters for the next cycle based on the comparison results. S4. Carry out tunnel excavation and support structure construction, and complete the initial support closure and secondary lining.

2. The construction method of the collaborative support structure for the tunnel portal section in a high-altitude shallow-buried fine sand layer according to claim 1, characterized in that, In step S1, the construction of a composite structural model with an advanced pipe roof as a rigid frame and horizontal high-pressure jet grouting piles as the co-bearing structure specifically includes the following steps: A mechanical role definition model was established, in which the advanced pipe shed was set as the main bending member and the horizontal high-pressure jet grouting pile was set as the water-stopping and supplementary load-bearing member. Numerical simulation calculations were performed based on engineering geological survey data to determine the design spacing, longitudinal overlap length, and stiffness matching coefficient of the advanced pipe shed and horizontal high-pressure jet grouting piles, and to establish the composite structure model.

3. The construction method of the collaborative support structure for the tunnel portal section in a high-altitude shallow-buried fine sand layer according to claim 1, characterized in that, In step S2, adjusting the grouting pressure, grout flow rate, and drill rod lifting speed of the jet grouting operation based on real-time collected ambient temperature and atmospheric pressure data specifically includes the following steps: An environmental monitoring station was established at the construction site to collect real-time data on ambient temperature and atmospheric pressure in the tunnel entrance area. The collected ambient temperature and atmospheric pressure data are input into a preset construction parameter mapping library, which contains the set grouting pressure, grout flow rate and drill rod lifting speed values ​​corresponding to different environmental conditions. Based on the mapping and matching results, control commands are automatically generated to drive the high-pressure grouting pump and drilling rig actuators, adjusting the jet grouting construction parameters to the set values ​​corresponding to the current environmental conditions.

4. The construction method of the collaborative support structure for the tunnel portal section in a high-altitude shallow-buried fine sand layer according to claim 3, characterized in that, The construction of the preset construction parameter mapping library specifically includes the following steps: Before formal construction, on-site process test piles were carried out to test the pile formation effect under different combinations of grouting pressure, grout flow rate and drill rod lifting speed under different atmospheric pressure and ambient temperature conditions. Optimal parameter combinations were selected to ensure that both the pile diameter and pile strength met the design requirements. The effective parameter combinations are correlated with the corresponding environmental conditions data and entered into the database to form the construction parameter mapping library.

5. The construction method of the collaborative support structure for the tunnel portal section in a high-altitude shallow-buried fine sand layer according to claim 1, characterized in that, In step S2, the process of adjusting the water-cement ratio and admixture dosage of the grout online based on feedback from grouting pressure and grout flow rate to form an interlocking horizontal jet grouting reinforcement layer specifically includes the following steps: Sensors installed on the grouting pipeline are used to monitor the grouting pressure and grout flow rate in real time during the high-pressure jet grouting process. The monitored grouting pressure and grout flow rate are compared with the preset benchmark diffusion range value to determine the diffusion resistance and loss of grout in the fine sand layer. Based on the comparison results, the water-cement ratio of the slurry and the amount of early strength agent and antifreeze agent added are adjusted online through an automatic slurry mixing system.

6. The construction method of the collaborative support structure for the tunnel portal section in a high-altitude shallow-buried fine sand layer according to claim 1, characterized in that, In step S3, the real-time data collection by pre-embedded sensors in the advanced pipe roof and horizontal high-pressure jet grouting piles, and the comparison of the monitoring data with the control threshold, specifically includes the following steps: Stress sensors and strain sensors are installed on the pipe body of the advanced pipe shed and on the stress-bearing parts of the horizontal high-pressure jet grouting pile. The system reads sensor values ​​in real time through an automated data acquisition system and calculates the current deformation of the surrounding rock and the stress value of the support structure. Retrieve the preset safety threshold corresponding to the current construction stage, and compare the calculated surrounding rock deformation and support structure stress value with the preset safety threshold.

7. The construction method of the collaborative support structure for the tunnel portal section in a high-altitude shallow-buried fine sand layer according to claim 1, characterized in that, In step S3, adjusting the excavation advance, support timing, and support parameters for the next cycle based on the comparison results specifically includes the following steps: When the comparison results show that the monitoring data reaches or exceeds the preset warning value, a warning signal is generated and the tunnel excavation advance length of the next cycle is reduced; Shorten the exposure time of the excavation face and the closure time of the initial support construction; Increase the stiffness parameters of the support structure, including increasing the installation spacing of the steel arch frames and increasing the thickness of the shotcrete.

8. The construction method of the collaborative support structure for the tunnel portal section in a high-altitude shallow-buried fine sand layer according to claim 1, characterized in that, In step S4, the tunnel excavation and support structure construction specifically include the following steps: The three-stage reserved core soil method was adopted to excavate the tunnel face in sections, retaining the core soil of the upper and middle stages to support the stability of the tunnel face; Following the excavation face, a steel frame is erected, and anchor pipes are installed at the arch foot of the steel frame. The ends of the anchor pipes are welded and fixed to the steel frame. Shotcrete seals the surface of the surrounding rock, making the initial support structure a closed loop; A cold-proof and heat-insulating layer is laid on the inner side of the initial support structure, and then secondary lining concrete is cast.

9. A collaborative support structure for the tunnel portal section in a high-altitude, shallowly buried silty fine sand layer, characterized in that... A construction method for the collaborative support structure at the portal section of a high-altitude shallow-buried silty fine sand layer tunnel as described in any one of claims 1-8 includes: The steel frame (1) is set at intervals along the longitudinal direction of the tunnel, the guide pipe (2) is fixedly set on the outer edge of the arch of the steel frame (1), the horizontal high-pressure jet grouting pile (3) is set through the guide pipe (2), the initial support concrete layer (4) covering the steel frame (1) and the surface of the surrounding rock, the cold protection and heat preservation layer (5) laid on the inner side of the initial support concrete layer (4), and the secondary lining (6) built on the inner side of the cold protection and heat preservation layer (5). The guide tubes (2) are arranged at intervals along the arch outline of the steel frame (1), and each guide tube (2) is welded and fixed to the outside of the steel frame (1). One end of the horizontal high-pressure jet grouting pile (3) passes through the guide pipe (2) and extends forward into the rock strata. The horizontal high-pressure jet grouting pile (3) forms an advanced reinforcement curtain along the tunnel arch. The initial support concrete layer (4) wraps around the root of the steel frame (1) and the guide pipe (2); The cold-proof insulation layer (5) is set close to the inner surface of the initial support concrete layer (4), and the secondary lining (6) is cast on the inner surface of the cold-proof insulation layer (5).

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