Method for relieving stress concentration of shallow-buried soft rock tunnel portal section

Abstract

The present application provides a kind of shallow soft rock tunnel portal section stress concentration relief method, belong to shallow soft rock tunnel construction technical field, the present application is formed by surface deep hole grouting curtain reinforcement ring to improve the strength of surrounding rock, adopts step method to control excavation disturbance with pipe roof advance support, burying multiple point displacement meter and surrounding rock pressure box collects surrounding rock radial displacement data and surrounding rock contact pressure data, based on monitoring data to determine the construction timing of secondary lining, establish the double game optimization model with the maximum stress uniformity and the maximum deformation control efficiency as the target, iteratively solve to obtain the thickness distribution optimization solution and stiffness distribution optimization solution, according to the optimization solution, the secondary lining thickness value and the steel configuration density of each calculation unit from the portal section to the tunnel body section are determined and the secondary lining is constructed, solve the technical problem that shallow soft rock tunnel portal section stress concentration leads to secondary lining cracking.

Description

Technical Field

[0001] This invention belongs to the field of shallow-buried soft rock tunnel construction technology, and specifically relates to a method for relieving stress concentration at the tunnel entrance section of a shallow-buried soft rock tunnel. Background Technology

[0002] In the construction of shallow-buried soft rock tunnels, the portal section, due to its shallow burial depth, low surrounding rock strength, and insufficient stress release, often employs measures such as grouting reinforcement, advanced support, and staged excavation to control surrounding rock deformation. Traditional methods improve the bearing capacity of the surrounding rock through pre-reinforcement, utilize pipe roofs and small guide pipes to form an advanced support system, and combine this with bench excavation to reduce disturbance. Monitoring data is used to determine the timing of secondary lining construction. However, in current tunnel engineering, due to significant differences in geological conditions between the portal and tunnel sections, the axial distribution of surrounding rock pressure is extremely uneven, and the stress concentration areas differ significantly from the bearing requirements of stable sections. Traditional methods, employing a uniform thickness for secondary lining design, fail to differentiate configuration based on the actual distribution of surrounding rock pressure. In other words, existing technologies present a technical problem: stress concentration at the portal section of shallow-buried soft rock tunnels leads to cracking of the secondary lining. Summary of the Invention

[0003] In view of this, the present invention provides a method for relieving stress concentration at the entrance section of a shallow-buried soft rock tunnel, which can solve the technical problem of secondary lining cracking caused by stress concentration at the entrance section of a shallow-buried soft rock tunnel in the prior art.

[0004] This invention is implemented as follows: It provides a method for alleviating stress concentration at the tunnel entrance of a shallow-buried soft rock tunnel. Before excavation of the tunnel entrance, deep-hole grouting is performed on the surface to form a curtain reinforcement ring. During excavation, a step-by-step excavation method is used, with pipe roof pre-support and small-diameter pipe grouting. After the initial support is completed, multiple displacement gauges and rock pressure cells are installed from the tunnel entrance to the tunnel body to collect radial displacement data and contact pressure data of the surrounding rock. Based on the radial displacement data, the timing of secondary lining construction is determined, and the rock pressure distribution and deformation amplitude values ​​of each part from the tunnel entrance to the tunnel body are calculated. A two-layer game optimization model of secondary lining thickness and support stiffness is established. Through iterative solution, the thickness distribution optimization solution and stiffness distribution optimization solution are obtained. The secondary lining thickness value of each calculation unit is determined according to the thickness distribution optimization solution, and the reinforcement density of each area is determined according to the stiffness distribution optimization solution before secondary lining is constructed. This achieves uniform stress distribution and coordinated deformation control, thereby alleviating stress concentration at the tunnel entrance.

[0005] Specifically, the deep hole grouting pre-reinforcement step involves grouting holes with a depth of 5 to 8 meters above the tunnel top and a spacing of 2 to 3 meters between holes. Cement-water glass dual-liquid fast-setting grout is used, and the grouting pressure is gradually increased to 2.0 MPa.

[0006] The curtain reinforcement ring refers to a continuous consolidation and reinforcement area formed in the surrounding rock above the tunnel roof by grouting through deep holes on the ground surface, which is used to improve the overall strength of the surrounding rock in the shallow buried section.

[0007] The cement-water glass dual-liquid fast-setting grout refers to a grout material in which cement grout and water glass solution are separately delivered by a dual-liquid grouting pump and mixed at the grouting orifice or inside the hole, with a setting time of 30 seconds to 5 minutes.

[0008] Specifically, the steps of the stepped excavation method are as follows: the advance per cycle is controlled at 0.5 to 0.8 meters, the pipe roof length is 15 to 20 meters, the external insertion angle is 5 to 10 degrees, the small pipe length is 3 to 5 meters, and the circumferential spacing is 0.3 to 0.5 meters.

[0009] The aforementioned pipe roof advanced support refers to a support structure formed by pre-installing steel pipes along the outer side of the excavation outline in front of the excavation face, with cement grout injected into the steel pipes to form a load-bearing unit.

[0010] The small-diameter pipe grouting refers to a support method in which small-diameter steel pipes are inserted into radial holes around the excavation face and then grouting is performed for reinforcement.

[0011] The multi-point displacement gauge refers to a monitoring device consisting of multiple displacement sensors buried along the radial borehole, with a monitoring section spacing of 5 to 10 meters and 4 to 6 measuring points arranged on each section.

[0012] The surrounding rock pressure cell refers to a pressure sensor embedded in the contact surface between the initial support and the surrounding rock, used to monitor the pressure value exerted by the surrounding rock on the support structure in real time.

[0013] Specifically, the step of determining the timing of secondary lining construction involves initiating the calculation of secondary lining thickness allocation when the daily deformation rate is less than 0.2 mm / d for 7 consecutive days and the cumulative deformation reaches more than 85% of the predicted value.

[0014] The aforementioned two-layer game optimization model refers to a nested optimization model consisting of an upper-layer model and a lower-layer model. The upper-layer model optimizes the secondary lining thickness with the goal of maximizing stress uniformity, while the lower-layer model optimizes the steel reinforcement density with the goal of maximizing deformation control efficiency.

[0015] The maximum stress uniformity is calculated using a stress uniformity index, which is equal to 1 minus the sum of squares of stress deviations in each calculation unit divided by the number of calculation units multiplied by the square of the standard stress value.

[0016] The calculation of the maximum deformation control efficiency adopts the deformation control efficiency index as an evaluation method. The deformation control efficiency index is equal to the product of the steel reinforcement density and the secondary lining thickness, divided by the standard stiffness parameter, multiplied by the negative surrounding rock deformation amplitude value of the natural constant, and divided by the standard deformation value raised to the power of the standard deformation value.

[0017] Specifically, the iterative solution step involves the upper-level model optimizing the secondary lining thickness value and then passing it to the lower-level model. Under the constraint of the secondary lining thickness value, the lower-level model optimizes the reinforcement configuration density and requires the deformation control efficiency index to be greater than 0.6.

[0018] Specifically, the step of determining the thickness of the secondary lining involves setting the thickness of the stress concentration area at the tunnel entrance to 60 to 80 cm, the thickness of the transition section to 50 to 60 cm, and the thickness of the stabilizing section to 40 to 50 cm.

[0019] Specifically, the step of determining the reinforcement density involves setting the reinforcement density in stress concentration areas to 150 to... The reinforcement density of the transition section is 100 to... The reinforcement density of the stable section is 80 to .

[0020] This invention achieves coordinated optimization of secondary lining thickness and reinforcement configuration through a two-layer game-theoretic optimization model. The upper-layer model optimizes the thickness distribution with the goal of maximizing stress uniformity, making the transmission of surrounding rock pressure more uniform across the secondary lining. The lower-layer model optimizes the reinforcement density with the goal of maximizing deformation control efficiency, thus enhancing deformation control under the constraint of a fixed thickness distribution. Through iterative solutions, differentiated thickness schemes are obtained: 60-80 cm for stress concentration areas, 50-60 cm for transition sections, and 40-50 cm for stable sections. Simultaneously, gradient reinforcement densities are configured from 150-200 kg / m³ to 80-100 kg / m³, achieving uniform transmission of surrounding rock pressure and effective constraint on deformation. In summary, this invention solves the technical problem mentioned in the background art of secondary lining cracking caused by stress concentration at the tunnel entrance section of shallow-buried soft rock tunnels. Attached Figure Description

[0021] Figure 1 This is a flowchart of the method of the present invention.

[0022] Figure 2 This is a time history curve of surrounding rock deformation monitoring.

[0023] Figure 3 This is a curve showing the distribution of surrounding rock pressure along the tunnel axis.

[0024] Figure 4 This is a diagram illustrating the iterative convergence process of a two-level game optimization model.

[0025] Figure 5Distribution diagram of optimized allocation results for secondary lining thickness. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below.

[0027] like Figure 1 The diagram shows a flowchart of a method for alleviating stress concentration at the entrance section of a shallow-buried soft rock tunnel, provided by the present invention. This method includes the following steps:

[0028] S10. Before excavation at the tunnel entrance, deep-hole grouting pre-reinforcement is carried out on the ground surface to form a curtain reinforcement ring. The grouting hole depth is 5 to 8 meters above the tunnel top, and the hole spacing is 2 to 3 meters. Cement-water glass double-liquid fast-setting grout is used, and the grouting pressure is increased in stages to 2.0 MPa.

[0029] S20. During the excavation process, the step method is used for sectional excavation. The advance of each cycle is controlled at 0.5 to 0.8 meters. At the same time, pipe roof advance support and small pipe grouting are installed. The pipe roof length is 15 to 20 meters, the external insertion angle is 5 to 10 degrees, the small pipe length is 3 to 5 meters, and the circumferential spacing is 0.3 to 0.5 meters.

[0030] S30. After the initial support is completed, multiple displacement gauges and surrounding rock pressure cells are installed from the tunnel entrance to the tunnel body. The monitoring section spacing is 5 to 10 meters, and 4 to 6 measuring points are set up in each section. A real-time monitoring system for displacement-time curves is established to collect radial displacement data and surrounding rock contact pressure data.

[0031] S40. Determine the timing of secondary lining construction based on the radial displacement data of the surrounding rock. When the daily deformation rate is less than 0.2 mm / d for 7 consecutive days and the cumulative deformation reaches more than 85% of the predicted value, start the calculation of secondary lining thickness distribution. Calculate the surrounding rock pressure distribution value and surrounding rock deformation amplitude value of each part from the tunnel entrance section to the tunnel body section based on the surrounding rock contact pressure data and the surrounding rock radial displacement data.

[0032] S50. Based on the surrounding rock pressure distribution value and the surrounding rock deformation amplitude value, a two-layer game optimization model of secondary lining thickness and support stiffness is established. The upper model aims to maximize stress uniformity, and the lower model aims to maximize deformation control efficiency. The thickness distribution optimization solution and stiffness distribution optimization solution are obtained through iterative solution.

[0033] S60. Determine the secondary lining thickness value of each calculation unit based on the thickness distribution optimization solution. The thickness of the stress concentration area at the opening is 60 to 80 cm, the thickness of the transition section is 50 to 60 cm, and the thickness of the stable section is 40 to 50 cm. Determine the reinforcement density of each area based on the stiffness distribution optimization solution. The reinforcement density of the stress concentration area is 150 to 200 kg / m³, the reinforcement density of the transition section is 100 to 150 kg / m³, and the reinforcement density of the stable section is 80 to 100 kg / m³.

[0034] S70. The secondary lining is constructed according to the secondary lining thickness and steel reinforcement density of each calculation unit. After the pouring is completed, the cracks that appear are repaired by chemical grouting. The grouting pressure is 0.5 to 1.5 MPa to relieve stress concentration in the opening section.

[0035] The curtain reinforcement ring refers to a continuous consolidation and reinforcement zone formed in the surrounding rock above the tunnel roof through deep-hole grouting. It is used to improve the overall strength and self-stabilizing capacity of the surrounding rock in shallow-buried sections and to block groundwater seepage channels. The cement-water glass dual-liquid rapid-setting grout refers to a grout material in which cement grout and water glass solution are separately delivered by a dual-liquid grouting pump and mixed at the grouting hole or inside the hole. The setting time is 30 seconds to 5 minutes, and it is used to quickly seal fissure channels in water-rich soft rock.

[0036] The aforementioned pipe roof pre-support refers to a support structure formed by pre-installing steel pipes along the outer side of the excavation outline in front of the excavation face. Cement grout is injected into the steel pipes to form load-bearing units, and the outer insertion angle is the angle between the steel pipe axis and the horizontal plane. The aforementioned small-diameter pipe grouting refers to a support method in which small-diameter steel pipes are inserted into radial holes around the excavation face and then grouting is performed for reinforcement. This method is used to reinforce the loose surrounding rock near the excavation outline.

[0037] The multi-point displacement gauge refers to a monitoring device composed of multiple displacement sensors embedded in the radial borehole, used to measure displacement changes at different depths within the surrounding rock. The surrounding rock pressure cell refers to a pressure sensor embedded at the contact surface between the initial support and the surrounding rock, used to monitor the pressure exerted by the surrounding rock on the support structure in real time. The real-time displacement-time curve monitoring system refers to a monitoring and analysis system that collects data from the multi-point displacement gauges and plots displacement-time curves to determine the convergence trend of surrounding rock deformation.

[0038] The surrounding rock radial displacement data refers to the sequence of radial displacement measurements of the surrounding rock collected by multi-point displacement gauges during the monitoring period, in mm, and is used to calculate the daily deformation rate and cumulative deformation. The surrounding rock contact pressure data refers to the sequence of pressure measurements of the contact surface between the surrounding rock and the initial support collected by the surrounding rock pressure cell during the monitoring period, in MPa, and is used to calculate the surrounding rock pressure distribution value.

[0039] The daily deformation rate refers to the rate value obtained by dividing the change in radial displacement data of the surrounding rock within two adjacent monitoring intervals by the time interval, with the unit being mm / d. It is used to quantify the time effect of surrounding rock deformation. The cumulative deformation prediction value refers to the final deformation of the surrounding rock estimated in advance through numerical simulation or empirical formulas. When the measured cumulative deformation reaches more than 85% of the cumulative deformation prediction value, it indicates that the stress release of the surrounding rock is basically completed.

[0040] The surrounding rock pressure distribution value refers to the pressure distribution value calculated based on the surrounding rock contact pressure data along the tunnel's axial and circumferential distribution characteristics, expressed in MPa, and is used to characterize the degree of stress concentration at different locations. The surrounding rock deformation amplitude value refers to the difference between the maximum and minimum displacements of each monitoring section calculated based on the surrounding rock radial displacement data, expressed in mm, and is used to characterize the degree of unevenness in surrounding rock deformation.

[0041] The aforementioned two-layer game optimization model refers to a nested optimization model consisting of an upper-layer model and a lower-layer model. The decision variables of the upper-layer model influence the constraints of the lower-layer model, and the optimization results of the lower-layer model are fed back to the upper-layer model, forming a coupling relationship. The upper-layer model aims to maximize stress uniformity, with the secondary lining thickness of each computational unit as the decision variable. Optimizing the thickness distribution helps to achieve a more uniform distribution of surrounding rock pressure. The lower-layer model aims to maximize deformation control efficiency, with the reinforcement density of each region as the decision variable. It optimizes the reinforcement configuration under the thickness distribution conditions determined by the upper-layer model.

[0042] The maximum stress uniformity function is used to evaluate the uniformity of stress distribution in various parts of the secondary lining structure. The input includes the surrounding rock pressure distribution value, the secondary lining thickness value of each calculation unit, and the number of calculation units. The output is the stress uniformity index, which is expressed as follows: The stress uniformity index is equal to 1 minus the sum of the squares of the stress deviations of each calculation unit divided by the number of calculation units multiplied by the square of the standard stress value. The stress deviation of each calculation unit is the surrounding rock pressure distribution value divided by the secondary lining thickness value of each calculation unit, minus the standard stress value. The standard stress value is the average value of the stress deviations of all calculation units. The unit of the stress deviation of each calculation unit is MPa / m, and the unit of the standard stress value is MPa / m. The stress uniformity index is a dimensionless parameter with a value range of 0 to 1. The closer the stress uniformity index is to 1, the more uniform the stress distribution.

[0043] The deformation control efficiency maximum function is used to evaluate the constraint effect of the support structure on the deformation of the surrounding rock. The input includes the deformation amplitude value of the surrounding rock, the reinforcement density, and the secondary lining thickness value of each calculation unit. The output is the deformation control efficiency index, which is expressed as follows: The deformation control efficiency index is equal to the product of the reinforcement density and the secondary lining thickness value of each calculation unit, divided by the standard stiffness parameter, and then multiplied by the negative surrounding rock deformation amplitude value of the natural constant, divided by the standard deformation value raised to the power of the standard deformation value. The standard stiffness parameter is taken as 100 kg, the standard deformation value is taken as 50 mm, the unit of reinforcement density is kg / m³, the unit of secondary lining thickness value of each calculation unit is m, the unit of surrounding rock deformation amplitude value is mm, and the deformation control efficiency index is a dimensionless parameter. The larger the deformation control efficiency index, the better the deformation control effect.

[0044] The coupling term of the maximum stress uniformity function and the maximum deformation control efficiency function is the secondary lining thickness value of each calculation unit. After the upper-level model optimizes the secondary lining thickness value of each calculation unit, it passes the result to the lower-level model. The lower-level model optimizes the reinforcement configuration density under the constraint of the secondary lining thickness value of each calculation unit. The deformation control efficiency index of the lower-level model is fed back to the upper-level model as a constraint condition, requiring the deformation control efficiency index to be greater than 0.6. The collaborative optimization of the two-level models is achieved through iterative calculation.

[0045] The thickness allocation optimization solution refers to the optimal allocation scheme of the secondary lining thickness values ​​for each computational unit obtained through iterative solution of the two-layer game optimization model, which is used to guide the thickness determination during secondary lining construction. The stiffness allocation optimization solution refers to the optimal allocation scheme of the reinforcement configuration density for each region obtained through iterative solution of the two-layer game optimization model, which is used to guide the reinforcement configuration during secondary lining construction.

[0046] The computational unit refers to a discretized grid unit that divides the secondary lining structure along the axial and circumferential directions. Each computational unit corresponds to an independent secondary lining thickness value and reinforcement density, used to achieve fine-grained allocation of thickness and stiffness. The stress concentration region refers to the area where the surrounding rock pressure distribution value is greater than 1.5 times the average surrounding rock pressure distribution value, mainly distributed in the shallower parts of the tunnel entrance section. The transition section refers to the area where the surrounding rock pressure distribution value is between 1.2 and 1.5 times the average surrounding rock pressure distribution value, located between the stress concentration region and the stable section. The stable section refers to the area where the surrounding rock pressure distribution value is less than 1.2 times the average surrounding rock pressure distribution value, mainly distributed in the deeper parts of the tunnel body section.

[0047] The chemical grouting repair refers to the repair technology of pressure injection of epoxy resin or polyurethane chemical grout into secondary lining cracks. After the grout penetrates into the cracks, it solidifies to form a cementitious body and restores the integrity of the structure.

[0048] The specific implementation methods of the above steps are described in detail below.

[0049] The specific implementation of step S10 is as follows: First, the spatial location and burial depth of the tunnel entrance section are determined according to the tunnel excavation design drawings. Grouting holes are then laid out on the ground surface along the tunnel axis in a staggered pattern. The hole depth is controlled within 5 to 8 meters above the tunnel top, and the hole spacing is set to 2 to 3 meters. Three-dimensional spatial positioning technology is used to ensure that the grouting holes accurately penetrate the overburden layer and enter the deep surrounding rock. Subsequently, a dual-liquid grouting pump is used to separately deliver cement grout and water glass solution into the grouting holes. The two grouts mix at the hole opening to form a fast-setting grout. The grouting process uses a staged pressurization method, with an initial pressure... The pressure is set at 0.5 MPa and gradually increased to 2.0 MPa based on the grout return situation. Grouting is stopped when a continuous solidified body is formed around the grouting hole. The deep hole grouting pre-reinforcement is based on the theory of permeation consolidation. High-pressure grout fills the fissures in the surrounding rock and cements the loose rock blocks to form a curtain reinforcement ring above the tunnel top. The curtain reinforcement ring bears part of the load of the overlying rock layer and blocks the seepage path of groundwater, reducing the stress release rate of the surrounding rock during subsequent excavation. This method is designed for shallow buried soft rock with shallow burial depth and water-rich characteristics, and establishes a surface reinforcement barrier in advance to create stable geological conditions for the excavation of the tunnel entrance section.

[0050] The specific implementation of step S20 is as follows: Before excavation, pipe roof holes are drilled along the outer edge of the excavation outline in front of the tunnel face. The length of the pipe roof holes is 15 to 20 meters, and the outer insertion angle is 5 to 10 degrees. Steel pipes are inserted into the holes and cement grout is injected to form a pipe roof advance support structure. After the pipe roof is laid out, the step method is used for excavation. The excavation advance of the upper step is controlled at 0.5 to 0.8 meters. Immediately after excavation, small guide pipe holes are drilled around the excavation face. The length of the small guide pipes is 3 to 5 meters, and the circumferential spacing is 0.3 to 0.5 meters. Steel pipes are inserted into the holes and grouting is performed for reinforcement. Subsequently, shotcrete is sprayed and steel arches are erected to form the initial support structure. Support and lower bench excavation are carried out after the initial support of the upper bench is completed. The excavation advance is consistent with that of the upper bench. The bench method of sectional excavation is based on the stress release control principle. By reducing the volume of single excavation, the range of stress disturbance in the surrounding rock is reduced, and the exposure time of the surrounding rock is limited by shortening the advance of each cycle. The pipe roof pre-support forms a pre-support arch before excavation, which bears part of the surrounding rock load and constrains the deformation of the surrounding rock. The small pipe grouting forms a reinforcement ring outside the excavation outline, which improves the strength and self-stabilizing ability of the shallow surrounding rock. The triple protection system works together to effectively control the surface settlement and surrounding rock deformation of the tunnel entrance section.

[0051] The specific implementation of step S30 is as follows: Drill measuring holes radially along the monitoring section after the initial support is completed. The hole depth is 3 to 5 meters. Multiple displacement sensors are embedded in the holes at different depths, with a spacing of 0.5 to 1.0 meters between each sensor. A pre-reserved trench is reserved at the contact surface between the initial support and the surrounding rock, and surrounding rock pressure cells are installed. Four to six surrounding rock pressure cells are deployed at each monitoring section, located at the arch crown, arch waist, and sidewalls. The spacing between monitoring sections along the tunnel axis is 5 to 10 meters, with a denser spacing of 5 meters at the tunnel entrance. The segment spacing can be widened to 10 meters. Multi-point displacement gauges and surrounding rock pressure cells are connected to the data acquisition system, and the acquisition frequency is set to once every 2 hours. The data acquisition system automatically records and stores the radial displacement data and contact pressure data of the surrounding rock, and simultaneously establishes a real-time monitoring system for displacement-time curves. The monitoring arrangement is based on the spatial distribution law of the surrounding rock deformation field. The spatiotemporal evolution characteristics of the internal displacement and surface pressure of the surrounding rock are obtained through multi-section and multi-measuring point combination monitoring, which provides basic data support for subsequent deformation convergence determination and stress distribution calculation.

[0052] The specific implementation of step S40 is as follows: Radial displacement data of the surrounding rock at each monitoring section is extracted from the real-time displacement-time curve monitoring system. The displacement difference between two adjacent monitoring times is calculated and divided by the time interval to obtain the daily deformation rate. The daily deformation rate of each monitoring point is continuously statistically analyzed over 7 days to determine whether all values ​​are less than the threshold of 0.2 mm / d. Simultaneously, the cumulative deformation at each monitoring point is calculated. A three-dimensional finite element model of the tunnel excavation is established using numerical simulation software. The surrounding rock physical and mechanical parameters and excavation procedures are input, and the cumulative deformation prediction value is obtained through simulation calculation. The measured cumulative deformation value is compared with the cumulative deformation prediction value. When the measured cumulative deformation reaches more than 85% of the cumulative deformation prediction value... When the stress release of the surrounding rock is basically completed, the secondary lining thickness distribution calculation program is started. The contact pressure data of the surrounding rock at each monitoring section is extracted from the data acquisition system. The continuous surrounding rock pressure distribution value is calculated by interpolation along the tunnel axis and circumference. The maximum and minimum displacement values ​​of each section are calculated based on the radial displacement data of the surrounding rock at each monitoring section. The difference between the two is the deformation amplitude value of the surrounding rock. The judgment method is based on the rheological theory of surrounding rock and the deformation convergence criterion. The daily deformation rate is used to determine whether the surrounding rock has entered the stable deformation stage. The cumulative deformation ratio is used to determine the degree of stress release completion. This ensures that the surrounding rock is in a relatively stable state during the construction of the secondary lining and avoids cracking of the secondary lining due to continuous stress.

[0053] The specific implementation of step S50 is as follows: the secondary lining structure is divided into several calculation units along the axial and circumferential directions. The surrounding rock pressure distribution value and surrounding rock deformation amplitude value at the corresponding positions of each calculation unit are input. A two-layer game optimization model solver is established. The upper-layer model sets the decision variable as the secondary lining thickness value of each calculation unit, the objective function as the function for maximizing stress uniformity, and the constraints include a thickness range of 0.4 to 0.8 meters and a deformation control efficiency index greater than 0.6 fed back by the lower-layer model. The lower-layer model sets the decision variable as the reinforcement density of each region, the objective function as the function for maximizing deformation control efficiency, and the constraints include a reinforcement density range of 80 to 200 kg / m³ and the secondary lining thickness value of each calculation unit transmitted by the upper-layer model. The iterative solution process first optimizes the secondary lining thickness value of each calculation unit according to the surrounding rock pressure distribution value, calculates the stress deviation of each calculation unit and substitutes it into the calculation. The stress uniformity index is obtained by maximizing the stress uniformity function. The optimized secondary lining thickness values ​​of each calculation unit are then passed to the lower-level model. The lower-level model optimizes the reinforcement density based on the surrounding rock deformation amplitude and the received secondary lining thickness values ​​of each calculation unit. The deformation control efficiency index is calculated and substituted into the deformation control efficiency maximizing function. This index is then fed back to the upper-level model as a constraint. The iteration is repeated until the changes in both the stress uniformity index and the deformation control efficiency index are less than 0.01. The final optimized solutions for thickness and stiffness distribution are then output. This two-layer game optimization is based on Stackelberg game theory. The upper-level model, as the leader, prioritizes the thickness distribution scheme, while the lower-level model, as the follower, optimizes the stiffness configuration under the thickness constraint. Through coupling terms, the synergistic optimization of thickness and stiffness is achieved, enabling the secondary lining structure to have sufficient deformation control capability while maintaining uniform stress distribution.

[0054] The specific implementation of steps S60 and S70 is as follows: Based on the secondary lining thickness values ​​of each calculation unit in the thickness distribution optimization solution, a secondary lining thickness distribution map is drawn. The thickness of the stress concentration area at the opening section is determined to be 60 to 80 cm, the transition section thickness to be 50 to 60 cm, and the stable section thickness to be 40 to 50 cm. Based on the reinforcement density of each region in the stiffness distribution optimization solution, the amount of steel reinforcement is calculated and a reinforcement diagram is drawn. The reinforcement density in the stress concentration area is 150 to 200 kg / m³, and the reinforcement density in the transition section is 100 to 150 kg / m³. The reinforcement density of the stable section is 80 to 100 kg / m³. The secondary lining formwork is installed and the reinforcement is tied according to the thickness distribution diagram and reinforcement diagram. Concrete is poured and cured to the design strength. After demolding, the distribution of cracks on the secondary lining surface is checked. Cracks are repaired by chemical grouting. Before grouting, the surface of the cracks is cleaned and grouting nozzles are installed. The grouting pressure is controlled within the range of 0.5 to 1.5 MPa. After the grout fills the cracks, grouting is stopped and cured. Stress concentration in the opening section is relieved by optimizing the thickness and stiffness configuration by zone.

[0055] It should be noted that the key technical concept of this invention includes a method for determining the timing of secondary lining construction based on deformation rate convergence. This method uses a dual indicator of daily deformation rate over seven consecutive days and cumulative deformation percentage to determine the stress release state of the surrounding rock. Traditional methods rely on single time intervals or empirical judgments, making it difficult to accurately grasp the timing of soft rock rheological stability. This invention combines rate convergence characteristics and quantitative assessment of deformation completion to ensure that the surrounding rock is in a stable state during secondary lining construction. This avoids structural cracking under continuous stress due to premature construction and also avoids the risk of surrounding rock instability due to late construction, thus improving the scientificity and accuracy of secondary lining construction timing selection. A two-layer game-theoretic optimization model achieves coordinated configuration of thickness and stiffness. The upper model optimizes thickness distribution to make stress more uniform, while the lower model optimizes steel reinforcement configuration to improve deformation control efficiency. The two models form a mutual constraint and feedback mechanism through thickness coupling terms. Traditional methods use single-objective optimization or empirical configuration, which cannot simultaneously consider stress uniformity and deformation control requirements. This invention balances the two optimization objectives through a game-theoretic framework, achieving refined and rational allocation of materials. It optimizes material use while ensuring structural safety, increasing the thickness and strengthening stress concentration areas and thinning and weakening stable areas, consistent with actual stress characteristics. The triple protection system works together to control the deformation of the shallow buried section. The surface curtain reinforcement ring establishes a bearing barrier in advance, the pipe roof supports form a pre-supported arch, and the small pipe grouting reinforces the outer edge of the excavation outline. The three lines of defense advance layer by layer from the surface to the inside of the tunnel. Traditional methods often use a single support method, which is difficult to effectively control the large deformation and surface settlement of shallow buried soft rock. This invention significantly reduces the stress release rate and deformation development range of the surrounding rock through multi-layered protection in space and advanced reinforcement in time, creating stable conditions for subsequent secondary lining construction. The synergistic effect of the above key technologies forms a complete stress concentration mitigation technology system. The triple protection system controls excavation disturbance and deformation development, deformation monitoring and timing determination ensure the stability of the surrounding rock during secondary lining construction, and the dual-layer game optimization achieves precise configuration of thickness and stiffness. The three links are interconnected and progressively advance, controlling stress release from the source, monitoring deformation convergence during the process, and optimizing structural configuration during construction. Compared with the single measures or experience-based control of traditional methods, this invention fundamentally alleviates the stress concentration problem at the entrance section of shallow buried soft rock tunnels through the synergistic effect of multiple technologies, and improves the long-term stability and safety of the secondary lining structure.

[0056] It should be noted that this invention also solves the following technical problem: the difficulty in accurately determining the timing of secondary lining construction at the portal section of shallow-buried soft rock tunnels, leading to insufficient support effectiveness. This invention establishes a real-time displacement-time curve monitoring system by embedding multi-point displacement gauges, collecting radial displacement data of the surrounding rock and calculating the daily deformation rate. When the daily deformation rate is less than 0.2 mm for seven consecutive days and the cumulative deformation reaches more than 85% of the predicted value, it is determined that the stress release of the surrounding rock is basically complete. Initiating secondary lining construction at this time ensures that the deformation of the surrounding rock tends to stabilize, avoiding excessive deformation in the later stages that would generate additional loads on the secondary lining, and also allows for timely sealing of the surrounding rock to prevent weathering and deterioration, thus achieving quantitative determination of the construction timing. The invention also addresses the technical problem of insufficient pre-reinforcement effect due to the lack of systematic coordination between advance support and surrounding rock reinforcement measures at the portal section of shallow-buried soft rock tunnels. This invention improves the overall strength of the surrounding rock in shallow buried sections by forming a continuous curtain reinforcement ring through deep hole grouting on the ground surface. Combined with pipe roof advanced support to form a load-bearing arch frame and small pipe grouting to reinforce the loose surrounding rock near the excavation outline, the three measures form a progressive reinforcement system from the outside to the inside, establishing a stable load-bearing structure before excavation and effectively controlling the initial deformation of the surrounding rock.

[0057] Specifically, the principle of this invention is as follows: The solution to the problem of secondary lining cracking caused by stress concentration at the tunnel entrance of shallow-buried soft rock tunnels lies in the real-time acquisition of surrounding rock contact pressure data and radial displacement data using a surrounding rock pressure cell and multi-point displacement gauges. This accurately identifies the location and degree of stress concentration areas, providing a data foundation for differentiated design. The upper layer of the two-layer game optimization model controls stress deviation to a minimum by adjusting the secondary lining thickness of each calculation unit, avoiding excessive local stress concentration. The lower layer optimizes the reinforcement configuration under thickness constraints, enhancing the support structure's resistance to deformation. The two models achieve synergistic effects through coupling terms. This data-driven optimization method ensures sufficient bearing capacity and stiffness reserve in stress concentration areas, while avoiding material waste in transition and stable sections. This eliminates the risk of cracking caused by stress concentration from the structural design perspective, conforming to the fundamental principles of stress homogenization and deformation coordination in materials mechanics.

[0058] The following provides a specific embodiment 1 of the present invention, and the specific implementation of each step in this embodiment 1 is described in detail below.

[0059] The specific implementation of step S10 involves pre-reinforcing the tunnel entrance section with deep-hole grouting on the surface before excavation to form a curtain reinforcement ring. The grouting hole depth is 5 to 8 meters above the tunnel top, with a hole spacing of 2 to 3 meters. A cement-water glass dual-liquid fast-setting grout is used, and the grouting pressure is increased in stages to 2.0 MPa. The cement-water glass dual-liquid fast-setting grout is delivered by a dual-liquid grouting pump, and the cement grout and water glass solution are mixed at the grouting hole opening or inside the hole. The setting time is 30 seconds to 5 minutes. This grout is used to quickly seal fissure channels in water-rich soft rock, improving the overall strength and self-stabilizing ability of the surrounding rock in the shallow buried section.

[0060] The specific implementation of step S20 involves using a stepped excavation method during the excavation process, with each cycle's advance controlled at 0.5 to 0.8 meters. Simultaneously, pipe roof pre-support and small-diameter pipe grouting are installed. The pipe roof length is 15 to 20 meters, with an outward insertion angle of 5 to 10 degrees. The small-diameter pipes are 3 to 5 meters long, with a circumferential spacing of 0.3 to 0.5 meters. Pipe roof pre-support is a support structure formed by pre-installing steel pipes along the outer edge of the excavation outline in front of the excavation face. Cement grout is injected into the steel pipes to form load-bearing units. Small-diameter pipe grouting involves radially drilling around the excavation face, inserting small-diameter steel pipes, and then grouting for reinforcement.

[0061] The specific implementation of step S30 involves installing multi-point displacement gauges and surrounding rock pressure cells from the tunnel entrance to the tunnel body after the initial support is completed. The monitoring cross-section spacing is 5 to 10 meters, with 4 to 6 measuring points set up at each cross-section. A real-time displacement-time curve monitoring system is established to collect radial displacement data and surrounding rock contact pressure data. The multi-point displacement gauge is a monitoring device composed of multiple displacement sensors installed along the radial borehole, and the surrounding rock pressure cell is a pressure sensor installed at the contact surface between the initial support and the surrounding rock.

[0062] The specific implementation of step S40 is based on determining the timing of secondary lining construction using radial displacement data of the surrounding rock. When the daily deformation rate is less than 0.2 mm / d for seven consecutive days and the cumulative deformation reaches more than 85% of the predicted value, the calculation of secondary lining thickness allocation is initiated. (Daily deformation rate...) The calculation formula is:

[0063] ;

[0064] In the formula, This represents the daily deformation rate, expressed in mm / d. For the first The radial displacement data of the surrounding rock monitored this time, in mm; For the first The radial displacement data of the surrounding rock monitored this time, in mm; For the first The monitoring time is measured in days (d). For the first The monitoring time is measured in days (d). This is the serial number of the monitoring session, dimensionless. Cumulative deformation. The cumulative value of the radial displacement data of the surrounding rock during the monitoring period, in mm, is calculated using the following formula:

[0065] ;

[0066] In the formula, Cumulative deformation, in mm; For the first The radial displacement data of the surrounding rock monitored this time, in mm; These are the radial displacement data of the surrounding rock at the initial monitoring time, in mm. Cumulative deformation prediction values. The final deformation of the surrounding rock, estimated in advance through numerical simulation or empirical formulas, is expressed in mm. This indicates that the stress release in the surrounding rock is basically complete. Surrounding rock pressure distribution value. The values ​​are calculated based on the distribution characteristics of the surrounding rock contact pressure along the tunnel's axial and circumferential directions, and are expressed in MPa. The calculation unit number is dimensionless. The value represents the deformation amplitude of the surrounding rock. Calculated based on the radial displacement data of the surrounding rock, denoted as the first... The difference between the maximum and minimum displacements of each monitoring section, in mm. The monitoring section number is dimensionless, and the calculation formula is as follows:

[0067] ;

[0068] In the formula, For the first The deformation amplitude of the surrounding rock at each monitoring section is expressed in mm. For the first The maximum radial displacement of the surrounding rock at each monitoring section, in mm; For the first The minimum radial displacement of the surrounding rock at each monitoring section, in mm.

[0069] The specific implementation of step S50 involves establishing a two-layer game-theoretic optimization model of secondary lining thickness and support stiffness based on the surrounding rock pressure distribution and deformation amplitude values. The upper-layer model aims to maximize stress uniformity, while the lower-layer model aims to maximize deformation control efficiency. The thickness distribution optimization solution and stiffness distribution optimization solution are obtained through iterative solving. The stress uniformity index of the upper-layer model... The calculation formula is:

[0070] ;

[0071] In the formula, This is a stress uniformity index, a dimensionless parameter, with a value ranging from 0 to 1. The closer the value is to 1, the more uniform the stress distribution. The number of units is dimensionless. For the first The stress deviation of each calculation unit, in MPa / m, is calculated using the following formula: ; For the first The surrounding rock pressure distribution value of each calculation unit, in MPa; For the first The secondary lining thickness value for each calculation unit, in meters; The calculation unit number has a value range of 1 to 1. Dimensionless; The standard stress value is expressed in MPa / m, and the calculation formula is as follows: Deformation control efficiency index of the lower-level model The calculation formula is:

[0072] ;

[0073] In the formula, The deformation control efficiency index is a dimensionless parameter. A larger value indicates better deformation control. The density of the reinforcing steel is specified in units of... ; For the first The secondary lining thickness value for each calculation unit, in meters; For the standard cross-sectional area, the empirical value is... ; This is the standard stiffness parameter, with an empirical value of 100 kg; This represents the deformation amplitude of the surrounding rock, in mm. The standard deformation value is 50mm (empirical value). After optimizing the secondary lining thickness values ​​for each calculation unit in the upper model, the results are passed to the lower model. The lower model optimizes the reinforcement density under the constraint of this secondary lining thickness value. The deformation control efficiency index of the lower model is fed back to the upper model as a constraint condition, requiring… The two-layer model is optimized through iterative calculation.

[0074] The specific implementation of step S60 involves determining the secondary lining thickness value for each calculation unit based on the thickness distribution optimization solution. The thickness of the stress concentration zone at the opening is 60 to 80 cm, the transition zone is 50 to 60 cm, and the stable zone is 40 to 50 cm. The reinforcement density for each region is determined based on the stiffness distribution optimization solution, with the reinforcement density in the stress concentration zone being 150 to 200. The reinforcement density of the transition section is 100 to 150. The reinforcement density of the stable section is 80 to 100. Average surrounding rock pressure distribution value The calculation formula is:

[0075] ;

[0076] In the formula, This represents the average surrounding rock pressure distribution value, in MPa. The number of units is dimensionless. For the first The surrounding rock pressure distribution value for each calculation unit, in MPa. Stress concentration areas refer to regions that satisfy... The transition section refers to the area that meets the requirements. The stable segment refers to the region that satisfies The area.

[0077] The specific implementation of step S70 is to construct the secondary lining according to the secondary lining thickness value and steel reinforcement density of each calculation unit. After the pouring is completed, the cracks that appear are repaired by chemical grouting. The grouting pressure is 0.5 to 1.5 MPa. The chemical grouting repair uses epoxy resin or polyurethane chemical grout for pressure injection. After the grout penetrates into the crack, it solidifies to form a cement body to restore the integrity of the structure and relieve the stress concentration in the opening section.

[0078] It should be noted that the variables involved in this invention are explained in detail in Table 1.

[0079] Table 1. Variable Explanation Table

[0080]

[0081] To better understand and implement this invention, the following is a specific application scenario example 2: Before excavation of the tunnel entrance section, the technical team implemented deep-hole grouting pre-reinforcement on the surface. The grouting hole depth was set at 6 meters above the tunnel top, with a hole spacing of 2.5 meters, arranged in a quincunx pattern, totaling 48 grouting holes. A cement-water glass dual-liquid fast-setting grout was used, with a cement grout water-cement ratio of 0.8:1, a water glass concentration of 35 Baume degrees, and a setting time controlled at 90 seconds. The initial grouting pressure was 0.5 MPa, gradually increased according to the grout return situation, ultimately reaching 2.0 MPa, with a single hole grouting volume of 1.2 to 1.8 MPa. This forms a curtain reinforcement ring with a thickness of about 8 meters. The permeability coefficient of the curtain reinforcement ring is reduced to one-tenth of that of the original surrounding rock, effectively blocking the seepage path of groundwater.

[0082] The excavation stage employs a bench excavation method, with the upper bench height being 4.5 meters and the lower bench height being 3.5 meters. Each cycle's advance is controlled at 0.6 meters. Before excavation, pipe roof support is installed ahead of the tunnel face. The pipe roof uses seamless steel pipes with a diameter of 108mm and a wall thickness of 6mm, with a length of 18 meters, an outward insertion angle of 8 degrees, and a circumferential spacing of 0.4 meters. Cement mortar is injected into the pipe roof, with a cement usage of 150kg per meter of pipe roof. Immediately after the upper bench excavation, small guide pipe holes are drilled around the excavation face. The small guide pipes are 4 meters long, with a circumferential spacing of 0.4 meters, and use steel pipes with a diameter of 42mm and a wall thickness of 3.5mm. The grouting pressure in the holes is 1.5MPa, and the grouting volume per hole is 0.08 to 0.12 MPa. During the excavation process, the surface settlement rate at the tunnel entrance was controlled within 3 mm / d, which was about 70% lower than that of the adjacent section without the triple protection system.

[0083] After the initial support was completed, the technical team deployed 12 monitoring sections from the tunnel entrance to the tunnel body. The spacing between sections was 5 meters within the first 60 meters of the tunnel entrance and 10 meters within the tunnel body. Each monitoring section had a 4-meter-deep borehole drilled radially, with 5 multi-point displacement gauges installed at 0.8-meter intervals within each borehole. Rock pressure cells were also installed at 5 locations: the arch crown, the left and right arch waists, and the left and right sidewalls. Monitoring data was collected every 2 hours. Figure 2 As shown, after 45 days of continuous monitoring, the daily deformation rate of each monitoring section dropped to below 0.15 mm / d, and the cumulative deformation reached 87% of the numerical simulation prediction value. The technical team determined that the stress release of the surrounding rock was basically completed and started the calculation of the secondary lining thickness distribution.

[0084] Based on the surrounding rock contact pressure data from 12 monitoring sections, the technical team plotted the surrounding rock pressure distribution curve along the tunnel axis, as shown in the figure. Figure 3 As shown in Table 2, the surrounding rock pressure distribution values ​​are 0.35 to 0.52 MPa in the 0 to 40 meter range at the tunnel entrance, 0.28 to 0.38 MPa in the 40 to 80 meter range, and 0.18 to 0.28 MPa in the 80 to 120 meter range. The surrounding rock deformation amplitude reaches a maximum of 18 mm in the tunnel entrance section and decreases to 8 mm in the tunnel body section.

[0085] Table 2 Statistical table of surrounding rock deformation parameters at each monitoring section

[0086]

[0087] The technical team divided the secondary lining structure into 192 calculation units: one 5-meter unit along the axial direction and one 45-degree unit along the circumferential direction. The surrounding rock pressure distribution and deformation amplitude values ​​at corresponding locations in each calculation unit were input into the solver of a two-layer game-theoretic optimization model. The upper-layer model set the secondary lining thickness as the decision variable for each calculation unit, ranging from 0.4 to 0.8 meters, with the objective function being the function for maximizing stress uniformity. Constraints included a deformation control efficiency index greater than 0.6 fed back from the lower-layer model, and the lower-layer model setting the reinforcement density in each region as the decision variable, ranging from 80 to 200 kg / m³, with the objective function being the function for maximizing deformation control efficiency, and constraints being the secondary lining thickness values ​​for each calculation unit transmitted from the upper-layer model. Figure 4 As shown, after 18 iterations, the stress uniformity index converged to 0.82 and the deformation control efficiency index converged to 0.74. The changes in stress uniformity and deformation control efficiency during the optimization process were both less than 0.01. The solver output the final optimized solutions for thickness distribution and stiffness distribution.

[0088] Based on the optimized thickness distribution solution, the 0-40 meter range at the tunnel entrance is identified as the stress concentration zone, with a secondary lining thickness of 65-75 cm for each calculation unit and an average thickness of 70 cm. The 40-80 meter range is identified as the transition zone, with a secondary lining thickness of 52-58 cm for each calculation unit and an average thickness of 55 cm. The 80-120 meter range is identified as the stable zone, with a secondary lining thickness of 42-48 cm for each calculation unit and an average thickness of 45 cm. Figure 5 As shown, according to the stiffness distribution optimization solution, the reinforcement density in the stress concentration area is 180 kg / m³, with double-layer bidirectional reinforcement, main reinforcement diameter 20 mm, and spacing 150 mm. The reinforcement density in the transition section is 125 kg / m³, main reinforcement diameter 18 mm, and spacing 200 mm. The reinforcement density in the stable section is 90 kg / m³, main reinforcement diameter 16 mm, and spacing 250 mm.

[0089] The secondary lining construction was carried out according to the thickness distribution diagram and reinforcement diagram. The concrete strength grade was C30, and the slump was controlled between 120 and 160 mm. Pumping technology was used for pouring, with each pour being 12 meters long. The curing time was no less than 14 days. After demolding, an inspection revealed a small number of surface cracks in the stress concentration area. The cracks were 0.2 to 0.5 mm wide and less than 30 mm deep. The technical team used epoxy resin chemical grout for repair, with the grouting pressure controlled at 0.8 MPa. After grouting, the compressive strength of the cementitious body formed at the cracks reached 35 MPa, exceeding the strength of the concrete itself. After the repair was completed, the secondary lining structure was monitored for 3 months. No new cracks appeared in the stress concentration area, the deformation tended to stabilize, and the stress concentration at the opening section was effectively alleviated.

[0090] The advancement of this invention over traditional methods lies in the fact that traditional methods typically employ uniform thickness configurations or empirical thickening approaches for secondary lining at tunnel entrances. These methods fail to address the spatial variations in stress distribution, leading to insufficient thickness in stress concentration zones and material waste in stable zones. This invention, however, utilizes a two-layer game-theoretic optimization model to achieve a coordinated configuration of thickness and stiffness. Stress concentration zones are directionally thickened and strengthened, while stable zones are appropriately thinned, ensuring that the load-bearing capacity of the secondary lining structure matches the actual stress state. This fundamentally alleviates stress concentration. Furthermore, traditional methods often rely on fixed time intervals or single deformation indicators for determining the timing of secondary lining construction, making it difficult to accurately grasp the rheological stability of soft rock. At any given moment, this invention employs a dual indicator of daily deformation rate and cumulative deformation percentage to ensure that the stress release of the surrounding rock is fully completed during the construction of the secondary lining, thus avoiding structural cracking under continuous stress. Traditional methods often use single support measures for deformation control in shallow buried sections, which have limited effectiveness in controlling large deformations and surface settlement in soft rock. This invention establishes a triple protection system consisting of a surface curtain reinforcement ring, pipe roof pre-support, and small-diameter pipe grouting. This system progressively controls the stress release and deformation development of the surrounding rock from the surface to the tunnel interior, creating stable geological conditions for subsequent secondary lining construction. Through the synergistic effect of multiple technologies, it significantly improves the construction safety and long-term structural stability of the tunnel entrance section in shallow buried soft rock.

[0091] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for alleviating stress concentration at the entrance section of a shallow-buried soft rock tunnel, characterized in that, Before excavation of the tunnel entrance section, deep-hole grouting was carried out on the ground surface to form a curtain reinforcement ring. During the excavation process, the step method was used for sectional excavation and pipe roof pre-support and small pipe grouting were installed. After the initial support was completed, multiple displacement gauges and surrounding rock pressure cells were installed from the tunnel entrance section to the tunnel body section to collect radial displacement data and surrounding rock contact pressure data. Based on the radial displacement data of the surrounding rock, the timing of secondary lining construction was determined and the surrounding rock pressure distribution value and surrounding rock deformation amplitude value of each part from the tunnel entrance section to the tunnel body section were calculated. A two-layer game optimization model of secondary lining thickness and support stiffness was established. The thickness distribution optimization solution and stiffness distribution optimization solution were obtained by iterative solution. The secondary lining thickness value of each calculation unit was determined according to the thickness distribution optimization solution and the reinforcement configuration density of each area was determined according to the stiffness distribution optimization solution before the secondary lining was constructed. This achieved uniform stress distribution and coordinated deformation control, thereby alleviating stress concentration in the tunnel entrance section.

2. The method according to claim 1, characterized in that, The deep hole grouting pre-reinforcement step specifically involves grouting holes with a depth of 5 to 8 meters above the tunnel top and a spacing of 2 to 3 meters between holes. Cement-water glass dual-liquid fast-setting grout is used, and the grouting pressure is gradually increased to 2.0 MPa.

3. The method according to claim 2, characterized in that, The curtain reinforcement ring refers to a continuous consolidation and reinforcement area formed in the surrounding rock above the tunnel roof by grouting through deep holes on the ground surface, which is used to improve the overall strength of the surrounding rock in the shallow buried section.

4. The method according to claim 3, characterized in that, The cement-water glass dual-liquid fast-setting grout refers to a grout material in which cement grout and water glass solution are separately delivered by a dual-liquid grouting pump and mixed at the grouting orifice or inside the hole, with a setting time of 30 seconds to 5 minutes.

5. The method according to claim 4, characterized in that, The steps of the stepped excavation method are as follows: the advance per cycle is controlled at 0.5 to 0.8 meters, the pipe roof length is 15 to 20 meters, the external insertion angle is 5 to 10 degrees, the small pipe length is 3 to 5 meters, and the circumferential spacing is 0.3 to 0.5 meters.

6. The method according to claim 5, characterized in that, The aforementioned pipe roof advanced support refers to a support structure formed by pre-installing steel pipes along the outer side of the excavation outline in front of the excavation face, with cement grout injected into the steel pipes to form a load-bearing unit.

7. The method according to claim 6, characterized in that, The small-diameter pipe grouting refers to a support method in which small-diameter steel pipes are inserted into radial holes around the excavation face and then grouting is performed for reinforcement.

8. The method according to claim 7, characterized in that, The multi-point displacement gauge refers to a monitoring device consisting of multiple displacement sensors buried along a radial borehole, with a monitoring section spacing of 5 to 10 meters and 4 to 6 measuring points arranged on each section.

9. The method according to claim 8, characterized in that, The surrounding rock pressure cell refers to a pressure sensor embedded in the contact surface between the initial support and the surrounding rock, used to monitor the pressure value exerted by the surrounding rock on the support structure in real time.

10. The method according to claim 9, characterized in that, The specific step for determining the timing of secondary lining construction is to initiate the calculation of secondary lining thickness allocation when the daily deformation rate is less than 0.2 mm / d for 7 consecutive days and the cumulative deformation reaches more than 85% of the predicted value.

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