Active unloading reinforcement method for pipe jacking tunnel underpassing railway under ultra-shallow covering condition

Abstract

This invention relates to the field of underground engineering and tunnel construction technology, and discloses an active unloading reinforcement method for pipe jacking tunnels passing under railways under conditions of extremely shallow overburden. By acquiring parameters of the railway's influence zone, temporary supports are erected at both ends, and construction beams are fixed to the rails to construct a primary load transfer path. The parameters and stiffness data are input into a model to solve the benchmark theoretical sequence. Reinforcement columns and connecting beams are constructed on both sides of the tunnel to form a portal pier structure, generating a secondary load transfer path and isolating the low-stress construction zone. Within this zone, the pipe jacking machine excavates, controlling the soil chamber pressure at the target threshold. Field monitoring data is collected and aligned with the benchmark sequence to generate a dynamic deviation matrix. When the residual reaches the warning threshold, parameters are calculated and compensatory grouting is performed. This invention effectively controls the deformation and settlement of existing railway lines by physically supporting and cutting off the train's live load transfer path, combined with closed-loop data feedback to dynamically control ground erosion.

Description

Technical Field

[0001] This invention relates to the field of underground engineering and tunnel construction technology, specifically to an active unloading and reinforcement method for pipe jacking tunnels passing under railways under conditions of extremely shallow overburden. Background Technology

[0002] With the deepening development of urban underground space, the demand for tunnel boring machines (TBMs) passing under existing railway lines is gradually increasing. When the ratio of the overburden thickness to the outer diameter of the TBM is less than or equal to 0.8, the project is considered to be under ultra-shallow overburden conditions. Under these conditions, the disturbance effect of tunnel excavation on the upper railway subgrade is significant. The continuous and repeated action of train dynamic loads combined with the ground loss caused by TBM excavation can easily lead to settlement and deformation of the railway track exceeding the allowable range, directly affecting railway traffic safety.

[0003] Existing technologies for reinforcing pipe jacking tunnels passing under railways typically employ large-diameter pipe roof support, full-section deep-hole grouting reinforcement, or direct pile foundation replacement. However, these methods have significant structural and technological defects when dealing with extremely shallow overburden conditions. Large-diameter pipe roof support experiences significant ground disturbance during construction, making it prone to surface collapse under extremely shallow overburden conditions. Furthermore, a single pipe roof structure cannot withstand high-frequency train dynamic loads over extended periods. Full-section deep-hole grouting reinforcement is labor-intensive and costly, and the uniformity of grout diffusion in complex soil is difficult to control. This method essentially relies on passively increasing soil strength, failing to fundamentally transfer the load from above. Direct pile foundation replacement requires piling and abutment construction directly beneath the railway line, severely disrupting normal railway operations and making it virtually impossible to implement under conditions requiring uninterrupted railway operation. Some conventional shallow-overburden anti-settlement pipe jacking structures only address the strengthening of the pipe itself, failing to solve the problem of dynamic load penetration downwards.

[0004] Overall, existing technologies generally lack an integrated design for active load transfer and settlement path blocking, do not employ a side-mounted portal pier active unloading technical architecture, and lack corresponding multi-source collaborative stress calculation models as parameter guidance. This results in existing reinforcement systems being unable to accurately match the actual working conditions of different pipe jacking diameters, making it difficult to simultaneously meet the core engineering requirements of low construction disturbance, low operational interference, and high-precision settlement control under ultra-shallow overburden conditions. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides an active unloading and reinforcement method for pipe jacking tunnels passing under railways under ultra-shallow overburden conditions. This method solves the problem that existing technologies cannot actively transfer the dynamic load of trains and block the settlement transmission path under ultra-shallow overburden conditions, resulting in large disturbances during pipe jacking and difficulty in controlling railway settlement.

[0006] To achieve the above objectives, the present invention provides the following technical solution: an active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions, the method comprising the following steps: S100: Obtain the distribution parameters of the railway impact zone; set up temporary supports at both ends and erect construction temporary beams, and fix the construction temporary beams to the rails to construct a primary load transfer path; S200: Input the distributed parameters and temporary support stiffness data into the calculation model to solve the reference theoretical sequence; construct reinforcement columns and connecting beams on both sides of the tunnel according to the reference theoretical sequence to form a portal pier structure; use the portal pier structure to generate a secondary transfer load path to isolate the low stress zone; S300, operating the pipe jacking machine in a low-stress zone; collecting soil chamber pressure and adjusting jacking parameters to maintain soil chamber pressure within the target earth pressure threshold; S400 collects settlement data and aligns it with the benchmark theoretical sequence to generate a dynamic deviation matrix; when the residual eigenvalue in the dynamic deviation matrix reaches the warning threshold, it calculates the grouting parameters and performs compensatory grouting to adjust the formation stress.

[0007] Preferably, in step S100, obtaining the distribution parameters of the railway influence area specifically includes: Obtain the soil cover thickness and outer diameter of the proposed pipe jacking tunnel; Calculate the ratio of the soil cover thickness to the outer diameter of the jacking tunnel. If the ratio is not greater than 0.8, it is determined that the condition of ultra-shallow soil cover is met. Temporary supports are laid out longitudinally along the railway line, with a spacing of eight to ten meters between them and a burial depth of not less than two meters.

[0008] Preferably, in step S100, the erection of the temporary beam specifically includes: A rubber buffer pad is inserted at the contact surface between the bottom of the rail and the temporary beam, and elastic fasteners are used to fasten the rail, the rubber buffer pad and the temporary beam into a whole. The vertical support reaction force is obtained by superimposing the dynamic load of the train, the self-weight of the track structure, and the weight of the temporary beam itself; The actual stress of the foundation is calculated based on the vertical support reaction force, the self-weight of the temporary support structure, and the bottom cross-sectional area. When the calculated actual stress of the foundation exceeds the bearing capacity limit, increase the cross-sectional area of ​​the bottom surface of the temporary support or shorten the spacing between temporary supports.

[0009] Preferably, in step S200, the solution of the benchmark theory sequence specifically includes: Input the standard value of train dynamic load, moment of inertia of temporary beam section, elastic modulus of reinforced column and subgrade coefficient into the co-force calculation model; The Winkel elastic foundation beam theory is introduced to calculate the maximum deformation at mid-span of the temporary beam. Based on the elastic modulus, moment of inertia of the cross section and the subgrade coefficient of the temporary beam, the deflection distribution sequence of the entire temporary beam is calculated. Extract the total number of single-sided reinforcement columns, the weighted average effective unit weight of the soil around the pile, the effective embedment depth of the reinforcement column, and the horizontal cross-sectional area of ​​a single reinforcement column to calculate the axial force borne by a single reinforcement column.

[0010] Preferably, in step S200, the construction of the reinforced column and connecting beam to form a portal pier structure specifically includes: The spacing between the two rows of reinforcing columns shall be no less than the sum of the outer diameter of the jacking tunnel and the preset extension length. When the working stratum is a water-rich sand layer, the all-round high-pressure jet grouting method is used to construct the reinforcement column, and the jet grouting pressure is controlled in the range of 30 MPa to 40 MPa. The tops of the two rows of reinforcing columns were chiseled flat, and reinforcing bars with a depth of not less than 300 mm were inserted. The transverse connecting beams were then poured on site.

[0011] Preferably, in step S300, maintaining the earth pressure within the target earth pressure control range specifically includes: Obtain the weighted average effective unit weight of the soil layer above the excavation face, the vertical embedment depth from the ground to the center of the excavation face, the vertical additional stress value weakened by the portal pier structure, the unit weight of groundwater and the static head height of groundwater at the center of the excavation face. The target earth pressure is calculated based on the center elevation of the excavation face using Rankine's active earth pressure coefficient. The lower limit of the calibrated earth chamber pressure control range is 1.1 times the theoretical target earth pressure, and the upper limit is 1.2 times the theoretical target earth pressure.

[0012] Preferably, in step S300, the operation of the pipe jacking machine specifically includes: The actual pressure of the soil pressure chamber, the cutting torque of the cutterhead, and the thrust of the main top cylinder are collected based on the hardware system timestamp alignment. When the actual pressure of the soil pressure chamber is consistently lower than the lower limit of the control range and the cutting torque of the cutterhead decreases abnormally, it is determined that there is a weak stratum in front of the excavation. When the thrust of the main hydraulic cylinder increases rapidly in a short period of time and the actual pressure of the earth pressure chamber approaches the upper limit of the control range, it is determined that a local hard rock interlayer has been encountered. Based on the outer diameter of the cutterhead of the pipe jacking machine, the outer diameter of the pipe section of the pipe jacking tunnel, the longitudinal length of the single pipe section, and the grouting volume filling coefficient within the range of 1.1 to 1.5, the theoretical grouting volume of a single pipe section during the advancement cycle is calculated, and grouting is performed synchronously at the tail of the pipe section.

[0013] Preferably, in step S400, the calculation of the dynamic deviation matrix and the extraction of feature values ​​specifically include: Collect data on railway track surface settlement, vertical displacement of temporary supports and portal piers, and strain status of transverse connecting beams. Use underlying hardware synchronous triggers to align the timestamps of multi-dimensional sensors to the same millisecond-level time series. Extract the ratio of the actual measured displacement value to the allowable deformation limit value at the displacement measuring point, and extract the ratio of the strain increment to the material's ultimate strain threshold at the strain measuring point; By introducing the reliability weight coefficients of the corresponding data of displacement measuring points and strain measuring points, the ratios of each item are weighted and integrated, and the comprehensive deformation evaluation index is calculated as the residual characteristic value.

[0014] Preferably, in step S400, the parameters calculated when the warning threshold is reached specifically include: The pre-defined warning threshold is 0.8; When the comprehensive deformation assessment index is determined to be not less than the warning threshold, the compensation grouting control logic is automatically triggered. Based on the coordinates of discrete displacement measuring points and the corresponding settlement, the surface settlement surface is generated by spatial spline interpolation algorithm, and then the area parameters of the area with settlement anomalies are obtained by integral calculation.

[0015] Preferably, in step S400, the execution of compensation grouting specifically includes: Extract the difference between the measured maximum settlement at the center point of the settlement anomaly area and the target settlement to be restored; Multiply the difference by the aforementioned area parameter, and then multiply by the grouting efficiency compensation coefficient in the range of 1.2 to 2.0 to obtain the theoretical compensation grouting volume in a single intervention cycle. After a monitoring cycle and feedback review, the comprehensive deformation assessment index stabilizes and falls back to a safe range, maintaining the current pipe jacking machine's thrust and soil removal speed. If the index continues to increase, the screw conveyor's soil removal speed is reduced and the jack thrust and external grouting pressure are increased.

[0016] This invention provides an active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions. It has the following beneficial effects: 1. This invention constructs a primary load transfer path by setting up temporary supports at both ends of the railway line's affected section and erecting construction beams on these supports, which are then fixed to the railway rails. This technical feature physically severs the direct transmission channel between the train's dynamic load and the track's self-weight and the subgrade soil directly above the proposed pipe jacking tunnel. The vertical dynamic load generated by the train's operation is then laterally transmitted through the support system to the stable foundations on both sides, avoiding instability at the excavation face caused by direct pressure from the live load above on the shallow overburden, and eliminating external vibration interference for subsequent underground excavation.

[0017] 2. This invention establishes a collaborative stress calculation model by inputting the stiffness parameters of the support system into a finite element framework. Based on this, a portal pier structure is constructed on both sides of the tunnel axis, consisting of reinforcing columns and transverse connecting beams, generating a secondary load transfer path. The portal pier structure bears the static earth pressure of the overlying soil and the residual load transferred by the connecting beams, and guides these loads to the underlying stable strata. By isolating a low-stress construction zone within the underpass area through the solid retaining structure, the pipe jacking machine can operate in a relatively constant static earth pressure environment, reducing the probability of surface heave or collapse during pipe jacking under ultra-shallow overburden conditions.

[0018] 3. This invention acquires railway settlement and component stress data at high frequency, aligns the measured data with the benchmark theoretical sequence to generate a dynamic deviation matrix, and performs compensatory grouting by resolving parameters when the residual eigenvalue reaches the warning threshold. This closed-loop data feedback mechanism enables proactive intervention in ground deformation. The system can dynamically calculate the required targeted grouting location and volume based on the comprehensive stress state of the structure, promptly using fluid media to compensate for ground loss caused by mechanical excavation, maintaining the triaxial stress balance of the soil surrounding the pipe section, and controlling railway track surface settlement within the physical limits for safe operation. Attached Figure Description

[0019] Figure 1 This is a flowchart of the active unloading and reinforcement method for pipe jacking tunnels passing under railways under ultra-shallow overburden conditions, according to the present invention. Figure 2 This is a schematic diagram of the overhead line support system and the primary load transfer path of the present invention; Figure 3 This is a diagram showing the side-mounted active unloading portal pier structure and the secondary load transfer path of the present invention. Figure 4 This is a schematic diagram of the micro-disturbance excavation and earth pressure balance control of the pipe jacking tunnel according to the present invention; Figure 5 This is a schematic diagram of the dynamic monitoring and compensation grouting closed-loop control of the present invention. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] See attached document Figure 1 , Figure 1This is a flowchart of an active unloading reinforcement method for a pipe jacking tunnel passing under a railway under extremely shallow overburden conditions, according to an embodiment of the present invention. The present invention provides an active unloading reinforcement method for a pipe jacking tunnel passing under a railway under extremely shallow overburden conditions, the method comprising the following steps: S100: Obtain the spatial distribution parameters of the railway line construction impact zone above the proposed pipe jacking tunnel. Based on these parameters, temporary supports are installed at both ends of the railway line construction impact zone. Construction temporary beams are erected on these temporary supports and fixed to the railway rails, thus suspending the railway track structure. A primary load transfer path is constructed through the construction temporary beams and temporary supports, channeling the train's dynamic load and the track structure's self-weight into this path, thereby cutting off the direct transmission of these loads to the underlying subgrade soil.

[0022] S200 extracts the spatial distribution parameters of the primary load transfer path and the bearing stiffness data of the temporary supports from S100. The extracted data, along with the standard value of the train dynamic load, the moment of inertia of the construction beam section, the elastic modulus of the pile, and the subgrade coefficient, are input into the finite element calculation framework to establish a collaborative stress calculation model. The benchmark theoretical sequence for solving the axial force of the portal pier pile, the bending moment of the transverse connecting beam, and the deflection of the construction beam is obtained through the collaborative stress calculation model.

[0023] The design parameters and layout coordinates of the pile reinforcement columns are determined based on the benchmark theoretical sequence. Two rows of vertical pile reinforcement columns are constructed on both sides of the tunnel axis. A transverse connecting beam is constructed at the top of the two rows of pile reinforcement columns, connecting the beam to the pile reinforcement columns to form a portal pier structure. The loads on the primary transfer load path in S100 and the self-weight of the subgrade soil are imported into the portal pier structure to generate a secondary transfer load path. The portal pier structure isolates a low-stress construction zone within the underpass area.

[0024] S300: Obtain the spatial boundary coordinates of the low-stress construction zone in S200. Operate the pipe jacking machine for tunneling within the low-stress construction zone defined by the spatial boundary coordinates. Control the jacking speed of the pipe jacking machine within the preset speed range, and simultaneously inject thixotropic mud during the tunneling process for drag reduction. Real-time acquisition of the soil chamber pressure of the pipe jacking machine and the static earth pressure of the surrounding strata is used to adjust the jacking parameters, ensuring that the soil chamber pressure is maintained within the control threshold range of the pre-calculated theoretical target earth pressure.

[0025] In S400, during the tunneling cycle of S300, high-frequency data are collected on railway track settlement, actual deflection data of the temporary construction beam in S100, and actual stress data and pile top displacement data of the portal pier structure in S200. The collected actual data streams are time-series aligned and the difference is calculated with the benchmark theoretical sequence output in S200 to generate a dynamic deviation matrix.

[0026] Extract the residual eigenvalues ​​from the dynamic deviation matrix and determine whether they reach the preset warning threshold. When the residual eigenvalues ​​reach the warning threshold, calculate the target location, grouting pressure, and grouting volume for compensation grouting based on the dynamic deviation matrix. Inject the medium through the preset compensation grouting holes behind the pipe section wall of the pipe jacking machine according to the calculated parameters, dynamically adjusting the static earth pressure of the surrounding strata in S300 to bring the subsequently collected residual eigenvalues ​​to a safe range.

[0027] See attached document Figure 2 , Figure 2 This is a schematic diagram of the overhead support system and primary load transfer path according to an embodiment of the present invention. In this embodiment, the active unloading reinforcement method, before performing the bottom excavation operation, constructs an independent physical support system on the ground to change the load transfer direction generated by the train operation, thereby creating boundary conditions for the tunnel excavation below that are free from interference from the live load above.

[0028] S110, obtain the spatial distribution parameters of the section affected by the railway line construction above the proposed pipe jacking tunnel. As a prerequisite for implementing this reinforcement method, obtain the overburden thickness and the outer diameter of the proposed pipe jacking tunnel, and calculate their ratio. Typically, when the ratio of the overburden thickness to the outer diameter of the pipe jacking tunnel is less than or equal to 0.8, an effective soil arching effect cannot be formed within the soil, and the live load above will be directly transmitted downwards, leading to instability of the excavation face. Based on this, it is determined that the current working environment meets the conditions for ultra-shallow overburden. Based on the obtained spatial distribution parameters, temporary supports are arranged longitudinally along the railway line at both ends of the section affected by the railway line construction. In this embodiment, the temporary supports are constructed using reinforced concrete cast-in-place structures. To ensure the uniformity of the longitudinal span's load-bearing capacity and to accommodate the operating clearance of the construction machinery below, the spacing between the temporary supports along the longitudinal direction of the railway line is set to eight to ten meters. The horizontal cross-sectional dimensions of the temporary supports are set to one meter by one meter, and the corresponding burial depth is set to be greater than or equal to two meters. This burial depth ensures sufficient contact between the base and the soil to diffuse stress. For the conversion and reinforcement treatment of insufficient bearing capacity of the foundation soil under different geological conditions, those skilled in the art can use conventional replacement or grouting processes. The specific foundation reinforcement methods are well-known technologies in this field and will not be elaborated here.

[0029] After the basic support structure is completed (S120), a temporary construction beam is erected on top of the deployed temporary supports. As a preferred method, a standard railway D-type temporary construction beam or a customized large-section composite H-type steel structure is used for assembly to meet the ultra-high section bending stiffness required for large-span load-bearing. The length of the temporary construction beam along the railway line must completely cover the section affected by the pipe jacking construction, and extend at least two meters to both ends of the affected section. This extension parameter is set to ensure that the support anchor points at both ends fall into stable soil outside the excavation disturbance range, preventing unexpected displacement of the support points due to subgrade settlement. After the temporary beam is in place, it is connected to the railway rails. Considering the dynamic impact characteristics of train operation, a rubber buffer pad with a thickness of 20 mm is sandwiched between the bottom of the railway rail and the contact surface of the temporary construction beam. Elastic fasteners are used to secure the railway rails, rubber buffer pads, and temporary construction beams into a single unit. The rubber buffer pad provides deformation compensation between rigid contact surfaces, absorbs the impact kinetic energy generated by the high-speed operation of the train, and reduces the proportion of high-frequency vibration waves transmitted to the supporting structure below.

[0030] S130 utilizes a combination of construction beams and temporary supports to construct a primary load transfer path. Without overhead work, the train's dynamic load and the track structure's self-weight are vertically transferred from the railway rails to the railway subgrade soil. After the construction beams are erected and secured, the train's dynamic load and the track structure's self-weight are supported by the construction beams. The load, under the influence of the beams' bending stiffness, is then transferred laterally along the longitudinal axis to the temporary supports at both ends, and finally guided into the stable soil layer at the bottom by the temporary supports. This operation physically severs the direct transmission channel of dynamic load to the subgrade soil directly above the proposed pipe jacking tunnel.

[0031] In practical engineering applications, the base area of ​​temporary supports is often subject to physical limits due to the land boundaries of the construction site. To verify the structural safety of a single load transfer path and ensure that the temporary supports do not settle, the system needs to perform anti-settlement calculations based on static load transfer and foundation bearing capacity theory. Before performing the foundation contact surface pressure calculation, it is necessary to clarify that the core purpose of anti-settlement is to eliminate plastic shear failure of the base caused by stress concentration on the bearing surface. The specific process of verifying the actual stress of the temporary support base based on static equilibrium conditions is shown in the formula: ; In the formula, This represents the actual stress generated at the contact surface between the foundation of the temporary support and the soil. This represents the vertical support reaction force transmitted to the top of a single temporary support pier when the temporary construction beam is under load and in operation. This support reaction force is calculated by superimposing the train dynamic load, the self-weight of the track structure, and the weight of the temporary construction beam itself, as calculated by the standard railway dynamic load model. This represents the structural self-weight of a single temporary support pier; This represents the bottom cross-sectional area of ​​the temporary support. Since this cross-sectional area is a physical characteristic of the structure, its value is always a positive real number due to construction specifications, thus automatically avoiding overflow anomalies caused by the denominator approaching zero in division operations. To ensure absolute stability under overhead conditions, the calculated actual stress of the foundation must be less than or equal to a preset bearing capacity limit. In this embodiment, the set bearing capacity limit is 200 kilonewtons per square meter. This limit is determined based on empirical values ​​of the allowable bearing capacity of the hard plastic clay layer or dense sand layer at the bottom of conventional railway subgrades, reflecting the safety envelope of the stratum's compressive strength. When the actual stress of the foundation calculated according to the above formula exceeds this bearing capacity limit, the system will output a structural warning and adjust the stress distribution parameters in the opposite direction by increasing the bottom cross-sectional area of ​​the temporary support or shortening the spacing between temporary supports until the calculation is repeated. The values ​​meet the safety limit requirements, thereby establishing a closed-loop optimization logic for the support structure parameters.

[0032] See attached document Figure 3 , Figure 3 This is a diagram of the side active unloading portal pier structure and the secondary load transfer path according to an embodiment of the present invention. In this embodiment, after completing the construction of the primary load transfer path, the active unloading reinforcement method further combines the engineering geology and the spatial topology of the proposed tunnel to establish a secondary load transfer path through a combination of pre-calculation and physical construction, thereby isolating a low-stress construction zone in the deep excavation area.

[0033] S210. To accurately guide the construction parameters of the physical structure and avoid the risk of ground disturbance caused by blind construction, in this embodiment, the system extracts the bearing stiffness data of the temporary piers and the span parameters of the construction beams based on the previous elevated state, and imports them into the finite element calculation framework to establish a collaborative stress calculation model of the portal piers, construction beams, and the ground. During the model initialization phase, core boundary conditions and material physical properties need to be input into this collaborative stress calculation model. The input parameters specifically include the standard value of the train dynamic load, the moment of inertia of the construction beam section, the elastic modulus of the pile, and the subgrade coefficient. The physical relevance of the above input parameters is based on the following: the standard value of the train dynamic load, as the external main control source, directly determines the extreme value of the transmitted excitation force boundary; the moment of inertia of the construction beam section and the elastic modulus of the pile constitute the spatial structural stiffness matrix of the support system; and the subgrade coefficient, as a parameter characterizing the foundation resistance, together with the structural stiffness, determines the final earth pressure distribution ratio. Through the nonlinear coupling of the aforementioned multi-source mechanical boundary and material parameters, the computational model can simulate the stress attenuation and structural response process when dynamic live load is transmitted downwards.

[0034] S220 utilizes the established collaborative stress calculation model to solve the baseline theoretical sequence for calculating the axial force of the portal pier pile, the bending moment of the transverse connecting beam, and the deflection of the temporary construction beam. Since the deformation of the temporary beam directly affects the ride comfort of the existing railway line above, to accurately describe the deflection behavior of the bending member under the combined action of distributed load and elastic foundation reaction, the system introduces Winkler's general theory for elastic foundation beams to verify the maximum mid-span deformation. The specific calculation process for the temporary beam deflection is shown in the formula: ; In the formula, This indicates the elastic modulus of the temporary construction beam; The moment of inertia of the cross section of the temporary construction beam; This represents the vertical deflection of the temporary construction beam at point x along the longitudinal coordinate system. Indicates the equivalent subgrade coefficient provided by the foundation or substructure; This represents the uniformly distributed load per unit length acting on the temporary construction beam, encompassing the equivalent transformation value of the track structure's self-weight and the train's dynamic load. Based on the numerical integral solution of this differential equation, the deflection distribution sequence of the entire temporary construction beam is obtained. Simultaneously, for the vertical transmitted compressive stress borne by the portal pier, the system extracts the axial force of the portal pier pile body using the section method; the calculation process is shown in the formula: ; In the formula, This indicates the axial force borne by a single pile-reinforced column; This represents the total vertical compressive load transferred from the transverse connecting beam to the single-sided reinforced column group; This represents the total number of pile reinforcement columns arranged on one side. This value is limited by the physical entity layout and must be a constant positive integer to ensure that the denominator of the division operation is always strictly greater than zero in order to maintain the mathematical completeness of the algorithm. represents the weighted average effective unit weight of the soil surrounding the pile; h represents the effective embedment depth of the pile-reinforced column. This represents the horizontal cross-sectional area of ​​a single pile-reinforced column. To avoid biased judgments due to reliance on a single extreme value, the system extracts the calculated maximum deflection value, maximum axial force value, and maximum mid-span bending moment value of the transverse connecting beam. This multi-dimensional parameter sequence is then weighted and comprehensively compared with the corresponding preset deflection limit, bearing capacity limit, and bending limit for verification. In this embodiment, for high-standard lines such as high-speed railways, the deflection limit is strictly set to one millimeter. The range of this one-millimeter value is forcibly calibrated according to the allowable deviation for long-wave irregularities of ballastless track in the National Railway Line Maintenance Rules, aiming to eliminate the risk of track slab delamination or cracking from the source of physical deformation. When the calculation results are verified, this benchmark theoretical sequence is output as the control benchmark for the physical construction.

[0035] S230: After obtaining the verified calculation parameters, determine the topological layout coordinates of the portal pier in physical space. Based on the predetermined excavation trajectory of the proposed pipe jacking tunnel, construct two rows of vertical pile reinforcement columns on both sides of the tunnel axis. To prevent the reinforcement piles from encroaching on the excavation outline during subsequent pipe jacking machine excavation and causing mechanical cutting interference, the spacing between the two rows of pile reinforcement columns is set to be greater than or equal to the sum of the outer diameter of the pipe jacking tunnel and the preset extension length. This preset extension length comprehensively considers the thickness of the pipe jacking machine shell, the thickness of the surrounding grouting layer, and the range of the plastic zone of soil disturbance caused by unloading at the excavation face, and is usually set between 1.5 meters and 2 meters. The horizontal net distance between the center of a single row of pile reinforcement columns and the axis of the pipe jacking tunnel must be strictly calibrated according to the stress influence radius output by the benchmark theoretical sequence to ensure that the curtain formed by the reinforcement columns can effectively block the compressive stress transmitted inward.

[0036] For S240, considering the differences in lithology at different engineering sites, the actual drilling and pile driving of the reinforced piles require matching multi-condition construction methods. When the working strata are detected to be complex strata such as water-rich sand layers or silty soft soil, the all-round high-pressure jet grouting method is adopted as a preferred method to construct the reinforced piles. The all-round high-pressure jet grouting method uses ultra-high pressure jets to cut and destroy the surrounding soil structure, while forcibly injecting cement grout to mix with soil particles. To ensure continuous seepage prevention and bearing capacity after pile driving, the jet grouting pressure is set at 30 MPa to 40 MPa. This high-pressure range is designed to overcome the in-situ lateral pressure caused by the natural effective unit weight of deep water-rich soil, ensuring effective splitting and penetration by the jet cutting; the diameter of the formed independent consolidated pile is controlled between 800 mm and 1200 mm. The center-to-center distance between adjacent piles is set at 70% to 80% of the pile diameter. This ratio establishes an effective column interlocking section margin, allowing adjacent consolidated bodies to interlock and form a seamless water-proof and pressure-bearing curtain. When the working strata are suitable strata with good self-stabilizing capacity, such as stiff plastic clay or weathered rock, the method of manually excavating and pouring concrete is used to construct the pile reinforcement columns. During manual excavation, wellpoint dewatering is implemented simultaneously to control the groundwater level at 0.5 meters below the excavation bottom. The borehole wall is supported by reinforced concrete. After the reinforcing cage is suspended inside the borehole, high-strength concrete is poured. The longitudinal reinforcement ratio of the reinforcing cage must be greater than or equal to 1.5% to resist the nonlinear bending moment generated by the unilateral earth pressure difference and prevent brittle tensile fracture of the pile. Regardless of the working condition, the bottom of all pile reinforcement columns must penetrate deep into the underlying stable strata. The penetration depth is calculated based on the geological bearing capacity characteristics to ensure sufficient end-bearing support for the pile ends.

[0037] In S250, after the construction of the two rows of reinforced pile columns is completed and the concrete or consolidated body reaches the design strength, the soil at the elevation of the proposed transverse connecting beam is excavated in layers under the protection of the temporary construction beam system, exposing the tops of the two rows of reinforced pile columns. Subsequently, the pile tops are chiseled and leveled, and the transverse connecting beam is constructed on top of the two rows of reinforced pile columns. The transverse connecting beam spans directly above the proposed pipe jacking tunnel and is a large-section reinforced concrete structure cast in situ. To achieve a rigid connection between the transverse connecting beam and the reinforced pile columns and to prevent plastic hinge yielding at the joint under eccentric compression, workers implement a rebar anchoring process at the top of the piles. The depth of the embedded reinforcing bars must be greater than or equal to 300 mm. This anchoring depth threshold is derived from the surface bond stress integral of the reinforcing bars to ensure that the pull-out bearing capacity of the reinforcing bars under ultimate load conditions is always greater than its material yield strength, and it is consolidated with the original structure using high-strength epoxy resin anchoring adhesive. When laying bidirectional reinforcing steel mesh inside the transverse connecting beam, vibrating wire or fiber optic strain gauges are pre-embedded at key stress nodes and securely tied to the embedded steel bars before pouring concrete. Through the connection and anchoring of the transverse connecting beam, the relatively independent pile reinforcement columns on both sides are integrated into an inverted U-shaped portal pier structure, and the overall lateral stiffness and torsional performance are reasonably improved to meet the spatial coordinated stress requirements.

[0038] S260 involves placing the assembled portal pier structure within a stable foundation outside the area covered by the previously constructed temporary construction beam. Based on the principle of structural spatial stiffness distribution, the portal pier structure bears the dynamic load of the train transmitted from the temporary supports and the self-weight of the temporary construction beam, while also bearing the hydrostatic pressure of the overlying subgrade soil. The portal pier structure transmits these loads downwards along the pile shaft, bypassing the shallow overlying soil zone where the pipe jacking tunnel is located, and directly discharging them into the deep, stable strata. Through this reshaping of the secondary load transfer path, the vertical additional stress originally directly pressing on the top of the pipe jacking tunnel is completely unloaded, forming a low-stress construction zone within the physical space enclosed by the portal pier structure, free from external dynamic interference from live loads. This unloaded state provides a mechanical environment guarantee for subsequent large-section pipe jacking with micro-disturbance cutting. Regarding the adjustment of the mix proportions of conventional grouting materials in complex strata, those skilled in the art can adaptively optimize them based on the on-site hydrological conditions. The material selection methods are well-known in the field and will not be elaborated upon here.

[0039] See attached document Figure 4 , Figure 4This is a schematic diagram of micro-disturbance excavation and earth pressure balance control in a pipe jacking tunnel according to an embodiment of the present invention. In this embodiment, thanks to the low-stress construction zone isolated by the previously constructed secondary load transfer path, the excavation face of the proposed pipe jacking tunnel is protected from the direct impact of the dynamic load from the overhead train. To prevent the soil ahead from collapsing or excessively heaving during excavation, the system needs to establish micro-disturbance cutting operation parameter control logic based on the soil mechanics state to ensure the mechanical compatibility between underground excavation and the above-ground support system.

[0040] S310: Obtain the soil physical parameters and overburden depth data in front of the pipe jacking machine cutterhead. These parameters are chosen as input because the natural unit weight and pore water pressure of the soil directly determine the basic magnitude of the in-situ stress field, while the overburden depth constitutes the geometric boundary for calculating the soil arching effect and overburden load. Based on the principle of soil mechanics limit equilibrium, the excavation face support pressure is set to provide a support reaction force that counteracts the in-situ static earth pressure. Considering the curtain effect of the portal pier structure weakens the transmission of vertical earth pressure in physical space, the system needs to recalculate the theoretical target earth pressure at the center elevation of the excavation face. The specific calculation process for the theoretical target earth pressure is shown in the formula: ; In the formula, This represents the theoretical target earth pressure at the center elevation of the excavation face; This represents the Rankine active earth pressure coefficient of the stratum. This represents the weighted average effective unit weight of the soil layers above the excavation surface; Indicates the vertical embedment depth from the ground surface to the center of the excavation face; This represents the value of the vertical additional stress weakened by the obstruction of the portal pier structure. Specifically, it was obtained by extracting the difference in normal stress on the excavation surface before and after applying the portal pier structure from the collaborative stress calculation model in S200. Indicates the specific gravity of groundwater; This represents the static water head height at the center of the excavation face. Due to the presence of the pre-unloading structure, this calculation process eliminates the equivalent uniformly distributed load term of train dynamic load in traditional earth pressure calculations, retaining only the residual matrix earth pressure and pore water pressure. As a preferred method, the control threshold for the earth pressure chamber of the pipe jacking machine is set to 110% to 120% of the theoretical target earth pressure. The selection of this threshold range is based on reserving a relaxation margin for ground stress release and maintaining a slightly positive pressure compaction state of the soil ahead during mechanical excavation disturbance.

[0041] S320 controls the pipe jacking machine to start tunneling operations within the physical protection boundary of the portal pier. During the dynamic process of cutting and soil removal, the system outputs working condition judgments based on weighted logic of multi-dimensional mechanical parameters, avoiding one-sided misjudgments caused by relying solely on the single extreme value fluctuation of the pressure sensor in the earth pressure chamber. Specifically, the system synchronously collects and aligns with multi-source data under the same microsecond-level time series and the same tunneling working conditions based on the timestamp of the hardware system. The collected parameters include the actual pressure of the earth pressure chamber, the cutting torque of the cutterhead, and the thrust of the main jacking cylinder. When the actual pressure of the earth pressure chamber is continuously lower than the lower limit of the control threshold, and the cutting torque of the cutterhead shows an abnormal decay with no resistance during idling, it is jointly judged that there is a risk of weak strata or unstable cavities ahead of the excavation. When the thrust of the main jacking cylinder rises sharply in a short period of time, and the actual pressure of the earth pressure chamber approaches the upper limit of the control threshold, it is judged that a local hard rock interlayer has been encountered or the soil removal port of the screw conveyor has been blocked. Based on the above multi-dimensional joint evaluation results, the system dynamically maintains the mass conservation of material entering and leaving the excavation face by adjusting the rotation speed of the screw conveyor and the jacking speed of the main jacking cylinder.

[0042] S330, as the tunnel jacking segment is gradually pushed into the ground during the excavation process, a circumferential construction gap will be generated between the outer wall of the segment and the cutterhead excavation outline. To immediately stop the tendency of the surrounding soil to displace into this gap, in this embodiment, thixotropic mud or cement grout is injected simultaneously at the tail of the segment. Based on the principle of geometric volume conservation and combined with the ground loss compensation mechanism, the core purpose of the grouting operation is to replace the over-excavation space generated by mechanical cutting with an equal volume of fluid medium, thereby maintaining the triaxial force balance of the soil surrounding the segment. The theoretical grouting volume calculation process for a single segment is shown in the formula: ; In the formula, This represents the theoretical grouting volume required during the advancement cycle of a single pipe segment; Indicates the grouting volume filling coefficient; Indicates the outer diameter of the cutterhead of the pipe jacking machine; Indicates the outer diameter of the pipe section in the pipe jacking tunnel; This represents the longitudinal length of a single pipe jacking tunnel segment. In the geometric logic of this formula, due to the physical interference constraints of the mechanical casing structure, The value is strictly greater than The value of eliminates the risk of abnormal calculations where the area difference within the brackets is negative or close to zero from the algorithm's underlying layer, ensuring that the theoretical grouting volume is a positive real number. In this embodiment, the grouting volume filling coefficient is set to a range of 1.1 to 1.5. This coefficient is determined based on the consideration that high-pressure grout will permeate and be lost into the surrounding porous formations during injection, requiring over-injection to compensate for this volume loss, thereby ensuring complete and dense encapsulation of the circumferential gaps.

[0043] S340, as the micro-disturbance tunneling continues, the settlement data of the portal pier and the three-dimensional coordinates of the spatial attitude of the pipe jacking machine are periodically checked. Through a closed-loop mapping feedback mechanism, it is verified whether the surface deformation under the active unloading reinforcement system is strictly controlled within the safe allowable range. For laser attitude guidance and hydraulic correction control during the pipe jacking process, those skilled in the art can use a conventional combination of automatic guide target plates and correction cylinders. The specific equipment assembly and attitude correction methods are well-known technologies in this field and will not be elaborated here.

[0044] See attached document Figure 5 , Figure 5 This is a schematic diagram of dynamic monitoring and compensation grouting closed-loop control according to an embodiment of the present invention. In this embodiment, as the pipe jacking tunnel is excavated with micro-disturbance, an automated dynamic monitoring network covering the entire construction impact area is established to monitor the spatial state evolution of the overlying railway line and the active unloading reinforcement system in real time. Based on the acquired real-time measurement data, this embodiment establishes a stratum response feedback mechanism to actively intervene and adjust for possible excessive settlement in a closed loop.

[0045] S410 deploys a multi-source sensor monitoring network at the construction site to continuously collect physical state parameters of the engineering environment. The collected data specifically include railway track settlement, vertical displacement of temporary supports and portal piers, and strain state of transverse connecting beams. The selection of these parameters as input data is based on the following: railway track settlement directly represents the control benchmark for train operation safety; support structure displacement reflects the foundation bearing stability of the secondary load transfer path; and structural strain reveals the degree of mechanical fatigue and damage accumulation within the reinforced concrete structure. To eliminate time phase deviations caused by differences in sampling frequencies between different acquisition devices, the system uses underlying hardware synchronization triggers to strictly align the timestamps of the multi-dimensional sensors to the same millisecond-level time series. This strict temporal and spatial alignment logic ensures that the datasets for subsequent collaborative computation are all under the same objective physical conditions.

[0046] S420, based on synchronously acquired multi-source time-series monitoring data, introduces a weighted state assessment algorithm to quantitatively calculate the overall structural safety margin under excavation disturbance. Before the specific calculation steps, it is necessary to clarify that the technical purpose of this assessment algorithm is to avoid false alarms caused by local extreme values ​​due to environmental noise interference from a single sensor by integrating distributed heterogeneous monitoring data, thereby comprehensively and objectively reflecting the overall stress safety status of the reinforced system. The specific calculation process of the comprehensive deformation assessment index is shown in the formula: ; In the formula, This represents the comprehensive deformation assessment index; This indicates the total number of valid displacement monitoring points; This represents the total number of valid strain monitoring points; Indicates the first The data reliability weighting coefficient corresponding to each displacement measurement point; Indicates the first The data reliability weighting coefficients corresponding to each strain measurement point are all summed to one. The specific values ​​of the above weighting coefficients are calculated and calibrated in advance by technicians based on the nominal measurement accuracy of each sensor and the importance of the structural node where it is located, using the analytic hierarchy process. Indicates the first The actual measured displacement value of each displacement measuring point; This indicates the allowable deformation limit value of the corresponding displacement measuring point; Indicates the first The strain increment at each strain measurement point; This represents the material's ultimate strain threshold for the corresponding component. In the underlying logic of this algorithm, each permissible limit value is a real number greater than zero, constrained by the physical rigidity of national railway line safety maintenance regulations. This property automatically avoids the overflow anomaly caused by the denominator approaching zero in division operations at the mathematical level. In this embodiment, the permissible deformation limit value of ballastless track is strictly defined as two millimeters. The system calculates this comprehensive deformation assessment index in real time. When the index is greater than or equal to a preset warning threshold, it is determined that the current system faces the risk of exceeding the deformation limit. As a preferred approach, this warning threshold is set to 0.8, determined based on reserving a 20% elastic deformation buffer space for the overall structure, ensuring that the system can intervene in advance before the structural material undergoes irreversible plastic yielding.

[0047] S430: When the system determines there is a risk of excessive deformation, it automatically triggers the compensation grouting control logic. As a preferred method, radial dual-liquid grouting technology is used to reinforce and compensate for formation voids. Based on the theory of formation volume loss, the core physical mechanism of compensation grouting is to inject consolidation material into the pores or fissures of the target soil by pumping a fluid medium under high pressure. The volume expansion effect generated during grout solidification and the compaction effect of the slab counteract the formation volume shrinkage caused by pipe jacking excavation. The calculation process for the required dynamic compensation grouting volume is shown in the formula: ; In the formula, This represents the theoretical compensation grouting volume required within a single intervention cycle; This represents the grouting efficiency compensation coefficient; This represents the equivalent area of ​​the region where settlement anomalies occur. This equivalent area is obtained by integral calculation based on the coordinates of discretely distributed displacement measurement points and their settlement amounts, after generating the surface settlement surface through a spatial spline interpolation algorithm. This represents the measured maximum settlement at the center point of the area; This represents the target settlement amount required to maintain track smoothness. To prevent excessive grouting from causing reverse uplift of the strata and damaging track smoothness, this target settlement amount is usually taken as 50% of the corresponding allowable deformation limit. The grouting efficiency compensation coefficient is determined based on the fact that high-pressure grout will inevitably experience friction loss and ineffective seepage in the actual stratum network, thus requiring volumetric compensation. Its value range is calibrated to 1.2 to 2.0 based on the soil permeability coefficient on site.

[0048] After performing the compensation grouting operation (S440), the system continuously and cyclically executes the above-mentioned steps of data acquisition, alignment, and status assessment at the measuring points, forming a complete closed-loop feedback between equipment control parameters and the mechanical response of the strata. If, after a monitoring cycle of feedback verification, the comprehensive deformation assessment index stably falls back to within the safe range, the current thrust and soil removal speed of the pipe jacking machine are maintained. If the index continues to increase, the system issues an intervention command to the main control unit of the pipe jacking machine, linking the reduction of the soil removal speed of the screw conveyor and appropriately increasing the thrust of the jacks, while simultaneously increasing the external grouting pressure. For the deployment of the reference network and the integration of the IoT remote communication module for conventional automated measuring stations, those skilled in the art can perform the work based on mature wireless data transmission protocols and surveying and mapping specifications. The specific equipment communication networking methods are well-known technologies in this field and will not be elaborated here.

[0049] Specific application examples are described. This example uses a newly constructed municipal utility tunnel passing under an existing high-speed railway as an example. The proposed tunnel will be constructed using the slurry-balanced pipe jacking method, with a pipe jacking machine outer diameter of 3.62 meters. The soil cover thickness in the section passing under the railway is 2.45 meters. Calculations show that the ratio of soil cover thickness to the outer diameter of the pipe jacking machine is 0.67, meeting the requirements for ultra-shallow soil cover. The strata are mainly silty clay, and the groundwater level is buried at a depth of 1.2 meters.

[0050] At both ends of the affected section of the railway line, temporary reinforced concrete supports were poured at longitudinal intervals of 9.5 meters. The temporary supports had a cross-sectional dimension of 1.4 meters by 1.5 meters and an embedment depth of 3.2 meters. D24 type construction beams were erected on top of the temporary supports. 20 mm thick rubber buffer pads were placed between the beams and the bottom of the rails and secured with elastic fasteners to complete the construction of the primary load transfer path.

[0051] After extracting the parameters of the piers and temporary beams and importing them into the finite element model for calculation, two rows of reinforced piles were constructed 3.5 meters from the centerline on both sides of the tunnel axis using the all-around high-pressure jet grouting method. The jet grouting piles had a diameter of 1050 mm, a center-to-center distance of 780 mm between adjacent piles, and a grouting pressure of 36.5 MPa. The pile tips penetrated into the slightly weathered rock layer. Subsequently, layered excavation exposed the pile tops, and reinforcing steel bars with a depth of 320 mm were implanted. A transverse connecting beam with a cross-section of 1.45 m x 1.25 m was then cast in situ, forming a portal pier structure. At this point, the static earth pressure of the overlying soil and the residual load transferred by the temporary beams were borne by the portal pier structure, completing the construction of the secondary load transfer path.

[0052] Based on the model, the difference in normal stress before and after the application of the pier structure was extracted, and the calculated reduction in vertical additional stress was 38.6 kPa. Combined with the in-situ earth pressure, the theoretical target earth pressure at the center elevation of the excavation face was calculated to be 53.4 kPa. The earth pressure chamber control threshold was set to 110% to 120% of the theoretical value, i.e., 58.7 kPa to 64.1 kPa. The jacking speed of the pipe jacking machine was controlled at 35 mm / min, and thixotropic mud was injected simultaneously, with a grouting filling coefficient set to 1.35.

[0053] During tunneling, the system frequently collected settlement and stress data. The allowable deformation limit of the high-speed rail surface was set at 2.0 mm. Within a certain monitoring period, the system calculated a comprehensive deformation assessment index of 0.82, with a maximum measured settlement of 1.64 mm at the center. The system automatically reduced the screw conveyor speed and injected a two-component consolidation grout into the abnormal settlement area with a grouting efficiency compensation coefficient of 1.45. After subsequent time-series data alignment and feedback, the comprehensive deformation assessment index stabilized and dropped back to 0.65.

[0054] To verify the actual engineering parameter performance of the active unloading reinforcement method in this embodiment, a traditional control group (conventional surface grouting reinforcement combined with earth pressure balance pipe jacking) was set up under the same geological conditions. Within a 10-meter range on both sides of the railway centerline where the pipe jacking machine passes under the railway, continuous monitoring data streams of the traditional scheme and the scheme of this application (recorded in three independent advancement sections) were captured, and the statistical results are shown in Table 1.

[0055] Table 1 Comparison of Measured Construction Parameters between Traditional and Applicable Schemes According to the data in Table 1, the active unloading reinforcement method provided in this application is superior to traditional reinforcement schemes in terms of various mechanical response and deformation control indicators under ultra-shallow overburden conditions.

[0056] The maximum rail surface settlement in the traditional control group reached 3.67 mm, exceeding the allowable limit of 2.0 mm for railway operation. The maximum rail surface settlement in the three tests conducted using the proposed scheme were 1.18 mm, 1.24 mm, and 1.09 mm, respectively, all within the safe threshold. This is mainly because the proposed scheme utilizes a combination of temporary construction beams and portal pier structures to create both primary and secondary load transfer paths. The vertical dynamic load generated by train operation and the track's self-weight are laterally transferred to the stable foundation through the support system, preventing the load from directly compressing the overlying soil and cutting off the mechanical transmission channel for plastic shear failure in the shallow soil.

[0057] The earth pressure range at the center of the excavation face reflects the degree of external disturbance to the soil at the working face. The traditional control group exhibited an earth pressure range as high as 48.3 kPa, indicating that the train's live load penetrated the shallow soil and directly impacted the excavation face. The earth pressure range in the test group of this application was reduced to the range of 8.7 kPa to 10.1 kPa. This data confirms that the portal pier structure effectively shielded the low-stress construction zone isolated within the underpass area. By eliminating the equivalent uniformly distributed load term of the train's dynamic load, the pipe jacking machine operated in a relatively constant static earth pressure environment, thereby reducing the probability of instability at the excavation face.

[0058] Regarding thrust control and ground compensation, the traditional control group exhibited a thrust fluctuation variance of 1452.8 square kN and a ground volume loss rate of 1.94%. In this application, the thrust fluctuation variance decreased to between 218.4 and 251.3 square kN, and the ground volume loss rate was controlled between 0.43% and 0.51%. This is attributed to the system's dynamic deviation matrix calculation and compensation grouting closed-loop control mechanism. When the residual characteristic value showed an abnormal growth trend, the system intervened by adjusting the screw conveyor's soil discharge speed and the synchronous grouting parameters after the pipe section, maintaining the triaxial force balance of the soil surrounding the pipe section. The maximum compressive strain data of the support system indicate that the overall deformation stiffness of the reinforced concrete portal pier meets the ground resistance distribution requirements, and no material ultimate strain exceeded the limit, ensuring the construction safety of the pipe jacking tunnel under the railway throughout the entire construction cycle.

[0059] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions, characterized in that, The method includes the following steps: S100: Obtain the distribution parameters of the railway impact zone; set up temporary supports at both ends and erect construction temporary beams, and fix the construction temporary beams to the rails to construct a primary load transfer path; S200: Input the distributed parameters and temporary support stiffness data into the calculation model to solve the reference theoretical sequence; construct reinforcement columns and connecting beams on both sides of the tunnel according to the reference theoretical sequence to form a portal pier structure; use the portal pier structure to generate a secondary transfer load path to isolate the low stress zone; S300, operating the pipe jacking machine in a low-stress zone; collecting soil chamber pressure and adjusting jacking parameters to maintain soil chamber pressure within the target earth pressure threshold; S400 collects settlement data and aligns it with the benchmark theoretical sequence to generate a dynamic deviation matrix; when the residual eigenvalue in the dynamic deviation matrix reaches the warning threshold, it calculates the grouting parameters and performs compensatory grouting to adjust the formation stress.

2. The active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions as described in claim 1, characterized in that, In step S100, obtaining the distribution parameters of the railway influence area specifically includes: Obtain the soil cover thickness and outer diameter of the proposed pipe jacking tunnel; Calculate the ratio of the soil cover thickness to the outer diameter of the jacking tunnel. If the ratio is not greater than 0.8, it is determined that the condition of ultra-shallow soil cover is met. Temporary supports are laid out longitudinally along the railway line, with a spacing of eight to ten meters between them and a burial depth of not less than two meters.

3. The active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions as described in claim 1, characterized in that, In step S100, the erection of the temporary beam specifically includes: A rubber buffer pad is inserted at the contact surface between the bottom of the rail and the temporary beam, and elastic fasteners are used to fasten the rail, the rubber buffer pad and the temporary beam into a whole. The vertical support reaction force is obtained by superimposing the dynamic load of the train, the self-weight of the track structure, and the weight of the temporary beam itself; The actual stress of the foundation is calculated based on the vertical support reaction force, the self-weight of the temporary support structure, and the bottom cross-sectional area. When the calculated actual stress of the foundation exceeds the bearing capacity limit, increase the cross-sectional area of ​​the bottom surface of the temporary support or shorten the spacing between temporary supports.

4. The active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions as described in claim 1, characterized in that, In step S200, the solution of the baseline theoretical sequence specifically includes: Input the standard value of train dynamic load, moment of inertia of temporary beam section, elastic modulus of reinforced column and subgrade coefficient into the co-force calculation model; The Winkel elastic foundation beam theory is introduced to calculate the maximum deformation at mid-span of the temporary beam. Based on the elastic modulus, moment of inertia of the cross section and the subgrade coefficient of the temporary beam, the deflection distribution sequence of the entire temporary beam is calculated. Extract the total number of single-sided reinforcement columns, the weighted average effective unit weight of the soil around the pile, the effective embedment depth of the reinforcement column, and the horizontal cross-sectional area of ​​a single reinforcement column to calculate the axial force borne by a single reinforcement column.

5. The active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions as described in claim 1, characterized in that, In step S200, the construction of the reinforced column and connecting beam to form a portal pier structure specifically includes: The spacing between the two rows of reinforcing columns shall be no less than the sum of the outer diameter of the jacking tunnel and the preset extension length. When the working stratum is a water-rich sand layer, the all-round high-pressure jet grouting method is used to construct the reinforcement column, and the jet grouting pressure is controlled in the range of 30 MPa to 40 MPa. The tops of the two rows of reinforcing columns were chiseled flat, and reinforcing bars with a depth of not less than 300 mm were inserted. The transverse connecting beams were then poured on site.

6. The active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions as described in claim 1, characterized in that, In step S300, maintaining the target earth pressure control range specifically includes: Obtain the weighted average effective unit weight of the soil layer above the excavation face, the vertical embedment depth from the ground to the center of the excavation face, the vertical additional stress value weakened by the portal pier structure, the unit weight of groundwater and the static head height of groundwater at the center of the excavation face. The target earth pressure is calculated based on the center elevation of the excavation face using Rankine's active earth pressure coefficient. The lower limit of the calibrated earth chamber pressure control range is 1.1 times the theoretical target earth pressure, and the upper limit is 1.2 times the theoretical target earth pressure.

7. The active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions as described in claim 1, characterized in that, In step S300, the operation of the pipe jacking machine specifically includes: The actual pressure of the soil pressure chamber, the cutting torque of the cutterhead, and the thrust of the main top cylinder are collected based on the hardware system timestamp alignment. When the actual pressure of the soil pressure chamber is consistently lower than the lower limit of the control range and the cutting torque of the cutterhead decreases abnormally, it is determined that there is a weak stratum in front of the excavation. When the thrust of the main hydraulic cylinder increases rapidly in a short period of time and the actual pressure of the earth pressure chamber approaches the upper limit of the control range, it is determined that a local hard rock interlayer has been encountered. Based on the outer diameter of the pipe jacking machine cutterhead, the outer diameter of the pipe section of the pipe jacking tunnel, the longitudinal length of a single pipe section, and the grouting volume filling coefficient within the range of 1.1 to 1.5, the theoretical grouting volume of a single pipe section during the advancement cycle is calculated, and grouting is performed synchronously at the tail of the pipe section.

8. The active unloading reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions as described in claim 1, characterized in that, In step S400, the calculation of the dynamic deviation matrix and the extraction of feature values ​​specifically include: Collect data on railway track surface settlement, vertical displacement of temporary supports and portal piers, and strain status of transverse connecting beams. Use underlying hardware synchronous triggers to align the timestamps of multidimensional sensors to the same millisecond-level time series. Extract the ratio of the actual measured displacement value to the allowable deformation limit value at the displacement measuring point, and extract the ratio of the strain increment to the material's ultimate strain threshold at the strain measuring point; By introducing the reliability weight coefficients of the corresponding data of displacement measuring points and strain measuring points, the ratios of each item are weighted and integrated, and the comprehensive deformation evaluation index is calculated as the residual characteristic value.

9. The active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions as described in claim 8, characterized in that, In step S400, the parameters calculated when the warning threshold is reached specifically include: The pre-defined warning threshold is 0.8; When the comprehensive deformation assessment index is determined to be not less than the warning threshold, the compensation grouting control logic is automatically triggered. Based on the coordinates of discrete displacement measuring points and the corresponding settlement, the surface settlement surface is generated by spatial spline interpolation algorithm, and then the area parameters of the area with settlement anomalies are obtained by integral calculation.

10. The active unloading and reinforcement method for pipe jacking tunnels passing under railways under extremely shallow overburden conditions as described in claim 9, characterized in that, In step S400, the execution of compensation grouting specifically includes: Extract the difference between the measured maximum settlement at the center point of the settlement anomaly area and the target settlement to be restored; Multiply the difference by the aforementioned area parameter, and then multiply by the grouting efficiency compensation coefficient in the range of 1.2 to 2.0 to obtain the theoretical compensation grouting volume in a single intervention cycle. After a monitoring cycle and feedback review, the comprehensive deformation assessment index stabilizes and falls back to a safe range, maintaining the current pipe jacking machine's thrust and soil removal speed. If the index continues to increase, the screw conveyor's soil removal speed is reduced and the jack thrust and external grouting pressure are increased.

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