Method for evaluating influence of surface loading effect on existing subway segments

Abstract

The invention discloses a method for evaluating the influence of a surface loading effect on an existing subway segment, and relates to the field of subway segment danger evaluation, and the method comprises the steps: building a similar model, carrying out an experiment according to the similar model, carrying out the statistics of experiment data, and obtaining the influence rule of the surface loading effect on the surrounding rock pressure and deformation mode of a tunnel segment according to the experiment data; describing a calculation method of the corrected surrounding rock pressure acting on the tunnel segment by adopting a surrounding rock pressure amplification coefficient beta and a surrounding rock pressure transfer coefficient eta under the surface loading condition; based on a shield segment deformation control standard in a rail transit specification, a four-level displacement influence partition table is constructed, and the influence on the tunnel segment under the ground surface loading condition is evaluated according to experimental data. According to the method, the influence rule of the surrounding rock pressure and the deformation mode of the tunnel segment is predicted and evaluated through the similar model, accurate early warning is carried out, a new design concept is provided for segment structure design, and a scientific design basis is provided for urban planning designers.

Description

Technical Field

[0001] The invention relates to the field of subway segment hazard assessment, and in particular to a method for assessing the impact of surface loading on existing subway segments. Background Art

[0002] As the main load-bearing unit of the tunnel structure, the stability and safety of the shield segment are crucial to the entire project.

[0003] However, in recent years, many shield tunnel structure damage accidents caused by sudden ground loading have occurred in China, indicating that adjacent sudden ground loading is an important factor threatening the safety of existing shield tunnel structures.

[0004] Ground loading can cause additional stress and deformation of shield segments, and may even lead to structural damage and failure. Therefore, studying the impact of sudden loading on shield segments is of great significance to ensure the safe operation and long-term stability of underground projects.

[0005] Therefore, there is an urgent need for a method to evaluate the impact of surface loading on existing subway segments. Summary of the invention

[0006] In order to solve the above problems, this application proposes a method for evaluating the impact of surface loading on existing subway segments, comprising the following steps:

[0007] S1. Construct a similarity model based on the tunnel segment ring information and collapsible loess information;

[0008] S2. Conduct experiments based on similar models and collect experimental data, and obtain the influence of surface load on surrounding rock pressure and deformation mode of tunnel segments based on the experimental data;

[0009] S3. The calculation method of the modified surrounding rock pressure acting on the tunnel segment under the surface loading condition is described by using the surrounding rock pressure amplification coefficient β and the surrounding rock pressure transfer coefficient η;

[0010] S4. Based on the shield segment deformation control standard in the rail transit specification, a four-level displacement influence zoning table is constructed, and the impact on the tunnel segment under surface loading conditions is evaluated based on experimental data.

[0011] Preferably, the similarity model includes a basic component unit, a loess simulation unit and a loading monitoring unit;

[0012] The basic component unit includes an indoor model test box and segment rings;

[0013] The loess simulation unit includes artificially prepared collapsible loess;

[0014] The load monitoring unit includes a hydraulic servo actuator, a pressure plate, an earth pressure gauge and a displacement gauge.

[0015] Preferably, the indoor model test box is made of transparent organic glass, and three detachable steel supports are used to maintain the stability of the model box. The segment ring includes 6 segments, and the segment material is made of ABS material with an elastic modulus E=0.86 GPa by 3D printing. The segment material includes 1 capping block, 2 adjacent blocks, and 3 standard blocks. The bolt connection between the segment materials is connected by wire tying.

[0016] Preferably, the preparation process of artificially preparing collapsible loess includes:

[0017] To obtain the test soil, two original soil samples were taken every 2 meters, and disturbed soil samples were taken from several layers in the actual stratum by drilling the original soil samples. The disturbed loess was spread flat, dried, crushed and sieved to obtain the test soil;

[0018] Preparation of artificial collapsible loess: adding CaO particles, industrial salt and gypsum powder to the test soil to prepare artificial collapsible loess;

[0019] Preferably, three-dimensional gapless joint supports are installed at both ends of the hydraulic servo actuator, the three-dimensional gapless joint supports include a plurality of joints, the joints are connected by flanges, the bearing plate is composed of two upper and lower organic glasses and a connecting rod, the upper organic glass is tightly buckled on the bottom of the actuator, and the lower organic glass is placed on the soil surface;

[0020] The displacement meter is arranged inside the middle ring of the tunnel to measure the vertical convergence displacement and the lateral convergence displacement respectively;

[0021] The earth pressure gauge is arranged outside the segment ring and is used to monitor the earth pressure at the arch crown, arch bottom, left and right arch shoulders, left and right arch waists, and left and right arch feet.

[0022] Preferably, the specific contents of the influence of the surface load on the surrounding rock pressure and deformation mode of the tunnel segment in S2 include the mechanical response law of the upper load on the existing subway segment, the mechanical response law of the load position on the existing subway segment and the mechanical response law of the load burial depth on the existing subway segment;

[0023] The mechanical response law of the upper load to the existing subway segment is:

[0024] As the upper load P value increases, the confining pressure at the measuring point increases, and the distribution of the confining pressure change at the measuring point is symmetrical on the left and right, with the same distribution law;

[0025] As the upper load increases, the confining pressure at the measuring point increases overall, with the same distribution pattern;

[0026] When the tunnel depth C = 0.15m (6m), as the upper load P above the shield tunnel gradually increases, the vertical convergence displacement Sh and the lateral convergence displacement Sv of the shield tunnel both show an increasing trend;

[0027] When there is no pile load directly above, the change of shield tunnel is the smallest;

[0028] When the upper load P = 40 kPa, the vertical convergence displacement of the shield tunnel reaches the warning value of 10 mm;

[0029] When the upper load is P = 20 kPa, the lateral convergence displacement reaches the monitoring and warning value of 5 mm, and it is necessary to pay attention to the structural state of the shield tunnel;

[0030] When the upper load is P = 75kPa, the vertical convergence displacement reaches the control value of 20mm; when the upper load is P = 100kPa, the lateral convergence displacement reaches the warning value of 10mm;

[0031] When the lateral convergence displacement or vertical displacement deformation of the segment exceeds 20mm, there is a safety risk in the shield tunnel structure.

[0032] Preferably, the mechanical response law of the loading position to the existing subway segment includes:

[0033] The load position is taken as the control variable, and the load eccentricity is defined as the horizontal distance from the center of the load carrier to the center of the shield tunnel. When the ground load is located directly above the tunnel, the eccentricity is 0D, and D is the diameter of the shield tunnel. The eccentricity of the test load is e=0D, e=0.5D, e=1D, and e=2D respectively.

[0034] The confining pressure of the tunnel vault, non-loaded side arch waist, arch angle, and arch bottom shows an overall downward trend with the increase of eccentric distance;

[0035] The decreasing trend of tunnel confining pressure on the eccentrically loaded side is smaller than that on the non-eccentric side;

[0036] When e = 0.5D, the confining pressure on the eccentric side of the tunnel increases slightly compared with that when the load is e = 0D. The increase values ​​of the spandrel, waist and arch foot on the tunnel loading side are 0.2, 0.08 and 0.25 kPa respectively. Then, the confining pressure begins to decrease as e increases further.

[0037] Taking the eccentricity of the load as the control variable, the horizontal distance between the ground load center and the tunnel center is set to e = 0D, e = 0.5D, e = 1D, and e = 2D. Under the conditions of P = 75kPa and burial depth C = 6m:

[0038] When the pile load is located directly above the shield tunnel, the vertical convergence displacement of the tunnel is 20 mm, which exceeds the control value specified in the specification;

[0039] When the eccentricity of the load is 1.2D, the vertical convergence displacement of the shield tunnel is 10mm. Attention should be paid to the structural state of the tunnel. As the eccentricity of the ground load increases, the vertical convergence displacement of the shield tunnel becomes smaller and smaller.

[0040] When the eccentricity of the pile load is greater than 1.2D, the vertical convergence displacement of the shield tunnel is less than 10mm, which meets the specified value in the specification;

[0041] Under the same upper load, when the burial depth C = 14m, as the eccentricity of the pile load increases, the vertical convergence displacement of the shield segment also shows the same law;

[0042] When the pile load is located directly above the shield tunnel, the vertical convergence displacement of the tunnel is 15 mm, reaching the warning value standard;

[0043] When the eccentricity of the pile load is 1.5D, the vertical convergence displacement of the shield tunnel is 10mm;

[0044] When the eccentricity of the load is greater than 1.5D, the vertical convergence displacement of the shield tunnel is less than 10mm, which meets the specified value in the code.

[0045] Preferably, the specific content of the mechanical response law of the existing subway segment to the buried depth of the pile load is:

[0046] When the tunnel depth increases, the tunnel confining pressure under sudden load decreases significantly, but the confining pressure shape is similar;

[0047] The influence of sudden load on tunnel confining pressure decreases with the increase of burial depth. The reason is that the additional vertical stress of soil generated by local ground load will decrease sharply with the increase of burial depth C.

[0048] As the tunnel depth increases, the change in the longitudinal deformation of the tunnel gradually decreases, and the tunnel is less affected by the ground load from above.

[0049] Preferably, the specific contents of the calculation method of using the surrounding rock pressure amplification coefficient β and the surrounding rock pressure transfer coefficient η to describe the modified surrounding rock pressure acting on the tunnel segment under the surface loading condition are:

[0050] According to the traditional soil column method theory, the overburden of the tunnel is regarded as the vertical load acting on the lining:

[0051] σ v =γH;

[0052] In the formula, σ v : surrounding rock pressure at the top of the tunnel; γ: bulk density of rock mass; H: buried depth of tunnel;

[0053] According to the error between the theoretical value and the measured value of the soil column method, the surrounding rock pressure amplification coefficient β and the surrounding rock pressure transmission coefficient η are introduced;

[0054]

[0055] Where: σ is the tunnel surrounding rock pressure after loading; σ0 is the initial tunnel surrounding rock pressure;

[0056]

[0057] Where: △σ is the tunnel surrounding rock pressure increment after loading; P is the surface loading;

[0058] Substituting the amplification coefficient β and the transfer coefficient η into the formula, a correction formula for the surrounding rock pressure when the shield tunnel is shallowly buried in collapsible loess strata is proposed:

[0059] q=γH+Δσ=γH+np=βγH.

[0060] Preferably, the four-level shift affects the following contents of the partition table:

[0061] When the convergence displacement is greater than 20 mm, it is the Grade I affected area;

[0062] When the convergence displacement is greater than 10 mm and less than 20 mm, it is the Grade II affected area;

[0063] When the convergence displacement is greater than 5 mm and less than 10 mm, it is the Grade III affected area;

[0064] When the convergence displacement is less than 5 mm, it is the Grade IV affected area;

[0065] When the convergence displacement is in the IV level impact zone, safety is not affected;

[0066] When the convergence displacement is in the Level I impact zone, tunnel monitoring should be closely strengthened to protect the structural safety.

[0067] In summary, compared with the traditional technology, the present invention adopts a method for evaluating the impact of surface loading on existing subway segments, in which a certain proportion of CaO particles, industrial salt and gypsum powder are added to loess to prepare loess with a certain collapsible property, which is used to simulate the collapsible loess material of the similar model test, and further based on the 3D printing and wire torsion test method, a construction technology of staggered assembled shield segments is constructed, thereby realizing the construction of a similar model test.

[0068] Through similar model tests, the influence of surface load on the surrounding rock pressure and deformation mode of existing shield segments under different load amounts, different eccentricities and different tunnel depths was proposed.

[0069] A calculation method for the modified surrounding rock pressure acting on the existing shield segments under surface loading conditions was proposed using the surrounding rock pressure amplification coefficient β and the surrounding rock pressure transfer coefficient η, and its rationality was verified.

[0070] Based on the shield segment deformation control standard in rail transit specifications, a four-level displacement influence zone is proposed, and the evaluation of the impact of surface loading conditions on existing shield segments can be obtained through a table lookup method.

[0071] The technical method of the present invention is further described in detail below through the drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 This is a 3D display diagram of a similar model of the present invention;

[0073] Figure 2 This is a comparison diagram of tunnel confining pressure caused by different pile load sizes in the present invention;

[0074] Figure 3 This is the confining pressure variation diagram of the tunnel lining of the present invention, Figure 3 (a) is the change of surrounding rock pressure at the arch top and arch bottom, Figure 3 (b) is the diagram of the surrounding rock pressure changes on the left and right spandrels;

[0075] Figure 4 is the displacement convergence value of the present invention when the burial depth is 6m, Figure 4 (a) is the vertical convergence displacement of the structure, Figure 4 (b) is the lateral convergence displacement of the structure;

[0076] Figure 5 is the displacement convergence value of the present invention when the burial depth is 14m, Figure 5 (a) is the vertical convergence displacement of the structure, Figure 5 (b) is the lateral convergence displacement of the structure;

[0077] Figure 6 is the measured value of surrounding rock pressure under eccentric loading in the present invention, Figure 6 (a) is the diagram of the surrounding rock pressure changes at the arch top and arch bottom. Figure 6 (b) is the diagram of the surrounding rock pressure changes on the left and right spandrels;

[0078] Figure 7 It is the convergence displacement value of the pipe segment when the eccentricity increases according to the present invention.

[0079] Reference numerals

[0080] 1. Loading plate. DETAILED DESCRIPTION

[0081] The technical method of the present invention is further described below by means of the accompanying drawings and embodiments. It should be noted that unless otherwise specifically stated, the relative arrangement of the components and steps, numerical expressions and numerical values ​​described in these embodiments do not limit the scope of the present application.

[0082] The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the present application, its application, or uses.

[0083] Technologies, systems, and devices known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, the technologies, systems, and devices should be considered part of the specification.

[0084] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not limiting. Therefore, other examples of the exemplary embodiments may have different values.

[0085] Unless otherwise defined, technical or scientific terms used in the present invention shall have the common meanings understood by one having ordinary skills in the field to which the present invention belongs.

[0086] The present invention provides a method for evaluating the impact of surface loading on existing subway segments, comprising the following steps:

[0087] S1. Construct a similarity model based on the tunnel segment ring information and collapsible loess information;

[0088] like Figure 1 As shown, the similarity model includes a basic component unit, a loess simulation unit and a loading monitoring unit;

[0089] The basic component unit includes an indoor model test box and segment rings;

[0090] The loess simulation unit includes artificially prepared collapsible loess;

[0091] The load monitoring unit includes a hydraulic servo actuator, a pressure plate, an earth pressure gauge and a displacement gauge.

[0092] The indoor model test box is made of transparent organic glass, and three detachable steel supports are used to maintain the stability of the model box. The segment ring includes 6 segments, and the segment material is made of ABS material with an elastic modulus E=0.86GPa through 3D printing. The segment material includes 1 capping block, 2 adjacent blocks, and 3 standard blocks. The bolt connection between the segment materials is connected by wire tying.

[0093] The preparation process of artificially preparing collapsible loess comprises:

[0094] To obtain the test soil, two original soil samples were taken every 2 meters, and disturbed soil samples were taken from several layers in the actual stratum by drilling the original soil samples. The disturbed loess was spread flat, dried, crushed and sieved to obtain the test soil;

[0095] Preparation of artificial collapsible loess: adding CaO particles, industrial salt and gypsum powder to the test soil to prepare artificial collapsible loess;

[0096] The hydraulic servo actuator is provided with three-dimensional gapless joint supports at both ends, the three-dimensional gapless joint supports include a plurality of joints, the joints are connected by flanges, the bearing plate is composed of two pieces of upper and lower organic glass and a connecting rod, the upper organic glass is tightly buckled on the bottom of the actuator, and the lower organic glass is placed on the soil surface;

[0097] The displacement meter is arranged inside the middle ring of the tunnel to measure the vertical convergence displacement and the lateral convergence displacement respectively;

[0098] The earth pressure gauge is arranged outside the segment ring and is used to monitor the earth pressure at the arch crown, arch bottom, left and right arch shoulders, left and right arch waists, and left and right arch feet.

[0099] S2. Conduct experiments based on similar models and collect experimental data, and obtain the influence of surface load on surrounding rock pressure and deformation mode of tunnel segments based on the experimental data;

[0100] The specific contents of the influence of surface loading on the surrounding rock pressure and deformation mode of tunnel segments in S2 include the mechanical response law of upper load on existing subway segments, the mechanical response law of loading position on existing subway segments, and the mechanical response law of loading burial depth on existing subway segments.

[0101] The mechanical response law of the upper load to the existing subway segment is:

[0102] like Figure 2 As shown in the figure, with the increase of the upper load P value, the confining pressure at the measuring point continues to increase with the continuous increase of the pile load, and the distribution of the confining pressure change at the measuring point is symmetrical on the left and right, and the distribution law is the same;

[0103] like Figure 3 As shown in the figure, with the increase of upper load, the confining pressure at each point is generally increasing, and the distribution law is basically the same;

[0104] like Figure 4 As shown in the figure, when the tunnel depth C = 0.15m (6m), as the upper load P above the shield tunnel gradually increases, the vertical convergence displacement Sh and the lateral convergence displacement Sv of the shield tunnel both show an increasing trend;

[0105] When there is no pile load directly above, the change of the shield tunnel is the smallest, the initial vertical convergence displacement of the shield segment is 4.4mm, and the initial lateral convergence displacement is 3.7mm;

[0106] When the upper load P = 40 kPa, the vertical convergence displacement of the shield tunnel reaches the warning value of 10 mm;

[0107] When the upper load is P = 20kPa, the lateral convergence displacement reaches the 5mm monitoring warning value. At this time, attention should be paid to the structural status of the shield tunnel;

[0108] When the upper load is P = 75 kPa, the vertical convergence displacement reaches the control value of 20 mm;

[0109] When the upper load is P = 100 kPa, the lateral convergence displacement reaches the warning value of 10 mm;

[0110] and Figure 5 When the tunnel burial depth C = 0.35m (14m), the initial vertical convergence displacement of the shield segment is 8.8mm, and the initial lateral convergence displacement is 4.2mm.

[0111] When the upper load is P = 25kPa, the vertical convergence displacement reaches the 10mm warning value. When the upper load is P = 35kPa, the lateral convergence displacement reaches the 5mm monitoring warning value. When the upper load is P = 100kPa, the vertical convergence displacement reaches the 20mm control value, and when the upper load is P = 145kPa, the lateral convergence displacement reaches the 10mm warning value.

[0112] When the lateral convergence displacement or vertical displacement deformation of the segment exceeds 20mm, there is a safety risk to the shield tunnel structure. Appropriate preventive measures should be taken to avoid greater structural damage to the shield tunnel.

[0113] like Figure 6 As shown in Figure 2, the mechanical response of the loading position to the existing subway segments includes:

[0114] Taking the load position as the control variable, the load eccentricity is stipulated as the horizontal distance from the center of the load carrier to the center of the shield tunnel. When the ground load is located directly above the tunnel, the eccentricity is 0D, D is the diameter of the shield tunnel, and the eccentricity of the test load is e=0D, e=0.5D, e=1D, and e=2D respectively. The load size and tunnel burial depth are 7.5kPa and 0.15m respectively, and the corresponding load size and tunnel burial depth of the actual working sinking are 75kPa and 6m respectively.

[0115] The confining pressures of the tunnel vault, non-loaded side arch waist, arch angle, and arch bottom show an overall downward trend as the eccentric distance increases;

[0116] The decreasing trend of tunnel confining pressure on the eccentrically loaded side is significantly smaller than that on the non-eccentric side;

[0117] When e = 0.5D, the confining pressure on the eccentric side of the tunnel increases slightly compared with that when the load is e = 0D. The increase values ​​of the spandrel, waist and arch foot on the tunnel loading side are 0.2, 0.08 and 0.25 kPa respectively. Then, the confining pressure begins to decrease as e increases further.

[0118] like Figure 7 As shown in the figure, the eccentricity of the load is taken as the control variable, and the horizontal distance of the ground load center offset from the tunnel center is set to e = 0D, e = 0.5D, e = 1D, and e = 2D. Under the conditions of P = 75kPa and burial depth C = 6m;

[0119] When the pile load is located directly above the shield tunnel, the vertical convergence displacement of the tunnel is 20 mm, which has exceeded the control value specified in the specification;

[0120] When the eccentricity of the load is 1.2D, the vertical convergence displacement of the shield tunnel is 10mm, and attention should be paid to the structural state of the tunnel. As the eccentricity of the ground load increases, the vertical convergence displacement of the shield tunnel becomes smaller and smaller;

[0121] When the eccentricity of the pile load is greater than 1.2D, the vertical convergence displacement of the shield tunnel is less than 10mm, which meets the specified value in the specification;

[0122] Under the same upper load, when the burial depth C = 14m, as the eccentricity of the pile load increases, the vertical convergence displacement of the shield segment also shows the same law;

[0123] When the pile load is located directly above the shield tunnel, the vertical convergence displacement of the tunnel is 15 mm, reaching the warning value standard;

[0124] When the eccentricity of the pile load is 1.5D, the vertical convergence displacement of the shield tunnel is 10mm;

[0125] When the eccentricity of the load is greater than 1.5D, the vertical convergence displacement of the shield tunnel is less than 10mm, which meets the specified value in the code.

[0126] The specific content of the mechanical response law of the existing subway segment to the buried depth of the pile load is as follows:

[0127] The depth of the load is taken as the control variable, corresponding to two working conditions with actual tunnel depths of 6 and 14m. When the tunnel depth increases, the tunnel confining pressure decreases significantly under sudden loads, but the shape of the confining pressure is similar; taking the change of confining pressure at the top measuring point of the tunnel as an example, when the tunnel depth changes from 6m to 14m, the confining pressure value decreases from 60kPa to 20kPa, and the decrease is very large, indicating that the influence of sudden loads on the tunnel confining pressure decreases with the increase of the burial depth. The reason is that the additional vertical stress of the soil caused by the local ground load will decrease sharply with the increase of the burial depth C. When the tunnel depth increases, the tunnel confining pressure decreases significantly under sudden loads, but the shape of the confining pressure is similar;

[0128] The influence of sudden load on tunnel confining pressure decreases with the increase of burial depth. The reason is that the additional vertical stress of soil generated by local ground load will decrease sharply with the increase of burial depth C.

[0129] When the upper load P = 75kPa, the burial depth increases from 6m to 14m. The vertical convergence displacement of the tunnel gradually decreases with the increase of the tunnel burial depth, and the vertical convergence displacement decreases from 20mm to 15mm. This shows that as the tunnel burial depth increases, the change in the longitudinal deformation of the tunnel is also gradually decreasing, and the tunnel is less affected by the ground load from above. The main reason is that the additional stress acting on the tunnel due to the ground load decreases with the burial depth of the shield tunnel.

[0130] Finally, we can conclude that:

[0131] (1) The confining pressure of the tunnel lining increases with the continuous increase of the pile load. When the pile load is directly above, the distribution of the confining pressure at the measuring point is roughly symmetrical, and the tunnel structure deforms in a "lateral elliptical" manner. When the pile load is applied to the side, the shield tunnel undergoes an approximate "oblique elliptical" deformation. As the eccentricity increases, the influence on the deformation of the shield tunnel decreases.

[0132] (2) When a sudden load occurs directly above the tunnel, the maximum confining pressure of the tunnel occurs below the center of the ground load, and the confining pressure gradually decreases with the increase of the e value. When eccentric loading occurs, the tunnel structure deforms into an "oblique ellipse".

[0133] When the tunnel burial depth increases, the tunnel confining pressure decreases significantly under sudden load, but the shape of the confining pressure is similar; the tunnel confining pressure shows an overall downward trend with the increase of the eccentric distance e, and the downward trend of the tunnel confining pressure on the eccentric side is significantly smaller than that on the non-eccentric side. When the eccentric distance e ranges from 0 to 0.5D, the confining pressure on the loaded side of the tunnel has a small area that shows an increasing trend and then decreases.

[0134] S3. The calculation method of the modified surrounding rock pressure acting on the tunnel segment under the surface loading condition is described by using the surrounding rock pressure amplification coefficient β and the surrounding rock pressure transfer coefficient η;

[0135] The theoretical calculation results of the arch surrounding rock pressure show that the relative errors of the calculated values ​​and the measured values ​​by the soil column method, Terzaghi method, Bill-Bauman formula, Xie Jiawei and rock column theoretical methods are 0.48%~11.9%, 11.0%~43.2%, 10.2%~42.5%, 26.7%~18.5%, and 10.2%~42.5%, respectively. Compared with other theoretical methods, the soil column method has a smaller error and is more stable.

[0136] The specific contents of the calculation method of the modified surrounding rock pressure acting on the tunnel segment under the surface loading condition are described by the surrounding rock pressure amplification coefficient β and the surrounding rock pressure transfer coefficient η:

[0137] According to the traditional soil column method theory, the overburden of the tunnel is regarded as the vertical load acting on the lining:

[0138] σ v =γH;

[0139] In the formula, σ v : surrounding rock pressure at the top of the tunnel; γ: bulk density of rock mass; H: buried depth of tunnel;

[0140] According to the error between the theoretical value and the measured value of the soil column method, the surrounding rock pressure amplification coefficient β and the surrounding rock pressure transmission coefficient η are introduced;

[0141]

[0142] Where: σ is the tunnel surrounding rock pressure after loading; σ0 is the initial tunnel surrounding rock pressure;

[0143]

[0144] Where: △σ is the tunnel surrounding rock pressure increment after loading; P is the surface loading;

[0145] Substituting the amplification coefficient β and the transfer coefficient η into the formula, a correction formula for the surrounding rock pressure when the shield tunnel is shallowly buried in collapsible loess strata is proposed:

[0146] q=γH+Δσ=γH+np=βγH.

[0147] When the correction formula is applied to other working conditions for calculation, the average relative error is 0.79% to 8.2%.

[0148] S4. Based on the shield segment deformation control standard in the rail transit specification, a four-level displacement influence zoning table is constructed, and the impact on the tunnel segment under surface loading conditions is evaluated based on experimental data.

[0149] The four-level shift affects the partition table at several times:

[0150] When the convergence displacement is greater than 20 mm, it is the Grade I affected area;

[0151] When the convergence displacement is greater than 10 mm and less than 20 mm, it is the Grade II affected area;

[0152] When the convergence displacement is greater than 5 mm and less than 10 mm, it is the Grade III affected area;

[0153] When the convergence displacement is less than 5 mm, it is the Grade IV affected area;

[0154] When the convergence displacement is in the IV level impact zone, safety is not affected;

[0155] When the convergence displacement is in the Level I impact zone, tunnel monitoring should be closely strengthened to protect the structural safety.

[0156] Finally, it should be noted that the above embodiments are only used to illustrate the technical method of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical method of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical method to deviate from the spirit and scope of the technical method of the present invention.

Claims

1. A method for evaluating the impact of surface loading on existing subway segments, characterized in that: The following steps are involved: S1. Construct a similarity model based on the tunnel segment ring information and collapsible loess information; S2. Conduct experiments based on similar models and collect experimental data, and obtain the influence of surface load on surrounding rock pressure and deformation mode of tunnel segments based on the experimental data; S3. The calculation method of the modified surrounding rock pressure acting on the tunnel segment under the surface loading condition is described by using the surrounding rock pressure amplification coefficient β and the surrounding rock pressure transfer coefficient η; S4. Based on the shield segment deformation control standard in the rail transit specification, a four-level displacement influence zoning table is constructed, and the impact on the tunnel segment under surface loading conditions is evaluated based on experimental data.

2. The method for evaluating the impact of surface loading on existing subway segments according to claim 1 is characterized in that: The similarity model includes a basic component unit, a loess simulation unit and a loading monitoring unit; The basic component unit includes an indoor model test box and segment rings; The loess simulation unit includes artificially prepared collapsible loess; The load monitoring unit includes a hydraulic servo actuator, a pressure plate, an earth pressure gauge and a displacement gauge.

3. The method for evaluating the impact of surface loading on existing subway segments according to claim 2 is characterized in that: The indoor model test box is made of transparent plexiglass, and three detachable steel supports are used to maintain the stability of the model box. The segment ring includes 6 segments of material, and the segment material is made of ABS material with an elastic modulus E=0.86GPa through 3D printing. The segment material includes 1 capping block, 2 adjacent blocks, and 3 standard blocks. The bolt connection between the segment materials is connected by wire binding.

4. The method for evaluating the impact of surface loading on existing subway segments according to claim 2 is characterized in that: The preparation process of artificially preparing collapsible loess comprises: To obtain the test soil, two original soil samples were taken every 2 meters, and disturbed soil samples were taken from several layers in the actual stratum by drilling the original soil samples. The disturbed loess was spread flat, dried, crushed and sieved to obtain the test soil; Artificial collapsible loess was prepared by adding CaO particles, industrial salt and gypsum powder to the test soil.

5. The method for evaluating the impact of surface loading on existing subway segments according to claim 2 is characterized in that: The hydraulic servo actuator is provided with three-dimensional gapless joint supports at both ends, the three-dimensional gapless joint supports include a plurality of joints, the joints are connected by flanges, the bearing plate is composed of two pieces of upper and lower organic glass and a connecting rod, the upper organic glass is tightly buckled on the bottom of the actuator, and the lower organic glass is placed on the soil surface; The displacement meter is arranged inside the middle ring of the tunnel to measure the vertical convergence displacement and the lateral convergence displacement respectively; The soil pressure gauge is arranged outside the segment ring and is used to monitor the soil pressure at the arch crown, arch bottom, left and right arch shoulders, left and right arch waists, and left and right arch feet.

6. The method for evaluating the impact of surface loading on existing subway segments according to claim 1 is characterized in that: The specific contents of the influence of surface loading on the surrounding rock pressure and deformation mode of tunnel segments in S2 include the mechanical response law of upper load on existing subway segments, the mechanical response law of loading position on existing subway segments, and the mechanical response law of burial depth on existing subway segments; The mechanical response law of the upper load to the existing subway segment is: As the upper load P value increases, the confining pressure at the measuring point increases, and the distribution of the confining pressure change at the measuring point is symmetrical on the left and right, with the same distribution law; As the upper load increases, the confining pressure at the measuring point increases overall, with the same distribution pattern; When the tunnel depth C = 0.15m, as the upper load P above the shield tunnel gradually increases, the vertical convergence displacement Sh and the lateral convergence displacement Sv of the shield tunnel both show an increasing trend; When there is no pile load directly above, the change of shield tunnel is the smallest; When the upper load P = 40 kPa, the vertical convergence displacement of the shield tunnel reaches the warning value of 10 mm; When the upper load is P = 20 kPa, the lateral convergence displacement reaches the monitoring and warning value of 5 mm, and it is necessary to pay attention to the structural state of the shield tunnel; When the upper load is P = 75kPa, the vertical convergence displacement reaches the control value of 20mm; when the upper load is P = 100kPa, the lateral convergence displacement reaches the warning value of 10mm; When the lateral convergence displacement or vertical displacement deformation of the segment exceeds 20mm, there is a safety risk in the shield tunnel structure.

7. The method for evaluating the impact of surface loading on existing subway segments according to claim 6 is characterized in that: The mechanical response laws of the loading position to the existing subway segments include: The load position is taken as the control variable, and the load eccentricity is defined as the horizontal distance from the center of the load carrier to the center of the shield tunnel. When the ground load is located directly above the tunnel, the eccentricity is 0D, and D is the diameter of the shield tunnel. The eccentricity of the test load is e=0D, e=0.5D, e=1D, and e=2D respectively. The confining pressures of the tunnel vault, non-loaded side arch waist, arch angle, and arch bottom show an overall downward trend as the eccentric distance increases; The decreasing trend of tunnel confining pressure on the eccentrically loaded side is smaller than that on the non-eccentric side; When e = 0.5D, the confining pressure on the eccentric side of the tunnel increases slightly compared with that when the load is e = 0D. The increase values ​​of the spandrel, waist and arch foot on the tunnel loading side are 0.2, 0.08 and 0.25 kPa respectively. Then, the confining pressure begins to decrease as e increases further. Taking the eccentricity of the load as the control variable, the horizontal distance between the ground load center and the tunnel center is set to e=0D, e=0.5D, e=1D, e=2D. Under the conditions of P=75kPa and burial depth C=6m: When the pile load is located directly above the shield tunnel, the vertical convergence displacement of the tunnel is 20 mm, which exceeds the control value specified in the specification; When the eccentricity of the load is 1.2D, the vertical convergence displacement of the shield tunnel is 10mm. Attention should be paid to the structural state of the tunnel. As the eccentricity of the ground load increases, the vertical convergence displacement of the shield tunnel becomes smaller and smaller. When the eccentricity of the pile load is greater than 1.2D, the vertical convergence displacement of the shield tunnel is less than 10mm, which meets the specified value in the specification; Under the same upper load, when the burial depth C = 14m, as the eccentricity of the pile load increases, the vertical convergence displacement of the shield segment also shows the same law; When the pile load is located directly above the shield tunnel, the vertical convergence displacement of the tunnel is 15 mm, reaching the warning value standard; When the eccentricity of the pile load is 1.5D, the vertical convergence displacement of the shield tunnel is 10mm; When the eccentricity of the load is greater than 1.5D, the vertical convergence displacement of the shield tunnel is less than 10mm, which meets the specified value in the code.

8. The method for evaluating the impact of surface loading on existing subway segments according to claim 6 is characterized in that: The specific content of the mechanical response law of the existing subway segment to the buried depth of the pile load is as follows: When the tunnel depth increases, the tunnel confining pressure under sudden load decreases significantly, but the confining pressure shape is similar; The influence of sudden load on tunnel confining pressure decreases with the increase of burial depth; The additional vertical stress of the soil generated by the local ground load will decrease sharply as the burial depth C increases; As the tunnel depth increases, the change in the longitudinal deformation of the tunnel gradually decreases, and the tunnel is less affected by the ground load from above.

9. The method for evaluating the impact of surface loading on existing subway segments according to claim 6 is characterized in that: The specific contents of the calculation method of the modified surrounding rock pressure acting on the tunnel segment under the surface loading condition are described by the surrounding rock pressure amplification coefficient β and the surrounding rock pressure transfer coefficient η: According to the traditional soil column method theory, the overburden of the tunnel is regarded as the vertical load acting on the lining: s v =γH; In the formula, σ v : surrounding rock pressure at the top of the tunnel; γ: bulk density of rock mass; H: buried depth of tunnel; According to the error between the theoretical value and the measured value of the soil column method, the surrounding rock pressure amplification coefficient β and the surrounding rock pressure transmission coefficient η are introduced; Where: σ is the tunnel surrounding rock pressure after loading; σ0 is the initial tunnel surrounding rock pressure; Where: △σ is the tunnel surrounding rock pressure increment after loading; P is the surface loading; Substituting the amplification coefficient β and the transfer coefficient η into the formula, a correction formula for the surrounding rock pressure when the shield tunnel is shallowly buried in collapsible loess strata is proposed: q=γH+Δσ=γH+np=βγH.

10. The method for evaluating the impact of surface loading on existing subway segments according to claim 1, characterized in that: The four-level shift affects the partition table at several times: When the convergence displacement is greater than 20 mm, it is the Grade I affected area; When the convergence displacement is greater than 10 mm and less than 20 mm, it is the Grade II affected area; When the convergence displacement is greater than 5 mm and less than 10 mm, it is the Grade III affected area; When the convergence displacement is less than 5 mm, it is the Grade IV affected area; When the convergence displacement is in the IV level impact zone, safety is not affected; When the convergence displacement is in the Level I impact zone, tunnel monitoring should be closely strengthened to protect the structural safety.

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