Centrifugal test method for testing influence of foundation pit excavation on existing tunnel at pit bottom

Abstract

The invention discloses a centrifugal test method for testing the influence of foundation pit excavation on an existing tunnel at the bottom of a pit, and relates to the technical field of soil consolidation experiments. According to the method disclosed by the invention, by taking upper-layer soil as sandy soil and lower-layer soil as soft saturated silty clay as a research object, a test result is more in line with actual geological conditions of China coastal areas and Yangtze river basins in which soft clay is widely distributed, so that the applicability and engineering guidance value of a research result are improved; the test method developed by the invention can monitor tunnel displacement and the change process of hyperstatic pore water pressure and soil pressure of surrounding soil along with time in real time, and provides an effective evaluation and detection method for engineering safety by monitoring displacement of various weak key points of the tunnel caused by excavation of an upper foundation pit in real time; through the test method provided by the invention, the excavation unloading mechanism and characteristics of the existing shield tunnel overlying foundation pit and the deformation and stress evolution law of the tunnel and the foundation soil body in the foundation pit excavation and pit bottom exposure stages can be researched.

Description

Technical Field

[0001] The invention relates to the technical field of soil consolidation experiments, in particular to a centrifugal test method for testing the influence of foundation pit excavation on an existing tunnel at the pit bottom. Background Art

[0002] Excavation causes stress release and redistribution in the foundation soil, accompanied by the generation of negative excess pore water pressure. As the negative excess pore water pressure gradually dissipates, the soil undergoes reverse consolidation deformation, leading to time-dependent additional stress and deformation in the tunnel, which can, in severe cases, cause structural damage. Soft clays, widespread in coastal areas of China and the Yangtze River basin, are characterized by high water content, high compressibility, low bearing capacity, and low permeability. In these areas, soil consolidation characteristics are particularly prominent, and their impact on existing subway tunnels during excavation cannot be ignored.

[0003] Existing research has largely focused on the mechanical response of excavation pits in sandy soils, but has often overlooked the influence of soil consolidation properties in weak strata on the time-varying effects of tunnels. Furthermore, existing model tests typically focus only on the stress and deformation characteristics of the soil and tunnel after excavation, failing to delve into the coupling mechanism between soil consolidation properties and tunnel stress and deformation during the excavation process and during the pit bottom exposure phase.

[0004] Therefore, in order to study the unloading mechanism and characteristics of the foundation pit excavation above the existing shield tunnel, the deformation and stress evolution law of the tunnel and foundation soil during the foundation pit excavation and pit bottom exposure stages, reveal the intrinsic connection between the soil consolidation characteristics and the mechanical response of the tunnel, solve the practical problems faced during the construction process, ensure the safety and smooth progress of the project, form a mature key control technology system for the excavation of the foundation pit above the existing formed shield tunnel, and develop a centrifugal simulation test for the excavation of the foundation pit above the existing shield tunnel.

[0005] The specific problems are as follows:

[0006] 1. Existing tests mostly focus on the analysis of sandy or unsaturated soil layers, which makes it difficult to truly reflect the response characteristics of soft clay layers;

[0007] 2. Existing studies can usually only obtain static data at a certain moment after the excavation is completed, and cannot fully reflect the temporal evolution of soil and tunnel responses;

[0008] 3. The stress level of conventional small-scale model tests is much lower than that of the prototype, resulting in the model test being unable to accurately reflect the mechanical properties of the prototype;

[0009] Therefore, a new solution to the above problems needs to be proposed. Summary of the Invention

[0010] The purpose of the present invention is to provide a centrifugal test method for testing the influence of foundation pit excavation on the existing tunnel at the bottom of the pit, so as to solve the problem of turtle essence.

[0011] To achieve the above-mentioned object, the present invention provides the following technical solution: a centrifugal test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit, comprising at least the following steps:

[0012] S1: Determine the model rate, where factors related to the model rate include but are not limited to the actual size of the on-site foundation pit, the buried depth of the shield tunnel, the model similarity relationship, the size of the model box, and the model material production;

[0013] S2: Determine parameters, including but not limited to pile support equivalent parameters, support insertion depth, soil layer parameters, and soil pressure sensor parameters;

[0014] S3: Based on the experimental simulation after the parameters are determined, the foundation pit excavation plan is determined.

[0015] Furthermore, the dimensions of the model box are length×width×height=880mm×595mm×400mm. According to a scale ratio of 1 / 100, the depth of the model box corresponds to a foundation thickness of 40m.

[0016] Furthermore, the equivalent parameters of the pile support are that the retaining piles are used as the retaining structure in the actual project, and aluminum plates are used instead according to the bending stiffness of the material. The conversion formula is:

[0017]

[0018] Where: E is the elastic modulus, δ is the material thickness, v is the Poisson's ratio, m1 represents the model material that is the same as the prototype, and m2 is the model alternative material. The thickness of the alternative material plate can be calculated from the above formula:

[0019]

[0020] Where: n is the model rate.

[0021] Furthermore, the support insertion depth is D, and the following calculation process is obtained according to existing literature:

[0022] D≈0.67h

[0023] Where h is the tunnel excavation depth.

[0024] Furthermore, the soil layer parameters include the thickness of the two soil layers and the soil mechanical property parameters;

[0025] The thickness of the two soil layers refers to the upper sand layer and the lower clay layer;

[0026] The soil mechanical property parameters are selected according to actual requirements.

[0027] Furthermore, the soil pressure sensor needs to be arranged before the soil pressure sensor parameters are obtained;

[0028] The arrangement of the earth pressure sensor takes into account the weight difference between the sensor and the soil. The sensor will be dislocated during the centrifugation process. However, excessive fixation is equivalent to reinforcing the soil. In addition, the earth pressure has a direction, and the pore pressure is assumed to be equal in all directions. For this reason, considering all factors, the sensor is only fixed vertically so that its measuring surface cannot rotate vertically relative to the soil. In addition, because the test focuses more on the magnitude of the vertical uplift or settlement of the soil, it focuses more on the magnitude of the vertical force.

[0029] The soil pressure sensor parameters include but are not limited to the monitoring of the uplift deformation of the foundation pit bottom and its development process over time, the flexural deformation of the tunnel and its development process over time, the soil pressure at the bottom of the foundation pit and in the surrounding soil, the pore water pressure and its development history over time, and surface settlement.

[0030] Furthermore, the arrangement of measuring points for monitoring the deformation of the foundation pit bottom and its development over time is based on the symmetry of the deformation, so as to monitor the displacement uplift profile and uplift value of the pit bottom and arrange an appropriate number of measuring point channels;

[0031] The measurement points of the flexural deformation of the tunnel and its development over time are arranged as follows: the tunnel is located directly below the foundation pit and the tunnel is parallel to the long side of the foundation pit.

[0032] Furthermore, the magnitude and time history of the earth pressure and pore water pressure at the bottom of the foundation pit and the surrounding soil are determined based on the symmetry of the deformation and the close distance between the pit bottom and the tunnel top. If the vertical distance between the pit bottom and the tunnel top is not enough to arrange two sensors, an additional monitoring section is added along the length of the foundation pit.

[0033] Determine the number of sections that need to be monitored, and determine the corresponding number of monitoring channels based on the needs of the monitoring sections.

[0034] Furthermore, the surface settlement requires analysis of the cross-sectional position, and the position and number of corresponding monitoring channels are determined according to the cross-sectional position.

[0035] Furthermore, the step S3 at least includes the following steps:

[0036] S3.1: Soil preparation, by preparing the soil for specific stratum soil parameters;

[0037] S3.2: Model and sensor embedding: embed the support model, tunnel model and sensors in the soil according to the model ratio and various parameters, and reserve the foundation pit location;

[0038] S3.3: Excavation of the foundation pit. The excavation of the foundation pit adopts the method of unloading equivalent weights. Equivalent weights are applied above the foundation pit to simulate the actual load state before the foundation pit is excavated. The applied load should match the actual engineering scenario to achieve a reasonable simulation effect. The process of foundation pit excavation is simulated by gradually reducing the equivalent weights. The load is gradually removed according to the preset steps to reflect the changes in the actual foundation pit excavation stage.

[0039] S3.4 Data acquisition: During each unloading process, sensors are used to record the stress, strain, and displacement changes of the soil, observe the responses of the soil and support structure, record tunnel displacement and other data, and obtain the intrinsic connection between the tunnel and the soil.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] 1. The present invention uses sandy soil as the upper layer and soft saturated silty clay as the lower layer as the research object, so that the test results are more consistent with the actual geological conditions of China's coastal areas and the Yangtze River Basin where soft clay is widely distributed, thereby improving the applicability and engineering guidance value of the research results;

[0042] 2. The test method developed by the present invention can monitor tunnel displacement and the changes in excess pore water pressure and earth pressure of the surrounding soil over time in real time. By monitoring the displacement of various weak key points in the tunnel caused by the excavation of the upper foundation pit in real time, it provides an effective assessment and detection method for engineering safety.

[0043] 3. The test method proposed in this invention can be used to study the unloading mechanism and characteristics of the foundation pit excavation overlying an existing shield tunnel, as well as the deformation and stress evolution of the tunnel and foundation soil during the foundation pit excavation and pit bottom exposure stages, revealing the intrinsic relationship between soil consolidation characteristics and tunnel mechanical response.

[0044] 4. The centrifuge experiment proposed in this invention can simulate stress states close to those of the prototype structure in a small-scale model through a high-gravity environment, overcoming the stress field mismatch defects in conventional model experiments. By applying a high gravity field, the centrifuge experiment can reduce such errors, making the experimental results more engineering-viable. This capability makes the experimental results closer to reality, helping to accurately assess the impact of foundation pit excavation on the tunnel. Centrifuge experiments can also simulate and accelerate the time effects of stratum consolidation and seepage, providing deformation response data consistent with actual conditions. This capability is difficult to achieve through traditional experiments in a short period of time. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0046] Figure 1 This is a schematic diagram of the arrangement of pit bottom uplift displacement measurement points of the present invention;

[0047] Figure 2 This is a diagram showing the arrangement of measurement points for tunnel uplift deformation according to the present invention;

[0048] Figure 3 This is a diagram showing the arrangement of measuring points for the pore pressure and soil pressure sensors of the present invention;

[0049] Figure 4 Schematic diagram of surface settlement measuring points of the present invention. DETAILED DESCRIPTION

[0050] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0051] The method proposed in this paper involves constructing a geotechnical centrifuge model test. This involves placing a geotechnical model in a high-speed centrifuge and subjecting it to centrifugal accelerations greater than the acceleration due to gravity, compensating for the loss of the geotechnical structure's deadweight due to scale reduction. The centrifuge test model applies centrifugal inertial forces exceeding times the force of gravity to the geotechnical model, compensating for the deadweight stress loss caused by scale reduction by 1 / n, achieving the same stress level as the prototype and reproducing the original geotechnical structure's properties within the model.

[0052] A centrifugal test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit, comprising at least the following steps:

[0053] S1: Determine the model rate. The factors related to the model rate include but are not limited to the actual size of the foundation pit on site, the buried depth of the shield tunnel, the model similarity relationship, the size of the model box, and the model material production;

[0054] In this example, we assume a foundation pit project with a length of 811 meters, a standard section width of 28.8 meters, a depth of 8.6-10.5 meters, a retaining pile diameter of 0.8 meters, a shield tunnel distance of 5-8 meters from the pit bottom, a shield tunnel segment outer diameter of 6 meters, and a segment thickness of 0.3 meters. After comprehensive consideration, the test model ratio was set at 1 / 100, resulting in a centrifuge acceleration of 100 g (total weight of one ton) during the test. The actual shape of the proposed foundation pit is a rectangle with length: width: height = 800 meters: 30 meters: 20 meters.

[0055] S2: Determine parameters, including but not limited to equivalent parameters of pile support, support insertion depth, soil layer parameters, and soil pressure sensor parameters;

[0056] S3: Based on the experimental simulation after the parameters are determined, the foundation pit excavation plan is determined.

[0057] The dimensions of the model box are length × width × height = 880 mm × 595 mm × 400 mm. According to the 1 / 100 scale ratio, the depth of the model box corresponds to a foundation thickness of 40 m.

[0058] The equivalent parameters of pile support are that the retaining piles are used as the retaining structure in the actual project, and aluminum plates are used instead according to the bending stiffness of the material. The conversion formula is:

[0059]

[0060] Where: E is the elastic modulus, δ is the material thickness, v is the Poisson's ratio, m1 represents the model material that is the same as the prototype, and m2 is the model alternative material. The thickness of the alternative material plate can be calculated from the above formula:

[0061]

[0062] Where: n is the model rate.

[0063] In this embodiment, based on the setting of a certain pit project proposed above, in this centrifugal test, the underground retaining piles use C35 underwater concrete with an elastic modulus of 3.15×104Mpa, a Poisson's ratio of 0.2, and a pile diameter of 800mm; the elastic modulus of the aluminum plate is 7×104Mpa, the Poisson's ratio is 0.3, and the model rate n=100. It can be calculated that the thickness of the model aluminum plate is.

[0064] The support insertion depth is D, and the following calculation process is obtained based on existing literature:

[0065] D≈0.67h

[0066] Where h is the tunnel excavation depth.

[0067] (1) According to the Technical Specifications for Construction Foundation Pit Engineering, the relationship between the embedded depth D of the support and the height h of the exposed portion is D = (0.8-1.2) h, where h can be considered as the tunnel excavation depth. To avoid excessive embedded depth of the support, which may cause the support to contact the tunnel or penetrate the tunnel, the ratio of excavation depth to support embedded depth is 3:2, i.e., D ≈ 0.67 h, as shown in the existing literature (Chen Renpeng, AL-MADHAGI ASHRAF, Meng Fanyan. Centrifugal model test study on the impact of foundation pit excavation on the side tunnel and the effect of partition wall [J]. Chinese Journal of Geotechnical Engineering, 2018, 40(S2): 6-11.).

[0068] (2) According to the Technical Specifications for Safety Protection of Excavation of Foundation Pit Above Existing Urban Rail Transit, when the relative clearance between the external foundation pit operation and the existing tunnel structure is less than 1.0D, it can be judged as very close. At the same time, in the example of this research project, the distance between the shield tunnel and the bottom of the foundation pit is 5-8m. In order to maximize the impact of excavation on the tunnel and make the tunnel deformation monitoring data more obvious, the clearance between the pit bottom and the tunnel is selected as 6m (1 times the tunnel diameter).

[0069] (3) Consider the size of the model box to avoid wasting space in the model box due to the thickness of the soil under the tunnel being too high.

[0070] Therefore, after comprehensive consideration, first of all, the net distance between the pit bottom and the tunnel top is 6m, the tunnel excavation depth can be h=8m, and the support embedding depth D=0.67h≈5m.

[0071] Soil layer parameters include the thickness of the two soil layers and soil mechanical properties;

[0072] The thickness of the two soil layers refers to the upper sand layer and the lower clay layer;

[0073] According to the above analysis, the excavation depth of the foundation pit is , the distance between the pit bottom and the tunnel is 6m, and the tunnel diameter is 6m. So far, the total soil layer thickness required is 20m.

[0074] Considering that the height of the model box can simulate a 40m foundation, the thickness of the soil layer at the bottom of the tunnel is temporarily set to 10m, and some space is left on the top of the model box for operation.

[0075] At this point, the total foundation soil thickness is 30m, and the upper sand layer can be set to 10m and the lower clay layer to 20m thick.

[0076] The mechanical properties of soil are selected according to actual requirements.

[0077] Before obtaining soil pressure sensor parameters, soil pressure sensor layout is required;

[0078] The arrangement of the earth pressure sensor takes into account the weight difference between the sensor and the soil. The sensor will be dislocated during the centrifugal process. However, excessive fixation is equivalent to reinforcing the soil. In addition, the earth pressure has a direction, and the pore pressure is assumed to be equal in all directions. For this reason, considering all factors, the sensor is only fixed vertically so that its measuring surface cannot rotate vertically relative to the soil. In addition, because the test focuses more on the magnitude of the vertical uplift or settlement of the soil, the magnitude of the vertical force is more important.

[0079] The soil pressure sensor parameters include but are not limited to the uplift deformation (contour) of the foundation pit bottom and its monitoring over time, the flexural deformation (contour) of the tunnel and its development over time (the strain gauge monitoring channel is not included in the total channel), the soil pressure at the foundation pit bottom and in the surrounding soil, the pore water pressure and its development over time, and surface settlement.

[0080] The arrangement of measuring points for monitoring the deformation (contour) of the bottom of the foundation pit and the development process over time is based on the symmetry of the deformation. Figure 1 The simplification shown is to arrange a suitable number of measuring point channels in order to monitor the displacement uplift profile and uplift value of the pit bottom;

[0081] The measurement points of the tunnel's flexural deformation (profile) and its development over time (the strain gauge monitoring channel is not included in the total channel) are arranged so that the tunnel is located directly below the foundation pit and parallel to the long side of the foundation pit; Figure 2 As shown in the figure, five monitoring sections are arranged along the length of the tunnel, and four strain gauges are arranged circumferentially in each monitoring section to measure the deformation and bending moment of the tunnel.

[0082] The magnitude and time history of the earth pressure and pore water pressure at the bottom of the foundation pit and the surrounding soil are determined based on the symmetry of the deformation and the close distance between the pit bottom and the tunnel top. If the vertical distance between the pit bottom and the tunnel top is not enough to arrange two sensors, an additional monitoring section is added along the length of the foundation pit.

[0083] Determine the number of sections that need to be monitored, and determine the corresponding number of monitoring channels based on the needs of the monitoring sections;

[0084] Considering the symmetry of deformation and the fact that the distance between the pit bottom and the tunnel top is temporarily set at 5m, which is 5cm after being reduced 100 times, it is expected that only two sensors can be arranged between the pit bottom and the tunnel top in the test. Figure 3 As shown in Figure 1, the section is parallel to the narrow side of the foundation pit and located in the center of the foundation pit. A total of one monitoring section requires 20 monitoring channels.

[0085] Surface settlement requires analysis of the cross-section location, and the location and number of corresponding monitoring channels are determined based on the cross-section location;

[0086] like Figure 4 As shown, when the cross section is parallel to the narrow side of the foundation pit and located in the center of the foundation pit, there is a total of 1 cross section and 4 monitoring channels are required.

[0087] S3 includes at least the following steps:

[0088] S3.1: Soil preparation, by preparing the soil for specific stratum soil parameters;

[0089] S3.2: Model and sensor embedding: embed the support model, tunnel model and sensors in the soil according to the model ratio and various parameters, and reserve the foundation pit location;

[0090] S3.3: Excavation of the foundation pit. The excavation of the foundation pit adopts the method of unloading equivalent weights. Equivalent weights are applied above the foundation pit to simulate the actual load state before the foundation pit is excavated. The applied load should match the actual engineering scenario to achieve a reasonable simulation effect. The process of foundation pit excavation is simulated by gradually reducing the equivalent weights. The load is gradually removed according to the preset steps to reflect the changes in the actual foundation pit excavation stage.

[0091] S3.4 Data acquisition: During each unloading process, sensors are used to record the stress, strain, and displacement changes of the soil, observe the responses of the soil and support structure, record tunnel displacement and other data, and obtain the intrinsic connection between the tunnel and the soil.

[0092] In summary:

[0093] (1) Existing model tests are mainly aimed at sandy soil layers or unsaturated soil layers. The present invention is based on saturated silty clay with weak soil properties, making the research results more suitable for China's coastal areas and the Yangtze River Basin where soft clay is widely distributed.

[0094] (2) Centrifuge experiments can reduce errors by applying a high gravity field, making the experimental results more engineering-feasible. This capability makes the experimental results closer to the actual situation and helps to accurately evaluate the impact of foundation pit excavation on the tunnel.

[0095] (3) The test method developed by the present invention can monitor the tunnel displacement and the changes in the excess pore water pressure and soil pressure of the surrounding soil over time in real time, and can monitor the displacement of various weak key points of the tunnel caused by the excavation of the upper foundation pit in real time.

[0096] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

Claims

1. A centrifuge test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit, characterized by: At least the following steps are included: S1: Determine the model rate, where factors related to the model rate include but are not limited to the actual size of the on-site foundation pit, the buried depth of the shield tunnel, the model similarity relationship, the size of the model box, and the model material production; S2: Determine parameters, including but not limited to pile support equivalent parameters, support insertion depth, soil layer parameters, and soil pressure sensor parameters; S3: Based on the experimental simulation after the parameters are determined, the foundation pit excavation plan is determined.

2. A centrifugal test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of a pit according to claim 1, characterized in that: The dimensions of the model box are length×width×height=880mm×595mm×400mm. According to a 1 / 100 scale ratio, the depth of the model box corresponds to a foundation thickness of 40m.

3. The centrifugal test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit according to claim 1, characterized in that: The equivalent parameters of the pile support are that the retaining piles are used as the retaining structure in the actual project, and aluminum plates are used instead according to the bending stiffness of the material. The conversion formula is: Where: E is the elastic modulus, δ is the material thickness, v is the Poisson's ratio, m1 represents the model material that is the same as the prototype, and m2 is the model alternative material. The thickness of the alternative material plate is calculated from the above formula: Where: n is the model rate.

4. The centrifugal test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit according to claim 1 is characterized by: The support insertion depth is D, and the following calculation process is obtained according to existing literature: D≈0.67h Where h is the tunnel excavation depth.

5. The centrifugal test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit according to claim 1, characterized in that: The soil layer parameters include the thickness of the two soil layers and the mechanical properties of the soil; The thickness of the two soil layers refers to the upper sand layer and the lower clay layer; The soil mechanical property parameters are selected according to actual requirements.

6. The centrifugal test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit according to claim 1, characterized in that: The soil pressure sensor parameters need to be arranged before being acquired; The arrangement of the earth pressure sensor takes into account the weight difference between the sensor and the soil. The sensor will be dislocated during the centrifugation process. However, excessive fixation is equivalent to reinforcing the soil. In addition, the earth pressure has a direction, and the pore pressure is assumed to be equal in all directions. For this reason, considering all factors, the sensor is only fixed vertically so that its measuring surface cannot rotate vertically relative to the soil. In addition, because the test focuses more on the magnitude of the vertical uplift or settlement of the soil, it focuses more on the magnitude of the vertical force. The soil pressure sensor parameters include but are not limited to the monitoring of the uplift deformation of the foundation pit bottom and its development process over time, the flexural deformation of the tunnel and its development process over time, the soil pressure at the bottom of the foundation pit and in the surrounding soil, the pore water pressure and its development history over time, and surface settlement.

7. A centrifuge test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit according to claim 6, characterized in that: The arrangement of measuring points for monitoring the deformation of the foundation pit bottom and its development over time is based on the symmetry of the deformation to monitor the displacement uplift profile and uplift value of the pit bottom and arrange an appropriate number of measuring point channels; The measurement points of the flexural deformation of the tunnel and its development over time are arranged as follows: the tunnel is located directly below the foundation pit and the tunnel is parallel to the long side of the foundation pit.

8. The centrifugal test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit according to claim 6, characterized in that: The earth pressure and pore water pressure at the bottom of the foundation pit and in the surrounding soil, as well as their temporal development history, were determined based on the symmetry of the deformation and the close proximity of the pit bottom and the tunnel top. If the vertical distance between the pit bottom and the tunnel top was insufficient to accommodate two sensors, an additional monitoring section was added along the length of the foundation pit. Determine the number of sections that need to be monitored, and determine the corresponding number of monitoring channels based on the needs of the monitoring sections.

9. The centrifuge test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit according to claim 6, characterized in that: The surface settlement requires analysis of the cross-section position, and the position and number of corresponding monitoring channels are determined according to the cross-section position.

10. The centrifugal test method for testing the impact of foundation pit excavation on an existing tunnel at the bottom of the pit according to claim 1, characterized in that: The S3 at least includes the following steps: S3.1: Soil preparation, by preparing the soil for specific stratum soil parameters; S3.2: Model and sensor embedding: embed the support model, tunnel model and sensors in the soil according to the model ratio and various parameters, and reserve the foundation pit location; S3.3: Excavation of the foundation pit. The excavation of the foundation pit adopts the method of unloading equivalent weights. Equivalent weights are applied above the foundation pit to simulate the actual load state before the foundation pit is excavated. The applied load should match the actual engineering scenario to achieve a reasonable simulation effect. The process of foundation pit excavation is simulated by gradually reducing the equivalent weights. The load is gradually removed according to the preset steps to reflect the changes in the actual foundation pit excavation stage. S3.4 Data acquisition: During each unloading process, sensors are used to record the stress, strain, and displacement changes of the soil, observe the responses of the soil and support structure, record tunnel displacement and other data, and obtain the intrinsic connection between the tunnel and the soil.

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