Tunnel deformation and confining pressure evaluation method based on bilateral foundation pit excavation

Abstract

The invention relates to a tunnel deformation and confining pressure evaluation method based on bilateral foundation pit excavation, and belongs to the technical field of underground engineering. According to the method, through an unloading model of foundation pit excavation, a calculation formula of the deformation amount of the enclosing structure at any position of the side wall of the foundation pit is deduced by utilizing an enclosing structure deformation curve of a left foundation pit and an enclosing structure deformation curve of a right foundation pit; according to the deformation of the enclosure structure, soil body displacement caused by the side wall of the foundation pit is further calculated, and the transverse additional stress of the tunnel is obtained; calculating the horizontal displacement and the vertical displacement of the tunnel between the two foundation pits in combination with the additional stress value; and according to the existing initial confining pressure calculation theory, the confining pressure of the tunnel segment structure is calculated. The tunnel longitudinal displacement and transverse confining pressure change caused by excavation of the foundation pits on the two sides can be predicted, theoretical support is provided, the risk that the tunnel longitudinal displacement is too large is early warned, safety accidents are prevented, engineering construction is guided and prevented, and the construction risk cost is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of underground engineering, and in particular relates to a tunnel deformation and confining pressure assessment method based on double-side foundation pit excavation. Background Art

[0002] With the continuous acceleration of urban rail transit construction and the increasing development and utilization of underground space resources, more and more foundation pit projects are appearing beside existing shield tunnels. As the current engineering environment becomes more and more complex, double-sided foundation pit excavation is sometimes encountered in engineering practice. During the excavation process, it is inevitable to cause displacement of the surrounding soil layer. Once the displacement exceeds the safety range, it may cause deformation or even damage to the shield tunnel, posing a serious threat to the safety and stability of urban transportation. The deformation requirements of shield tunnels are extremely strict, the relative bending shall not be greater than 1 / 2500, and the maximum absolute displacement of the structure shall not exceed 20mm. For the special working condition of double-sided excavation, the corresponding evaluation of the shield tunnel is the key to controlling the displacement of the tunnel structure, so it is necessary to conduct relevant research on it.

[0003] Although domestic and foreign scholars have formed a relatively complete theoretical research method for the influence of single-sided foundation pit excavation on the deformation of adjacent shield tunnels, the research on the influence of double-sided foundation pit excavation on the deformation of shield tunnels is still not perfect, and it is impossible to accurately predict the transverse and longitudinal displacement and confining pressure of the tunnel. Therefore, it is necessary to further study the laws of the transverse and longitudinal displacement and confining pressure of the tunnel and propose solutions. Summary of the invention

[0004] The purpose of the present invention is to provide a tunnel deformation and confining pressure assessment method based on double-sided foundation pit excavation in view of the deficiencies of the prior art.

[0005] The object of the present invention is achieved through the following technical solution: A method for evaluating tunnel deformation and confining pressure based on double-sided foundation pit excavation, comprising the following steps:

[0006] Step S1: Based on the actual data of the left foundation pit, right foundation pit, retaining structure and shield tunnel obtained from the design data, an unloading model based on the excavation of the left foundation pit and the right foundation pit is established. The model is simplified to be in the middle of the left foundation pit and the right foundation pit of the shield tunnel, and the excavation depth of the left foundation pit and the right foundation pit is d i , the buried depth of the enclosure structure is d 0i , width is B i ; The outer diameter of the shield tunnel is D t , the radius is R, the buried depth of the center axis is h t , the horizontal clearance from foundation pit 1 is s i ; i represents the number of foundation pit 1, i=1 represents the right foundation pit, i=2 represents the left foundation pit;

[0007] Step S2: establishing the displacement field of the soil excavated by the left foundation pit and the right foundation pit on both sides, and obtaining the additional stress value at the axis of the shield tunnel under the double-sided excavation;

[0008] Step S3: Calculate the longitudinal displacement of the tunnel according to the additional stress value at the axis obtained in step S2;

[0009] Step S4: Calculate the confining pressure of the tunnel according to the soil displacement field calculated in step S2;

[0010] Step S5: According to the longitudinal displacement of the tunnel calculated in step S3, determine whether the longitudinal displacement of the tunnel is less than the maximum displacement value of the tunnel design; according to the confining pressure of the tunnel calculated in step S4, determine whether the confining pressure of the tunnel is less than the maximum confining pressure value of the tunnel design.

[0011] Furthermore, the step S2 is specifically as follows:

[0012] The soil displacement field is calculated as follows:

[0013]

[0014] Where: j = 1, 2 (j = 1 means that the foundation pit 1 causes the displacement of the surrounding soil on the near side of the tunnel, and j = 2 means the far side); S xij represents the horizontal soil displacement around the tunnel caused by the foundation pit 1 (when j = 1, it represents the horizontal displacement of the near side of the tunnel caused by the foundation pit), S zij represents the vertical soil displacement around the tunnel caused by the near foundation pit 1;

[0015] The additional stress value q at the axis under double-sided excavation is calculated as follows:

[0016]

[0017] Where: K1 is the horizontal soil spring stiffness, K2 is the vertical soil spring stiffness.

[0018] Furthermore, the step S3 is specifically as follows:

[0019] Calculate w2 as follows:

[0020]

[0021] Where: k1 and k2 are the stiffness of the first and second layer springs respectively, G is the shear modulus of the soil, and w2 is the deformation of the second layer spring;

[0022] According to the calculated w2, the tunnel longitudinal displacement w is calculated as follows: i :

[0023]

[0024] Where: k1 and k2 are the stiffness of the first and second layers of springs, G is the soil shear modulus, l is the length of each tunnel section, (w2) i is the deformation of the second layer of spring;

[0025] Furthermore, the step S4 is specifically as follows:

[0026] The initial confining pressure of the tunnel is calculated according to the following formula:

[0027]

[0028] Where P1 is the overlying soil and water pressure; P2 is the reaction force at the base of tunnel 3; P3 is the horizontal soil and water pressure at the arch top; P4 is the additional horizontal soil and water pressure at the arch bottom; P G is the deadweight of the lining; P e The formation resistance is regarded as 0 in this patent and is not considered;

[0029] The tunnel confining pressure is calculated according to the following formula:

[0030]

[0031] Where θ is the clockwise angle of the tunnel cross section lining position starting from the top as 0, Δv zt is the difference between the displacement of the soil above the tunnel and the displacement at the tunnel axis, Δv zb Δv is the difference between the displacement of the soil under the tunnel and the displacement at the tunnel axis; xr is the difference between the displacement of the soil on the right side of the tunnel and the displacement at the tunnel axis, Δv zl is the difference between the displacement of the soil on the left side of the tunnel and the displacement at the tunnel axis; the displacement of the soil around the tunnel is given by S xij , S zij The beneficial effects of the present invention are:

[0032] 1. The present invention takes into account the shape change of the excavation retaining structure, and establishes the soil displacement field under double-sided foundation pit excavation based on the image source method. The existing tunnel is placed in the displacement field to establish the response mechanism of the tunnel, and the tunnel confining pressure distribution formula under double-sided foundation pit excavation is derived, which provides theoretical guidance for the circumferential deformation evaluation of the tunnel during actual construction.

[0033] 2. The present invention idealizes the existing tunnel into a Euler-Bernoulli beam, and the soil-tunnel interaction can be simulated as a three-parameter Kerr model. It stipulates that the existing tunnel and the surrounding soil deform together, and there is no local separation phenomenon, which simplifies the calculation model and makes it easier to understand.

[0034] 3. The present invention establishes a method for evaluating tunnel deformation and confining pressure based on double-sided foundation pit excavation, which is applicable to double-sided synchronous excavation of foundation pits and staggered synchronous excavation of foundation pits on both sides, and is also applicable to engineering evaluation of single-sided foundation pit excavation schemes. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 flow chart

[0036] Figure 2 Theoretical calculation model diagram

[0037] Figure 3 Diagram of the mechanism of the impact of foundation pit excavation on the side tunnel

[0038] Figure 4 Horizontal and vertical displacement diagram of the left tunnel under double-side excavation

[0039] Figure 5 Vertical displacement field of soil under double-sided foundation pit excavation

[0040] Figure 6 Lateral displacement field of soil under double-sided foundation pit excavation

[0041] Figure 7 Comparison of tunnel confining pressure before and after double-sided foundation pit excavation

[0042] Explanation of the accompanying drawings: 1-left foundation pit; 2-protection structure; 3-tunnel; 4-right foundation pit; 5-general foundation pit; 6-side wall soil. DETAILED DESCRIPTION

[0043] The present invention will be further described below by way of examples and drawings. Figure 2 As shown in the figure, before excavation, all points of the normally consolidated soil are in a static earth pressure state. After the excavation of the single-sided foundation pit 5, the unloading of the soil in the general foundation pit 5 destroys the original stress equilibrium state of the side wall soil 6, causing displacement of the surrounding soil and the retaining structure 2, generating an unloading stress increment, thereby causing displacement of the tunnel 3. The impact of double-sided excavation on the tunnel 3 is more complicated. Based on the papers "Han TR, Wu G, Lu Y, et al. Behavior of shield tunnel lining with excessive deformation under grouting treatment: field experiment [J]. Journal of Construction Facility Performance, 2021, 35(6): 1657." and "Lu Y, Han T R. Resilience of subway tunnel structure: Monitoring deformation caused by surrounding engineering activities and repair treatment effects [J]. Urban Lifeline, 2023, 1(8): 1-19.", the measured data results of the case study on the impact of large-scale excavation in soft soil areas on subway shield tunnel 3 during the reconstruction of a certain city in China are used as a case study to evaluate and analyze the specific steps as follows:

[0044] The plane excavation dimensions of the left foundation pit 1 beside tunnel 3 are L1=190m, B1=150m, and the excavation depth is d1=21.05m. The plane excavation dimensions of the right foundation pit 4 are L2=160m, B2=110m, and the excavation depth is d2=21.05m. The soil layers and thicknesses from top to bottom are overburden (0-5m), clay (5-25m), and fine sand (25m). The soil-related calculation parameters are taken as the arithmetic average according to the thickness of each soil layer. The soil layer is regarded as a uniform soil layer during the calculation process. First, the displacement curve of the retaining structure 2 has been calculated.

[0045] The parameter d i d 0i , B i Substituting into formulas (1) to (6), the displacement field of the soil excavated on both sides is obtained, which is divided into the vertical soil displacement field and the lateral soil displacement field, as well as the additional stress at the 3-axis of the tunnel.

[0046] The parameter D t , R, h t 、E s Substituting , v into formulas (7) to (21), the vertical displacement and horizontal displacement of the double-side excavation tunnel 3 are obtained, and the inter-ring confining pressure transfer value is calculated.

[0047] The parameters γ1, γ2, γ3, γ T 、z1、z2、 Δ e , k s Substituting into formulas (22) to (27), we first obtain the initial confining pressure of tunnel 3, and then calculate the additional confining pressure value of tunnel 3 after redistribution under double-sided excavation through (28) to (30).

[0048] The following is a detailed description of the specific implementation of the embodiment of the present invention in conjunction with the accompanying drawings.

[0049] Figure 3Figure 3 is the horizontal displacement and vertical displacement diagram of the left tunnel under double-sided excavation. The positive value represents that the horizontal displacement of tunnel 3 moves toward the direction of the excavation area of ​​foundation pit I1, and the vertical displacement moves downward. The calculated value and the maximum horizontal displacement of tunnel 3 are at y=168m, which is 21.22mm, corresponding to the center of the excavation area of ​​foundation pit I1. The horizontal displacement value decreases on both sides until it approaches 0 at the edge of the excavation area. The calculated value, whether vertical or lateral, is basically consistent with the actual measured value, verifying the accuracy of the present invention. The maximum vertical displacement of tunnel 3 is at y=222m, and the calculated displacement value is 44.17mm, which is similar to the horizontal displacement law, decreasing from the maximum value to 0 at the edge of the excavation area. Due to the superposition of horizontal stress, the excavation of foundation pit II 4 excavation area offsets part of the unloading stress of foundation pit I1 excavation area, while the superposition of vertical displacement increases the downward unloading stress as a whole. At the same time, the excavation area of ​​foundation pit I1 and foundation pit II The center positions of the 2 excavation areas differ by 50m, which affects the positions of the maximum horizontal and vertical displacements, and the tunnel displacement curve is no longer left-right symmetrical.

[0050] Figure 4 and Figure 5 is the calculated soil displacement diagram under double-sided excavation, Figure 5 It represents the vertical displacement field under double-sided excavation. Within the height range corresponding to the retaining structure 2, the overall soil displacement shows settlement, which is greater than 30 mm. An arc-shaped uplift area appears within 5 m at the bottom of the left and right retaining structures 2. The lateral displacement field is greatly affected by the displacement difference of the retaining structure 2. The maximum displacement of the left retaining structure 2 is 1.2 times that of the right retaining structure 2, which makes the zero-value line of the soil displacement deviate to the right and form an arc shape with the right retaining structure 2. Unlike the excavation of the single-sided foundation pit 5, the settlement value of the vertical soil displacement field under double-sided excavation is much larger than the lateral soil displacement field as a whole. This is mainly because the effects caused by the excavation on both sides are the same in the vertical direction and opposite in the lateral direction. The response is the superposition of vertical soil displacement and the reduction of lateral soil displacement.

[0051] Figure 6 This is a comparison diagram of the confining pressure of Tunnel 3 before and after double-sided excavation. When the initial confining pressure is in the case of large longitudinal deformation of Tunnel 3, the lateral earth pressure coefficient calculated by the Rankine earth pressure is large, and the initial confining pressure presents a "fat gourd" shape. After the double-sided excavation is completed, the top and bottom confining pressures of Tunnel 3 increase and the waist confining pressure decreases. The maximum reduction in confining pressure on the left waist is 74.91 kN, and the maximum reduction on the right waist is 18.91 kN.

[0052] at last Figure 7The present invention provides a method for evaluating tunnel deformation and confining pressure based on double-sided foundation pit excavation, which includes the following steps: first, determining the evaluation project; then collecting the data of the existing left foundation pit 1 and right foundation pit 4 related to the project; calculating through the Matlab program, drawing the vertical and horizontal displacement diagrams of the tunnel 3 under double-sided excavation, and the lateral confining pressure distribution diagram of the tunnel 3; comparing the displacement obtained by the study with the first-level control standard in the Technical Code for Safety Protection of Urban Rail Transit Structures; if the vertical and horizontal displacement values ​​of the tunnel 3 are less than 20 mm, it is reasonable; if not, it means that the excavation method is unreasonable, and the tunnel 3 needs to be maintained by reinforcing the existing tunnel 3, grouting correction, adjusting the support system in the foundation pit 5, etc.

Claims

1. A method for evaluating tunnel deformation and confining pressure based on double-sided foundation pit excavation, characterized in that: The steps include: Step S1: Based on the actual data of the left foundation pit (1), the right foundation pit (4), the retaining structure (2) and the shield tunnel (3) obtained from the design data, an unloading model based on the excavation of the left foundation pit (1) and the right foundation pit (4) is established. The model is simplified to the excavation depth of the left foundation pit (1) and the right foundation pit (4) being d between the left foundation pit (1) and the right foundation pit (4) of the shield tunnel (3). i , the buried depth of the enclosure structure (2) is d 0i , width is B i ; The outer diameter of the shield tunnel (3) is D t , the radius is R, the buried depth of the center axis is h t , the horizontal clearance from foundation pit 1 is s i ; i represents the number of foundation pit 1, i=1 represents the right foundation pit (1), i=2 represents the left foundation pit (4); Step S2: establishing displacement fields of soil excavated from the left foundation pit (1) and the right foundation pit (4) on both sides, and obtaining additional stress values ​​at the axis of the shield tunnel (3) under the double-sided excavation; Step S3: Calculate the longitudinal displacement of the tunnel (3) according to the additional stress value at the axis obtained in step S2; Step S4: Calculate the confining pressure of the tunnel (3) according to the soil displacement field calculated in step S2; Step S5: judging whether the longitudinal displacement of the tunnel (3) is less than the maximum displacement value of the tunnel design according to the longitudinal displacement of the tunnel (3) calculated in step S3, and judging whether the confining pressure of the tunnel (3) is less than the maximum confining pressure value of the tunnel design according to the confining pressure of the tunnel (3) calculated in step S4.

2. The method for evaluating tunnel deformation and confining pressure based on double-sided foundation pit excavation according to claim 1, characterized in that: The step S2 is specifically as follows: The soil displacement field is calculated as follows: Where: j = 1, 2 (j = 1 means that the foundation pit 1 causes the displacement of the surrounding soil on the near side of the tunnel (3), and j = 2 means the far side); S xij represents the horizontal soil displacement around tunnel (3) caused by the foundation pit 1 (when j = 1, it represents the horizontal displacement of the near side of tunnel (3) caused by the foundation pit), S zij represents the vertical soil displacement around the tunnel (3) caused by the near foundation pit 1; The additional stress value q at the axis under double-sided excavation is calculated as follows: Where: K1 is the horizontal soil spring stiffness, K2 is the vertical soil spring stiffness.

3. The method for evaluating tunnel deformation and confining pressure based on double-sided foundation pit excavation according to claim 1, characterized in that: The step S3 is specifically as follows: Calculate w2 as follows: Where: k1 and k2 are the stiffness of the first and second layer springs respectively, G is the shear modulus of the soil, and w2 is the deformation of the second layer spring; Based on the calculated w2, the longitudinal displacement w of the tunnel (3) is calculated as follows: i : Where: k1 and k2 are the stiffness of the first and second layers of springs, G is the soil shear modulus, l is the length of each section of the tunnel (3), (w2) i is the deformation of the second layer of spring.

4. The method for evaluating tunnel deformation and confining pressure based on double-sided foundation pit excavation according to claim 1, characterized in that: The step S4 is specifically as follows: The initial confining pressure of tunnel (3) is calculated according to the following formula: Where P1 is the overlying soil and water pressure; P2 is the reaction force at the base of tunnel 3; P3 is the horizontal soil and water pressure at the arch top; P4 is the additional horizontal soil and water pressure at the arch bottom; P G is the deadweight of the lining; P e The formation resistance is regarded as 0 in this patent and is not considered; The confining pressure of tunnel (3) is calculated according to the following formula: Where θ is the clockwise angle of the tunnel (3) cross section lining position starting from the top as 0, Δv zt is the difference between the displacement of the soil above tunnel (3) and the displacement at the axis of tunnel (3), Δv zb is the difference between the displacement of the soil under the tunnel (3) and the displacement at the axis of the tunnel (3); Δv xr is the difference between the displacement of the soil on the right side of tunnel (3) and the displacement at the axis of tunnel (3), Δv zl is the difference between the displacement of the soil at the left side of tunnel (3) and the displacement at the axis of tunnel (3); the displacement of the soil around the tunnel is given by S xij , S zij Calculated.

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