A safety risk assessment and construction method for pile foundation settlement caused by shield tunneling

Abstract

The present invention belongs to the technical field of shield construction, and provides a safety risk assessment and construction method for pile foundation settlement caused by shield tunneling. The method of the present invention comprises: in the current shield construction, predicting the settlement prediction values of the pile foundations at several different positions according to a pre-established finite element numerical model, and verifying them in combination with the actual settlement values; adjusting the pile-tunnel horizontal distance and the pile foundation length of the pile foundation, predicting the settlement values of the pile foundations at different positions according to the finite element numerical model, and referring to the pile foundation settlement safety risk level, generating a settlement isovalue two-dimensional map divided into the safety risk levels of the pile foundations at different positions; according to the predicted safety risk level of the pile foundation, shield construction is carried out after executing the corresponding protection measures plan. The present invention verifies the numerical model by summarizing the shield construction parameters, and then generates a two-dimensional isovalue map of the pile foundation settlement safety risk level division, which is timely and reliable for the safety risk level of the adjacent pile foundations in subsequent construction.

Description

Technical Field

[0001] The present invention relates to the technical field of shield construction, and in particular to a safety risk assessment and construction method for pile foundation settlement caused by shield tunneling. Background Art

[0002] Shield tunnel excavation will inevitably cause soil disturbance and stratum loss, causing deformation of nearby structures, especially for elevated bridge pile foundations. Due to their use requirements, they are highly sensitive to deformation and any carelessness can lead to serious consequences. Therefore, it is extremely important to accurately evaluate the pile foundation safety risk level during shield construction and take relevant control measures in advance.

[0003] In recent years, many scholars have conducted extensive research on the problem of shield tunneling approaching pile foundations. Wu Xianguo et al. (Journal of Railway Engineering Society, 2012, 29(07):87-92.) used the horizontal distance between pile and tunnel as 3m and the pile length exceeding the tunnel depth by 3-5m as the limit, and combined with the pile foundation type, to divide the risk level of pile foundations; Wang Lifeng (Rock and Soil Mechanics, 2014, 35(S2):319-324.) used the soil mechanical parameters, the horizontal distance between pile and tunnel, the pile top load and the stratum stress loss as parameters to define the pile foundation proximity value and divide the pile foundation risk; Wang Chuang et al. (Journal of Civil Engineering, 2017, 50(S2):174-181.) divided the risk level of pile foundations by numerical simulation; Selemetas et al. (Géotechnique, 2017, 67(9):823-836.) studied the influence of shield construction on pile foundation settlement through field measurements. The above studies have been relatively thorough in studying the impact of shield tunneling on pile foundation deformation and the assessment of pile foundation safety risk levels. However, all of them analyzed and studied the pile foundation settlement deformation solely through field measurements or numerical simulations. In actual projects, due to the complexity of on-site construction conditions, the assessment of pile foundation safety risk levels solely through numerical simulations is not very accurate, and field measurement data are often only obtained after the shield tunneling passes through the pile foundation, which lacks timeliness. Summary of the Invention

[0004] In response to the defects in the existing technology, the present invention provides a method for assessing the safety risks of pile foundations caused by shield tunneling, so as to solve the problem that the safety risk level of pile foundations cannot be evaluated and corresponding control measures cannot be taken according to the corresponding safety risk level during the existing shield construction process.

[0005] The present invention provides a safety risk assessment and construction method for pile foundation settlement caused by shield tunneling, comprising:

[0006] In current shield construction, the settlement prediction values of pile foundations at several different locations are predicted based on the pre-established finite element numerical model and verified with the measured settlement values;

[0007] Adjusting the horizontal distance between the pile and the tunnel and the length of the pile foundation, predicting the settlement values of the pile foundation at different positions according to the finite element numerical model, and generating a two-dimensional settlement contour map with the safety risk levels of the pile foundation at different positions with reference to the pile foundation settlement safety risk levels;

[0008] According to the predicted safety risk level of the pile foundation, shield construction is carried out after implementing the corresponding protection measures plan.

[0009] Optionally, the finite element numerical model is generated based on construction parameters, which include excavation face support pressure, shield tail grouting pressure, and soil loss rate.

[0010] Optionally, the security risk levels include risk zone I, risk zone II, risk zone III, and risk zone IV, which are determined by the following method:

[0011] According to the maximum settlement value C specified by national and / or local standards max , determining the settlement values corresponding to the first rate value and the second rate value of the maximum settlement value are the first-level early warning control value C1 and the second-level early warning control value C2 respectively; wherein the first rate value is greater than the second rate value;

[0012] Determine the risk level according to the interval of the sinking threshold value S;

[0013] When the sinking threshold S≥C max Risk zone I; C max >S≥C1 is risk zone II; C1>S≥C2 is risk zone III; C2>S is risk zone IV;

[0014] The risk levels corresponding to Risk Zone I, Risk Zone II, Risk Zone III and Risk Zone IV decrease in sequence.

[0015] Optionally, the horizontal coordinates of the settlement contour two-dimensional map are the pile-tunnel horizontal distance Z and the tunnel diameter D. t The ratio of the vertical axis is the pile length L p The ratio of the tunnel depth H.

[0016] Optionally, the implementing corresponding protection measures in settlement areas with different safety risk levels includes:

[0017] When S is in risk zone I, take primary protection measures for the pile foundation;

[0018] When S is in risk zone II, secondary protection measures are taken for the pile foundation;

[0019] When S is in risk zone III, monitoring is performed during shield construction;

[0020] When S is in risk zone IV, the pile foundation settlement is monitored during shield tunneling.

[0021] Optionally, the first-level protection measures include soil reinforcement, isolation pile scheme and pile foundation replacement scheme; the second-level protection measures include soil reinforcement and isolation pile scheme.

[0022] By adopting the above technical solution, this application has the following beneficial effects:

[0023] The present invention verifies the numerical model through on-site data collection of early construction and summary of shield construction parameters, and then establishes a two-dimensional equivalent map of the pile foundation settlement safety risk level division under the shield construction condition by changing the pile foundation position parameters in the numerical model. The pile foundation safety risk level is pre-divided, and corresponding engineering measures are taken according to the corresponding risk level to ensure the safety and normal use of the pile foundation, achieving good results, and having timeliness and reliability for the safety risk level of adjacent pile foundations in subsequent construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly describes the drawings required for the specific embodiments or the description of the prior art. Similar elements or parts are generally identified by similar reference numerals throughout the drawings. Elements or parts in the drawings are not necessarily drawn to scale.

[0025] Figure 1 A flow chart showing a safety risk assessment and construction method for pile foundation settlement caused by shield tunneling provided by an embodiment of the present invention;

[0026] Figure 2 A comparison chart showing the predicted values and measured values of the finite element numerical model provided by an embodiment of the present invention is shown;

[0027] Figure 3 A schematic diagram of a two-dimensional settlement contour map provided by an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0028] The following embodiments of the technical solution of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention and are therefore only examples and are not intended to limit the scope of protection of the present invention.

[0029] It should be noted that, unless otherwise specified, the technical or scientific terms used in this application should have the common meanings understood by those skilled in the art to which the present invention belongs.

[0030] This embodiment takes a section of the Nantong Metro as an example: the section is constructed using the shield method. The section tunnel is buried at a depth of 14m. The left-line tunnel is excavated first. After the left-line excavation is completed, the right-line is constructed. The distance between the center axes of the left and right tunnels is 13-17m. Elevated pile foundations and bridge pile foundations are densely distributed along the tunnel. There are four groups of pile foundations along the section. The shield first passes through the JG01 and JG02 pile foundations, and after a period of excavation, passes through JG03 and JG04. The basic conditions of the pile foundations are shown in Table 1:

[0031] Table 1

[0032] Pile number Pile length / m Horizontal distance of pile tunnel / m JG01 73 10.05 JG02 60 8.05 JG03 21 6.92 JG04 27 6.40

[0033] like Figure 1 As shown, this embodiment provides a safety risk assessment and construction method for pile foundation settlement caused by shield tunneling, including:

[0034] S1. In the current shield construction, the settlement prediction values of the pile foundation at several different locations are predicted based on the pre-established finite element numerical model, and verified with the actual settlement measurement values.

[0035] This embodiment is based on the measured values of JG01 and JG02 pile foundations, verifies the finite element numerical model and divides the pile foundation safety risk levels, predicts the safety risk levels of JG03 and JG04 pile foundations, and takes corresponding protective measures.

[0036] It should be noted that field data collection is primarily achieved through the following methods: using a level and indium steel ruler, initial values are collected when the shield excavation face is 200 meters away from the cross-section where the pile foundation is located. Measurements are taken at least twice and the average is taken. Monitoring then begins when the shield excavation face is within 200 meters of the pile foundation cross-section and continues until the pile foundation settlement and deformation stabilize. The heights of the measuring points H1, H2, H3, etc., obtained after adjustment, are subtracted from the initial value H0, ΔH1 = H1 - H0, ΔH2 = H2 - H0, etc., to obtain the pile foundation settlement values at different locations on the shield excavation face.

[0037] S2. Adjust the horizontal distance between the pile and the tunnel and the length of the pile foundation. Predict the settlement values of the pile foundation at different locations based on the finite element numerical model. Refer to the pile foundation settlement safety risk level to generate a two-dimensional settlement contour map that divides the safety risk levels of the pile foundation at different locations.

[0038] The settlements of JG01 and JG02 after the pile foundation deformation stabilized were -1.5mm and -1.4mm respectively. The generated finite element numerical model was verified based on the measured values of JG01 and JG02 pile foundations as follows Figure 2As shown in the figure, it can be seen that the numerical results are in good agreement with the measured results, which verifies the rationality of the model. Then the horizontal distance between the pile tunnel and the length of the pile foundation are adjusted to calculate the settlement values of the pile foundation at different positions. And combined with the safety risk level, the following is generated: Figure 3 The settlement isovalue 2D diagram shown in the figure has divided the risk level of the pile foundation at different positions in the current process according to the safety risk level. The horizontal axis of the settlement isovalue 2D diagram is the horizontal distance Z between the pile and the tunnel and the tunnel diameter D. t The ratio of the vertical axis is the pile length L p The ratio of the settlement to the tunnel burial depth H is used to generate a two-dimensional settlement contour map in the form of a ratio, which is universally applicable to different tunnel burial depths and pile-tunnel distances in different projects.

[0039] Specifically, the finite element numerical model is generated based on the construction parameters, including the excavation face support pressure, shield tail grouting pressure, and soil loss rate.

[0040] The construction parameters during shield tunneling through pile foundations are summarized, mainly including the excavation face support pressure, shield tail grouting pressure, and soil loss rate. The working conditions in actual projects are analyzed in detail and applied to the numerical model. Numerical simulation software is used to establish a numerical model of engineering shield tunneling through pile foundations. The above construction parameters reflect the impact of the shield during the shield tunneling process.

[0041] In one possible implementation, the security risk levels include risk zone I, risk zone II, risk zone III, and risk zone IV, which are determined by the following method:

[0042] According to the maximum settlement value C specified by national and / or local standards max , determining the settlement values corresponding to the first rate value and the second rate value of the maximum settlement value are the first-level early warning control value C1 and the second-level early warning control value C2 respectively; wherein the first rate value is greater than the second rate value;

[0043] Determine the risk level according to the interval of the sinking threshold value S;

[0044] When the sinking threshold S<C max Risk zone I; C max >S≥C1 is risk zone II; C1>S≥C2 is risk zone III; C2>S is risk zone IV;

[0045] The risk levels corresponding to Risk Zone I, Risk Zone II, Risk Zone III and Risk Zone IV decrease in sequence.

[0046] In this embodiment, the division of safety risk levels refers to the Technical Specifications for Urban Rail Transit Monitoring and the Jiangsu Province Urban Rail Transit Engineering Monitoring Regulations. For a first-level municipal bridge pier, the maximum settlement value should not exceed -15 mm, and 80% and 65% of the cumulative settlement value of the pile foundation are used as the first-level early warning control value C1 and the second-level early warning control value C2, which are -12 mm and -9.5 mm respectively. Based on the settlement threshold value S of the above-mentioned pile foundation, the pile foundation safety risk level is divided into four areas, S ≥ -15 mm is a very high risk area (Zone I), -15 mm>S ≥ -12 mm is a relatively high risk area (Zone II), -12 mm>S ≥ -9.5 mm is a general risk area (Zone III), and -9.5 mm>S is a relatively low risk area (Zone IV), forming the division of safety risk levels in this embodiment.

[0047] S3. Based on the predicted safety risk level of the pile foundation, shield construction shall be carried out after implementing the corresponding protection measures plan.

[0048] Optionally, in step S3, corresponding protection measures are implemented in settlement areas with different safety risk levels, including:

[0049] When S is in risk zone I, first-level protection measures are taken for the pile foundation;

[0050] When S is in risk zone II, secondary protection measures are taken for the pile foundation;

[0051] When S is in risk zone III, monitoring is performed during shield construction;

[0052] When S is in risk zone IV, the pile foundation settlement is monitored during shield tunneling.

[0053] Among them, the first-level protection measures include soil reinforcement, isolation pile scheme and pile foundation replacement scheme; the second-level protection measures include soil reinforcement and isolation pile scheme.

[0054] Soil reinforcement plan: Since shield excavation disturbs the surrounding strata and causes stratum loss, it will cause the stratum to deform. The deformation of the stratum is transmitted to the area around the pile foundation, causing the pile foundation to deform. By reinforcing the soil around the pile foundation, the soil's ability to resist deformation is improved and the impact of tunnel excavation on the pile foundation is reduced. Soil reinforcement mainly adopts grouting reinforcement method, which injects cement slurry or cement and water glass mixed slurry into the soil.

[0055] Isolation pile solution: Before tunnel construction, bored cast-in-place piles or cement-soil mixing piles are constructed around the pile foundation to protect the pile foundation. The bottom of the isolation piles must be buried below the bottom of the shield tunnel, at least 1.2 times the tunnel burial depth. At the same time, a crown beam is used on the top of the isolation piles to connect them into a whole.

[0056] Pile foundation replacement plan: add pile holes and cast-in-place piles around the existing pile foundation, and build a pedestal, which is connected to the existing pedestal by implanting steel bars to form a whole. It shares the upper load with the original pile foundation or completely replaces the original pile foundation to bear the upper load alone. The newly built pile foundation must meet the bearing capacity requirements and the pile length must be greater than the original pile foundation length.

[0057] According to the above generated two-dimensional settlement equivalent map, JG03 and JG04 are pre-classified into safety risk levels. The buried depth of the tunnel passing through JG03 and JG04 is 14m, the pile lengths are 21m and 27m respectively, and the horizontal distances between the piles and tunnels are 6.92m and 6.40m. According to the above map, their safety risk level is zone IV. During the shield tunneling process, they only need to be monitored and no engineering protection measures are required. During the shield tunneling process, no engineering measures were taken for JG03 and JG04. After the shield construction was completed, the final settlements of JG03 and JG04 measured on site were -2.7mm and -3.1mm respectively, and the safety risk level was zone IV. It can be seen that, compared with the use of Figure 3 The risk level classification results are consistent with those of the numerical calculations. The final settlements of JG03 and JG04 obtained are -3.2 mm and -3.9 mm, which are close to the measured results and are relatively safe.

[0058] Optionally, the method provided in this embodiment can also include strengthening settlement monitoring while protecting the pile foundation. For example, if the excavation surface is within ±50m of the pile foundation cross section, monitoring is performed once a day; if it is within ±100m beyond ±50m, monitoring is performed once every two days; if it is within ±200m beyond ±100m, monitoring is performed once every five days; and if it is beyond ±200m, monitoring is performed once every 15 days, until the pile foundation settlement and deformation stabilize.

[0059] In summary, a method combining field measurement data and numerical simulation was used to pre-classify the pile foundation safety risk levels in a certain section of the Nantong Metro. Appropriate engineering measures were taken according to the corresponding risk levels to ensure the safety and normal use of the pile foundation, achieving good results.

[0060] The above embodiments are merely used to provide a detailed introduction to the technical solutions of the present application. However, the descriptions of the above embodiments are only intended to help understand the methods of the embodiments of the present invention and should not be construed as limiting the embodiments of the present invention. Any changes or substitutions that can be easily conceived by those skilled in the art should be included within the scope of protection of the embodiments of the present invention.

Claims

1. A safety risk assessment and construction method for pile foundation settlement caused by shield tunneling, characterized in that: include: In current shield construction, the settlement prediction values of pile foundations at several different locations are predicted based on the pre-established finite element numerical model and verified with the measured settlement values; Adjust the pile-tunnel horizontal distance and pile length of the pile foundation, predict the settlement values of the pile foundation at different positions according to the finite element numerical model, and generate a settlement contour two-dimensional map with the safety risk levels of the pile foundation at different positions with reference to the pile foundation settlement safety risk level; the horizontal coordinate of the settlement contour two-dimensional map is the horizontal distance Z between the pile and the tunnel diameter D t The ratio of the vertical axis is the pile length L p Tunnel depth H The ratio of According to the predicted safety risk level of the pile foundation, shield construction is carried out after implementing the corresponding protection measures plan; The finite element numerical model is generated based on construction parameters, including excavation face support pressure, shield tail grouting pressure, and soil loss rate; The safety risk levels include Risk I, Risk II, Risk III and Risk IV, which are determined by the following method: Maximum settlement values as specified by national and / or local standards , determine the settlement values corresponding to the first rate value and the second rate value of the maximum settlement value as the first-level early warning control values and secondary warning control value ; wherein the first rate value is greater than the second rate value; Determine the risk level according to the interval of the sinking threshold value S; When the sinking threshold S≥ Risk zone I; >S≥ Risk zone II; >S≥ Risk zone III; >S is risk zone IV; The risk levels corresponding to Risk Zone I, Risk Zone II, Risk Zone III and Risk Zone IV decrease in sequence.

2. The method according to claim 1, characterized in that The shield construction is carried out after implementing the corresponding protection measures according to the predicted safety risk level of the pile foundation, including: When S is in risk zone I, take primary protection measures for the pile foundation; When S is in risk zone II, secondary protection measures are taken for the pile foundation; When S is in risk zone III, monitoring is performed during shield construction; When S is in risk zone IV, the pile foundation settlement is monitored during shield tunneling.

3. The method according to claim 2, characterized in that The first-level protection measures include soil reinforcement, isolation pile scheme and pile foundation replacement scheme; the second-level protection measures include soil reinforcement and isolation pile scheme.

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