Water-rich soft soil stratum deep foundation pit dewatering excavation risk analysis method

Abstract

The invention relates to a water-rich soft soil stratum deep foundation pit dewatering excavation coupling analysis method. The method comprises the steps that an SOIL module on a large-scale commercial finite element software ABAQUS calculation platform is used, a Biot consolidation control equation is adopted to describe the coupling effect of soft soil stress-pore unsteady flow under foundation pit dewatering excavation, a finite element model is established, and in combination with field monitoring experience of foundation pit excavation construction on an adjacent environment influence range, an in-pit dewatering mode is adopted to determine the soft soil stress-pore unsteady flow coupling effect under foundation pit dewatering excavation. The calculation time and the calculation precision are comprehensively considered, and step-by-step refined simulation in the foundation pit construction process is achieved. According to the method, the fluid-solid coupling effect in the dewatering excavation process of the deep foundation pit in the water-rich soft soil stratum can be accurately simulated, the influence of foundation pit excavation on the surrounding environment is effectively predicted, and the engineering safety and reliability are improved.

Description

Technical Field

[0001] The present application relates to the field of building construction, and in particular to a coupling analysis method for dewatering and excavation of a deep foundation pit in a water-rich soft soil stratum. Background Art

[0002] With the rapid development of urban rail transit, the construction of subway underground stations inevitably encounters water-rich soft soil layers with complex engineering geological conditions and sensitive surrounding environment. The deformation problem caused by foundation pit construction often dominates. At present, the design and construction of subway underground station foundation pits have gradually transformed from conventional strength control to deformation control. Therefore, it is an urgent and important topic to study the coupled deformation law and control measures of deep foundation pit dewatering excavation in water-rich soft soil layers.

[0003] Water-stop curtains can be divided into bottom-drop water-stop curtains and suspended water-stop curtains according to whether they are inserted into the aquifer waterproofing bottom plate. A suspended water-stop curtain means that the curtain does not penetrate the entire aquifer into the relatively impermeable layer, but enters a certain depth of the aquifer, combined with dewatering in the pit, to form an internal dewatering and external stop groundwater treatment method. Many scholars at home and abroad have conducted research on suspended water-stop curtains. The surface settlement outside the pit caused by foundation pit dewatering decreases as the depth of the water-stop curtain entering the confined aquifer increases. This has been pointed out in the relevant studies of Sun Wenjuan, Cheng Yun, Liu Guofeng, Pujades, etc. The cost of building a bottom-drop water-stop curtain is high and the construction is difficult. The suspended water-stop curtain has obvious advantages in terms of project cost, construction quality and construction period. Feng Xiaola et al. compiled a three-dimensional finite element calculation program based on the design and construction principles of deep foundation pit dewatering, and conducted a three-dimensional seepage finite element analysis on the deep foundation pit dewatering project of the Wuhan Yangtze River Tunnel Wuchang Shield Shaft. The model results were compared with the measured values ​​of the project, proving the applicability of the model and calculation method. The model results of the foundation pit with or without a suspended water-stop curtain were compared, and the effect of the suspended water-stop curtain on foundation pit dewatering was quantitatively explained. Zhang Yangsong et al. analyzed the effect of water-stop curtains of different design depths on surface settlement using an actual foundation pit project as an example, and believed that when the depth of the water-stop curtain increases, the maximum surface settlement gradually decreases, and the maximum lateral displacement of the supporting structure also increases. The difference between the two gradually tends to zero, that is, the deeper the curtain, the smaller the surface settlement caused by consolidation. Wang et al. took the foundation pit project of Yishan Road Station of Shanghai Metro Line 9 as the background, simulated the water-stopping effect of the water-stop curtain through parameter inversion of the on-site pumping test, and the study showed that as the length of the filter tube decreased and the depth of the water-stop curtain increased, the water pressure change of the confined aquifer decreased, and the length of the filter tube showed a linear relationship with the water pressure change outside the pit.

[0004] The statistical analysis empirical method based on a large amount of local measured data is currently a relatively feasible method to guide the design of foundation pit dewatering, but there is still uncertainty in engineering risk prevention. It is necessary to analyze and deconstruct the overall mechanism of the environmental impact of deep foundation pit dewatering in water-rich soft soil strata, and coordinate engineering risks and social benefits as a whole. Summary of the invention

[0005] In order to facilitate the analysis of the coupling of dewatering and excavation of deep foundation pits in water-rich soft soil strata, the present application provides a coupling analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata.

[0006] The present application provides a coupling analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil layers, which adopts the following technical scheme: A coupling analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil layers, comprising the following steps: Step 1: Select the fluid-solid coupling calculation principle for soft soil deformation: Through the SOIL module on the large-scale commercial finite element software ABAQUS computing platform, the Biot consolidation control equation is used to describe the coupling effect of soft soil stress-pore unsteady flow under foundation pit dewatering excavation, which better describes the elastic-plastic deformation behavior of soft soil under complex loading and unloading paths; Step 2: Establish the finite element model: Combined with the on-site monitoring experience of the impact of foundation pit excavation on the adjacent environment, the finite element model was established by adopting the pit dewatering method and comprehensively considering the calculation time and calculation accuracy. Step 3, simulation of foundation pit construction process: Based on the principle of dewatering on demand, a step-by-step and detailed simulation is adopted for the foundation pit dewatering excavation process.

[0007] By adopting the above technical solution, the Biot consolidation control equation and the SOIL module on the ABAQUS computing platform can more accurately describe the stress-pore unsteady flow coupling effect of soft soil under foundation pit dewatering excavation, especially the elastic-plastic deformation behavior of soft soil under complex loading and unloading paths, which improves the prediction accuracy of the model; combined with the on-site monitoring experience of the impact range of foundation pit excavation construction on the adjacent environment, the pit dewatering method is adopted to establish a finite element model, which can evaluate the impact of different construction schemes on foundation pit deformation and surface settlement in the design stage, optimize the foundation pit design scheme, and reduce construction risks; by simulating the foundation pit construction process in a step-by-step and refined manner, ensure that each step of construction is carried out within a controllable range, avoid foundation pit instability and damage to the surrounding environment caused by improper construction, and improve construction efficiency and safety. Through the above steps, it is more convenient to analyze the coupling of deep foundation pit dewatering excavation in water-rich soft soil strata.

[0008] Preferably, the consolidation control equation in step 1 is as follows: Where x and z represent horizontal and vertical directions, respectively; u is displacement; p is excess pore water pressure; G and μ are shear modulus and Poisson's ratio, respectively; γ and γw are the specific gravity of soil and water, respectively; k is permeability; and εv is volumetric strain.

[0009] By adopting the above technical solution and using the consolidation control equation, the stress-pore water pressure changes of soft soil during the excavation of foundation pit dewatering can be accurately simulated to ensure that the model can truly reflect the actual situation. The consolidation control equation model can better describe the elastic-plastic deformation behavior of soft soil under complex loading and unloading paths, thereby improving the accuracy of model prediction. Through the precise calculation of fluid-solid coupling of soft soil deformation, it is helpful to optimize the design and construction plan of the foundation pit, reduce surface settlement and deformation of retaining structures caused by precipitation, and improve the safety and economy of the project.

[0010] Preferably, the step 1 also includes a continuity equation: Where w is the source and sink term of the volume strain of soft soil and the accumulation of excess pore water pressure caused by foundation pit dewatering. w is positive during groundwater recharge and negative during groundwater outflow. The stress-strain relationship of soft soil is simulated using the modified Cambridge model.

[0011] By adopting the above technical solution and introducing the continuity equation, the volume strain and excess pore water pressure changes of soft soil during foundation pit dewatering can be described more accurately. The continuity equation can better simulate the influence of groundwater flow on soil mechanical properties by considering the source and sink terms of the volume strain of soft soil and the accumulation of excess pore water pressure caused by foundation pit dewatering. This helps to more accurately predict the deformation and settlement of soil during foundation pit excavation and dewatering, thereby providing a more reliable basis for foundation pit design and construction. At the same time, the use of the modified Cambridge model to simulate the stress-strain relationship of soft soil can more realistically reflect the elastic-plastic behavior of soft soil under complex loading and unloading paths, thereby improving the practicality and reliability of the model.

[0012] Preferably, the parameters of the equation in step 1 can all be determined by a conventional indoor triaxial shear test.

[0013] By adopting the above technical solution and measuring through conventional indoor triaxial shear tests, the practicality and accuracy of the model are improved, which helps to more accurately predict the deformation and settlement during the foundation pit dewatering excavation process.

[0014] Preferably, when the finite element model in step 2 is established, the geometric dimensions of the foundation pit and the interval are based on the on-site monitoring experience of the impact of the foundation pit excavation construction on the adjacent environment. The foundation pit excavation size is 24.9m (width) × 18.6m (depth), and a 0.8m thick underground continuous wall is used as the retaining structure and water-stop curtain, with a wall depth of H = 39m.

[0015] By adopting the above technical solutions, and combining the geometric dimensions of the foundation pit and interval as well as the on-site monitoring experience, a finite element model is established to ensure that the model can truly reflect the actual engineering situation and improve the prediction accuracy of the model. A 0.8m thick underground continuous wall is used as a retaining structure and a water-stop curtain, with a wall depth of H = 39m. While ensuring the stability of the foundation pit, it can effectively control the surface settlement outside the pit and reduce the impact on the surrounding environment. Reasonable foundation pit size and retaining structure design can help avoid excessive deformation caused by foundation pit excavation and precipitation, reduce construction risks, and ensure the safety of foundation pit construction. Through accurate model simulation, the foundation pit retaining structure and construction plan can be optimized in the design stage to avoid unnecessary over-design, thereby saving engineering costs.

[0016] Preferably, the model calculation width of the finite element model in step 2 needs to be outside the range affected by precipitation. Usually, when the distance from the edge of the foundation pit to the side boundary of the model is greater than 5 times the depth of the foundation pit, the influence of the truncation boundary can be ignored. The precipitation impact radius was estimated empirically, and the depth of this model was 100m and the plane was 300×200m; Constrain the horizontal displacement of the side boundary of the model, and the horizontal and vertical displacement of the bottom boundary respectively; the bottom of the model is set as an impermeable boundary, and the side boundary is a permeable boundary (to maintain hydraulic recharge); The underground continuous wall and the surrounding soil are set to tangential friction contact, normal hard contact, friction coefficient of 0.3, and impermeable.

[0017] By adopting the above technical solutions, the surface settlement calculated by the numerical model and the commonly used foundation pit design software Beijing Lizheng was compared. The surface settlement outside the pit considered by this model for precipitation excavation fluid-solid coupling is highly consistent with the measured value, and the calculation results can better reflect the characteristics of the impact of foundation pit precipitation on the environment. Due to the fluid-solid coupling effect, precipitation not only causes consolidation settlement of the soil, but also causes the water head difference inside and outside the pit due to the water-isolating and bypassing effect of the underground continuous wall, which makes the active zone have downward permeability, resulting in the maximum surface settlement of fluid-solid coupling effect is about 2 times that of the case of excavation alone.

[0018] Preferably, the number of subdivision units of the finite element model in step 2 is 264792.

[0019] By adopting the above technical solution and finely dividing the grid, the model can more accurately capture the stress-pore unsteady flow coupling effect of soft soil during the excavation of foundation pit dewatering, so as to better predict and evaluate the impact of foundation pit construction on the surrounding environment. In addition, a large number of subdivided units helps to improve the resolution of the calculation results, reduce numerical errors, ensure that the model calculation results are highly consistent with the actual situation, and ensure the accuracy and reliability of the finite element model.

[0020] Preferably, the step-by-step refinement simulation in step 3 includes the following steps: S31. The initial geostress field of the model is applied using the Geostatic module, and the initial water level is set at the side boundary of the foundation pit to simulate the hydraulic recharge at the boundary; S32. During the excavation process, the free water level boundary is set to move with the excavation depth to simulate the ground dewatering. The calculation time for each excavation layer is 10 days.

[0021] By adopting the above technical solution, the excavation process of foundation pit dewatering can be simulated in a refined step-by-step manner, which can accurately simulate the soil deformation and groundwater flow caused by foundation pit dewatering and excavation. By using the Geostatic module to apply the initial geostress field of the model, it is ensured that the model is consistent with the actual situation in the initial state, thereby improving the reliability of the model. At the same time, by setting the free water level boundary to move with the excavation depth during the stratum excavation process to simulate stratum dewatering, the calculation time for each excavation layer is 10 days, which can accurately capture the changes in groundwater levels during each step of the excavation process, thereby more realistically reflecting the impact of foundation pit dewatering on the surrounding environment. This refined simulation method can not only improve the accuracy of model predictions, but also provide a reliable reference basis for actual projects, which helps to optimize the design of foundation pit dewatering and reduce engineering risks.

[0022] Preferably, in step 3, the construction conditions of the foundation pit excavation and dewatering process include: Initial ground stress balance and underground continuous wall construction; Set up precipitation wells to lower the water level; Excavate to -2 meters and install the first support; Excavate to -8 meters and install the second support; Excavate to -13 meters and install the third support; Excavate to -18 meters and install the fourth support; Excavate to the bottom of the pit, install the fifth support, and cast the base plate.

[0023] By adopting the above technical solution, the construction conditions of the foundation pit excavation and dewatering process can describe in detail the specific construction steps in the foundation pit dewatering excavation process, ensuring the accuracy and precision of each step. In particular, timely installation of corresponding supports after each excavation helps to control the deformation and stability of the foundation pit and reduce surface settlement caused by precipitation. In addition, through step-by-step refined simulation, various parameters in the foundation pit excavation and dewatering process can be better predicted and controlled, improving the safety and efficiency of construction.

[0024] Preferably, the method also includes model verification, in which the accuracy of the model is verified by comparing the measured surface settlement outside the pit with the theoretical value.

[0025] By adopting the above technical solutions, the model verification phase ensures the accuracy and reliability of the model by comparing the measured values ​​of surface settlement outside the pit with the theoretical values, and effectively reflects the actual impact of foundation pit dewatering and excavation on the environment. Specifically, the model can more realistically simulate the surface settlement caused by foundation pit dewatering, improve the accuracy of the prediction, help optimize the foundation pit design and construction plan, and reduce engineering risks.

[0026] In summary, the present application includes at least one of the following beneficial technical effects: 1. Through the SOIL module on the large-scale commercial finite element software ABAQUS computing platform, the Biot consolidation control equation is used to describe the coupling effect of soft soil stress-pore unsteady flow under foundation pit dewatering excavation, which can better describe the elastic-plastic deformation behavior of soft soil under complex loading and unloading paths and improve the accuracy of foundation pit deformation prediction; 2. A finite element model that comprehensively considers calculation time and accuracy was established, combined with the on-site monitoring experience of the impact of foundation pit excavation construction on the adjacent environment to ensure the rationality and practicality of the model, which is helpful to optimize the foundation pit design plan and reduce engineering risks; 3. Through the step-by-step and refined simulation of the foundation pit graded dewatering excavation process, the impact of each construction step on the foundation pit deformation can be accurately captured, which provides a scientific basis for real-time adjustment and optimization during the construction process, and effectively controls the foundation pit deformation and the safety of the surrounding environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a cross-section view of the foundation pit.

[0028] Figure 2 It is a two-dimensional refined finite element analysis model of foundation pit.

[0029] Figure 3 It is a comparison between the measured and theoretical values ​​of surface settlement outside the pit.

[0030] Figure 4 It is the lateral displacement of the underground continuous wall caused by graded precipitation excavation.

[0031] Figure 5 It is the relationship curve between surface subsidence, excavation and precipitation.

[0032] Figure 6 It is the influence of the excavation and insertion ratio of the underground continuous wall foundation pit on the surface settlement outside the pit.

[0033] Figure 7 It is the influence of the dewatering excavation coupling insertion ratio of the underground continuous wall foundation pit on the surface settlement outside the pit.

[0034] Figure 8 It is the influence of the insertion ratio on the maximum surface settlement and the location of the maximum settlement point. DETAILED DESCRIPTION

[0035] The following is combined with Figure 1-8 This application is described in further detail.

[0036] The present application embodiment discloses a risk analysis method for dewatering deep foundation pits in water-rich soft soil layers. Figure 1 The risk analysis method for deep foundation pit dewatering in water-rich soft soil strata includes the following steps: The first step is to select the fluid-solid coupling calculation principle for soft soil deformation The coupling effect of stress-pore unsteady flow is inevitable when deep foundation pits are excavated by dewatering in soft soil. Based on the SOIL module on the ABAQUS computing platform of the large-scale commercial finite element software, the Biot consolidation control equation is used to describe the coupling effect of soft soil stress-pore unsteady flow under foundation pit dewatering: Where x and z represent horizontal and vertical directions, respectively; u is displacement; p is excess pore water pressure; G and μ are shear modulus and Poisson’s ratio, respectively; γ and γw are the specific gravity of soil and water, respectively; k is permeability; and εv is volumetric strain.

[0037] The source and sink terms w of volume strain and excess pore water pressure accumulation of soft soil caused by foundation pit dewatering are added to the right side of the continuity equation. w is positive when groundwater is recharged and negative when it is discharged. The stress-strain relationship of soft soil is simulated using the modified Cambridge model. This model can better describe the elastic-plastic deformation behavior of soft soil under complex loading and unloading paths, and is therefore widely used in environmental geotechnical problems caused by soft soil foundation pit excavation. The main model parameters can be determined by conventional indoor triaxial shear tests.

[0038] The second step is to establish a finite element model Combined with the on-site monitoring experience of the impact of foundation pit excavation on the adjacent environment, the finite element model was established by adopting the pit dewatering method and comprehensively considering the calculation time and calculation accuracy. refer to Figure 1 Specifically, according to the geometric dimensions of the foundation pit and the interval, combined with the on-site monitoring experience of the impact of foundation pit excavation construction on the adjacent environment, the foundation pit excavation size is 24.9m (width) × 18.6m (depth), and a 0.8m thick underground continuous wall is used as the retaining structure and water-stop curtain, with a wall depth of H = 39m.

[0039] The calculation width of the model needs to be outside the range of precipitation influence. Usually, when the distance from the edge of the foundation pit to the side boundary of the model is greater than 5 times the depth of the foundation pit, the influence of the truncation boundary can be ignored. The precipitation influence radius is estimated empirically. The depth of this model is 100m and the plane is 300×200m. The horizontal displacement of the side boundary of the model, the horizontal and vertical displacements of the bottom boundary are constrained respectively; the bottom of the model is set as an impermeable boundary, and the side boundary is a permeable boundary (to maintain hydraulic replenishment). The underground continuous wall and the surrounding soil are set to tangential friction contact and normal hard contact. The friction coefficient is 0.3 and it is impermeable. The in-pit precipitation method is adopted. Considering the calculation time and calculation accuracy, a total of 264,792 units are divided, and the established calculation model is as follows Figure 2 shown.

[0040] The third step is to simulate the foundation pit construction process Based on the principle of dewatering on demand, a step-by-step and refined simulation is adopted for the excavation process of foundation pit dewatering. The specific steps are as follows: S31, the Geostatic module is used to apply the initial geostress field of the model, and the initial water level is set at the side boundary of the foundation pit to simulate the hydraulic recharge of the boundary.

[0041] S32, simulates ground dewatering by setting the free water level boundary to move with the excavation depth during the ground excavation process, and the calculation time for each excavation layer is 10 days.

[0042] The construction conditions of foundation pit excavation and dewatering process include: S321, initial ground stress balance and underground continuous wall construction; An initial geostress field is applied to the model to simulate the natural stress state around the foundation pit, and then the underground continuous wall is constructed.

[0043] S322, set up a precipitation well to lower the water level; A dewatering well is set up inside the foundation pit to lower the groundwater level by pumping water, creating conditions for subsequent excavation.

[0044] S323, excavate to -2 meters and install the first support; Excavate to a depth of -2 meters and install the first support to enhance the stability of the foundation pit.

[0045] S324, excavate to -8 meters and install the second support; Continue excavation to a depth of -8 meters and install the second support to further enhance the stability of the foundation pit.

[0046] S325, excavation to -13 meters, installation of the third support; Continue excavation to a depth of -13 meters and install the third support to ensure the stability of the foundation pit.

[0047] S326, excavation to -18 meters, installation of the fourth support; Continue excavation to a depth of -18 meters and install the fourth support to further strengthen the foundation pit.

[0048] S327, excavate to the bottom of the pit, install the fifth support, and cast the base plate.

[0049] Finally, excavate to the bottom of the pit, install the fifth support, and pour the bottom plate to complete the excavation process of the entire foundation pit.

[0050] Step 4: Verify the model: refer to Figure 3 By comparing the surface settlement calculated by the existing commonly used foundation pit design software Beijing Lizheng with the numerical model, the calculation result of Beijing Lizheng is much smaller than the measured value, mainly because Beijing Lizheng only considers excavation deformation and cannot calculate foundation pit precipitation. The surface settlement outside the pit considering precipitation excavation fluid-solid coupling is highly consistent with the measured value, and the calculation result can better reflect the characteristics of the impact of foundation pit precipitation on the environment. Due to the fluid-solid coupling effect, precipitation not only causes consolidation settlement of the soil, but also causes the water head difference inside and outside the pit due to the water-isolating and bypassing effect of the underground continuous wall, which makes the active zone have downward permeability, resulting in the maximum surface settlement of fluid-solid coupling effect is about 2 times that of the case of excavation alone.

[0051] Step 5: Analyze the results: refer to Figure 4 In the excavation stage, excavation was carried out with graded precipitation. After the first excavation, the underground continuous wall was in cantilever lateral displacement mode. Due to its limited lateral displacement, the cantilever lateral displacement of the underground continuous wall was obviously constrained after the construction of the first concrete support was completed. As the excavation surface gradually moved downward, the soil side pressure difference caused by the height difference of the soil body caused the retaining wall to move further in the pit, and the "kicking" deformation trend of the top of the underground continuous wall gradually increased. Since the soil at the support elevation is generally excavated first and then the support is erected, the retaining wall will inevitably deform before the support has a protective effect, and a large deep horizontal deformation will occur. The lateral displacement of the underground continuous wall in the precipitation stage has similar deformation characteristics to that in the excavation stage. Precipitation causes soil consolidation, and the underground continuous wall deforms into the pit. The seepage force and water pressure in the foundation pit caused by precipitation are greatly reduced, thereby aggravating the lateral displacement of the retaining structure in the passive zone.

[0052] refer to Figure 5 , the relationship between surface settlement and excavation and precipitation. The first excavation depth was relatively small, and the settlement outside the pit showed a triangular settlement. Starting from the second excavation, due to the lateral displacement of the underground continuous wall and the increase in the plastic uplift of the pit bottom soil during the excavation process, the soil around the foundation pit produced a plastic zone, causing the ground groove-shaped settlement. In the second excavation, in the main affected area, the vertical deformation of the soil gradually transitioned from a groove-shaped settlement to an uplift state. Overall, the smaller the excavation depth, the more obvious the trend of the vertical deformation of the soil transitioning from a groove-shaped settlement to an uplift state.

[0053] The surface settlement curve trend calculated by the software commonly used for foundation pit three-dimensional precipitation simulation is similar to that calculated by this method, and the agreement is good.

[0054] Whether it is the proportion of total surface settlement (precipitation settlement accounts for about 62% of total settlement) or the scope of settlement (the maximum surface settlement is about 0.62 he from the pit edge when only excavation is carried out, and the maximum surface settlement is about 0.88 he from the pit edge when precipitation is carried out), precipitation is much greater than excavation, and the surface settlement range around the foundation pit is controlled by the surface precipitation settlement range. Due to the resistance of the underground continuous wall, the precipitation settlement curve is concave.

[0055] The purpose of setting up a water-stop curtain is to prevent or reduce the seepage of groundwater inside and outside the foundation pit, which can change the direction of groundwater seepage, extend the seepage path, control the decline of groundwater level outside the pit, and effectively reduce the surface settlement caused by precipitation. The water-blocking and bypassing effect of the underground continuous wall and the stability of the foundation pit itself are closely related to its insertion depth. In order to comprehensively study the control characteristics of the underground continuous wall insertion ratio on surface settlement, the underground continuous wall insertion ratio of the foundation pit between 0.5 and 2.6 (insertion ratio 2.6 into the bottom) is studied and analyzed.

[0056] refer to Figure 6 When the underground continuous wall is not inserted into the bottom, the surface settlement curve outside the pit basically does not change with the insertion ratio. After entering the bottom, the main affected area of ​​the curve will float upward to a certain extent, and the change difference is about 3mm.

[0057] refer to Figure 7 Under the action of fluid-solid coupling, the surface settlement curve outside the pit floats up with the increase of the insertion ratio. When the insertion ratio is between 1.7 and 2.3, the changes of the three settlement curves are relatively small. After the underground continuous wall enters the bottom, its settlement curve changes more than the curve when the insertion ratio is 2.3, the size of the curve groove decreases suddenly, and the floating change is larger.

[0058] refer to Figure 8, the maximum surface settlement caused by the excavation of the foundation pit changes little with the insertion ratio, and the maximum settlement value has a 3mm mutation only after the underground continuous wall enters the bottom. This shows that when the insertion depth of the underground continuous wall meets the stability requirements of the foundation pit, the continued increase in the insertion depth has little effect on controlling the surface settlement caused by the excavation of the foundation pit, which can be basically ignored. The precipitation settlement is sensitive to the insertion ratio. Since the excavation settlement basically does not change with the insertion ratio, the total settlement is basically controlled by the precipitation settlement. For the maximum surface settlement, under the current engineering hydrogeological conditions, the maximum surface settlement decreases with the increase of the insertion ratio. When the insertion ratio is between 0.5 and 1.7, the maximum surface settlement is almost linearly related to the insertion ratio. When the insertion ratio is greater than 1.7, the maximum surface settlement hardly changes with the increase of the underground continuous wall insertion ratio. Only when the underground continuous wall enters the bottom, the maximum surface settlement value decreases significantly. This also shows that the control of the underground continuous wall on the precipitation and depth reduction outside the pit is an effective focus for reducing surface settlement.

[0059] In a specific embodiment, according to the safety level of the foundation pit, the maximum surface settlement control value outside the excavation pit of the station foundation pit is 50mm. When the insertion ratio is 1.4, the maximum surface settlement can be controlled at about 45mm. Since the maximum settlement outside the pit also involves many factors such as construction technology, cycle, and whether the underground continuous wall has leakage, the insertion of the underground continuous wall can be appropriately increased to 1.7 as a safety reserve. Taking the station foundation pit as an example, it is about 184m long, 20m wide, 16m buried, and about 448m in circumference. The cost of the underground continuous wall is about 1.792 million yuan / m. If the bottom-entry type is adopted, it will cost an additional 26.88 million yuan.

[0060] Step 6: Analyze the results In this example, a coupled finite element analysis model of deep foundation pit excavation and dewatering in water-rich soft soil was established. Specifically, a numerical simulation was conducted on the construction process of a deep foundation pit of a subway station in water-rich soft soil. Combined with the field measured data, the deformation characteristics of the foundation pit were studied. The main conclusions are as follows: (1) Excavation and dewatering of foundation pits have an obvious coupling effect. In addition to causing consolidation settlement of the soil, dewatering also causes downward permeability in the active zone due to the water-isolating and bypassing effect of the underground continuous wall. This causes the maximum surface settlement to be about twice that of the excavation condition alone.

[0061] (2) During the excavation process of the foundation pit, both precipitation and excavation will cause the underground continuous wall to deform horizontally into the pit. The early deformation of the retaining wall is inevitable during the excavation stage, which causes a large deep horizontal deformation; the seepage force caused by precipitation and the water pressure in the pit drop significantly, which aggravates the lateral deformation of the retaining structure in the passive area.

[0062] (3) The insertion ratio of the underground continuous wall has a significant effect on the head drop outside the pit. When the insertion ratio is between 0.5 and 1.7, the barrier effect of the underground continuous wall increases nonlinearly with the increase of the insertion ratio; when the insertion ratio is greater than 1.7, the maximum surface settlement hardly changes with the increase of the insertion ratio of the underground continuous wall. The maximum surface settlement decreases with the increase of the insertion ratio, and the overall change almost fits the relationship between the minimum head outside the pit and the insertion ratio.

[0063] The implementation principle of the risk analysis method and system for dewatering and excavation of deep foundation pits in water-rich soft soil strata in the embodiment of the present application is as follows: through the refined simulation of the dewatering and excavation process of deep foundation pits in water-rich soft soil strata, combined with the Biot consolidation control equation and the modified Cambridge model, the coupling effect of soft soil stress-pore unsteady flow under foundation pit dewatering and excavation can be accurately described. Through reasonable model setting and parameter selection, the deformation and surface settlement during foundation pit excavation can be effectively predicted, providing a scientific basis for foundation pit design and construction. Compared with traditional empirical and physical model methods, this method can more accurately simulate the complex mechanical behavior during foundation pit dewatering and excavation, and improve the safety and economy of the project.

[0064] The above are all preferred embodiments of the present application, and the protection scope of the present application is not limited thereto. Therefore, any equivalent changes made according to the structure, shape, and principle of the present application should be included in the protection scope of the present application.

Claims

1. A coupling analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata, characterized in that: The following steps are involved: Step 1: Select the fluid-solid coupling calculation principle for soft soil deformation: Through the SOIL module on the large-scale commercial finite element software ABAQUS computing platform, the Biot consolidation control equation is used to describe the coupling effect of soft soil stress-pore unsteady flow under foundation pit dewatering excavation, which better describes the elastic-plastic deformation behavior of soft soil under complex loading and unloading paths; Step 2: Establish the finite element model: Combined with the on-site monitoring experience of the impact of foundation pit excavation on the adjacent environment, the finite element model was established by adopting the pit dewatering method and comprehensively considering the calculation time and calculation accuracy. Step 3, simulation of foundation pit construction process: Based on the principle of dewatering on demand, a step-by-step and detailed simulation is adopted for the foundation pit dewatering excavation process.

2. The coupled analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata according to claim 1 is characterized by: The consolidation control equation described in step 1 is as follows: Where x and z represent horizontal and vertical directions, respectively; u is displacement; p is excess pore water pressure; G and μ are shear modulus and Poisson’s ratio, respectively; γ and γw are the specific gravity of soil and water, respectively; k is permeability; and εv is volumetric strain.

3. The coupled analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata according to claim 2 is characterized by: The step 1 also includes the continuity equation: Where w is the source and sink term of the volume strain of soft soil and the accumulation of excess pore water pressure caused by foundation pit dewatering. w is positive during groundwater recharge and negative during groundwater outflow. The stress-strain relationship of soft soil is simulated using the modified Cambridge model.

4. The coupled analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata according to claims 2-3 is characterized by: The parameters of the equation in step 1 can all be determined by conventional indoor triaxial shear tests.

5. The coupled analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata according to claim 1 is characterized by: When the finite element model in step 2 is established, the geometric dimensions of the foundation pit and the interval are taken into consideration, combined with the on-site monitoring experience of the impact of the foundation pit excavation construction on the adjacent environment. The foundation pit excavation dimensions are 24.9m (width) × 18.6m (depth), and a 0.8m thick underground continuous wall is used as the retaining structure and water-stop curtain, with a wall depth of H = 39m.

6. The coupled analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata according to claim 5 is characterized by: The model calculation width of the finite element model in step 2 needs to be outside the range affected by precipitation. Generally, when the distance from the edge of the foundation pit to the side boundary of the model is greater than 5 times the depth of the foundation pit, the influence of the truncation boundary can be ignored. The precipitation impact radius was estimated empirically, and the depth of this model was 100m and the plane was 300×200m; Constrain the horizontal displacement of the side boundary of the model, and the horizontal and vertical displacement of the bottom boundary respectively; the bottom of the model is set as an impermeable boundary, and the side boundary is a permeable boundary (to maintain hydraulic recharge); The underground continuous wall and the surrounding soil are set to tangential friction contact, normal hard contact, friction coefficient of 0.3, and impermeable.

7. The coupled analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata according to claim 6 is characterized by: The number of subdivided units of the finite element model in step 2 is 264792.

8. The coupled analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata according to claim 1 is characterized by: The step-by-step refinement simulation in step 3 includes the following steps: S31. The initial geostress field of the model is applied using the Geostatic module, and the initial water level is set at the side boundary of the foundation pit to simulate the hydraulic recharge at the boundary; S32. During the excavation process, the free water level boundary is set to move with the excavation depth to simulate the ground dewatering. The calculation time for each excavation layer is 10 days.

9. The coupled analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata according to claim 8 is characterized by: In step 3, the construction conditions of the foundation pit excavation and dewatering process include: Initial ground stress balance and underground continuous wall construction; Set up precipitation wells to lower the water level; Excavate to -2 meters and install the first support; Excavate to -8 meters and install the second support; Excavate to -13 meters and install the third support; Excavate to -18 meters and install the fourth support; Excavate to the bottom of the pit, install the fifth support, and cast the base plate.

10. The coupled analysis method for dewatering and excavation of deep foundation pits in water-rich soft soil strata according to claims 1-9, characterized in that: It also includes model verification, which verifies the accuracy of the model by comparing the measured surface settlement outside the pit with the theoretical value.

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