Analysis Of Influence Of Foundation Pit Dewatering And Excavation On Soil Settlement

With the rapid advancement of urbanization in China,the construction of high-rise buildings and underground space leads to more and more deep foundation pit projects with larger and larger volumes.In the process of deep foundation pit excavation,it is necessary not only to ensure the stability of the structure,but also to effectively control the settlement and deformation of the surrounding ground.Especially in the area rich in groundwater,the ground subsidence is more serious and its mechanism is more complicated due to the influence of groundwater.In this paper,indoor physical experiments are carried out to study the superposition effect of foundation pit dewatering and excavation on ground deformation,and to analyze the influence degree of single factor(dewatering or excavation)on surface settlement.According to the change of soil mechanical parameters during the test,the simulation was carried out considering the change of porosity.By comparing the results of the model without considering the change of porosity,the variation law of the formation stress field and displacement field was explored.Based on the excavation engineering of a subway station,a three-dimensional numerical model is established based on the variation of porosity.By comparing the surface settlement data of numerical simulation,theoretical calculation and engineering monitoring,the influence law of foundation pit dewatering and excavation on ground settlement is obtained.The main work and contents of this paper are as follows:(1)Two sets of control tests,"foundation pit dewatering and excavation under sandy soil foundation" and "foundation pit dewatering under sandy soil foundation",were carried out to analyze the variation rules of stress,porosity,settlement and permeability coefficient of sandy soil in the tests.The results show that: The sand stress at the bottom of the foundation pit is obviously affected by excavation.The time history curves of the sand stress in the two groups of tests show different trends,while the variation trend of the sand stress on both sides of the foundation pit is basically the same.At the same time,due to the release of sand stress caused by excavation,the stress in the foundation pit dewatering test increases by about 38.2%.According to the change of effective stress of sandy soil,the porosity of sandy soil in different depths was calculated,and the surface settlement curve under the change of porosity was drawn.The final result showed that the dewatering settlement accounted for 85.8% of the total settlement.The permeability coefficient of sand is more sensitive to the decrease of porosity,and it decreases by about 28.8% compared with that in the foundation pit dewatering test.(2)To solve the problem of porosity change of sand during the test process,the MIDAS calculation model was improved,and the simulation was carried out based on the test overview.The model results considering porosity change and the model results without porosity change were compared with test data.The results show that the trends of stress and settlement isopotential lines in the two numerical models are basically the same.Compared with the maximum stress value considering porosity change,the maximum stress value decreases by 4.63%,and the maximum surface settlement value decreases by 24.9%.The results of the model considering the change of porosity are closer to the experimental data,and the error decreases with the increase of the burial depth.(3)Based on the general situation of subway station engineering,the improved numerical simulation method is adopted to establish a three-dimensional model.The theoretical formula is used to respectively calculate the dewatering settlement and excavation settlement in the process of foundation pit construction,and the monitoring data are compared and analyzed.The results show that the coupling effect of dewatering settlement and excavation settlement during foundation pit construction is not considered in the theoretical calculation,and the error is large compared with the monitoring value.The simulation method without considering the change of porosity ignores the objective facts of the dynamic changes of soil mechanical parameters in the process of foundation pit dewatering and excavation.Compared with the monitoring data,the simplified calculation results are larger with an error of more than 15%.However,the surface subsidence results considering the porosity change are the closest to the monitoring data,with an error between 5% and 9%.