| In order to alleviate the pressure brought by land use and traffic,the construction of high-rise buildings and subway has become an inevitable trend,followed by more and more deep and large foundation pit projects.The deep foundation project of Dongfanghong Square Subway Station of Lanzhou Rail Transit Line 1 is located in red sandstone stratum with high groundwater level.It is of great significance to study the deformation law caused by dewatering and excavation.Based on this project,the deformation caused by pumping test of foundation pit and subsequent dewatering excavation is studied by theoretical deduction,field monitoring and numerical simulation.The main work and conclusions of this paper are as follows:(1)Combined with engineering examples,the pumping test is carried out,and groundwater level and land subsidence are monitored.The Dupuit infiltration curve is modified by curve fitting method,and the Dupuit infiltration curve,the modified infiltration curve and the measured value of water level drawdown are compared and analyzed.The measured results are more inclined to the revised infiltration curve,the error is less than 1m,which indicates that the revised infiltration curve has certain reliability.(2)Based on the revised infiltration curve,the calculation method of ground subsidence caused by foundation pit dewatering considering the influence of seepage force and lateral friction between wells and soil is put forward by theoretical deduction.It is concluded that the variation trend of settlement value is basically the same by comparing the calculated value with the measured value,which is concave first and convex later.The theoretical value is greater than the measured value,tending to be conservative,and the error is within 5mm.It has certain engineering practical value.(3)The position of maximum horizontal displacement of the pile gradually moves down with the increase of excavation depth of foundation pit,and finally it is near the bottom of the foundation pit,about 1 to 2m away from the bottom of foundation pit.The maximum settlement value of the surface appears at a distance of 5 to 7 m from the side of the foundation pit,about 0.29 to 0.41 times of the depth of foundation pit excavation.(4)When the foundation pit is excavated to red sandstone stratum,the phenomenon of soil and water flowing from pile to pile is easy to occur due to the characteristics of red sandstone disintegration in water and the effect of seepage force.Therefore,the construction quality of bored biting piles should be guaranteed in the course of construction,and the accidents caused by excessive deformation of foundation pit should be prevented.(5)The settlement of surrounding buildings caused by foundation pit dewatering and excavation conforms to the law of cubic curve.In addition,higher-order curve fitting is also analyzed,and it is concluded that the fitting data should not be too much and the exponents of the fitting curve should not be too high in order to reduce the error when the settlement of the building is predicted.The higher the accuracy of equation coefficients is required with the increase of the exponents of the fitting curve According to the actual situation,the appropriate model is selected.(6)Using the finite difference software FLAC3 D to simulate single well pumping,the simulation results of surface subsidence are compared with the theoretical values.It is concluded that the variation trend of the two values is basically the same.The theoretical values are larger than the simulation values,and the maximum error is about 8 mm.(7)The finite difference software FLAC3 D is used to establish the three-dimensional coupling model of foundation pit dewatering and excavation,and the simulation value is compared with the monitoring value.It is concluded that the effect of bored bite pile with internal support is better,which indicates that the design scheme is reasonable and can provide reference for the optimization design and construction of pile-support structure of similar deep foundation pit projects. |
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