Subway Station Sandy Cobble Foundation Pits Deformation Characteristics And Safety Grade Evaluation

With the increasing scale of subway construction, the project of foundation pits bring about more risks and disasters. The accidents of foundation pits collapse occurred in recent years around the world have a seriously impact on people’s lives and property safety. Research on the deformation characteristics of sandy cobble foundation pits and its affact on surrounding environment can improve its design, construction and monitoring, ensuring the safe construction of sandy cobble foundation pits and reducing kinds of disasters and accidents.Based on a sandy cobble foundation pits with rich water at a subway station in the project of Changsha Rail Transit Line1, based on the high strength, high permeability and low cohesion mechanical properties of such sandy cobble grounds, using methods such as field monitoring and numerical simulation, studying the deformation characteristics of sandy cobble foundation pits and its impact on the surrounding environment, the mainly research conclusions are as follows:(1) The FLAC3D three-dimensional calculation results show that the sandy cobble foundation pits in long strip also have the obvious effect of three-dimensional space; Compared to the horizontal displacement of pits in soft land area, the maximum of horizontal displacement on underground continuous wall is smaller, but it’s easy to have skirting damage; The distance between the position of maximum settlement around the surface of pits and the wall is about1/2of the depth of excavation of pits, the greatest impact of range is about2times of that of the depth of excavation of pits, and the surface subsidence curves are steeper compared with that around the soft land areas; Compared with the pits in soft land areas in the same scale, the amount of basement uplift is smaller and the maximum uplift occurs at about2m from the underground continuous wall.(2) Comparison results of the calculated values and the measured values of the finite elements show that Hardening-soil model can accurately simulate the deformation features of pit of sand and gravel, and its calculated results are basically the same as the three-dimensional calculation results of FLAC3D. Meanwhile the method proposed in the text of using uniform design and finite element to determine the unknown parameters of the constitutive model of stratums of sand and gravel is feasible.(3) For the stratums of sand and gravel, compared to the horizontal displacement, vertical displacement of underground pipeline can better reflect the security status of the pipeline. The deformation trend of underground pipelines is basically the same as that of the outside pits. The safe distance between underground pipelines and the pits is about1.5times of the depth of excavation.(4) The buildings and the pit interact mutually. When there exist buildings around pits, the maximum horizontal displacement of underground continuous wall and the maximum settlement value of the surface increase respectively, however, the embedded solid role of strata of sandy cobble to piles in buildings makes that the excavation of pits has smaller influence on buildings.(5) Selected the excavation depth, sandy cobble ground thickness and groundwater table height as safety grade evaluation indexes of sandy cobble foundation pits, which weight coefficients are calculated by the method of combinng the orthogonal experimental design, FEM computation and analytic hierarchy process.(6) Established the safety grade partition and the evaluation standard of the sandy cobble foundation pits, and then work out the matter-element extension evaluation model to calculate the safety degree of such sandy cobble foundations.