Research On The Key Design Technologies Of The Deep Foundation Pit In Cobble Stratum In Chengdu Area

In recent years, with the large-scale city construction and rail transit construction, there are many deep excavations in rich water cobble stratum in Chengdu region.The domestic and foreign research achievements on the clay or sandy layer of deep foundation pits are very rich, but for deep foundation pits in cobble stratum the deep and systematic research is very needed. In this paper, based on the background of Chengdu cobble stratum,and mainly relying on the station excavation engineerings in Chengdu metro, the Jichang Road tunnel engineering and Shuangliu tunnel engineering in Chengdu-Mianyang-Leshan passenger dedicated line, the tunnel engineerings n the Lidui park branch of Chengdu-Guanxian high-speed rail, several key design problems of deep foundation pit in cobble stratum in Chengdu area have been researched by theoretical analysis, literature research, granular discrete element method, finite element method,and field monitoring method.The work is summarized as follows:(1)By the granular discrete element method,the braces’erection time of the deep foundation pit was analyzed and four braces’erection time was decided.Based on the time,the excavation of the deep foundation pit in cobble stratum was simulated and the stress and deformation of supporting structures in deep foundation pit were analyzed and the safety of the deep foundation pit was evaluated.The three move patterns,referring to translational,rotation about top,rotation about bottom,was simulated by granular discrete element method,so the distributional characteristics and calculating method of earth pressure at rest,active earth pressure and passive earth pressure were obtained.The two problems that how fine particles from cobble layers with various densities in deep foundation pit engineering are influenced by pumping groundwater and what the law of the land subsidence caused by pumping groundwater is were studied by granular discrete element mothed.(2)The envelope curve of land subsidence caused by the excavation of the deep cobble foundation pit was obtained from the study on the field monitoring data,which was proved by granular discrete element method.This envelope curve was compared with that one from the deep soft soil foundation pit in Shanhai area and the differences was summed up.By the investigation of domestic and foreign research status,combining with the features of buildings and structures around the support projects,the division standards of the municipal pipeline,bridge pile foundation and building around the deep cobble excavation in Chengdu area were confirmed.Then the influence divisions of the building and structure were acquired by project cases and numerical simulation.(3)The deformation control standard of allowable tensile stress and allowable subsidence for the large diameter municipal pipeline crossing deep foundation pit were decided by theoretical analysis,which was also used to the engineering.The artificial exciting force was used to simulate the train vibration load which was added to the finite difference software of FLAC3D.So the subsidence law of the rail bridge crossing deep foundation pit was researched with the train vibration load in the course of the excavation of the foundation pit.The control standard of the rail bridge.which was also called speed limit standard, was obtained.The standard was used to the engineering.