Study On Some Problems In Top-down Method And Long-narrow Foundation Pit

With the acceleration of urbanization,the available space overground gradually tends to be saturated. The development and utilization of underground space has put forward new requirements for the support technology of foundation pit. The deformation analysis of deep foundation pit with top-down construction method and stability against basal heave in anisotropy soft clay and the passive earth pressure for the soils behind retaining wall,as well as basal heave stability in Long-narrow foundation pit, are all new problems in developing the underground space. FEM analysis method, analytical solution method and model tests method were used to solve the problems mentioned above. The main work and conclusions are as follows:1. FEM analysis was conducted to study the effect of different size of earth berms and engineering pile on deformation behavior of deep foundation pit with top-down construction method. The theoretical calculation formula for m of Hangzhou typical soil considering width,height and slope rate of earth berm and engineering pile was derived, which were applied in 3D FEM analysis program to support 3D FEM analysis method considering the influence of earth berm and engineering pile. It is obvious that earth berms and engineering pile in pit can reduce the deformation of retaining wall, surface subsidence and rebound significantly. The influence of engineering pile can be neglected while the engineering pile interval is ten times the pile diameter.2. As the strength of natural clay shows significant anisotropic properties,it is necessary to consider the effects of the anisotropy of soft clay when analyzing stability against basal heave of foundation pit. Assuming that the soil slip failure mode is a circular arc sliding mode and its center is the intersection point of retaining wall and the lowest support, a formula of the anisotropic clay strength based on the results of Hangzhou intact soft clay tests using hollow cylinder apparatus (HCA) and limit equilibrium method are adopted in the derivation of the equation of stability against basal heave of foundation pit. It is shown that it would overestimate the safety against basal heave if the anisotropy of strength is ignored or the Casagrande anisotropic strength formula is adopted.3. Facilities were developed to study the Long-narrow foundation pit such as subway station, utility tunnel. An extensive series model tests was conducted to discuss the influential factors on lateral passive earth pressures behind relative translational retaining walls. These factors include the width of backfill, interactive friction between wall and soil, and compactness of soil. The change in lateral earth pressure distribution from the at-rest condition to the passive condition was monitored by a set of pressure cells. The particle image velocimetry technique was employed to observe the development of a failure zone in the soils. The experiment results show the lateral earth pressures were significantly influenced by the width of backfill or interactive interface or compactness of soil. In case of infinite soils, as the decrease of soil width,the increase of interactive interface or soil compactness, the passive earth pressures also increase rapidly accompanied with the heights of the application points of the resultant earth pressure moves down gradually. Moreover, the lateral earth pressures decreased significantly after reaching to peak when the backfill was dense sandy. Contrast to the narrow foundation pit near existing retaining wall or bedrock, the lateral earth pressures in relative translational rigid retaining wall reach to passive limit state more easily.4.FEM analysis models were presented by Plaxis to study the foundation pit with different width. The basal heave stability model considering the effect of pit width was put forward,which was based on the distribution of slip surface was found from FEM analysis results. These influential factors on the basal heave stability were discussed in a case, including the foundation pit width and the embedment of retaining wall. The studies indicated that with the decrease of pit width and the increase of retaining wall’s embedment, the heave stability safety increased gradually. Moreover, the size effect is more obvious in sandy soil than clayey soil.