Deformation Behavior Of Deep Excavations Supported By Permanent Structure In Shanghai Soft Deposit

Due to its many advantages such as the capability of deformation control and being of benefit to sustainable development, the construction method of deep excavations supported by permanent structure is an efficient way to build multi-storied underground structures. It is a new type of retaining system which has developed quickly and become more and more widely used in Shanghai soft deposit in recent years. A major concern in the construction activities of this type of retaining system is to predict the wall deflections and surface ground settlements in the design stage. However, a fundamental study of the deformation behavior of this type of retaining system is relatively limited and infrequently reported in the literature. As deep excavations supported by permanent structure are in nature very complex, it is insufficient to analyze their deformation behavior by using conventional methods such as the method of beam on elastic foundation and the plane finite element method. This thesis evaluates the deformation behavior of deep excavations supported by permanent structure in Shanghai soft deposit, based on a series of 3-D finite element analysis as well as data analysis from a large number of case histories. The main contributions of this thesis are described in the following:(1) A three dimensional finite element model considering the interactions between the soil and structures is setup to simulate the construction procedures of a deep excavation supported by permanent structure. The elastic-plastic Modified Cam-Clay constitutive model is used to present the behavior of the soil. The interaction between the soil and the diaphragm wall is simulated by surface-based contact. Deformation behaviors and stress and strain responses due to deep excavation in homogeneous soil are investigated in details. Results of analyses considering and without considering the interfaces between the diaphragm wall and the soil are compared to investigate the influence of the wall-soil interface on the deformation caused by excavation. Effect of input parameters of the wall-soil interface and the Modified Cam-Clay model on the response of deep excavations is also discussed.(2) Extensive parametric studies are performed by a series of 3-D finite element analyses of deep excavations supported by permanent structure in typical Shanghai stratum to investigate their influence on excavation-induced deformations. The analyzed parameters include the thickness of the wall, the thickness of the horizontal slabs, the vertical strut spacing, cantilever excavation depth, soil reinforcement, the ratio between the excavation length and excavation width, and the wall connection. Moreover, some factors affecting the deformations of deep excavations such as the location of access openings, bermed excavation,'Island-type'excavation, blocked excavation and the existing of piles are also evaluated. General trends of the maximum lateral displacement of wall, maximum ground settlement, maximum lateral displacement of ground and bottom heave are obtained by collecting them from the numerical experiments. Relationships between these deformation variables are analyzed and their upper limits are also proposed. The general trends of these deformation variables provide a guideline to the analysis of data from a large number of case histories in the following chapter.(3) A database of 315 case histories of deep excavations in Shanghai soft deposit is presented. Deformation behavior of deep excavations supported by permanent structure is analyzed based on these deformation data. The range and mean value of the maximum lateral displacement of wall are proposed. Some factors which affect the maximum lateral displacement of wall are analyzed. These include the thickness of soft soil above wall toe, the embedded depth ration of wall, the system stiffness, the factor of safety against basal heave, the ratio between the sectional area of piles and the excavation area, and the location of first level strut. The range and mean value of the maximum ground settlement are also proposed. An equation is proposed to describe the boundary of ground settlement based on the analysis of ground settlement profile. Factors affecting the ground settlement are studied. Ground deformation behavior of deep excavations supported by permanent structures is compared with that of conventional bottom-up method. Ranges of bottom heave, vertical displacement of top of wall and piles are evaluated. Deformation behavior of other retaining system (constructed using bottom-up method) such as diaphragm wall, bored pile wall, sheet pile wall, SMW, soil cement columns, and compound soil nail wall are studied and are compared with that of deep excavations supported by permanent structure. Many figures obtained trough these theoretical analysis offer a practical approach for geotechnical engineers to predict deformations of deep excavations in Shanghai soft deposit.(4) Comprehensive monitoring systems were installed on three excavations which were supported by permanent structure. Emphases are put on the analysis of deformations of these three excavations. Results show that maximum lateral displacements of all the inclinometers in walls at different stages fall in the range proposed by theoretical analysis. Furthermore, mean values of measured maximum lateral displacements of wall agreed quite well with that proposed by theoretical analysis. This confirms the validity of the theoretical analysis results to predict maximum wall deformations. Monitored ground settlements, pipeline settlements, and buildings settlements fall within the range defined by the boundary equation proposed by theoretical analysis. This shows that the boundary equation can be used to evaluate the influence of deep excavation on adjacent facilities. Vertical deformations of the horizontal slabs are analyzed based on the monitored data. Time effects on lateral wall deflections and 3-D settlements of buildings caused by excavation are also studied.