Behavior Of Cellular Diaphragm Wall In Soft Deposit

Cellular diaphragm wall is a novel structure, which could be used as the horizontal retaining structure and the vertical bearing structure for deep excavation, simultaneously. Due to its advantages such as the capability of deformation control and the benefit of sustainable development, cellular diaphragm wall has been recently applied in several excavation cases. For this type of structure used as both the horizontal retaining and vertical bearing structure, a major concern is to predict the force, deformation and bearing behavior in the design and construction stages. However, a fundamental study of this type of structure is relatively limited and infrequently reported in the literature. The analysis method of the traditional retaining structure is insufficient for this type of structure. In order to extend and develope this type of novel structure in deep excavation, this thesis evaluates the force, deformation, and bearing behavior of cellular diaphragm wall in Shanghai soft deposit, based on a series of in-situ tests, indoor model tests, three dimensional (3D) finite element analysis, and field monitoring. The main contributions of this thesis are described in the following:1. Finite element method is used to analyze the typical retaining behavior of the cellular diaphragm wall. Both a slicing finite element model and a 3D finite element model are proposed to simulate the construction procedures of a deep unbraced excavation supported by the cellular diaphragm wall in typical Shanghai stratum. Force and deformation behaviors including the wall deflection, ground settlement, earth pressure, and bending moment of the wall due to deep excavation are investigated in details. 3D effects of the force and deformation behaviors are studied by comparing the results computed by the slicing model and 3D model. The plane strain ratio (PSR) is used to reflect the relationship of the maximum wall deflcetion between the plane strain and 3D finite element results. By performing a series of parametric studies, the influence of the geometry size of the cellular diaphragm wall, wall construction joint, soil reinforcement, pile spacing, and the bottom plate on the behavior of the cellular diaphragm wall are investigated. General trends of the maximum lateral displacement of wall and maximum ground settlement are obtained by analyze the computed results from the numerical experiments. The general trends provide a reference for the design and construction of the celluar diaphragm wall.2. The in-situ static load tests of two cellular diaphragm wall panels are performed. Vertical bearing behavior of the single diaphragm wall panel including the ultimate bearing capacity, the evolution of the skin friction and toe resistance, and the effect of the toe-grouting are studied according to the tested data. The load transfer behavior of single diaphragm wall panel in soft soil is analyzed by the load transfer method. A hyperbolic ideal elastic-plastic model is presented as the load transfer function of the wall-side, while a tri-linear model is introduced to simulate the load transfer behavior of wall end. The load transfer method is validated to be suitable for the bearing behavior analysis of the single diaphragm wall panel by comparing the calculated results with the tested data.3. Finite element method is used to analyze the bearing mechanism of the cellular diaphragm wall. Firstly, a 3D finite element model is used to simulate the in-situ static load test of single diaphragm wall panel. The parameters of this model is ajusted by comparing the computed results and tested data. Based on these parameters, a 3D finite element analysis of the static load test of the cellular diaphragm wall is carried out. By performing a series of parametric studies, the influence of the soil properties, the interface properties between the wall and soil, and the geometry size of the cellular diaphragm wall on the vertical bearing behavior of the cellular diaphragm wall are investigated. By introducing the concepts of the tip stress coefficient of the soil core and the tip stress coefficient of the partition wall, a general formula for the calculation of the vertical bearing capacity of the cellular diaphragm wall is proposed.4. Indoor model tests are conducted to test four 1/2-scale diaphragm wall specimens which panels are connected by the cross-plate joints. Three significant design parameters of the cross-plate joint are considered: the thickness, the length, and the number of the holes of the longitudinal plate of the joint. Effect of these design parameters on the vertical shear strength of the cross-plate joint are presented and discussed in detail. A numerical model is carried out to study the crack propagation and failure mechanism of the cross-plate joint. Through the results from experimental study and explicit finite-element modeling, the concept of the exerting ratio of the shear strength is introduced. Based on this concept, the shear resistance of Perfobond connectors derived by Oguejiofor and Hosain is modified to be suitable for the calculation of the shear strength of the cross-plate joint by considering the influence of the opening ratio of the longitudinal plate of the joint.5. Comprehensive monitoring systems are installed on an unbraced excavation which is retained by cellular diaphragm wall. Field monitoring results show that maximum lateral displacements of all the inclinometers in walls at different stages fall in the range proposed by theoretical analysis. Furthermore, monitored displacement evolution of the wall top and distribution of earth pressure and bending moment are similar to the theoretical analyzing results. This confirms the validity of the theoretical analyzing results to predict force and deformation behavior of the cellular diaphragm wall.