Study On Mechanism Of Negative Skin Friction On Rectangular Closed Diaphragm Wall As Bridge Foundation In Collapsible Loess Subgrade

Rectangular closed diaphragm wall foundation is a new type of bridge foundation which is developed in the past 30 years. The new type foundation has aroused extensive concern for its many advantages such as high bearing capacity, large stiffness, smaller work amount and lower cost. So, rectangular closed diaphragm wall foundation has a good prospect of application and extension.When pile foundations are applied in the loess area in western China, there is no rigid stratum for pile foundations in the loess layers with huge thickness. If rectangular closed diaphragm wall instead of pile foundation is applied, better benefit will be obtained for bridge engineering. But negative skin friction (NSF) will be an important problem for rectangular closed diaphragm wall which is used in collapsible subgrade.Model tests, numerical simulation and theoretical analysis were utilized in the research to study the bearing characteristics of rectangular closed diaphragm wall without NSF, the action mechanism of NSF, the bearing behaviors, of rectangular closed diaphragm wall subjected to NSF, and the calculation methods for NSF. Primary coverage and achievements as follows:1. Simplified mechanical model of tubular pile or closed diaphragm wall was proposed base on elasticity mechanics. An analytical solution of unit shaft resistance-axial strain relationship by semi-inverse method was obtained. This solution will provide theoretical basis for data processing of outer and inner axial strain which were measured in the model test of axially loaded cast-in-situ tubular piles or closed diaphragm wall.2. Two groups of model tests were done by the new developed testers, the first group was load tests, and the second is immersion tests. In the load tests, bearing performance, wall-soil-cap interaction, wall group effect of rectangular closed diaphragm wall were studied. It was observed that the bearing performance of single diaphragm wall is similar to cast-in-situ pile. The load transfer mechanism of rectangular closed diaphragm wall is much more complex than single diaphragm wall. Outer shaft resistance' and inner shaft resistance were not developed at the same time. And distribution curves of inner shaft resistance are similar to power function curves. The maximum inner shaft resistance takes place at the toe of wall.3. Action mechanism of negative skin friction and the bearing behaviors of rectangular closed diaphragm walls after immersion were studied through model tests. It was observed that distribution curves of negative skin friction are similar to parabola, the neutral plane depth of rectangular closed diaphragm walls is lower than that of single diaphragm wall, and additional settlement of rectangular closed diaphragm walls is smaller than that of single diaphragm wall in the same loess stratum under the same immersion condition.4. Modulus reduction method was put forward to simulate collapsible deformation of loess. According to the method, after reducing modulus and increasing unit weight of the loess, collapsible deformation will occur. In order to decrease the compress deformation of bottom soil layer, unit weight reduction method was applied. The results of FDM analysis show that modulus reduction method is theoretically reliable, simple in application and useful in practice.5. Behavior of negative skin friction on rectangular closed diaphragm wall was modeled by FLAC^3D on the basis of modulus reduction method. The influences of sectional size, sectional shape on behavior of negative skin friction were studied in numerical simulation analysis.6. Calculated method of negative skin friction and dragload, calculation formula of allowable bearing capacity were proposed for rectangular closed diaphragm wall in collapsible loess subgrade.7. An iterative approach was proposed for evaluating the settlement of rectangular closed diaphragm wall subjected to negative skin friction. In the iterative approach, shear stiffness index was deduced, and power function method was put forward to calculate the inner shaft resistance. Calculated results indicate the iterative approach is reasonable and feasible.