A Study On Seismic Response Of Composite Foundation With Cement Mixed Pile

Soft soils are widely distributed in the prosperous coastal areas in China. A great deal of structures will be constructed with the economical development and urbanization. Composite foundation is usually adopted to improve the engineering properties of soft site in engineering practice. Cement Deep Mixing Method (CDMM) as one of the good foundation treatment methods is widely used. Each strong earthquake brought great damages and losses to people in china, which lies in a high-frequency earthquake region. Therefore, study on seismic response of cement mixed pile foundation during earthquakes is of great theoretical and practical importance in seismic design of engineering.Referring to the test method of clay, some composite cemented soil samples are compounded to be performed the dynamic triaxial test. The dynamic stress-strain cures of composite cemented soil are gained by the undrained dynamic triaxial test. The effects such as replacement ratio of the cement and confining pressure on the dynamic shear module and blend ratio of composite cemented soil are also analyzed. The dynamic modules under various conditions are also gained. These test data provide credible parameters for the finite element simulation.The Finite Element software, ABAQUS is adopted in this paper. The subroutine of dynamic elastic-plasticity soil constitutive Davidenkov model has been developed and incorporated in ABAQUS to describe the dynamic stress-strain relationship of soils and composite cemented soils. A representative soft site in Nanjing city is selected to be studied. Two kinds of finite element model are developed to analyze the 2D elastic-plasticity seismic response of composite foundation with cement mixed pile by the total stress method. One of the two models is the equivalent composite module model, the other is pile-soil separation model. In the equivalent composite module model, the composite foundation is considered as homogeneous foundation having an equivalent composite module, not taking account the contact between the composite foundation and the soil. In the pile-soil separation model, the pile and soil are separated, and there are contact surfaces between them. Then, friction, slipping or separation between the contact surfaces may occur.In this paper, some factors such as the characteristics of the input ground motion, the depth and width of composite foundation, the pile module and site condition that affect the seismic response of composite foundation, are considered. In the pile-soil separation model, the affects by the replace ratio of the pile are also considered. Then some rules are presented: the frequency characteristics, acceleration peak value of the input ground motion have great influence on the seismic response of composite foundation. The peak acceleration response on the ground surface of the composite foundation is much smaller than that of free field site. But outside the composite foundation, the peak acceleration response on the ground surface may larger than the free site's. With the depth and the module of the pile increasing, the peak acceleration response on the composite foundation decrease, but the affect by the composite module is very small. There seems to be no relationship between the width of composite foundation and the peak acceleration response on the ground surface ofcomposite foundation. In the pile-soil separation model, the same rules can be concluded. We also can find the maximal shear strain of the soil between the piles decreasing with the replace ratio of pile increasing in the pile-soil separation model.In this paper, both the dynamic properties of composite cemented soil and seismic response of the composite foundation with cement mixed pile are studied, and some valuable conclusions are obtained, which provide basic data for the composite foundation design and theoretical references for the engineering practice.