Study On Behavior Of Pile-raft Foundation In Soft Soil And Its Design Method

The behavior of pile raft foundation in soft soil is a very important problem in geotechnical engineering. The design of pile raft foundation is also a key technology in civil engineering. In this paper, the behavior of pile raft foundation in soft soil was studied and the design method of which is presented.By considering that the slide takes place when the shear resistance around pile exceed the ultimate shear resistance, the behavior of single pile and pile group was studied. The pile-pile interaction factor decreases with the increase of load acted on pile top. It is found that elastic analysis method overestimates the interaction of pile-pile and pile-soil.The model to estimate the distribution of vertical stress in soil is developed. The vertical stress on the surface of pile toe is assumed to equal to the vertical stress on the bottom of raft multiplied with the coefficneit of load transfer for pile foundation. Boussinesq's theory is employed to estimate the vertical stress under pile toe. The coefficient of load transfer for pile foundation was obtained by the Regression Analysis with the datum calculated by FEM and elastic analysis method. It is founded that the vertical stress calculated by the presented method agrees well with that estimated by FEM and elastic analysis method.The curve of load vs. settlement of single pile in foundation is obtained, so the stiffness of single pile in pile foundation is presented. The settlement of pile top in pile foundation consist of pile compression and settlement of substratum. The pile compression was obtained by static load test or the theory of single pile, while the settlement of substratum was estimated by layer-wise method. It is founded that the stiffness of pile in foundation and settlement of foundation estimated by above method accord with the results of in-situ measurement. It is also shown that the method to estimate the settlement of pile foundation of National Code of Pile Foundation Design (NCPFD) overestimates the settlement of pile foundation. It is very difficult to determine the parameters of compressibility of substratum. Empirical formula based on the in-situ measurement (CPT and SPT) and compression test under high pressure arerecommended to estimate the compressibility of substratum.The model of pile foundation was developed based on the above theory and method. The pile was simplified as a nonlinear elastic spring. The raft is deelement bythe 16-node degenerated element. The element is particularly suitable to analysis the raft with irregular thickness and shape, and the horizontal deformation of raft can also be considered if needed. Compared with the other methods, it is rather easy and simple to estimate the pile stiffness. And the multi-layer and the uneven soil can also be considered.A formula of raft-supported soil relative stiffness is presented. The inner force, settlement of raft, pile force were studied with the increase of raft-supported soil relative stiffness. A formula to determine the reasonable thickness of raft is also presented.At the end of the paper, the applications of the method are introduced on three projects. It is shown that, compared with others, the design method is more easier to use and the parameters is easier to determine.