Research On Behavior Of Composite Foundation In Consideration Of Post-Peak Softening Characteristics Of Cemented Soils

Composite foundation is a kind of artificial foundation setting up reinforcement body in natural soil. Cemented soil pile is one type of composite foundation and it has been widely adopted in civil engineering projects in China. However, it is still beyong full understanding of pile-soil co-work mechanism and load transfer behavior and it is at a preliminary stage yet, which greatly hampers the promotion and further development of composite foundation technique. In this paper, the work mechanism and failure mode of cemented soil pile composite foundation are deeply and systematically studied, on which basis proposal of bearing capacity determination method of cemented soil pile composite foundation is put forward.The recent studies on composite foundation are summarized in the first part of the paper. According to the results of in-situ and lab tests, a non-linear stress-strain relationship model of cemented soil expressed in piecewise function is established, whose parameters can be determined from test results. The expressions of modulus can be conveniently obtained from this model as well. The agreement degree is very well adopting this model to fit in-situ and lab test results. Parametric analysis shows that the increase of cemented soil strength is mainly the consequence of adhesion increase in soil; when the surrounding pressure increases, the modulus of cemented soil linearly increases in the mathematical relationship of logarithm correspondingly, and the existence of surrounding pressure improves the ductility of cemented soil; the increase degree of cemented soil strength with high admixture ratio is greater than that of cemented soil with low admixture ratio, but when the former fails, the strength loss is also greater than that of the latter.According to related soil mechanics theories and calculation methods, a three-dimensional FEM computer program is developed in which bearing capacity and deformation analysis of single and group pile composite foundation can be performed. The cemented soil piles, original soil mass, foundation plate, cushion layer and pile-soil interface are discreted into finite calculation elements of different types respectively. Taking advantage of symmetry, one quarter of the whole analysis system is taken to be studied to save calculation capacity, and the load applying process is simulated by step loading method. In the program, the interaction effect between soil mass and pile is considered by setting up interface element, and the non-linear behavior of shaft, soil mass and the interface between pile and soil is considered. The influence of surrounding pressure increase on the modulus of material in cushion layer is considered by using incremental secant method while the influence of error accumulation using step loading method is improved by adopting mid-point incremental method.The three-dimensional FEM simulation analysis of a single pile composite foundation is performed by adopting the cemented soil model and simulation analysis method suggested in this paper. The calculated results well agree with the test data, which testifies the validity of the model. A great amount of parametric influence analysis of cemented soil pile composite foundation in two typical soils is carried out to reveal the influence of factors as load vs. settlement curve, axial forces of shaft, side friction, stress ratio of pile to soil, natural soil, cement admixture ratio and pile length etc. The load transfer mechanism of cemented soil pile composite foundation is estimated as well. The load vs. settlement curve of cemented soil pile composite foundation is a straight line at the beginning and a curve with sharp drop after failure. There is an obvious inflection point on the load vs. settlement curve, which is corresponding to the compressive failure state point of cemented soil pile top. Therefore, the bearing load capacity of cemented soil pile composite foundation can be determined by this point. Based on the simulation analysis results, the distribution curves of shaft axial forces and side friction developing with the increase of load along depth are discussed. Stress ratio of pile to soil is analyzed, especially when the foundation reaches failure state. Pile top deformation and shaft compression at failure state of pile top are studied as well. Determination of bearing capacity of cemented soil pile composite foundation based on failure mechanism is suggested.