Working Mechanism And Optimizing Design Of Multi-Element Composite Foundation

The multi-element composite foundation has been widely applied to the geotechnical engineering. But its theory is not further than the practice. Researching the deformation and stress and pore water pressure behaviors of multi-element composite foundation has a far-reaching effect for the optimizing design. This paper expresses the following aspects of the new progress and results of multi-element composite foundation.The paper firstly generalizes the current design methods of the bearing capacity and settlement for the multi-element composite foundation, and suggests the new research contents.The determination of bearing capacity and settlement is still the key question in the multi-element composite foundation. If the composite modulus of multi-element composite foundation is determined correctly, the settlement can be calculated by the layer-wise summation method. So an analytical formula of composite modulus of multi-element composite foundation was proposed by considering the interaction between the piles and soils. In the formula the horizontal deformation of the piles and soils is concerned. The formula applying to actual engineering, it is found that the calculation results are agreement with the actual measuring data.An analytical formula reflecting the change of pile-soil stress ratio with loading is also revealed. It is valid that the pile-soil stress ratio is affected by many factors such as loading and length-diameter ratio and replacement ratio and the pile-soil relative stiffness. According to the analytical formula, three types for the pile-soil stress ratio vs. load are discussed. That are: 1) the pile-soil stress ratios of two piles all increase when the load increases, 2) the pile-soil stress ratios of two piles all decrease when the load increases, 3) the pile-soil stress ratios of one pile increases when the another decreases.Considering the change of the density, water content and cohesion of soils, the adjusting factor of the bearing capacity of multi-element composite foundation is analyzed and determined renewedly. Using the new adjusting factor the bearing capacity is calculated. It is found that the calculating results are consistent with the actual data.In order to exploring the deformation, stress and pore water pressure behaviors in the multi-element composite foundation and validating the calculation results by the analytical formula, the many kinds of multi-element composite foundation in different piles and soil layers are calculated systematically by using the FLAC program. The ground structures include three types of homogeneous soil, layer ground (the upper is weak, the lower is hard) and another layer ground (the upper is hard, the lower is weak). Also the component piles include three types of the flexible and rigid piles, the granular column and flexible pile, the granular column and rigid piles. It is discussed that the affection of five factors, which are the deformation of piles, stress of piles, settlement of composite foundation, pore water pressure and pile-soil stress ratio.An analytical method of the multi-element composite foundation is proposed by considering the daub effect of drainage pile in the radial directionand the weakening effect of un-draining pile bottom preventing the vertical drainage of lower layer. Then the consolidation behaviors of different multi-element composite foundation are discussed based on the two methods such as numerical and analytical method. It is revealed that the consolidation process of the composite foundation with drainage pile and rigid or flexible pile. The results from the analytical method are in agreement with that from the numerical method. And it was discussed that the ratio between radius of drainage and pile length and the rigidity of pile affect the consolidation degree.Numerical examples show that the multi-element composite foundation has five kinds of effect, which are retainer action, cushion action, accelerative consolidation action, integrity action, stress equilibrium action and adapting stratum action.The working...