Study On Settlemtnt Calulation Method For Composite Ground Under Flexible Foundation

With its good economy and practicability, composite ground was widely used under flexible foundation such as embankment foundation.The existing theories of composite ground settlement are concentrated in the rigid foundation,while the settlement caculation mehhods under flexible foundation are limited. Composite ground under rigid foundation accord with“pile-soil equal strain hypothesis”. however, embankment foundation belongs to flexible foundation, and composite ground under flexible foundation no longer meets the equal strain hypothesis. Many studies show that, deformation between pile and soil under flexible foundation is inconsonant, and relative slip is inevitably under larger load; meanwhile, pile-soil interface shear stress is produced due to relative slipping, which caused settlement difference between pile and soil at the same depth. Therefore, a more reasonable settlement calculation of composite ground under flexible foundation must consider both relative displacement between pile and soil and non-synchronized settlement at the same depth of soil. however,existing methods taking both two points above into account are seldom.Firstly, bearing mechanism and deformation characteristic of composite ground under flexible foundation was discussed in this paper, and based on the assumption of pile-soil interface deformation coordination, effective pile length calculation formula under deep mixing pile composite ground was established by shear displacement method; Secondly, considering slip characteristics of composite ground under flexible foundation foundation the relationship between shear stress and shear displacement was assumed to meet to the ideal elastic-plastic model, and the settlement was regarded as three parts:elastic zone, plastic zone, and underlying stratum; thirdly, according to relationship between shear stress and relative displacement on the pile-soil interface, compression calculation method in plastic zone was established by mechanical analysis of pile and soil.And considering the characteristics of vertical soil stress variation, a calculation model in elastic zone which considering both vertical soil stress variation and piles-soil relative slip was built up based on improved displacement model of soil between piles by discussing current soil displacement modes. As a result, a new settlement calculation method of composite ground under flexible foundation was presented. Finally, two practical engineering calculation and comparative analysis with other methods indicates that this method is reasonable and feasible.The method to caculate effective length of pile of composite ground under flexible foundation reflected the relationship between load size and effective pile length; meanwhile, the method to caculate settlement of composite ground under flexible foundation took both relative slip between piles and soil and non-synchronized settlement at the same depth of soil into account, which accord with the deformation characteristics and pile-soil interaction characteristics of composite foundation under flexible foundation characteristics and had some reference value for caculating settlement of composite ground under flexible foundation.