Research On Deformation Stiffness Of Rigid Long-short-pile Composite Foundation

Long-short-pile composite foundation,a new type of foundation treatment form, appears basing on the development of the theory and practice in composite foundation.Because it both can satisfy the engineering foundation bearing capacity and deformation of the request, and can achieve better economic effect, it is used more and more widely in engineering。But, at present the load transfer mechanism and interaction mechanism of long-short-pile composite foundation are still in preliminary study phase, Related research also insufficiently thorough, design theory is still not perfect, still at the initial stage of exploration.Basing on the method of combining three-dimensional finite element numerical simulation with theory analysis,from load transfer mechanism of single pile composite foundation and same-long-pile composite foundation,research the composite foundation, especially long-short-pile composite foundation long pile side friction and bottom resistance, short pile side friction and bottom resistance interaction mechanism in the process; further,research the distribution mechanism and change process of the vertical load in long piles, short pile and soil; Then according to the load transfer mechanism, side friction and bottom resistance mechanism, with the deformation stiffness of single pile composite foundation and same-long-pile composite foundation, based long-short-pile composite foundation deformation stiffness. The preliminary conclusions are shown as follows:(1) The pile side friction is related with the pile side soil pressure in composite foundation.After appearing relative sliding between pile and soil, pile side friction increases with the increase of soil pressure; Soil pressure is related with vertical stress of soil,to, also with the transmission way of upper loads in soil;(2) Due to diffusion of upper loads to surrounding soils, the transfer depth of upper loads in single pile composite foundation is limited, so pile side friction as well as the pile side soil pressure is little affected by the upper loads; The influencing depth by upper loads, from corner pile, side pile to central pile in group pile composite foundation, increases gradually. The pile side friction,as well as the soil pressure,of the central pile in composite foundation without the soil out of raft is influenced by upper loads along the all pile length;(3) Besides the property fo pile-end soil,pile-end resistance is influenced by the pile side friction. Before the pile side friction reaches limit, the increased upper loads is undertaken by pile side friction and end resistance jointly,so pile-end soil stress increases slowly; After pile side friction reaches limit, and it can't bear the new increased loads, so the newly added upper loads can only be undertaken by pile-end soil,and it increases quickly;(4) In long-short-pile composite foundation, pile side friction is related with the soil pressure,and pile-end resistance is related with pile side friction and the upper loads. The side friction and end resistance of long pile is greater than short pile, and the pile-soil stress ratio of long pile is larger than short pile.And along with the decrease of the length of short pile, the end resistance and the pile soil stress ratio of long pile increases,while the short pile decreases;(5) In long-short-pile composite foundation, the reinforcement scope in horizontal direction of long pile is larger than short pile length short pile.And along with the decrease of the length of short pile, the horizontal reinforcement scope of short pile decreases gradually;(6) Finally, based on the simulation analysis and deformation stiffness of single pile and same-long pile composite foundation, this paper gives the calculation method of deformation stiffness of long-short-pile composite foundation.