Analysis On Instability Of Rigid Pile Composite Ground In Soft Clay

The technique of rigid pile composite foundation has been widely used in the practice of soft soil area because of its advantages of convenient construction, high bearing capacity and good stability. However, accident happens occasionally in such constructions. As a result, researching the failure mechanism to prevent the occurrence of similar accidents has been an urgent problem in engineering field.The main contents are as follows:(1) According to the centrifuge test, a3-D numerical model of embankment reinforced with rigid pile composite foundation which considered the pile-soil contact has been built with the ABAQUS software. Verify the rationality of the finite element model by comparing the result of analytical model to the centrifuge test.(2) The instability critical point of rigid pile composite foundation is discussed. It is found that the changes of p-s curve is consistent with the development of equivalent plastic strain zone, and regard the combination of the feature point of p-s curve with the equivalent plastic strain zone running through the bottom to the top as the criterion.(3) Analyse the influence of pile stiffness, embedded condition and pile spacing to the stress and deformation of the composite ground, and chose some typical examples to analyse it’s failure mechanism. The result shows:within a certain range, increasing the pile stifness can improve the stability of the composite ground, if the pile stiffness is too severe, the soil may flow around the pile body; increasing the pile embedded depth can effectively improve the stability of composite ground when the length of pile end embedded into the stratum is3～6.25times of the pile diameter, too long length will cause unnecessary waste, and with insufficient embedded depth, the pile may overturning; reducing the pile spacing can effectively improve the stability of composite ground when the pile spacing is2.5～7.5times of the pile diameter, with too large pile spacing, the failure mode of piles may change and with too small pile spacing, the strength of soil between the piles can’t be fully played(4) A3-D numerical model of embankment reinforced with pile net composite foundation has been built to anlayse the failure mechanism of the composite ground with a net on pile top, and then analyse the effect of geogrid rigidity, cushion thickness and geogrid position to the instability mechanisms of the embankment. The result reveals:the failure mode of composite ground with a net is similar to which without a net; the geogrid can increase the value of stress ratio of pile to soil inside composite foundation, reduce the scope of soil plastic zone and significantly improve the stability of composite foundation;the stability of embankment is best when the geogrid rigidity is between2000and4000kN/m, cusion thickness between0.3and0.6m, geogrid lay on top of the pile.