Research On Structure Optimization And Vertival Bearing Capacity Theory For The Multi-nodes Drilled Expanded Bored Pile

The diameter-changed bored piles are wildly used in engineering because of their high bearing capacity and readily construction.As an optimized advanced pile type developed from the branch and plate piles and DX piles,the multi-nodes drilled and expanded bored piles reach a reliable construct quality with a high construct efficiency and an obvious improvement of bearing capacity,however,there's an urgent investigation to pile shaft structure settings and vertical bearing mechanism.The article studied and completed the pile shaft structure settings and vertical bearing mechanism using the methods of numerical simulation and theoretical analysis.The main conclusions list as follows:1.Through theoretical derivation,it was confirmed that the profile geometrical shape of the enlarged-part was combined with astroid and arc,and therefore the specific mathematical model formula of the enlarged-part profile geometrical shape was deduced.As the comparison model tests observed,estimation has been made that the best ratio D/d of plate diameter to pile diameter should be 2?2.5,and the optimized number of enlarged-parts was 2?3 setting in the lower hard soil layers,which is the recommended position of enlarged parts.Furthermore,the minimum space of the adjacent enlarged-parts could be 2.5D-3.5D.2.By comparison,it was observed that skin friction at the upper affected area of enlarged-parts got degraded,while the lower affected area got strengthened when the pile was compressed.In a word,the enlarged-parts provide a softening effect on skin friction in adjacent area when compressed.By taking the softening effect stated above into account,a new bearing capacity calculation formula resisting compression was proposed,and the verification showed well.3.By use of the load transfer method,applying the hyperbolic model to pile shaft,pile base and enlarged-part base,a nonlinear method predicted for pile settlement was solved,and some parameters about skin friction was discussed.Through introducing cavity expansion theory into enlarged-part base,the force-displacement relationship at the enlarged-part base was deduced considering geometrical features,in addition,the distribution traits of horizontal inner pressure ph in compressed area was derived.Based on the results,the load transfer method was used again to recalculate the settlement curve.By comparison with the former approach,it was discovered that the new approach provided a more accurate prediction of early and middle period pile settlement relative to the former method,and meanwhile the bearing capacity calculated by the new method showed more sensitive to enlarged-parts features.The enlarged-part resistance calculated by the former method is lower than the actual value,but it already thinks about the soil plastic yielding and reaches to a limitation.While the new method lacks of plastic restriction,only applying to elastic conditions,leads to a higher prediction in the latter period about the enlarged-part resistance,and hence the late settlement calculated is small than the measured value.4.By comparison,it was observed that skin friction at the upper affected area of enlarged-parts got strengthened,while the lower affected area got degraded when the pile was pulled.In a word,the enlarged-parts provide a reinforcement effect on skin friction in adjacent area when pulled.By taking the reinforcement effect stated above into account,a new bearing capacity calculation formula resisting compression was proposed,and the verification showed well.