Research On The Bearing Behavior Of Bottom Uplift Pile In Layered Foundation

With more and more buildings bearing uplift force,it is a common form to provide uplift force through uplift pile foundation in engineering.The bottom uplift pile converts the uplift load into the uplift force at the pile bottom,and due to the influence of the Poisson effect,the bearing capacity and crack control of the bottom uplift pile are superior to normal uplift pile.Based on the research of normal uplift pile,this article analyzes the stress mechanism of bottom uplift pile through on-site tests,highlights the advantages of the bearing performance of bottom uplift pile,and considers the influence of Poisson effect on bottom uplift pile.A calculation method for the bearing capacity of bottom uplift pile in thick layered foundation is derived.Based on the load transfer method,the bearing mechanism and deformation characteristics of the bottom uplift pile were analyzed.Through on-site measurement data and ABAQUS finite element software,the characteristics of the lateral friction resistance and distribution of the bottom uplift pile were compared and analyzed.The specific content and conclusions are as follows:(1)Through on-site tests,the stress mechanism of normal uplift pile and bottom uplift pile was analyzed,and the development and distribution of pile side friction under the stress form of bottom uplift pile,as well as the variation of ultimate anti pull bearing capacity were discussed.Due to the influence of the Poisson effect,the phenomenon of "tightening as the pile is pulled out" occurs in the bottom uplift pile.When the bottom uplift pile is loaded,the pile body bears pressure,thereby reducing the occurrence of cracks in the pile body and improving the durability of the uplift pile.(2)Based on the assumption of the power exponent failure surface of the uplift pile,the stress analysis of the soil block microelement is carried out according to the principle of limit equilibrium,and the expression of the vertical force of the microelement is obtained,which is then integrated into the respective soil layers for calculation.Based on the Poisson effect of the pile,an incremental expression for the lateral friction resistance of the pile was derived,and a calculation method for the bearing capacity of the bottom uplift pile considering the influence of layered foundation and Poisson effect was finally obtained.The calculation results indicate that the reasonable range of parameter N for calculating the uplift bearing capacity of bottom uplift pile in layered foundations in Lanzhou Xinqu is 50≤N≤100 when using a power index failure surface.(3)On the basis of introducing the load transfer method to analyze the load displacement and deformation of the bottom uplift pile,the displacement coordination method was verified through on-site test data.Analyzed the deformation characteristics of the uplift pile and the surrounding soil when elastic deformation and elastic-plastic deformation occur in the bottom uplift pile,and derived the calculation formula for the displacement when the uplift pile and the surrounding soil jointly undergo elastic deformation and elastic-plastic deformation.Finally,three different mathematical models were fitted using on-site measured data,and the fitting ac curacy of each model was obtained.The fitting results of the models were analyzed.(4)By using ABAQUS finite element software simulation,the characteristics and differences between bottom uplift pile and normal uplift pile in terms of axial force distribution,frictional resistance development and distribution,and ultimate bearing capacity were compared and analyzed under the same pile top displacement limit conditions.The influence of soil properties at the pile bottom on the variation of pile side frictional resistance was also analyzed.