Study On Reliability Of Vertical Bearing Capacity And Resistance Factor Of Necking(cap) Piles

Due to the influence of construction technology,poor workability of concrete,groundwater and other adverse factors,the necking of foundation pile is easy to occur in the construction and use,which affects its vertical bearing capacity and reliability index,resulting in the deviation between the actual bearing capacity and the design value.At present,the probabilistic limit state design method based on reliability has been adopted in the design of superstructure.However,as the substructure,especially the pile foundation engineering design still adopts the traditional safety factor design method.In order to unify with the reliability design method of superstructure,it is very important to study the vertical bearing capacity,reliability index and resistance sub coefficient of single pile with reduced diameter(cap)and evaluate the safety of pile foundation scientifically and reasonably.Therefore,based on the National Natural Science Foundation Project "transparent soil test analysis of necking pile-soil interaction and Research on the influence of necking on bearing capacity reliability"(No.51978247)and Henan Province key research and development and promotion special project(scientific and technological research)"transparent soil model test research of necking defective pile-soil-cap interaction"(No.202102310242),in this paper,the vertical bearing capacity and reliability index of bearing capacity of single(cap)pile with necking are studied systematically,and the recommended value of resistance is given.The main contents are as follows:(1)Through the indoor model test,two groups of complete(cap)single piles and 54 groups of necking piles(cap)single piles are tested vertically hierarchical loading The experimental results show that: necking piles will change the bearing capacity of the pile and increase the settlement value of the pile top;the bearing capacity decreases with the decrease of the necking radial dimension and decreases with the increase of the necking axial dimension;For single pile without cap,the influence degree of reducing position on bearing capacity is middle necking > shallow necking > deep necking,while for single pile with cap,the influence degree is shallow necking > middle necking > deep necking.(2)Based on the bearing capacity test results,the variation law of pile bearing capacity reduction coefficient is analyzed.On this basis,the calculation formula of bearing capacity reliability index of(cap)single pile with necking is derived by using the full probability calculation formula.Combined with the engineering example,the influence of necking parameters(position,radial dimension and axial dimension)on the reliability index is analyzed.The results show that: The influence of necking parameters on the bearing capacity reduction coefficient and reliability index is the same,which decreases with the decrease of radial dimension,the increase of axial dimension and the deepening of necking position.Necking will reduce the reduction coefficient of bearing capacity,which will lead to the decrease of the reliability index of bearing capacity,however,the reduction coefficient and reliability index of some single pile without cap with necking in the middle increase abruptly.The reason is that the r necking resistance increases the vertical bearing capacity.(3)Based on the calculation results of reliability index,the target reliability index of bearing capacity of(cap)single pile with necking is derived by using the full probability formula,and the influence of necking on the target reliability of bearing capacity is analyzed through an example.The results show that the target reliability index of the necking pile increases with the increase of the radial dimension and the decrease of the axial dimension.The results show that the location of necking has a significant effect on the reliability,and the farther the necking is from the pile top,the larger the target reliability index is.For the shallow necking single pile and the middle necking single pile with large reduction degree,its target reliability is less than 3.0,and the safety of pile foundation is low;for other necking piles,although the target reliability is greater than 3.0,there are still potential safety hazards.(4)Based on the calculation results of the target reliability index,the calculation formula of the resistance partial factor considering the influence of the necking is given by using the first order second moment method.Combined with the example analysis,the influence of the necking parameter and the load ratio on the resistance factor is studied,and the recommended value of the resistance factor is given.The results are as follows: the resistance factor increases with the decrease of the radial dimension,the increase of the axial dimension and the deepening of the necking position,and the influence degree of the three factors on the resistance factor is in the order of necking radial dimension > necking axial dimension > necking position;In addition,the resistance factor decreases slightly with the increase of the load ratio,and the influence of the load ratio on the resistance factor is less than that of the reduced diameter.Finally,according to the calculation results of resistance factor,the recommended values of partial resistance factors of single pile without cap and single pile with cap are given,which provides theoretical support for the revision and improvement of relevant codes of pile foundation engineering.