Study On The Generation And Development Of Negative Friction Resistance Of Pile Foundation In Collapsible Loess Area

In the northwest area of our country,particularly the collapsible loess in Gansu Province is widely distributed and has a large thickness.The collapsible loess always have a large area of collapsible settlement in a short period of time after being infiltrated by water,which often seriously affect the bearing capacity of the pile foundation,causing collapse and inclination of buildings and so on.The experimental researches of collapsible loess in Lanzhou area are relatively few.Therefore,it is important to study the effect of being pre-infiltrated on the bearing behavior of pile foundation in collapsible loess region.This article combines with a specific engineering project to conduct on-site infiltrating and loading experiment on test pile based on a full understanding of the climate characteristics of Lanzhou area,the way of inundating testing pile foundation is letting water naturally infiltrating without maintaining the water head pressure.The experiment set up three kinds of working conditions: The pile foundation is subjected to intermittent infiltrating under the force of its own weight,and let the soil consolidate freely after stopping inundating testing pile foundation,then the loading and unloading test of the test piles is conducted,and at the same time,a brief analysis of the generation and development mechanism of negative friction resistance was studied.The following conclusions are drawn from the experiment finally: By simulating the process of discontinuous rainfall,we can find that the influence of rainfall on the negative friction resistance at each section of the pile fundament's side is small and the range of infiltrating impact does not exceed 8m,the collapsibility of soil after being infiltrated is stratified by stages;During the loading process,the partial lower layer of soil collapsed produces negative frictional resistance due to extrusion of the pile fundament;In this paper,the discontinuous water immersion test of the pile foundation under the action of its own weight shows that the water immersion has little effect on the value and distribution of the negative frictional resistance on the pile side after the negative frictional resistance has generated,but the settlement of the pile foundation increases significantly with the water immersion;During the entire experiment process,negative frictional resistance is always there and alternately distributed along the pile together with positive frictional resistance,resulting in multiple neutral points aroused,in order to determine the location of the neutral point expediently,the concept of equivalent neutral point is proposed.The ratio of the equivalent neutral point depth to the pile length fluctuates in the range of 0.257 to 0.40,which is much smaller than the value specified in the code.Therefore,in the actual design process of pile foundation,it is necessary to fully consider the influence of rainfall and adjust the length of the pile fundament according to the equivalent neutral point value.