Experimental Study On Working Behavior Of Bored Pile Under Vertical Load In Loess Area

The bored pile is a commonly used deep foundation type in the loess area.It can withstand both vertical pressure and vertical uplift.This paper analyzes the working properties of vertical compressive piles and uplift piles in loess field through the field single-pile compression and anti-pulling static load test and the measured data obtained from the addition of sliding micrometers during the static load test,and some conclusions of guiding significance to engineering practice can be obtained.The load-displacement curves of super-long bored compressive pile and uplift pile in loess area are both slowly changing,it shows pure friction pile characteristics;The axial force transfer rate of the pile body shows obvious “the top and the end of the pile quick,the middle of the pile slow” characteristics;the side friction resistance under various loads are all single peak curve,and the peak position gradually move down with the increase of the pile top load;The compressive piles are located at the depth of0.342~0.618 times the pile length,uplift piles are located at the depth of 0.281~0.562 times the pile length;The exertion value of side friction resistance of super-long pile with post grouting in loess area can reach 180.4~193.2kPa,ratio to the recommended value of the specification is about 1.38 ~ 1.57;The exertion value of side friction resistance of uplift pile can reach 127.5~129.3kPa,ratio to the recommended value of the specification is about 2.0,and ratio to the compressive pile is about 0.66~0.72;For compressive pile and uplift pile,the pile side friction resistance and pile-soil relative displacement are both in accordance with the hyperbolic model,the relationship between the initial shear stiffness and the depth of the soil on the side of the pile can be both quantitative reflected by the Janbu formula.In addition,finite element simulation of uplift piles shows good consistency withmeasured load-displacement curves,pile axial force curves,and pile side friction resistance curves;Through finite element analysis,it is found that to improve the bearing capacity of single pile by increasing pile diameter and pile length,it is necessary to select the appropriate ratio of length to diameter,and to improve the bearing capacity of single pile by increasing the compressive modulus of the soil around the pile and the elastic modulus of the pile,it is necessary to select the appropriate modulus ratio of pile and soil.