Research On Behavior Of Piles Buried In The Soil During Pit Excavation

Nowadays,it is so crowded in large and medium-sized cities that people's growing need for extra space can't be satisfied.As a result,developing the underground space in the large and medium-sized cities may be a good alternative,for example,the large underground shopping mall,large underground parking garage.Bottom-up excavation method is more and more widely used in large and medium-sized cities as it has many advantages:small influence on the surroundings,little affect on the traffic and so on.During Bottom-up excavation,as piles under the pit carry the weight of the basement and the upper structure,piles undergoes the cyclical loading and basal heave.Therefore,performance of piles under basal is complicated..Behavior of piles under basal during bottom-up excavation is discussed in detail herein by means of model experiments and finite element analysis.Results from model experiments show that under the same circumstance,bending moments along shorter pile are greater than those along longer pile and bending moments along piles in bottom-up method are greater than those along piles in top-down method on account of the P-A effect.Results from finite element analyses of single pile show that:lift at the top of pile with enlarged toe is comparatively small during excavation;when soil in the pit is reinforced through jet grouting,lateral displacement of retaining wall decreases apparently,but its effect on uplift at pile top is small;when jump layered excavation is adopted to reduce deflection of retaining wall,the struts must be installed promptly;although structures abovethe ground increase the stiffness of the whole structure(the basement and the topstructures),but it increases the lateral deformation of retaining wall as well because of P-A effect;for squeezed branch pile,there is no evident change in the uplift at pile top when the diameter of squeezed branch is increased by 1.2 times.Results from finite element analyses of pile group show that:variation in pile length is an effective way to diminish rebounds and differential rebounds of piles and keep differential rebounds unchanged during pit excavation;variation in pile radius help to diminish differential rebounds between piles,but their effect on differential rebounds are really small;for non-displacement pile,the impact of variation in pile space on rebounds at pile head can be neglected;reinforcements of soil inside pit through jet-grouting have little effect on rebounds and differential rebounds at pile head;for jump-layer excavation,changes in rebounds and differential rebounds are small,but lateral deflections of piles and retaining wall increase extensively;there is no much change in rebounds and differential rebounds of piles when numbers of slabs of basement increase;when stitffness and weight of top structure are considered,differential rebounds between pile and wall decrease apparently;large differential rebounds between piles around pit center induce uplift deformations at middle of slabs,which will conversely pull piles to deflect towards pit center;when piles and retaining wall can slip vertically,lateral deflections of piles decrease accordingly;when horizontal supporting of columns to slabs is weakened,there are sharp increases in rebounds and differential rebounds of piles,and so do the lateral deflections of piles towards pit center.