Study Of The Environmental Impact Caused By The Borehole Group Effect During The Pile Foundation Construction In Soft Soil

Soft soil with poor engineering geological condition is widely spread in the the Eastern Coastal Developed Regions of China where the engineering constructions frequently took palce in.A large amount of hollow boreholes left in the soil strata during the construction of bored piles and CFG piles could lead to the borehole group effect and caused surface settlement in the area.To deal with this problem,based on a measured ground surface settment data in engineering caused by borehole group effects,the following aspects were investigated.(1)The surface settlements caused by single hollow borehole was investigated by the finite element method(FEM)from four aspects: hole diameter,hole depth,the soil strength and different specific gravities of slurry(just for bored pile holes).For the single borehole condition,the settlement value was mainly influenced by the hole diameter and increased linearly as it increased.The distance between the maximum settlement position and the hole was mainly influenced by the hole depth and increased linearly as it increased.(2)The mechanism of soil defomation caused by hollow hole and superposition mechanism of many holes was investigated.Caused by the shrinkage of the borehole wall,the soil deformation finally reached a steady because of the horizontal and vertical stress arch.For the single borehole,the maxim deformation value and the distance between the maximum value position and ground surface were mainly influence by the hole depth.For the borehole group,the shrinkage of each hollow borehole became greater than that for the single hole because of interaction effects between the horizontal and vertical stress arch in the soil nearby,when the hole center distance was relatively small.It was also the main reason of the serious soil deformation caused by borehole group effect.(3)The ground surface settlement and deep soil defomation caused by borehole group effect were investigated.For the borehole group,the settlement increases logarithmically as the hole number increases,and when the bole number is larger than certain value,the settlement does not change.The maxim settlement and horizontal displacement caused by the borehole group effect were deep in the ground away from the surface,when the hole number was limited.As the hole number increased,the maxim settlement and horizontal displacement in the soil increased,but the distance between the maximum value position and ground surface decreased.The maximum value position moved to the ground surface,when the hole number was greater than 400,it was a triangle deformation mode.The main soil deformation area was above the elevation of the hole bottom as well as within the distance of double of the hole depth,it was the main influence area of the borehole group.(4)To simulate the massive pile holes in practical engineering,a method of multi-holes merging and the conversion coefficient ? were proposed,and the case was modelled by FEM using this method,and the settlements derived by the numerical simulation and field measurement were compared and analyzed.