Research Of Mechanism And Control Methodology Of The Influence Of Borehole-group Effect On The Environment In Soft Soil Area

The groundwater level in the soft soil area is high and the soil is poor,so its characteristics are sensitive to engineering activities.A large amount of hollow boreholes are left between the top of the piles and the ground surface after the construction of bored piles and CFG piles if bored piles and CFG piles are utilized as the foundations of buildings.Engineering cases show that a large amount of hollow boreholes have considerable influence on the surrounding environment,i.e.the borehole group effect.Various methods such as field monitoring,centrifuge tests and numerical simulation were used to study the deformation of surrounding soil caused by single hole or borehole group,and the essential mechanism of the borehole group effect is explored in this research.For the problem that the massive pile holes are difficult to simulate,a method of multi-holes merging and the equivalent method were proposed.Furthermore,some control measures are proposed based on the influence of the borehole group effect on the environment,including the pre-construction of diaphram walls before the construction of the pile foundation.The main contentare as follows:(1)In this investigation,the deformation of surrounding soil caused by a single hole or borehole group was conducted using Plaxis 3D.The simulation results show that the value of the surrounding surface settlement caused by group holes is logarithmically related to the number of holes.When the number of holes increases to a certain value,the settlement no longer increases significantly.When the number of holes is small,both the maximum vertical and horizontal deformation of the surrounding soil induced by the group hole effect are located at a certain depth from the ground surface.As the hole number increased,the maximum settlement and horizontal displacement of the soil increased,and the position of the maximum value moved to the ground surface and it exhibits a triangular deformation mode.The main influence area of the borehole group effect was above the elevation of the hole bottom as well as within 2 times the depth of the hollow bores from the edge of the hollow bores(2)In order to finely simulate of the borehole group effect,the centrifuge test and corresponding numerical simulation of a single hole and borehole group were carried out and then the parameter analysis were performed by numerical simulation in this study.The centrifuge test results further verified the considerable influence of the group hole effect on the environment.Based on the experimental and numerical simulation results,the intrinsic mechanism of the borehole group effect was revealed by analysis of the change in stress around the hole and the deformation of the surrounding soil.The simulation results show that stress changes containing the horizontal stress arch and the vertical stress transfer occurred when only one borehole was created,effectively limiting the shrinkage deformation of the hole perimeter.When a large amount of hollow boreholes exist and the spacing between the holes is small,the horizontal and vertical stress arches around the holes are disturbed due to the interaction between them.As a result,the shrinkage deformation of each hole in the case where a large amount of hollow boreholes exist is beyond that in the case where a single hole exist,which is the main reason for the significant deformation of the surrounding soil caused by the borehole group effect.(3)To alleviate the response of the surrounding environment induced by the borehole grout effect,several economical and effective control measures are proposed including the pre-construction of retaining structures,pre-construction the first level of struts,construction of isolation piles before the construction of the pile foundation and backfilling hollow holes after the construction of the pile foundation,To be specific,backfilling a certain number of rows of hollow holes can result in a considerable effect In addition,pre-construction the first level of struts can greatly increase the lateral stiffness of the retaining structures and further reduce the horizontal deformation of the retaining structures.The application of the isolation pile outside the excavation can also mitigate the settlement of the ground surface outside the isolation pile to a certain extent.(4)In order to the problem that the massive pile holes are difficult to simulate,a method of multi-holes merging and the equivalent method were proposed,which are verified by the field data.