Study Of The Mechanism And Controlling Method Of Borehole Group Effect During The Pile Foundation Construction In Soft Soil

Soft soil with high moisture content and high compressibility is widely spread in the regions near sea,rivers and lakes in China,and its characteristics are sensitive to engineering activities.A large amount of hollow boreholes existing between the top of the cast-in-place piles and the ground surface during the construction of the pile foundation would lead to the borehole group effect,making remarkable influence on the surrounding environment.In this investigation,an exploration of the borehole group effect was conducted using Plaxis 3D.Through deformation of inner wall of holes and stress changes,the mechanism of borehole group effect is searched.Influencing factors such as hole-forming orders,various distribution shapes,and consolidation process on the borehole group effect is analyzed.Pre-construction of diaphram walls before the construction of the pile foundation and backfilling hollow holes for a certain range immediately are proposed as two relatively economic methods to reduce the effect caused by borehole group.To simplify the massive calculation,the transfer coefficient for converting the multiple boreholes into one hole based on the soil properties is calculated.An equivalent method,which simplifies the borehole group into a fixed depth excavation is also explored.The main content includes the following aspects:(1)The essential mechanism of the borehole group effect is explored.There are stress changes containing the horizontal stress arch and the vertical stress transfer when only one borehole is created.The following two main factors should be considered as for borehole group effect.The first factor is the influence of the stress arch between adjacent holes,which is specifically performed as the influence on the distribution of the circumferential stress arch and the arch strength.The second one is the unloading effect of the second-time formed hole near existing hollow holes,which will lead to a tendency of the surrounding soil deforming to the hole.The borehole group effect is the product of the two factors above.(2)The influence of various hole-forming orders,various distribution shapes,and consolidation process on the borehole group effect is analyzed.The results indicate that the hole-forming order has a significant influence on the surrounding environment,which should be considered in combination with practical application.In the direction perpendicular to hole-forming direction,the center-to-periphery order induces the most significant surface settlement;While the side-to-side order influences the surface settlement most in the same direction as the hole-forming direction.In addition,different distribution shapes of group boreholes in a constant amount make a relatively difference on the borehole group effect.Square distribution results in the maximum surface settlement while the circle ones matter least.Furthermore,the borehole effect becomes more significant because of consolidation.(3)Two relatively economical controlling 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 and backfilling hollow holes for a certain range immediately.The results show that the installation ratio of diaphram walls is a key factor on influencing the control performance,and backfilling a certain amount of rows of hollow holes can result in a satisfying performance.(4)To simplify the calculation,the method is proposed for converting the multiple boreholes into one hole based on the soil properties considered in this paper.The conversion of same size of area fits best.An equivalent method is explored based on that converting method,which simplifies the borehole group into a fixed depth excavation.Group boreholes with a certain depth can be simplified by an equivalent pit with a fixed depth.