Study On Corner Effect Of Excavations In Clays

In recent years,the infrastructure development in China involves a growing number of deep excavations.Considering these excavations are generally located in densely populated and built-up areas,the surrounding deformation are supposed to estimated carefully.This problem is particularly highlighted in soft clay fields.As one of the most effective tools to analyze the deformation of surrounding soils caused by excavation,Finite Element Method(FEM)is widely accepted by designers and researchers.The excavations are generally of long rectangular shape in plan,leading to the common practice that the physical model of excavations are mostly simplified as plane strain cases in FEM analysis without considering the three dimensional characteristics,also to save computational time and resources.However,the corners of excavations have higher system stiffness due to the additional constrains from the side wall,resulting in less deformation.The characteristics of deformation and stress around the corners does not meet the plane strain condition,especially for excavations of small scales.Therefore,three dimensional analysis considering the corner effect should be conducted.The term Plane Strain Ratio(PSR)was expanded to quantify the corner effect of various excavation response.FEM software Plaxis was employed to build both the two-dimensional and three-dimensional models,and influences of various key design factors on corner effect is revealed.Both excavations in green field and with adjacent piles in complex built environment were considered.Finally,the Multivariate Adaptive Regression Splines(MARS)was used to develop the predictive model relating the PSR to key influential factors mentioned above,based on the numerical results.This study will be helpful for understanding and estimating corner effect of excavations.A short summary as well as the main conclusions arrived at in this thesis include:(1)The term PSR was expanded for quantifying the corner effect of retaining wall deflection(PSR_h)and ground surface settlement(PSR_v),respectively.The influences of excavation length L,width B,depth H_e,clay undrained shear strength ratio s_u/?_v'and stiffness factor of the wall?on both PSR_h and PSR_v were systematically investigated.It is found that both B and a are positive to corner effect while L and s_u/?_v'is the opposite.(2)Using embedded beam element to simulate piles,excavation responses with adjacent piles were examined.Firstly,the interaction between piles and the excavation in plane strain condition was considered.It is found the piles decrease the lateral wall deflection and the ground surface settlement while this decrease rises with pile diameter D.PSR was utilized for quantification of corner effect on pile response(PSR_p).The results show that the existence of piles has less influence on PSR_h and PSR_v compared with the green field cases,while the curvature of PSR_p profile decreases as the distance between pile and wall increases.(3)The main advantages as well as the model development process of MARS were demonstrated.Some examples were given to verify the ability of MARS to approximate functions and deal with multi-dimensional nonlinear geotechnical problems.Models to predict PSR were developed,followed by parametric relative importance plot via analysis of variance(ANOVA).In addition,the predictive performance of these models were validated by some additional FEM tests and the cases histories.