Numerical Simulation Study On Synergy Of Cement Soil Wall-Soil Nailing Composite Support Structure

The composite soil nailing wall is a composite foundation pit supporting structure formed by adding other auxiliary structures on the basis of soil nailing wall.There are complex interaction problems between its interior and exterior and the soil medium.In the previous design,the auxiliary structure is mostly used as a safety reserve,however,the experimental and actual monitoring data show that the auxiliary structure also has the functions of sharing load and limiting displacement.In order to further clarify the synergistic mechanism between the composite soil nailing wall support structure,the cement soil wall-soil nailing composite support structure is taken as the research object,and the FLAC3 D finite difference software is used to establish the three-dimensional model with the engineering example.Through the comparative analysis of monitoring data and simulation results,the basic action law of cement soil wall-soil nailing composite support structure is studied,and the applicability and feasibility of the three-dimensional model are verified.On the basis of studying the basic law of action,taking the maximum horizontal displacement of cement soil wall as a measure index,and the sensitivity of the influencing factors of structural design parameters is analyzed by orthogonal experiment.According to the basic action law and sensitivity analysis results,the concept of synergy is introduced,and the ratio of the width of the cement soil wall to the length of the vertical projection direction of the soil nail reinforcement area is referred to as “wall nail ratio”.The synergistic mechanism of soil nailing wall and cement soil wall in soil medium is studied by numerical simulation.Combined with other engineering examples,the reasonable range of wall nail ratio is given.The main conclusions are as follows:(1)The horizontal displacement curve of cement soil wall-soil nailing composite support structure is “convex belly” shape,and the maximum value is in the range of 0.7?0.9H;the surface settlement curve is “spoon” shape with the maximum value is within the range of 1?2H from the edge of the foundation pit;the axial force of the soil nail is “jujube core” shape with large front middle part and small two ends;the foundation pit uplift is “mound” shape with large intermediate ridge and small around;the slip surface is roughly curved,and the safety factor of each excavation step is improved by cement soil wall.(2)The sensitivity of structural design parameters to the maximum horizontal displacement of cement soil walls is as follows: soil nail length> cement soil wall width> soil nail horizontal spacing> cement soil wall shear strength> soil nail inclination angle> cement soil wall spatial position.The soil nail length is the most sensitive in the soil nail design parameters,and the cement soil wall width is the most sensitive in the cement soil wall design parameters.(3)In the design parameters of soil nail,the length of the soil nail is changed,and the maximum horizontal displacement of cement soil wall is small when the wall nail ratio is between 8% and 11%;the influence range of the soil nail inclination angle to the wall nail ratio is small,mainly concentrated between 8% and 11%,when the wall nail ratio is between 9% and 10%,the maximum horizontal displacement of cement soil wall is small.In the design parameters of cement soil wall,the width of cement soil wall is changed,the displacement is significantly reduce and the maximum horizontal displacement of the cement soil wall is small when the wall nail ratio is between 7% and 12%;by changing the spatial position of the cement soil wall,under the condition that the wall nail ratio is constant,the constraint of horizontal displacement is the most obvious when the cement soil wall is at the edge of the foundation pit,and the backward movement will affect the axial force of the soil nail.Through the analysis of the synergistic effect of the four structural design parameters and 16 engineering examples about composite soil nailing wall foundation support.In the engineering design,it is recommended to place the cement soil wall at the edge of the foundation pit,and the wall nail ratio is controlled between 8% and 10%.In this range,the support structure is reasonable in stress and the synergistic effect is the best.(4)The three-dimensional model can reflect the spatial effect of the foundation pit,and the analysis result is better than the two-dimensional model.The load sharing ratio changes with the dynamic excavation of the foundation pit,and the load sharing ratio of cement soil wall gradually decreases,and the load sharing ratio of soil nail wall gradually increases.