Stability Analysis Of Deep Foundation Pit Soil Nailing Supporting Structure Based On Finite Element Slip Surface Stress Method

Recently,soil nailing has been applied to a variety of geotechnical engineering projects for its low cost and convenient construction advantages.However,it is still not very clear about the working mechanism and the failure mechanism of the soil nailing technology.Most of the time,experiences guide the engineering design in practice.Currently,Limit equilibrium method and FEA(Finite Element Analysis) are the main methods to analyze the foundation pit stability.Limit equilibrium method is based on many assumptions,and it ignores the interaction effects between the soil and the soil nails.FEA method is based on a more rational stress and strain relationship for the soil and the soil nails.Thus,FEA method is more reasonable in both theory and practice.This paper has carried out the following research:First,FE slip surface stress method proposed by Prof.Shao was applied into deep foundation pit soil nailing stability analysis.The stress field for the deep foundation pit soil nailing is analyzed with Canadian Soil Engineering FE software GeoStudio.This paper adopted pattern search method to determine the minimum slope safety factor and the most dangerous slip surface.By comparing the results with those calculated by limit equilibrium method and strength reduction method,the feasibility on the application of the slip surface stress method into the deep foundation pit stability analysis were studied.Second,the mechanism of the soil nailing technology was discussed.The performance of the soil nailing and its influence factors are reviewed.The deformation-force characteristics of the soil nails is discussed.Further,the methods to analyzed the the foundation pit stability by soil nailing technology were discussed.Slip surface stress method was briefly compared with the other methods such as strength reduction method,limit equilibrium methods and its advantages were pointed out.Third,the finite element modeling of the deep foundation pit by soil nailing technology was briefly discussed.An analysis was performed on a case of the natural slope and a deep foundation pit by soil nailing.The results showed that the factor of safety for the natural slope obtained by slip surface stress method is very close to that obtained by the limit equilibrium method and the strength reduction method.However,the location of the most dangerous slip surface obtained by the finite element method(slip surface stress method and the strength reduction method) is close,the location of the most dangerous slip surface obtained by the limit equilibrium method is relatively shallow.For the case of the deep foundation pit by the soil nailing,the factor of safety obtained by the slip surface stress method and the strength reduction method is very close,the location of the dangerous slip surface is also consistent.Fourthly,a finite element simulation was performed on the deep foundation pit by soil nailing under plane strain conditions.A detained investigation was made on the design parameters of the soil nailing such as the design length,the design spacing,the design angle of the soil nails and their influence on the stability of the deep foundation pit excavation.