Deformation Control Mechanism And Stability Analysis Of The Retaining And Protecting System Combining Underpining Pile With Soil Nailing Wall

The construction technique of soil nailing wall is convenient and economical,however,it is difficult to control the lateral deformation of foundation pit and vertical deformation of slope top.The underpinning pile is an approach of foundation reinforcement and is used to control the settlement of adjacent building foundation.The combination of underpining pile and soil nailing wall is a new retaining and protecting system.It can both prevent vertical settlement of adjacent building foundation and limit the lateral deformation of foundation pit.With the settlement of adjacent buildings during pit excavation,the part of load originally borne by natural foundation transfers to underpining piles and lateral soil pressure of foundation pit reduces by the load of adjacent buildings.The underpining pile and soil nailing wall shares the lateral earth pressure and restricts the deformation of foundation pit and adjacent buildings,forming an interactive system of underpinning piles,soil nailing walls and upper structure.The preventative underpining of the adjacent building foundation by the combination of underpining pile and soil nailing wall belongs to active protection.At present,there are few researches on this new support structure,and the scientific research of this supporting structure has important practical significance based on the theory value and application prospect.This study was supported by National Natural Science foundation of China(41272281)and the Natural Science foundation of Shandong(No.ZR2012EEM016).This subject will take a way of theory analysis,field test and numerical analysis to achieve the research object of load distribution transfer path and its failure model based on deformation compatibility in the combined supporting system of underpinning pile soil nailing wall.The main research results of this paper are as follows:(1)Based on the elastic foundation beam theory,the force analysis for the micro element of the supporting pile is carried out,and a finite differential scheme for internal force and deformation analysis of underpining pile is established.In order to verify the rationality of this method,the project case is calculated by using numerical software,and the calculated results of the finite differential scheme and numerical software are compared with the measured results.The pile bending moment is distributed in S shape,and there is a negative bending moment at the top of each pile.The results of finite differential scheme are close to the measured results.The results of finite differential scheme and numerical software are basically consistent with the measured displacement curve showing a "bulge" shape.(2)The soil pressure distribution is calculated on the soil nail and underpinning pile based on the horizontal stiffness and the internal force and deformation calculation method of considering P – ? effect is proposed.The settlement on the slope top is obtained through the traditional finite element analysis,and the settlement on the pile top is obtained through the Mindlin solution.The comparison between two methods shows that considering P – ? effect calculation value is closer to the experimental value.The underpinning pile bears the part load of the existing building,which leads to the reduction of the earth pressure the side wall of the foundation pit.The horizontal displacement and bending moment of the pile body caused by the reduced earth pressure are greater than the additional displacement and additional bending moment caused by the pile top load P.At the same time,the additional horizontal displacement caused by the pile top load P will also leads to the soil nailing partial stress,which limits the lateral deformation of the foundation pit.(3)According to the field monitoring results,the displacement of underpining pile increases during the excavation of the foundation pit,and the displacement curve presents a "bulge type";The settlement on the slope top increases continuously along with the excavation step by step.The settlement on the slope top presents a "groove" curve distribution,and the settlement reaches the maximum 4-5m away from the foundation pit.The axial force of soil nail is smaller at both ends and larger in the middle.In the analysis of the influence parameters,the horizontal displacement and bending moment of pile body decrease with the increase of soil nail prestress.The displacement and bending moment decrease less obviously when the prestressing force exceeds 40 k N,so there is a critical prestress in the underpining pile with soil nailing method.(4)The vertical instability of underpinning pile,lateral instability of underpinning pile,anti-pull failure of soil and overall instability failure are summarized.The influence of different parameters on the overall instability of the foundation pit is obtained simultaneously.Based on the logarithmic spiral failure mechanism,the analytical expressions of the internal energy dissipation rate and the power of external forces acting on the foundation pit are derived,and the theoretical calculation method for the stability of the retaining structure under the upper limit analysis method is proposed.(5)According to the performance requirements of engineering structure,the design steps and calculation methods of underpinning pile and soil nailing wall are proposed.The calculation formula of internal force and deformation of underpinning pile and axial force of soil nailing are summarized.Based on the bearing capacity estimation of underpinning micro pile,the lateral friction resistance and the correction coefficient of the pile tip resistance are given,and the calculation formula of the bearing capacity of the underpining pile is proposed.The embedded depth should meet anti overturning checking calculation,re-hunch stability checking computation and overall stability checking computation.(6)Based on one project case,the calculation results of deformation and internal force combining underpining pile with soil nailing wall are compared with the pile-anchor support and the diaphram wall support under the same conditions.In the aspect of building settlement,the whole rigidity of diaphram wall structure is larger,which has the innate advantage in protecting existing buildings.It can be concluded that the underpining pile with soil nailing structure is superior to the pile-anchor supporting structure,and the underpining pile has the characteristics of reducing pile.In terms of lateral deformation,it can be seen that the lateral displacement of underpining pile with soil nailing method is the largest,however,the lateral displacement is within the reasonable range specified in the code.