Analysis On Mechanical Behavior Of Asymmetric Support Excavation Support Structure

In recent years,as the country’s development of underground space has become more and more important,the form of foundation pit support has become more and more complex,and asymmetric foundation pit support is one of them.Common asymmetric foundation pit support is divided into asymmetric overload and unbalanced excavation,but there are few asymmetric support applications and research on different types of foundation pit support.This paper takes the asymmetric support foundation pit of Qingshan North Road in Nanchang City as an example,and takes the asymmetric support foundation pits with different support forms on both sides as the research object.The main research contents are as follows:(1)The original design of the foundation pit support of Qingshan North Road is the symmetrical support of concrete occlusal piles on both sides,and the concrete piles on the north side are first constructed,but the location of the original rainwater tank culverts is inaccurate during the construction of the south side piles.It is too close to the integrated pipe gallery.Without the support of concrete piles,it has to be replaced by steel sheet piles to form asymmetric support.The bending stiffness of the support piles on both sides is nearly 10 times different.In the case of asymmetric support,the rigidity of the supporting structures on both sides is different,and the deformation is different.At the same time,the internal support not only produces compression deformation,but also produces translational deformation,resulting in inconsistent soil deformation on both sides of the foundation pit.In the current national technical regulations for foundation pit support,the earth pressure on the inner and outer sides of the support structure is calculated by Rankine earth pressure.The influence of soil deformation on earth pressure is not considered.It is obviously unscientific to apply directly to asymmetric support.The finite element method can accurately reflect the influence of soil deformation on earth pressure,and it is more scientific to apply to the analysis of asymmetric support foundation pit.Compared with the finite element results,it is found that the horizontal displacements on both sides are larger and evenly oriented in the pit,which is inconsistent with the actual measurement,and the stability coefficient is small,and the bending moment difference between the two sides is not large.The side bending moment of the finite element is nearly double that of the steel pipe pile side,and the result of the finite element is obviously more reasonable.In addition,using ABAQUS and PLAXIS to model and analyze the engineering examples,it is found that the pile displacement and pile bending moment values and trends calculated by the two finite element software are similar,and the displacement and bending moment of ABAQUS are small.The maximum deviation of steel sheet pile displacement is 8.8%,the maximum displacement of the bite pile is 13.85%,the maximum displacement of steel sheet pile is 9%,and the maximum displacement of the bite pile is 21.5%.This is because PLAXIS considers the coefficient of friction between piles and soils,but there is a binding relationship between piles and soils in ABAQUS.ABAQUS uses a plastic strain-destroying cloud map.PLAXIS uses a total displacement incremental cloud map,indicating that the method is different,so the ABAQUS stiffness side is first destroyed,and PLAXIS is the opposite.(2)Using the optimization method considering the influence of displacement on earth pressure,the calculation formula of earth pressure in the specification is improved,and the internal relationship is found and expressed by formula.Write the MATLAB program and compare the calculated earth pressure with the measured earth pressure or the finite element software calculation results.It is found that in the calculation of active earth pressure,the method above is more realistic in the bottom of the pit,and the standard method below the bottom of the pit is more reasonable;the passive earth pressure solved by this method is much more reasonable than the norm method.(3)The asymmetric support foundation pit is generally calculated by the overall model.When the separation model is used for calculation,the determination of the fixed point of the inner support is the key.As for the problem that the fixed point of the support in the asymmetric support pit is difficult to determine,the formula for calculating the fixed point of the inner support of the asymmetric support foundation pit is introduced by the specification.By changing the various mechanical parameters of the soil,the finite element software is used.The simulation analysis is compared with the analytical solution,and it is concluded that when the translational deformation is relatively large,the internal support fixed point can be calculated according to this formula,otherwise the fixed point is considered to be at the midpoint of the support.By changing the stiffness ratio of the two sides of the support,the asymmetric support foundation pit is studied.It is found that the greater the stiffness ratio of the two sides,the more unfavorable the stability of the foundation pit,and the effect of the stiffness ratio on the displacement on the side with greater stiffness.The impact on the bending moment is large.In the case of different bearing stiffness on both sides,the stiffness of the small side should not be lower than the bending stiffness of the pile calculated by the symmetric base on the other side to ensure that the inner support does not move on the inner support.Make the foundation pit safer.By changing the stiffness ratio of the two sides of the support,the asymmetric retaining foundation pit is studied.It is found that the larger the stiffness ratio of the two sides,the more unfavorable the stability of the foundation pit is,and the greater the influence of the stiffness ratio on the displacement on the fixed side of the stiffness is greater than the influence of the bending moment.In the case of different supporting stiffness on both sides,the bending stiffness of the small side of the pile should not be lower than that of the other side according to the symmetrical foundation pit,so as to ensure that the fixed point of internal bracing falls on the inner brace,which makes the foundation pit safer.