Study On Lateral Pressure Calculation Of Retaining Structure Of Foundation Pit Adjacent To Composite Foundation

Composite foundation is a kind of foundation form commonly used in high-rise buildings,in which longitudinal or transverse reinforcements are installed in natural soil,and the bearing capacity of composite foundation is improved by the synergistic loading of composite foundation piles and soil.When the composite foundation is excavated laterally,the interaction between the newly built foundation pit and the existing foundation causes the internal force and displacement of the composite foundation and the supporting structure to change regularly with the increase of the spacing(strip width).In this paper,numerical simulation and curve linear fitting are combined.By comparing the fitting results of centrifugal test and PLAXIS model,the scientific reliability of the numerical model and its parameters is verified.The following progress has been made:1.Through the PLAXIS numerical simulation of 13 groups of soil strips whose width is 0 to 12 s(s is pile spacing,s = 2000mm),the interaction mechanism and distance rule between composite foundation and adjacent excavation foundation pit are clarified,and the influence radius is determined:(1)Excavation of foundation pit and internal force of composite foundation influence each other:Excavation of foundation pit leads to decrease of earth pressure in active area and increase of passive area of supporting structure.With the increase of soil strip width from 0 to 3 s,the bending moment of pile near foundation pit increases,the ratio of axial force to pile-soil stress increases sharply,the bending moment of supporting structure decreases,and the shielding effect is obvious;with the continuous increase of soil strip width,the bending moment of pile,axial force and pile-soil stress ratio of composite foundation decreases,the range of change decreases,and the bending moment of supporting structure decreases gradually.When the strip width increases to 9 s,the influence between excavation foundation pit and composite foundation can be neglected.(2)Excavation of foundation pit and displacement of composite foundation affect each other:excavation of foundation pit leads to lateral unloading of composite foundation,displacement of pile top of composite foundation,stretching of soil strip,surface settlement,lateral displacement of supporting structure.With the increase of soil strip width from 0 to 3 s,the displacement of composite foundation pile decreases,the displacement of supporting structure and the settlement of foundation increase first and then decrease,and the displacement of supporting structure decreases greatly under the blocking effect.With the increase of soil strip width,the displacement and settlement of composite foundation pile,support structure and foundation decrease,and the variation range decreases.When the width of soil strip reaches 9s.The displacement and settlement of pile and supporting structure of composite foundation tend to be stable distribution.2.Setting the support structure as rigid body,comparing the numerical simulation results of lateral pressure acting on the support structure with the calculation results of the current code,it is proved that the calculation of the lateral pressure in the current code is too conservative.An empirical algorithm of lateral pressure is proposed,which is approximated to three continuous lines with concave depth of 5 m and 13 m by linear fitting.With the increase of strip width from 0 to 3s,the coeff-icient of earth pressure(K1,K2 and K3)of each section decreases,decreases first,then increases and then decreases.With the increase of strip width.,the coefficient of earth pressure tends to be stable gradually.