Calculation Of Earth Pressure And Deformation Characteristics Of Finite Soil In Simultaneous Excavation Of Adjacent Foundation Pits In Urban Compact Area

The deep foundation pit is a hot issue in the geotechnical engineering field,and the calculation of the earth pressure acting on the retaining structure is a key issue in the design of the foundation pit.The current traditional earth pressure is based on the assumption of a semi-infinite body,using Rankine earth pressure,Coulomb earth pressure or a modified earth pressure calculation method.With the rapid development of China's economy and the advancement of urbanization,it is not uncommon for several adjacent foundation pits to be constructed successively or simultaneously in large and medium-sized urban dense areas.At this time,the soil between adjacent foundation pits is limited soil,and the traditional soil pressure calculation method cannot be used in the design of the enclosure structure.Exploring the calculation method of the finite soil pressure between adjacent foundation pits and the deformation characteristics of the foundation pit under simultaneous excavation has important engineering value.The work and main conclusions of this paper are as follows:(1)On the basis of previous studies on finite soil pressure,the method of differential body force balance is adopted.Based on the limit equilibrium theory,using the force balance equation and differential equation,the calculation formula for the finite cohesive earth pressure between two adjacent enclosure structures under simultaneous excavation is derived.Theoretical analysis of the influence of soil width,cohesion,and soil gravity on active earth pressure,the study found that active earth pressure is positively correlated with finite soil width and gravity,and negatively correlated with cohesion.Combined with the small strain soil hardening model in the finite element software Plaxis,the rationality of the theoretical calculation is verified.(2)The PFC software was used to simulate the deformation characteristics of the finite sandy soil between the foundation pits when the two retaining walls were deformed at the same time.It was found that the sliding surfaces were symmetrically distributed,extending diagonally from the bottom of the pit on both sides to the ground,intersecting at a certain depth.It provides a basis for establishing an earth pressure model for the formation of soil arching effect due to the deflection of principal stress in the limited soil between adjacent foundation pits under simultaneous excavation.Using the thin-layer element method,the soil between the foundation pits is divided into a rectangular area and two triangular areas.The stress analysis and stress analysis of the soil elements in the rectangular area and the triangle area are carried out respectively,the corresponding earth pressure coefficient and earth pressure expression are established,and the influence of the limited soil width and soil parameters on the earth pressure is analyzed.The passive earth pressure gradually decreases with the increase of the finite soil width,and the decreasing amplitude gradually decreases.As the internal friction angle increases,the active earth pressure decreases gradually,the active earth pressure resultant point moves downward,and the passive earth pressure gradually increases.Increase the ratio of the wall-soil friction angle to the inner-soil friction angle,the active earth pressure of the wall above 2/5 depth does not change much,and the active earth pressure of the wall below 2/5 depth gradually decreases.(3)Finite element numerical simulation is used to establish numerical models under different foundation pit spacing,soil parameters and construction sequence of foundation pit groups.During the construction of adjacent foundation pits,the deformation characteristics of the retaining structure on the side adjacent to the foundation pit and the retaining structure on the side away from the foundation pit are analyzed.Different excavation sequences have a greater impact on the deformation of the inner wall.When the rigidity of the ground connecting wall of two adjacent foundation pits is not equal,the ground connecting wall with less rigidity bears a smaller earth pressure.(4)Combining the actual measurement data of the construction of the ultra-deep foundation pit group,the law of monitoring data such as the enclosure structure,ground settlement,and supporting axial force during the successive construction of adjacent foundation pits is analyzed.The rationality of the theoretical analysis and numerical simulation in this paper is verified,and the corresponding experience is also provided for the subsequent construction of ultra-deep foundation pits.The excavation of adjacent foundation pits will have a superimposed effect on the settlement of the soil between the pits.The delayed excavation of the foundation pit will cause the second deformation of the first excavated foundation pit.