Study On Failure Mode And Design Method Of Soil-rock Dual Deep Foundation Pit

With the deeper development of urban underground space,the problem of soil-rock dual-element deep foundation pit engineering tends to become normal when the excavation depth is greater than the buried depth of bedrock.At present,the analysis and design of soilrock dual deep foundation pit at home and abroad either follow the method of soil foundation pit or choose the theory of rock slope.The fundamental reason is that the failure mode of this kind of foundation pit has not been clearly defined,and the rock mass structure has not been understood and paid attention to,which leads to excessive safety redundancy in the support design of earth-rock dual deep foundation pit and waste a lot of building materials.Therefore,this paper ADAPTS to the requirements of the development of high-quality foundation pit engineering,takes "scientific utilization of foundation pit rock mass structure"as the goal,adopts the research methods of theoretical analysis,field investigation and numerical simulation to study the failure mode of soil-rock dual deep foundation pit,and forms the design method of soil-rock dual deep foundation pit engineering accordingly.The main achievements are as follows:(1)The concept of soil-rock dual deep foundation pit is defined,and the stability of the following horizontal rock mass is divided into soil-rock dual deep foundation pit.It is defined as the foundation pit in which the upper part is soil and the lower part is rock mass within the depth range influenced by excavation and support system.Based on the stability of the underlying recumbent rock mass,the soil rock dual element deep foundation pit is divided into:the soil rock dual element deep foundation pit with the underlying recumbent rock mass stable and the soil rock dual element deep foundation pit with the underlying recumbent rock mass unstable.It is emphasized that the "key" of soil-rock dual deep foundation pit lies in "rock",and the stability of rock mass is the premise of the targeted design of this kind of foundation pit.(2)The paper reveals four failure modes and their development rules for the stability of the underlying rock mass.For soil-rock dual element deep foundation pit with stable underlying rock mass,it is determined that the failure mode gradually changes from circular arc failure to circular plane failure,section failure and sliding shear failure with the increase of the overlying soil layer thickness,and the regular curve of failure mode change with soil layer thickness is obtained,which is used as the criterion of foundation pit failure mode.The formula of rock mass tangential stability and the analytical solution of foundation pit stability are derived.(3)The spatial analysis model of the recumbent structural plane of the horizontal rock mass under the soil-rock dual deep foundation pit is established.For the soil-rock bielement deep foundation pit where the underlying rock mass cannot be self-stabilized,the analysis model of the influence of three kinds of structural planes on the stability of the foundation pit is established based on the inclination structural plane at different depths of the side wall of the foundation pit.Considering the supporting role of the adjacent foundation wall of the structural plane,the influence of the structure on the overall stability of the soil-rock foundation pit is analyzed.The preliminary findings are as follows:The failure mode of this kind of foundation pit does not necessarily have the overall sliding failure along the structural plane,but is related to the comprehensive factors such as the type,dip Angle,size of structural body and spatial effect of the inward inclined structural plane of the foundation pit.There may be three failure modes:"arc-structural plane","arc-tangential angle-structural plane",and "arc-rock wall surface failure".And the space effect has the constraint effect on the deformation and failure of foundation pit.(4)Put forward the general design method of soil-rock dual deep foundation pit.The design process of soil and rock dual element deep foundation pit engineering is defined,the accurate survey method of rock mass structural plane is put forward,and the stability criterion of the underlying rock mass is put forward.Based on the stability degree of rock mass and the overall failure mode of foundation pit,the targeted support structure types and structural measures of overlying soil layer and rock mass structural plane are defined,and the support design method of soil and rock dual element foundation pit engineering is established,so as to make reasonable use of the structural ability of rock mass and improve the theory and technology of foundation pit engineering.