Study On The Ultimate Bearing Capacity Of Surrounding Soil And Rock Underlying Gripper Of Shaft Boring Machine

With the development of deep underground space and the exploitation of resources,the shaft boring machine(SBM)has become the future development trend in the construction of deep and large shafts because of its advantages of high efficiency,good safety and environmental protection.In the process of SBM driving,the gripper is supported on the shaft side to fix the SBM and provide the driving force for the SBM.If the rock and soil mass around the shaft can not provide enough bearing capacity under the action of gripper,it will cause the problems of insufficient driving force,difficult to control the attitude,and even instability of the shaft side.Considering that the first SBM in our country has not been put into use just after its commissioning,there is no construction experience and related theory of shaft ultimate bearing capacity,so it is necessary to carry out the research of the ultimate bearing capacity of shaft surrounding rock and soil under the action of SBM gripper.On the basis of the first SBM(MSJ5.8/1.6D)independently developed in China,the stability and bearing capacity of the shaft side underlying gripper in SBM are studied by the method of theoretical analysis and numerical simulation.The main work and research results are as follows:1)After research on shaft drilling method,SBM drilling technology and equipment and related research results,this paper analyzes the mechanical transfer mechanism and the stress state of the rock and soil mass surrounding the shaft underlying gripper during the SBM driving,which lays a foundation for the subsequent study of the ultimate bearing capacity of shaft side.2)By using the GDEM numerical simulation software of coupling finite element and discrete element method,considering the influence of three factors,i.e.driving depth,shaft diameter and surrounding rock grade,the three-dimensional numerical simulation of the effect of gripper on the surrounding rock and soil of shaft in the process of driving is studied,and the distribution and characteristics of displacement,stress,strain and failure units in the surrounding rock and soil of shaft underlying gripper are obtained.The calculation results show that:(1)the main failure mode of the surrounding rock and soil of shaft underlying gripper is shear slip failure,and the simplified failure surface is obtained;(2)with the increase of driving depth,the failure unit of shaft is obviously reduced,the displacement underlying gripper is reduced,and the ultimate bearing capacity is slightly increased;(3)with the increase of the shaft diameter,the failure unit of the shaft is slightly increased,the plastic shear strain and displacement increased sharply,but the ultimate bearing capacity is obviously reduced;(4)with the increase of the surrounding rock grade,the failure unit of the shaft is slightly increased.Compared with other factors,the influence of surrounding rock grade is the biggest.3)Based on the failure mode of shaft obtained by numerical simulation,the failure surface is assumed to be the combination of logarithmic helix and linear rotation surface,and the stress distribution on the failure surfaces are analyzed.Then,based on the MohrCoulomb criterion,the ultimate bearing capacity formula of surrounding soil of shaft underlying gripper is derived by using the plastic limit equilibrium method.The change rule of bearing capacity coefficient and the influence rule of basic factors are analyzed.Finally,the theoretical calculation value in typical soil layer is compared and analyzed with the numerical simulation results to illustrate the rationality of the formula.4)Based on the Hoek-Brown criterion,the numerical simulation analysis of the failure of the shaft rock mass underlying gripper is carried out,and the distribution law of the failure unit,shear strain and displacement in the shaft rock mass is obtained.The results show that the failure mode of the surrounding rock of shaft underlying gripper is mainly shear sliding failure.Based on this,the simplified failure surface of the sliding surface is determined.In addition,the Hoek-Brown criterion parameters have significant impact on rock failure.The higher the degree of rock disturbance and fragmentation,the more failure units,the greater the shear strain and displacement.5)According to the failure mode and shape of the failure surface of the shaft rock obtained from the numerical simulation,it is assumed that the sliding failure surface is composed of five planes.Based on the Hoek-Brown failure criterion,the stresses on each failure surface are analyzed,and the calculation formula of the ultimate bearing capacity of the shaft rock underlying gripper is derived by using the plastic limit equilibrium theory of the rock mass.Besides,the influence law of the basic parameters in the calculation formula is analyzed.Finally,for the same rock stratum,the ultimate bearing capacity of the foundation obtained according to the specification is compared with the ultimate bearing capacity of the shaft rock mass calculated by the derived formula.In the typical rock stratum,the calculation results of the formula are compared with the numerical simulation results,which shows the rationality of the formula in this paper.The research results of this paper not only provide a prediction and evaluation method for the ultimate bearing capacity of surrounding rock and soil of shaft underlying SBM gripper in the construction of deep and large shaft,but also have guiding significance for the design and construction risk control of SBM.