Tensile fracture mechanism of drilling shaft under the special engineering conditions of thick alluvium and thin bedrock

Based on the background of a coal mine auxiliary shaft under construction in Anhui Huainan mining area,China,the process and characteristics of drilling shaft rupture under tension in thick alluvium and thin bedrock are analyzed,the mechanical model of vertical shear stress for bending deformation of overlying strata in shaft ingate is established,the vertical displacement function of surrounding strata is given,and the analytical solution of vertical tensile stress of shaft is deduced by using the principle of minimum potential energy and the theory of elastic mechanics.The analysis shows that the bending deformation of the overlying strata on the shaft ingate is the cause of the vertical shear stress acting on the shaft.The tensile stress generated by the shear tensile stress acting on the shaft accumulates to a threshold value from bottom to top.When the resultant force of the threshold value and the shaft selfweight stress exceeds the ultimate tensile strength of the shaft,the first rupture failure occurs at the joint of the drilling shaft,and then,with the development of the bending deformation of the rock stratum,the shaft above the failure continues to be subjected to the vertical shear tensile stress,resulting in the second tensile failure,which leads to the flooding accident of water and sand bursting into the shaft in the bottom aquifer.The stability of the surrounding rock of shaft ingate has a significant influence on the stress state of the upper drilling shaft.The thinner the overburden bedrock is,the more likely the drilling shaft will be broken,the elastic modulus ratio of bedrock to weathered bedrock and the elastic modulus ratio of weathered bedrock to bottom aquifer have little influence on the location of shaft rupture and the maximum vertical displacement at the top of corresponding shaft ingate.By adopting such technical approaches as equal strength design for vertical tension of drilling shaft,wall base at the bottom of drilling shaft,1-2 horizontal isolation joints from shaft ingate to bedrock at the bottom of drilling shaft,surface L-grouting to reinforce soft surrounding rock of shaft ingate,the current relevant design specifications are improved to ensure the safety of drilling shaft operation.