厚表土薄基岩特殊工程条件下的钻井井壁受拉破断机理

为揭示厚表土薄基岩特殊工程条件下的钻井井壁受拉破断机理,以安徽淮南矿区某在建煤矿副井为背景,分析厚表土薄基岩钻井井筒受拉破断过程与特征,建立马头门上覆岩层弯曲变形竖向剪切拉应力力学模型,给出井筒围岩分层竖向位移函数,采用最小势能原理及弹性力学理论推导出井筒竖向拉应力解析解。分析表明,井筒马头门上覆岩层受施工多次扰动影响发生弯曲变形是产生作用于井筒之上的竖向剪切拉应力的致因;该剪切拉应力产生的作用于井筒之上的拉力由下而上积累到某一阈值,该阈值与井筒自重应力的合力超过钻井井筒极限抗拉强度时,在钻井井筒接头处发生第1次拉断破坏,其后,随着岩层弯曲变形发展,拉断处以上井筒继续受竖向剪切拉应力作用而发生第2次拉断破坏,并导致底部含水层水砂溃入井筒发生淹井事故。马头门围岩的稳定性对改变上部钻井井筒受力状态有重要影响,其上覆基岩越薄影响越大,越易发生钻井井壁拉断破坏,基岩与风化基岩弹性模量比和风化基岩与底含弹性模量比对井筒发生拉断破坏时的位置,以及对应马头门顶部最大竖向位移影响均较小。通过采用钻井井筒竖向受拉等强设计、钻井井筒底部设置壁座、马头门至钻井井筒底部基岩段设置1~2道水平隔离缝、地面L型注浆加固马头门软弱围岩等技术途径,完善现行相关设计规范,确保钻井井筒运行安全。

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

In order to reveal the tensile fracture mechanism of drilling shaft lining 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.