中文摘要：

岩体的完整性及节理裂隙的发育程度是影响围岩稳定性的主要因素。潜在失稳块体将引起围岩掉块、剥落、滑移甚至坍方等失稳现象。以小三岔口隧道为依托,基于块体理论研究,借助围岩结构分析和数值模拟相结合的方法,从岩体结构特征入手,定位隧道开挖潜在失稳区域,寻找可动块体,确定关键块体,深入分析围岩失稳变形模式,揭示无支护条件下围岩的变形、受力特征并提出针对性施工建议。研究结果显示：岩体节理裂隙发育、节理面相互切割成不利组合和岩块结构面结合性差是围岩失稳的根本原因;关键块体的运动是造成围岩失稳的直接原因。

英文摘要：

The integrity of rock and development degree of joint and fracture are the two main factors which influence the stability of surrounding rock,the potential failure block will cause problems such as fall-block,spalling,sliding and even collapse. Taking Xiaosanchakou tunnel as an example,by the means of combination of surrounding rock structure analysis and numerical simulation method based on the theory of block,the potential unstable region was posited,the movable block was found and the key block was determined from the angle of rock mass structure characteristic. And an in-depth analysis was make on instability deformation of surrounding rock,reveals the deformation and stress characteristics of surrounding rock without supporting conditions. The results showed that the development of jointed rock mass,bad combination caused by joint surface cutting each other and poor rock mass structural plane combination are the fundamental reasons of surrounding rock instability; the movement of key blocks is the direct cause of instability of surrounding rock.