中文摘要：

作为具有大跨度、超浅埋等特点的千年古地下洞室群，龙游石窟吸引着众多前来研究的岩石力学家和工程地质学家。近些年来，龙游石窟的3号洞顶板多处出现了开裂和离层。安装在3-1号岩柱周围起支撑洞室作用钢柱上的应变片数值也发生了变化。据分析，这些现象出现的一个重要原因是3—2号岩柱强度降低引起洞室围岩受力条件的改变。利用FLAC^3D数值软件，借鉴强度折减法，对3-2号岩柱强度降低引起上述变化的情况进行了模拟。数值模拟的结果可以较好地解释上述现象。在此基础上，进～步对3号洞顶板和其他岩柱的应力变化等进行了数值分析。结果表明：①3—2号岩柱的强度已有所下降，并有继续降低的趋势；②随着3—2号岩柱强度的降低，顶板的裂缝将不断增长、增多。其结果可为3-2号岩柱强度降低过程中洞室围岩力学行为的科学分析、预测及石窟的保护等提供依据。

英文摘要：

Longyou grottos, the ancient underground grottos with the characteristics of shallow embedding, large span and some others, have been attracting many experts of rock mechanics and engineering geology coming to research. Recently, many positions of the roof of grotto No.3 have the phenomenon of cracking and exfoliating. The data of strain meters installed on the steel pillars which have the function of supporting the grotto are also changed. What the strength of rock pillar No.3-2 decreasing leads to the adjusting of strength condition of rock around is thought to be one of the main reasons causing the changes mentioned above. With the aid of FLAC3D, the paper simulates the changes above caused by the strength decreasing of rock pillar No.3-2, using the main fundamentals of strength reduction technique for reference. The results of the numerical simulation explain the changes get by monitoring. Based on which, the paper analyzes the change of stress of the roof and other rock pillars of grottos No.3 in the process of deformation and failure of rock pillar No.3-2. By the research, the authors get the points： ①②The strength of rock pillar No.3-2 has been decreased a little; and still it is in the process of continuous decreasing. ② The cracks on the roof will be longer and more in the process. The conclusions provide a basis for analyzing reasonably, forecasting the strength state of grottos and protecting the grottos in the process of deformation and failure of rock pillar No.3-2.