端部岩桥直剪破坏试验及断裂条件

Failure of rock bridge at the end and fracture condition under direct shear tests

以重庆鸡尾山岩质滑坡为代表的前缘"关键块体"型滑坡具有较强的隐蔽性,研究"关键块体"前缘锁固段岩桥的破坏性质对滑坡预警具有重要意义。以鸡尾山滑坡地质条件为基础,在岩样端部预制不同长度边缘裂缝,填入软弱材料,形成软弱夹层节理,在不同法向压力下进行直剪试验,分析了前缘端部岩桥的裂纹扩展规律、块体剥落特征信息和岩桥断裂条件,提出了临滑阶段剪应力变化率k值。试验结果表明:软弱夹层节理长度对岩桥破坏模式和块体剥落剧烈程度有重要影响,且端部岩桥越长,破坏前临滑阶段剪应力增速k值越小。端部岩桥破坏模式为剪切破坏、拉剪破坏和张拉破坏,且不同破坏模式决定了相同节理岩桥块体剥落的剧烈程度。岩桥块体剥落点与破坏点剪应力比值百分数平均值为79.5%～92.2%,且端部岩桥临滑阶段时间快慢依次为短节理慢、中间节理居中和长节理快。端部岩桥3种破坏模式满足一定断裂条件,且呈现出3个阶段和裂纹稳定扩展。通过本直剪试验研究揭示的端部岩桥破坏特征信息和断裂条件,可对前缘"关键块体"锁固段型岩质边坡失稳破坏评价提供理论依据。

Since the Jiweishan landslide in Chongqing in the front edge of the 'key block' is of certain concealment, it is of significance for the early warning of landslide to study the failure property the rock bridge in the front locking section of 'key block'. Based on the geological condition of Jiweishan, after prefabricating different length edge cracks, infilling soft materials and forming the joints of soft interlayer at the ends of the sample, direct shear tests were conducted on the prepared samples under different confining pressures. The propagation rules of crack, the characteristic information of the block spalling and the fracture condition of the rock bridge were analysed and the k value of changed rate of shear stress was proposed in a would-be sliding stage. The length of joint of the soft interlayer has a certain effect on the failure modes and the extent of block spalling, and the longer the end of rock bridge is, the smaller the k value of the growth rate of the shear stress is. The failure modes of the rock bridge at the end include shear failure, tensile-shear failure and tensile failure, however the extent of block spalling of the same joint is different under varied coalescent modes. The ratio of shear stress of spalling point to that of failure point is in the range of 79.5%-92.2%. The short joint is slower than the middle joint, but the long joint is faster than the middle joint in the time of would-be sliding stage. The three types of failure modes of the end rock bridge meet certain fracture conditions and present three stages and the crack growth stably. The conducted direct shear tests reveal the failure and characteristic information of rock bridge at the end and the fracture condition, and thus this study can provide a theoretical basis for the instability and failure estimate of the rock slope containing the front edge of the locking section which is called 'key block'.