中部锁固岩桥三轴加卸荷力学特性及裂纹扩展研究

Mechanical properties and crack model of central rock bridge in triaxial unloading test

锁固型高陡岩质边坡内部岩桥破坏机制复杂,研究边坡中部锁固段的破坏规律及其对边坡整体变形破坏机制具有重要意义。为表征滑坡后缘拉裂缝和前缘蠕滑破坏,在完整岩样端部预制裂纹形成中部岩桥,开展3种不同长度岩桥试样的三轴加载和三轴加卸荷试验,分析2种应力路径下的应力–应变特征、强度特征和裂纹扩展模式,从断裂力学角度揭示了裂纹扩展机制。结果表明:随围压和岩桥长度的增加,试样峰值强度和对应的应变增大,且三轴加卸荷峰值和应变均大于三轴加载;应力–应变曲线呈现出"突发式破坏"和"峰后回升"现象,部分试样还表现出"双峰值"特征;岩桥试样呈现贯通岩桥、贯通试样上端面、向外环向破坏、向内环向破坏及贯通试样下端面等5类裂纹扩展模式;岩桥试样在下部节理尖端应力集中处产生张拉裂纹和剪切裂纹,大部分裂纹起裂角集中在40°~50°范围。中部岩桥三轴加卸载力学试验表明,边坡锁固段并非一次剪断破坏,可能呈现逐次多级破坏模式,本研究获得的岩桥裂纹扩展及破坏机制,可为锁固型岩质边坡开挖卸荷的破坏机制和变形特征提供理论支撑。

The failure mechanism of the internal rock bridge of the high-steep rock slope is complex. It is important to study the failure of central locking section and the failure mechanisms of slope. The rock bridge was formed by prefabricated cracks at the ends of the rock simple in order to represent the tensile failure in the trailing edge of slope and shear failure in the toe of slope. The conventional triaxial loading test and triaxial loading-unloading test were carried out on three rock bridges of different lengths. The characteristics of stress-strain curves, strength-deformation and the crack propagation types under two types of stress paths were analyzed, and the mechanism of crack propagation was discussed based on the fracture mechanics. The results of test show that the peak strength of specimens and the corresponding strain increased with the increasing of confining pressure and rock bridge length. The peak strain and peak value of rock bridges specimen under the triaxial loading-unloading are greater than those from the conventional triaxial loading test. The ＂burst failure＂ and ＂post-peak rise＂ characteristic appeared in the stress-strain curve, and the stress-strain curves of some specimens show the ＂double peak ＂ phenomena. There are five types of crack propagation in the rock bridge specimens： the coalescence rock bridge, the coalescence specimens upper end-face, the inward circumferential failure, the outward circumferential failure and the coalescence specimens lower end-face. The tension cracks and shear cracks are generated at the tip of the lower joint of rock bridge specimen where the stress concentration occurs. Most of the crack initiation angles are between 40° to 50 °. The lab test revealed that the failure mode of locking section in rocky slope was a multi-stage process.