不同应力路径下小跨度隧洞围岩的宏细观破坏机理

Macro and micro failure mechanism of surrounding rock of small span tunnel under different stress paths

为探究小跨度隧洞围岩在不同应力路径下的破坏机制,依据现场工况设计相似模型试验可以直观了解围岩的破坏过程,同时数值模拟技术为洞悉岩体内部的破坏机理提供了可视化手段。本文依托某隧洞工程设计了相似模型的加、卸荷破坏试验,并利用颗粒流分析软件PFC进行数值仿真。对比宏、细观两种尺度下的试验发现,隧洞两侧直墙中部岩体的切向应力峰值最低,该处易受拉破坏。拱肩和拱脚处的围岩因应力集中而松动,该处产生的裂隙与直墙处裂隙贯通后将岩体切割成块是隧洞破坏的主要形式。开挖卸荷的应力路径下,隧洞整体的承载能力更强,但破坏时剥落的岩体更加破碎,围岩的损伤深度更大且内部剪切裂纹占比更多,最终形成的破坏面接近“V”型,同时开挖卸荷还会形成开挖扰动区,使拱顶和拱底处岩体的切向应力峰值降低,部分岩体的承载能力下降。

According to the different stress paths,similar model test and PFC simulation test of tunnel surrounding rock are designed to compare the failure mechanisms at macroscopic and mesoscopic scales.The following conclusions are drawn.1)Excavation unloading will disturb the surrounding rock to form a certain excavation damaged zone.2)Under the loading path,the stress of surrounding rock failure is 1.500 MPa;under the unloading path with initial stress of 60% σ_(Zmax) and 100% σ_(Zmax),the failure stress is 1.583 and 1.833 MPa respectively in the model test.3)In terms of the failure mode of rocks under different stress paths,tensile fractures first appear in two sides of the vertical walls;thereafter,the spandrel and arch foot are loosened due to the stress concentration.The fractures gradually coalesce with those occurring in the vertical walls.4)In the process of excavation unloading,the proportion of shear cracks is 35.3%,and the rock is subject to strong shear effect.The final failure surface is approximately V-shaped.5)The tangential peak stress on the vertical walls at the free face is the lowest;the vertical walls at the free face show the poorest bearing capacity and are easily subjected to tensile failure.