# Analysis of Collapse Mechanism and Treatment Evaluation of a Deeply Buried Hard Rock Tunnel
## 3 Mechanism Analysis of Tunnel Collapse
### Analysis of Collapse Causes  
* The regional structure is mainly the fault structure, and the surrounding rock is mainly breccia tuffaceous sandstone, tuff, and sedimentary tuff of the Guantou formation of the Lower Cretaceous system. The lithology changes greatly, with strong rock alteration and well-developed joint fissures; the rock mass is of a fragmentary mosaic structure. The stress release during the excavation of surrounding rock tends to cause extrusion and internal bulging deformation, and the pressure stress concentration causes tensile fracture damage.
* The site mainly consists of eroded and denuded low mountain and hilly landforms. The tunnel passes through the surface reservoir, and the groundwater is sensitive to the tunnel construction. Under the influence of groundwater erosion and softening, the shear strength of the rock mass is reduced, and the cohesion between rock layers is reduced, which may induce shear slip near the structural zone.
* The tunnel uses a full-section blasting method, which significantly disturbs and crushes the surrounding rock mass. When the tensile stress on the structural surface is greater than its shear strength, the unstable block will be destroyed and shear slip will occur along the weak structural surface, causing the exposed broken block to flake off.
* Due to the failure of timely support after tunnel excavation, the surrounding rock was exposed for a long time, which lead to fracture development and stress state changes in the plastic loosening zone of the surrounding rock. As a result, the surrounding rock lost its stability and fell off.
* The excavation condition of surrounding rock is inconsistent with the geological survey report. Geological prospecting data show that this section is identified as grade III surrounding rock by geology, and the initial spray and hanging net support design was adopted based on geological prospecting data. In fact, under the influence of structure, the surrounding rock is more broken, the joints are more developed, and the blasting disturbance has a greater impact, which is more likely to result in the instability of surrounding rock due to the insufficient support strength.  
Based on the comprehensive analysis, the main factors causing the collapse of the tunnel were as follows: geological structure, groundwater influence, construction method, support measures and timing, survey and design problems. A generalized model of surrounding rock instability has been established (see Figure 3). It has revealed that the main cause of small-scale collapses is the combined action of the face extrusion and bulging deformation and the tension fracture of the inclined structure at the top stratum. In the process of balance arch formation at the arch roof, the support strength is not enough, and the stress release causes compression failure and collapse of the crushed block. The collapse can be classified as a local small-scale instability of the surrounding rock caused by the massive rock mass compression and shear failure along the structural plane.