Failure mechanism analysis and yielding support control method for asymmetric large deformation tunnels in squeezing rock: A case study

Deep tunnel excavation in squeezing rock has strong tendency to asymmetric large deformation under the influence of active fracture zones. Issues, including steel arch twisting, shotcrete cracking and spalling and anchor failure could lead to the damage of the primary support and compression collapse in the reinforced concrete of the secondary lining. In order to explore the mechanism of asymmetric large deformation and control its potential threats, the research team has carried out a series of in-situ tests, including ground stress test, rock sample composition analysis, rock structure surface identification, high-density electrical resistance method test, and surrounding rock loosening zone test in the excavation surface on the Daliangshan Tunnel of Yunlin Highway in Yunnan Province. The research findings show that the geologic tectonics and volcanism could cause an intrusion of the granite into the schist to form the mechanical genesis mechanism of the extruded tectonic stress and lithologic contact zone. According to the yielding principle and the New Austrian Tunnelling Method, the research team has raised a two-scheme proposal for controlling the extrusion deformation of the tunnel based on a comparative study on the two schemes. The results show that the yielding support control scheme could have positive impact on the control of asymmetric large deformation section and the stress distribution of the tunnel surrounding rock. After the adoption of yielding support control method, the stress state of surrounding rock showed reasonable improvement by 32.5% less in the degree of its unevenness at its the largest bearing capacity. This support scheme has also cut down the costs of tunnel support by 10% compared to double-layer rigid primary support scheme. The research method has already been applied to the construction of large deformation section in the Daliangshan tunnel and the nearby Tianshengqiao tunnel.