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| Mechanical responses and instability failure mechanisms of surrounding rock of tunnels in boulder-cobble mixed stratum |
| ZAN Wenbo1，2，LAI Jinxing1，CAO Xiaoyong3，QIN Yiwen1，FENG Zhihua1，4 |
| (1. School of Highway，Chang¢an University，Xi¢an，Shaanxi 710064，China；2. School of Civil Engineering，Shaanxi Polytechnic Institute，Xianyang，Shaanxi 712000，China；3. CCCC First Highway Consultants Co.，Ltd.，Xi¢an，Shaanxi 710075，China；4. Hebei Provincial Communications Planning and Design Institute，Shijiazhuang，Hebei 050011，China) |
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Abstract The boulder-cobble mixed(BCM) stratum is mainly composed of boulder and cobble and mixed with sand gravel and clay，in which the tunnel excavation is prone to cause a large-scale collapse at the arch due to the poor cementation of the BCM stratum. To systematically study the mechanical response mechanisms and deformation laws of surrounding rock during the process of excavation and instability failure，the model materials with similar particle size distribution were prepared based on a tunnel crossing through the BCM stratum in Tibet，and laboratory similar model test was implemented to analyze the stress and displacement fields comprehensively in the case of no support system. The results show that the BCM surrounding rock is of relatively good self-stability after excavation，but the capacity decreases sharply when encountering water. Three progressive arch collapses expand gradually in turn and develop upward in the form of a sharp arch. At the excavation stage，the radial stress relaxation of the surrounding rock is obvious while the circumferential stress concentration occurs within a certain depth range. The radial and circumferential stresses of the surrounding rock increase sharply before the first collapse and decrease continuously at the arch in the later collapse. For sidewall rock，the radial stress almost changes in a sinusoidal form and the circumferential stress increases all the time or increases first and then decreases. At the excavation stage，the deformation of the surrounding rock is generally low，and the settlement within the range from 15 cm in front of and 25 cm in back of the monitoring section caused by excavation accounts for more than 90% of the total settlement. The settlement of the vault increases sharply once encountering water，and the settle zone of the vault is exactly consistent with the collapse area in each stage. However，the deformation of the side wall is insignificant at the stages of excavation and water-induced failure. The results of model test and field monitoring are basically consistent with each other. The stress and displacement transfer of the surrounding rock is synchronous and interactive，and the staged failure process is mainly attributed to that the surrounding rock forms a protective circle similar to an arch structure around the tunnel through self-regulation. The formation of each arch structure has a certain hindrance to the development of surrounding rock deformation，and the first arch structure¢s obstruction effect is greater than the subsequent ones. The research results are valuable to guide the determination of support design parameters and the prevention and treatment of the BCM stratum tunnel collapse.
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2021, Vol. 40 
