Study On The Evolution Of Excavation Damage Of Tunnels In Granite Under Complex Stress Paths

The excavation of deep buried tunnel in brittle surrounding rock causes the stress redistribution,which leads to the stress concentration around the tunnel.When the stress exceeds the crack initiation stress,the complex stress path experienced by the surrounding rock will promote the initiation and propagation of cracks,cause rock mass damage,and ultimately lead to brittle spalling failure of the surrounding rock under a lower stress level.The brittle failure of deep buried tunnel not only affects the operation of engineering construction and the long-term safe of underground engineering,but also changes the permeability characteristics of rock masses.Especially for the permeability-sensitive underground engineering such as high-level radioactive waste repository,it is of great significance to study Excavation Damage Zones(EDZs).Based on the construction of the high-level radioactive waste disposal Beishan underground research laboratory(URL),this paper focuses on the brittle damage of rocks and the damage evolution of the surrounding rock of deep-buried caverns under complex stress paths.The following studies are carried out:(1)For the Beishan granite,three key stress thresholds in the crack evolution process are determined,the pre-peak damage evolution process of the granite is studied,and the permeability changes of the granite after different stress levels are analyzed.Research shows that the ratios of crack closure stress,crack initiation stress,crack damage stress to uniaxial compressive strength of Beishan granite is about 0.25,0.50,and 0.83,respectively.The damage parameters determined by the lateral residual strain can better characterize the brittle damage caused by crack propagation in granite.The Beishan granite damaged by different stress levels still has lower porosity and permeability,indicating that it is difficult to describe the changes in the permeability of the rock mass in EDZs in advance through the indoor permeability test.(2)The applicability of the existing crack initiation stress determination method is analyzed,and on this basis,two new methods to determine the crack initiation stress are proposed.It mainly evaluates the applicability of commonly used crack volumetric strain method,lateral strain response method,and cumulative acoustic emission hits method to different types of rocks.The lateral strain interval response method is proposed,which has higher accuracy and smaller dispersion,and is suitable for high-porosity sandstone.The cumulative acoustic emission hits curve slope method is proposed.This method has good applicability and accuracy for Beishan granite and marble with low porosity,as well as cyan sandstone and red sandstone with high porosity.Two newly proposed methods are used to determine the two crack initiation stresses of the Beishan granite.At the same time,it is clear that the initiation and propagation of cracks in the granite have two stages through the amplification cyclic loading and unloading tests.These two crack initiation stresses respectively represent the initiation of cracks on the weak surface and in the hard minerals.(3)A detailed three-dimensional stress path analysis was carried out for the excavation of a deep-buried granite tunnel,and the influence mechanism of the complex stress path on the damage evolution of the granite tunnel was studied.Studies have shown that the contour line where the deviator stress is equal to the first crack initiation stress near the excavation surface is in good agreement with the final spalling depth,and the contour line where the deviator stress is equal to the second crack initiation stress in the tunnel is in good agreement with the final spalling width.The stress direction rotates greatly during excavation,but the stress rotation beyond the crack initiation stress can degrade the strength of rock mass.In order to quantify the complex stress path in excavation,the concept of cumulative effective rotation angle is proposed,and then the excavation damage in rock mass is quantified to a certain extent.Based on the cumulative effective rotation angle and the two crack initiation stresses of granite,a stress path method is proposed to characterize the evolution of the excavation damage of the deep-buried granite tunnel and predict the extent of the EDZs.(4)The stress path method was used to predict the EDZs in the ROOM 418 tunnel and the access ramp of Beishan URL.In the ROOM 418 tunnel,the predicted results of the stress path method are in good agreement with the actual observations.Under the unfavorable conditions of this paper,the Beishan access ramp will not have brittle spalling damage,but there will be a certain range of excavation damage.It is necessary to pay attention to the changes in the permeability of the rock mass in the EDZs.