Deformation-based support selection for tunnels in strainburst-prone ground

Abstract

Since the 1980s, many contributed to major advances in the state-of-the-art of ground control in burst-prone ground and innovative support selection procedures as well as a sophisticated tool box full of effective rock retention and support components and systems. Most of these advances and the design of support in burst-prone ground, however, are based on the fundamental assumptions that seismic events create ground motions that damage excavations, and thus load and potentially damage the ground support. It is therefore often implied that the source of energy that causes damage stems from mining-induced seismic events. Kaiser and Cai (2013a, b) critically reviewed the support design guiding principles and identified three deficiencies with serious practical implications: As a consequence, support needs to cope with large localised deformations and thus all support components in a support system must satisfy a deformation-based design criterion. This paper builds on the above-mentioned articles and presents a deformation-based design concept for the control of damage caused by seismically triggered strainbursts. First, the impact of bulking deformations sustained during the failure of brittle rock masses is illustrated on the well-known ground–reaction-curve concept whereby geometric bulking deformations caused by mining-induced stress changes can be integrated. Second, it is demonstrated that the deformation demand on support is dominated by brittle failure processes in the inner shell (defined by the minor principal stress contour (at 3≤UCS/10)). Field observations from extensometers support these findings. This is consistent with findings from discrete element modelling results presented by Garza-Cruz et al. (2014). Next, the concept of excavation deformation potential (EDP) is introduced to assess the influence of mine stiffness on the vulnerability of an excavation to strainbursting. It is suggested that the EDP is a key parameter for the identification of potential strainburst locations. This hypothesis is then tested qualitatively on extensive strainbursting encountered during the excavation of five tunnels at the Jinping II project (China). It is concluded that potential strainburst locations correspond to locations with elevated deformation potential. From a support design perspective, it is concluded that strainburst damage can be most effectively be controlled by a support system consisting of a robust retention elements in combination with stiff rock mass reinforcements, minimising bulking, and with yielding bolts, satisfying a deformation rather than an energy demand criterion.