Quasi-three-dimensional physical model tests on a cavern complex under high in-situ stresses

Physical model tests offer a powerful method to explore the influence of material nonlinearities, plastic flow, and spalling failure in structures constructed in discontinuous rocks. We report an experimental setup using a stiff modular loading frame, hydraulically applied simulated loads and in vivo excavation to represent cavern construction at depth. The model experiments are instrumented with fiber Bragg displacement sensors and monitored by digital imaging of fiducial points via an endoscope enabling the effects of rock reinforcement in suppressing failure to be followed. The Shuangjiangkou hydropower station in the Dadu River in China is taken as a prototype for physical model tests. The cavern complex contains a power house, a transformer house, a tail water surge chamber as well as other openings under high in-situ stresses. For conditions of high horizontal stresses, model experiments show that failure is limited under stress/strength ratios less than 1.5. As stresses to strength ratios increase above 1.5 model deformations increase and failure initiates by spalling. Attempts to match observed behavior with continuum numerical models including plastic failure and flow are successful only at moderate overstresses. As stresses are build to induce spalling, the continuum models do not adequately represent the mode nor the extent of failure. Simple analytical models accommodating spalling failure are able to replicate the depth and extent of failure around the opening.