Excavation damaged zone division and time-dependency deformation prediction: A case study of excavated rock mass at Xiaowan Hydropower Station

Abstract Rock mass excavation is accompanied by rock deformation during unloading, which degrades the quality of rock mass and significantly affects the safety and stability of rock mass engineering. Taking the excavated rock mass from the dam foundation of Xiaowan Hydropower Station as the study object, long-time single hole acoustic tests (SHAT) were conducted in the dam abutment rock mass, with the changes in the acoustic wave velocity recorded. Then, a new method quantifying the excavation damaged zone (EDZ) was proposed according to the cumulative degradation rate of acoustic wave velocity, and a time-dependency model for evaluating the evolution of the excavation heavily damaged zone (EHDZ) was established. The results indicated that the cumulative degradation rate curves of the wave velocity along the boreholes exhibited significant differences in time and space. The depth corresponding to the inflection point on the curve can be regarded as the bottom boundary of the EHDZ, which could be used to generate time-dependency curves, and demonstrate the spatiotemporal evolution process for the EHDZ. The time-dependency curves can be classified into three categories. The differences between the classified curves are related to the depth of the initial EHDZ (EHDZ0), rock mass elastic modulus (E), and unloading stress (σu). The time-dependency model for the EHDZ evolution process follows a negative exponential function. The EHDZ0 depth, E, and σu can determine the coefficients of the model. The model provides an effective method to predict EHDZ depths at different times after rock mass excavation. The prediction results are of considerable significance for researches on the unloading failure and the selection of rock mass reinforcement, which would ensure the stability and safety of the rock mass engineering during construction and operation.