A method to determine relevant geomechanical parameters for evaluating the hydraulic erodibility of rock

Abstract

Among the methods used for evaluating the potential hydraulic erodibility of rock, the most common methods are those based on the correlation between the force of flowing water and the capacity of a rock to resist erosion, such as Annandale's and Pells' methods. The capacity of a rock to resist erosion is evaluated based on erodibility indices that are determined from specific geomechanical parameters of a rock mass. These indices include unconfined compressive strength (UCS) of rock, rock block size, joint shear strength, a block's shape and orientation relative to the direction of flow, joint openings, and the nature of the surface to be potentially eroded. However, it is difficult to determine the relevant geomechanical parameters for evaluating the hydraulic erodibility of rock. The assessment of eroded unlined spillways of dams has shown that the capacity of a rock to resist erosion is not accurately evaluated. Using more than 100 case studies, we develop a method to determine the relevant geomechanical parameters for evaluating the hydraulic erodibility of rock in unlined spillways. The UCS of rock is found not to be a relevant parameter for evaluating the hydraulic erodibility of rock. On the other hand, we find that the use of three-dimensional (3D) block volume measurements, instead of the block size factor used in Annandale's method, improves the rock block size estimation. Furthermore, the parameter representing the effect of a rock block's shape and orientation relative to the direction of flow, as considered in Pells' method, is more accurate than the parameter adopted by Annandale's method.