Abstract

A series of flume experiments was performed as a part of a comprehensive study of rock erosion in spillway channels. The flume provided a controlled environment in which different flow rates and block configurations could be examined. The results of the experiments suggest that there is a strong influence of block orientation with respect to flow direction on the block erodibility threshold. Similarly, increasing flow turbulence intensity and/or increasing block protrusion height were also found to significantly lower the block erodibility threshold

1. INTRODUCTION

Scour of rock occurs as a result of hydraulically driven removal of individual blocks (i.e., plucking or quarrying). This is a critical issue in many engineering projects, particularly for dams, bridges, and tunnels, where excessive erosion can threaten the stability of the structure. The same phenomenon accounts for geomorphic evolution of channels in hard, jointed rocks. Numerous investigations from both engineering and geomorphology perspectives have examined the erosion of rock blocks subject to a variety of flow conditions including open-channels [1 – 5], hydraulic jumps [6, 7], knick-points [8], and plunging jets/plunge pools [9 – 18]. A common focus has been the evaluation of the role of discontinuities bounding the block in transmitting hydraulic pressures to the underside of the block promoting ejection from the surrounding rock mass.

A lesser emphasis has been placed on the influence of the discontinuity orientations on kinematic modes of block removal. In nearly all studies, simplified rectangular or cubic block geometries were used with vertical and horizontal discontinuities thus limiting failure modes to predominantly 1D lifting or 1D sliding. For more complex block geometries, as commonly encountered in nature, the 3D orientations of the block bounding discontinuities strongly influence block removability, kinematics and stability [19]. Accordingly numerous kinematic failure modes exist. These include pure translational modes (lifting, 1-plane sliding, 2-plane sliding), pure rotational modes (about a corner, about an edge) or some combination of translational and rotational modes (Figure 1). The orientation of the block with respect to the direction of loading (e.g., flow direction) will determine the applicable mode of failure. From a scour perspective, this indicates the threshold for block erodibility (i.e., the flow condition causing removal of the block) will vary with the direction of the hydraulic forces.

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