EROSION OF FRACTURED MATERIALS

The natural fracture patterns that often exist in soil and rock materials are believed to have a major influence on how these materials erode. Material that normally would not be expected to erode when exposed to flowing water can fail along existing fracture planes. Conventional strength testing does not necessarily account for these potential failure surfaces. The objective of this study was to investigate the dominant parameters that cause failure of a fractured block matrix. A study was conducted by placing a matrix of blocks downstream of an overfall. The discharge over the overfall was increased until the block matrix failed due to the forces transmitted by the impinging flow. The block size, block orientation, and overfall height were varied systematically over a range of flow rates. Test results are presented, and the dominant failure mechanisms are described. The failure discharge was observed to decrease as the overfall height increased. This result was expected because the larger drops allow the flow to impact the surface with more energy. The failure discharge was also observed to increase if the block was placed with its long axis oriented vertically. This orientation distributed the block weight over a smaller area, thus requiring an increased pressure to dislodge the block. The repeatability of these fractured material tests was also examined, as was the block layer thickness. In addition to the hydraulic erosion of fractured materials, preliminary results on the measurement of pressures below a block matrix are discussed. This article provides fundamental research information concerning scour hole development and headcut erosion in fractured materials.