A Study of the Fragmentation of Rock by Impingement with Water and Solid Impactors

Based on findings in other research areas it was reasoned that water slugs of substantial size (up to 1/2 lb) traveling at moderate velocities (< 800 fps) could impinge on rock with fragmenting pressure. To substantiate this reasoning, a compressed air launching gun was designed and built and an extensive series of impact tests was made on a steel-mounted pressure transducer and on sandstone and limestone targets. In a very few tests pressure values measured on the steel target exceeded the nominal compressive load limits of the rocks, but on actual rock targets significant fragmentation failed to occur. The basic difficulty was related to an inability to maintain a suitable air-water interface on a traveling water slug containing high kinetic energy. This, together with higher-than-anticipated dynamic failure values in the rook, prevented effective fragmentation. Pilot tests which replaced the water with spherical slugs of steel (3/16 to 2 in. diam.) resulted in spalling fragmentation and excellent specific energy values. Minimum values of specific energy appeared to occur with velocities below 500 fps. However, the evidence for the 2-inch spheres was limited by the fact that the 12-inch target cubes (sandstone, granite, basalt) shattered at velocities below 400 fps. The tests suggest that moderate-velocity large slugs of water are not satisfactory for fragmentation of rock, but heavy large-sized solid impactors are very effective at moderate velocities.