While significant advances have been made in excavation of rock, largely in improvements in drilling, blasting, loading, and haulage, conventional operations have been improved until little feasible increase in efficiency or rate of advance appears possible. Tunnel-boring machines have been developed which will excavate at rapid rates in soft to medium-hard rock. Hypervelocity impact is being intensively investigated as a possible means of cutting and breaking rock, as well as lasers, electric current, explosive drilling, high frequency vibration, etc. The current development and use of high-velocity small-diameter water jets, metallic jets, and projectiles of various materials involve ultra-high-pressure phenomena of fundamental interest.
Rock disintegration by water jets has been investigated in the USSR for over a decade. Large-diamete jets of low velocity are used to mine coal, and high-velocity capillary jets will cut and break rocks under laboratory conditions. Velocity ranges for breaking and cutting are dependent upon rock and jet properties, and pulsed jets break some rocks more easily than continuous jets. Spectacular laboratory results have been reported for breakage of unconfined blocks of some types of rocks. Compressors, pressure multipliers, gas explosion chambers, and water cannons have been used to generate jets. get properties, such as continuity, unit pressure, total pressure, and stability have been measured. Results agree with theory, although the latter has been established only for relatively low-velocity flow. Diameter, speed of traversing, and frequency of pulsed jets affect rock response. Investigations in Britain have only recently begun consideration of harder rocks and higher velocities, although water jet cores have been found to be more continuous at lower velocities than previously believed. Lower limits of jet velocity for rock cutting were established for the rocks investigate. Shaped-charge jets employing liners have proved very effective for armor peneration, and have found industrial application in tapping iron furnaces and perforating oil well casings. Cursory attempts to use them for mining or excavating rock in the late 1940's were discouraging, largely because of high costs. Only two basic studies of rock penetration have been reported since.
Hypervelocity projectile impact was studied mainly because of the interest in the impact of meteorites upon space vehicles and upon rocks on the moon. The phenomena of particle impact on rock and metal are similar in some respects; most metals tend to flow, and craters in rock are larger because of the brittleness and low tensile strength of rock. Projectiles are usually accelerated by means of light gas guns, although other means are employed.
Hypervelocity impact studies involve pressures from one atmosphere to those in the megabar range for gases, liquids, and solids. Hence, it is desirable to be able to define equations of state for the substances involved throughout the pertinent ranges of temperature and pressure. Most high pressure data for water and minerals are for static conditions, although shock parameters are tabulated for both. Several equations of state have been proposed for gases, including the virial equation, KWB equation, and a modified Abel equation, all for application to gaseous products of the detonation of high (chemical) explosives.
