INTRODUCTION
Tround International Inc. is currently developing a hard rock drill that uses a salvo of three frangible projectiles to prefracture the rock ahead of a conventional tri-cone bit. This paper reports on a series of experiments conducted for Tround to study the fragmentation enhancement possible through the use of an optimally timed and spaced salvo of projectiles. In this study the complex three-dimensional problem was simplified to a two dimensional salvo impact problem in a transparent brittle plate of glass. The use of high speed photography and dynamic photoelasticity was then used to study the development of the impact damage zone and the various interactions of the stress waves.
One of the most promising new techniques for rock excavation that has come out of some 30 or more novel hard rock excavation techniques studies under the old NSF/RANN program (Crouch, 1973 and Wang, 1975) is projectile impact. Projectile impact is a practical way to get large amounts of energy efficiently to the rock face. Two research groups, Sandia Laboratories (Newsom, et. al, 1976) and Physics International (Watson and Godfry, 1972) went beyond the normal laboratory confines and conducted field experiments in the early 70's demonstrating the feasibility and advantages of using projectile impact as a hard rock fragmentation or comminution technique. All previous research, with the exception of the Sandia-Tround Tera-drill program, has been concerned with the fragmentation due to the impact of a single projectile, see for example Vanzant, 1973, Bauer and Calder, 1969, and Kabo, et. al., 1977. It is an accepted and proven fact that in hard rock blasting improved fragmentation and rock removal occurs with proper choice of delay between adjacent charges (Winzer, et. al., 1979). This paper reports on a series of experiments conducted for Tround International Inc. to study the fragmentation enhancement possible through the use of an optimally timed and spaced salvo of projectiles. Tround is using a salvo of three frangible projectiles to prefracture hard rock ahead of a conventional tri-cone bit for deep drilling application (Dardick, 1977). Sandia Labs found that 100% drilling rate increases could be expected with such a technique even with a non-optimized salvo (Newson, et. al., 1976). This paper discusses the fragmentation enhancement possible by optimizing the time delay and spacing between two projectiles in a salvo. Past studies of projectile impact in rock have been primarily conducted on a post-mortem basis. In order to study the dynamic interaction between adjacent projectiles, it was decided in this study to simplify the complex three-dimensional problem and study edge impact on a transparent two-dimensional plate. By using high speed photography and the technique of dynamic photoelasticity, it was possible to follow the development of the damage zone and individual cracks as well as observing the stress waves within the material. Admittedly, the two-dimensional modeling of the actual problem magnifies some of the fragmentation damage as compared to the three-dimensional situation, but a good qualitative understanding of the problem can be obtained.
