Dynamic stress waves produced in rock by explosive blasts

Abstract

This research project was directed toward a comprehensive study of dynamic stress waves produced in rock by explosive blasts. The object of the study was to compare the relative rock breaking performances of different explosives and to assess the ground vibration influence that blasting has on the structural stability of underground openings and rock slopes. The borehole method was used to estimate the relative performances of explosives; therefore, it was necessary to measure the internal body waves generated in the rock by the explosive blasts. The dynamic acceleration forces developed along rock surfaces during an explosive rock breaking blast need to be measured to provide an estimate of absolute ground vibration levels that may cause damage or instability to structures.The first phase of this rock breaking project was to develop an instrumentation system for monitoring explosion generated dynamic stress waves. The system had to be capable of monitoring and permanently recording both the internal body waves that propagate from a blasthole within rock and the lower frequency surface waves that travel along free surfaces. An accurate and reliable instrumentation system was developed that proved suitable for monitoring body, as well as surface waves in both underground and open-cut mines.As explosion generated stress waves represent a study of dynamic phenomena the dynamic elastic constants of rock need to be calculated. Two novel techniques, one to provide a quick and simple measurement of compressional wave bar velocity and the other an empirical estimate of the shear wave velocity of a dry, consolidated rock are presented. By employing either one of these techniques together with the easily measured rock density and velocity of propagation of P waves all the dynamic elastic constants can be calculated.The relative rock breaking performances of two aluminised water gel (slurry) explosives, specially developed for underground production blasting, were compared with the more commonly used 75 per cent bulk strength ammonium gelatine dynamite and ammonium nitrate - fuel oil explosives. Based on relative performance tests conducted at an underground mine it was concluded that the two slurry explosives were not expected to fracture more rock by passage of the dynamic stress wave than the gelatine dynamite; however, an improved fracture pattern would be expected around a slurry blasthole when compared to that produced around an ANFO blasthole.It was possible to confirm the generally accepted square root and cube root charge weight scaling exponents for surface and body waves respectively from field tests conducted at an open-cut coal mine. This confirmation was based on multiple linear regression and analysis of variance statistical techniques. It was also determined from the field tests in coal that an exponential law relationship that incorporates spherical wave dispersion provides a better estimate of dynamic stress levels close to a blasthole than the power law or exponential law relationships.As well as monitoring small scale blasts generally for the purpose of estimating relative explosive performances in rock or developing ground vibration prediction equations, several large underground production and open-cut blasts were monitored. Surface wave prediction equations were estimated from the monitoring of these large blasts for the underground and the open-cut mine. Several recommendations to reduce ground vibrations, and thereby possibly reduce slope instability problem.s at the open-cut coal mine are made.Favreau's (1969) mathematical model of dynamic stress waves generated by an instantaneously detonated spherical charge in an ideal medium was extended to linear charges and modified to more realistically represent a real rock and explosive situation. The model computed particle velocity levels close to the borehole for linear charges similar to those measured at field tests conducted as part of this rock breaking project.