ABSTRACT
A computer program has been written to simulate ground vibrations induced by multiple-hole surface blasts. The program is an inexpensive and relatively easy to use tool for predicting the principal blast vibration characteristics that determine damage and annoyance potential: peak particle velocity, frequency content and pulse duration. The program generates a complete ground motion history by superposing, at any surface position, the vibrations induced by each individual explosive charge. It requires specification of (hence allows accounting for) the blast geometry, initiation sequence (including random delays), position with respect to blast, pulse characteristics induced by each charge, and propagation laws. A methodology is outlined to determine the necessary variables either on the basis of site investigations (e.g., velocity, attenuation, single-hole input pulse), or by back-calculation from measured blast vibration records. The latter procedure has been applied to blasts from two coal strip mines. Main difficulties encountered are back calculating individual input pulses, propagation (especially dispersion) parameters and delays. A normal distribution of delays] based on published results, has been shown to influence the results and the response of a single degree of freedom structure significantly. Broadening of the wave types included, improved input pulse determination and propagation laws, and more detailed verifications with field results are required in order to make the program more directly applicable to practical situations.
INTRODUCTION
At distances from a near surface blast greater than 1500 feet, Rayleigh waves usually carry about 70% of the vibration energy measured on the surface (Richart et al., 1970). Owing to dispersion and attenuation, waves generated by single hole blasts characteristically possess multiple cycles, long signal durations, and low frequencies. The duration of the single charge Rayleigh pulses generally is greater than the delay times used in blasting, so that waves generated by individual charges superpose. Most methods available and widely used to predict vibrations induced by a blast are based on the maximum charge weight per delay and only predict peak particle velocity (e.g., Ghosh and Daemen, 1983; Wiss and Linehah, 1978; Siskind et al., 1980; Holmberg and Petsson, 1978; Shoop and Daemen, 1983). Such scaled distance laws provide no information on the frequency content of the ground motions, nor on the total pulse duration. Studies by the U.S. Bureau of Mines and others (Siskind et al., 1980; Petsson et a1.,1980; Medearis, 1978; etc.) show that frequency of the ground vibration can play an important role is assessing the potential damage to a structure from ground vibration induced by blasting. Dowding (1985, Ch. 6) presents a comprehensive and rational scheme for predicting dominant frequencies of pulses induced by individual or by substantially delayed holes. The purpose of this study was to develop a simple model based on superposition and actual field measurements that more completely predicts the essential characteristics of blast vibrations (amplitude, duration, and frequency content). Details of the model, as well as a computer program to perform the calculations, can be found in Barkley and Daemen (1983).
PREVIOUS STUDIES
Studies by Wiebols and Cook (1965) and Aimone (1982) concentrate on determining strain or stress within the blasted mass, so that the mechanisms leading to rock fragmentation during blasting can be better explained.
