Fragmentation mechanism in crater blasting

• Published in: International journal of rock mechanics and mining sciences & geomechanics abstracts Vol. 30; no. 4; pp. 413 - 429
• Main Authors: Fourney, W.L., Dick, R.D., Wang, X.J., Wei, Y.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.08.1993; New York, NY Pergamon
• Subjects: Applied sciences; Buildings. Public works; Exact sciences and technology; Geotechnics; Soil mechanics. Rocks mechanics; Finite element method; Three dimensional model; Crater; P wave; Blast wave; Displacement field; Two dimensional model; Model test; Blasting operation; Crush; Wave reflection; Fragmentation
• ISSN: 0148-9062
• DOI: 10.1016/0148-9062(93)91723-V

This paper presents the results of a series of model tests that were conducted to investigate the mechanism of fragmentation in a cratering situation. In particular, the size of the crushed zone as a function of charge size was investigated using both PMMA and Homalite models. The increase in stress level traveling out into the polymeric models as charge sizes were increased was also investigated These were of interest since one of the possible mechanisms being investigated was spall and the magnitude of the reflected wave depends both upon the energy consumed in creating the crushed zone and the magnitude of the stress wave leaving the charge site. These tests were conducted in 2-D models and the charge size was varied from 100 mg of PETN to 600 mg. The results obtained showed that over this range of charge size the size of the crushed zone quickly reached a size where the ratio of the crushed zone radius to the borehole radius was about seven and then remained constant for further increases in charge. The stress level in the outgoing P-wave continued to increase with charge size. This increase, however, was less than would be expected. For a four-fold increase in charge, the maximum stress in the outgoing wave only increased by about 40%. Additional tests were conducted in Homalite 100 to investigate the fragmentation mechanism. A multiple spark gap camera in conjunction with a specially designed smoke diversion device was used to view the fracture formation after the charge was detoned. A number of different tests were conducted and all showed that strong multiple spalling occurred such that the material within the spall zone was fractured to the point where very little residual strength remained. The material in the near vicinity of the borehole was also greatly weakened by the system of radial crack created as the P-wave propagated away from the explosive source and the system of circumferential cracks created as the PP-wave traveled back across this system of radial cracks. The mechanism of fragmentation indicated from these tests was one where the material between the borehole and the free surface is greatly weakened by the stress waves over the first 50 or so μsec after detonation. It is proposed that this greatly weakened area is then acted upon by the residual pressure in the borehole to create the final crater. The feasibility of this mechanism was checked by using a finite element code to determine the displacement of several points in the crater region and to compare these displacements with displacements measured in 3-D tests conducted in models made from a very high strength—low porosity cement. The agreement with the measure displacements was good, indicating that the mechanism is reasonable.