FRAGMENTATION MECHANISM IN CRATER BLASTING

This paper presents the results of a series of model tests that were c onducted to investigate the mechanism of fragmentation in a cratering situation. In particular, the size of the crushed zone as a function o f charge size was investigated using both PMMA and Homalite models. Th e increase in stress level traveling out into the polymeric models as charge sizes were increased was also investigated. These were of inter est since one of the possible mechanisms being investigated was spall and the magnitude of the reflected wave depends both upon the energy c onsumed in creating the crushed zone and the magnitude of the stress w ave 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 resu lts obtained showed that over this range of charge size the size of th e crushed zone quickly reached a size where the ratio of the crushed z one radius to the borehole radius was about seven and then remained co nstant for further increases in charge. The stress level in the outgoi ng P-wave continued to increase with charge size. This increase, howev er, was less than would be expected. For a four-fold increase in charg e, the maximum stress in the outgoing wave only increased by about 40% . Additional tests were conducted in Homalite 100 to investigate the f ragmentation mechanism. A multiple spark gap camera in conjunction wit h a specially designed smoke diversion device was used to view the fra cture formation after the charge was detonated. A number of different tests were conducted and all showed that strong multiple spalling occu rred such that the material within the spall zone was fractured to the point where very little residual strength remained. The material in t he near vicinity of the borehole was also greatly weakened by the syst em of radial crack created as the P-wave propagated away from the expl osive source and the system of circumferential cracks created as the P P-wave traveled back across this system of radial cracks. The mechanis m of fragmentation indicated from these tests was one where the materi al between the borehole and the free surface is greatly weakened by th e stress waves over the first 50 or so musec after detonation. It is p roposed that this greatly weakened area is then acted upon by the resi dual pressure in the borehole to create the final crater. The feasibil ity of this mechanism was checked by using a finite element code to de termine 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 th at the mechanism is reasonable.