An Investigation of Combined Thermal Weakening and Mechanical Disintegration of Hard Rock

The research under modified Contract No. H0220068 has been devoted to experimental thermal-mechanical fragmentation of Missouri red granite in place, and to supporting theoretical analyses. The results of the previous year\u27s experimental work showed that thermal stresses are several times more effective in fragmenting hard rock when they are created within the rock rather than upon the surface. Also, large blocks {4-foot cubes) are not adequate to simulate the response of in situ rock. Based upon laboratory tests an experimental round was designed analogous to an explosive blasting round with coiled wire heating elements placed in drill holes. Three displacement relief faces were required for effective fracture at a 5 kw power level for a 10 inch burden. However, the heaters clogged and failed, and although the rock was effectively fractured, the fragments required considerable mechanical effort to remove them. Electric arcs at 12 kw, utilizing carbon electrodes were employed to create thermal inclusions, and in holes 14 inches in depth with equal burden. The rock was fractured and easily removed, as was also the case with 20 inches depth and overburden. Experiments are in progress to optimize round geometry and energy levels. Major problems are the brittleness of the carbon electrodes and maintaining of a stable arc as the heating is begun. Both of these problems are being solved. Theoretical analyses of temperature and thermal stress distribution are progressing well and procedures are being refined for use of more accurate boundary conditions, temperature-dependent rock properties, and other input parameters. Laboratory experiments are being conducted to determine stress and temperature distribution for a cylindrical geometry for basalt. Calculations of projected rates of advance and excavation costs indicate that for a slot type round the process is technically and economically feasible. --Summary, page 1a