Thermal Spallation Excavation of Rock

The Spa1lation Process Because of the low thermal conductivity of many hard rocks, rapid heating of these rocks produces a thin surface layer in which the temperatures attain high values. Thermal expansion of this surface layer is constrained by the reminder of the still cool rock, and when stresses within the surface rock become high enough, the surface rock breaks away from the cooler rock behind it and flies or falls off as a thin flake called a spall. Then the next, newly exposed surface is heated, and the process continues. This process is the basis of spallation drilling. The hot gases from a jet burner provide the heat for spallation to occur, and their high velocity provides a scouring action that transfers heat to the rock and removes the spalls as rapidly as they form. Spallation is a process which works in very hard rock. It is dependent upon the thermal expansion coefficient and the thermal diffusivity of the rock but is also affected by any discontinuities in the rock. To date the efforts which have been made to evaluate the various rock according to their spallability has been minimal. As the success of this process is dependent upon the characteristics of rock it is expected that the study of rock mechanics will prove to be of greater value to this program than to the other mechanism for drilling and excavating rock. Commercial Uses of SPALLATION In the 19408s, the Linde Air Products Division of Union Carbide (UC 1 began developing spallation for use in mining taconite ore, which is presently the chief source of iron in the United States. In this work UC developed a jet-piercing tool that burned fuel oil with oxygen to produce spallation and contained mechanical cutters to remove rock that was not amenable to spallation. The UC jet-piercing machines have since produced about 40 million feet of shallow blast holes used for emplacing explosives in the taconite mines. During this work it was found that hole diameters could be increased by merely reducing the advance rate of the burners and that existing holes could be enlarged by making another pass through the hole with the same burner. The Browning Engineering Go. of Hanover, N.H., has developed a hand-held spallation burner to cut slots in granite. It has been used for a quarter of a century and is now standard equipment for quarrying granite throughout the world. This burner, which resembles a small jet engine oriented with its exhaust pointed downward, is the forerunner of a flame jet burner used to spall experimental holes in granite at maximum rates in excess of 100 ft/hr when operating in hard, competent granite. It uses No. 2 fuel oil, which is burned with compressed air. The system uses water to cool the burner and the exhaust gases. These gases, along with the steam produced from the cooling water, blow the spalls from the hole. Experimental Work Theoretical and experimental work has been accomplished by the Massachusetts Institute of Technology and the Los Alamos National Laboratory. This work is reported in Refs. (3) and (4). To verify the experimental results of this work laboratory scaled down field tests were conducted using two we1 1 characterized granites from quarries in Barre, VT and Westerby, RI, under defined heating conditions. In the laboratory tests a propane - oxygen heating torch was used to direct a flame at the granite surface and the spal 1 ing process was examined at various heating rates with a high-speed video taping system operated at 200 frame per second. This produced a time-lapse sequence where the onset of the spallation process was easily distinguished. Also the heat flux from the torch to a flat surface at various stand off distances and flows was measured. A similar set of tests was conducted using the more easily quantified and uniform heat source of a 1.5 kw GO2 laser. This allowed accurate