The Influence of Temperature Dependent Properties of Thermal Rock Fragmentation
This investigation is concerned with a thermal rock fragmentation system employing heat to create subsurface thermal inclusions. The effectiveness of this system as applied to three different rock types is studied. The rock types are Dresser basalt, Charcoal gray granite, and Sioux quartzite. The influence of temperature dependent material properties is investigated. The actual three-dimensional problem of in situ rock fragmentation involves parallel rows of equally spaced holes drilled to a constant depth. A heat source at the bottom of each hole creates a thermal inclusion resulting in a stress field causing fracture. Axisymmetric models of the fragmentation system were obtained by considering the typical planes of symmetry and nature of the actual three-dimensional problem. These models were used to study the fracture characteristics of the three hard rock types and to investigate the influence of temperature dependent material properties. The temperature and stress solutions were obtained using finite element approximations. Fracture predictions were based on the Griffith and the McClintock-Walsh modified Griffith fracture criteria. It was found that for any given fracture length there was a definite order in critical stress time among the three rock types. The results indicate that the heat transfer problem is governed by the quartz content of the particular rock type. The complete fracture problem was not governed by any one particular rock characteristic. The dimensionless critical stress time ratio, t[superscript * subscript f], was found to be related to the dimensionless fracture length ratio, L*, according to the equation, t* = (L*)³. This suggests that small hole spacings should be used for an optinum fragmentation configuration. Although critical stress times, for a given hole spacing, were different for each rock type the fracture patterns were found to be functions of the problem geometry only.
T. F. Lehnhoff and J. D. Scheller, "The Influence of Temperature Dependent Properties of Thermal Rock Fragmentation," International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, Elsevier, Jan 1975.
The definitive version is available at http://dx.doi.org/10.1016/0148-9062(75)91405-9
Mechanical and Aerospace Engineering
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