ABSTRACT ABSTRACT
Thermal conductivity measurements of intact rock specimens do not reflect the effects of natural rock discontinuities, and large scale insitu tests are location specific and costly. To allow extrapolation of laboratory measurements to rock masses with discontinuities, the effects of discontinuities on rock thermal conductivity were measured using divided-bar techniques. The introduced discontinuities lowered the thermal conductivity in all cases, and conductivity decreased systematically with increasing roughness of the discontinuity. Conductivity increased with increasing pressure on the discontinuity.
INTRODUCTION
Safe disposal of high level radioactive wastes in deep rock depositories, such as granite, depends upon knowledge of the containment characteristics of the host rock. The containment characteristics can be affected by the temperature loading of the rock from radioactive decay of the wastes. The temperature levels will depend upon the dissipation of this heat and thus upon the thermal conductivity of the rock. Discontinuities lower the overall thermal conductivity of the rock mass. Although the effects of discontinuities may be evaluated with large scale tests, few are usually made due to the cost and time. The construction of a repository and the imposition of thermal stresses by decaying waste may change the fracture characteristics; therefore, repository design for different rock conditions may involve extrapolation of site specific large scale results. Consequently, it is important to more fully understand the specific effects of discontinuities on the rock thermal conductivity. This paper presents and discusses the results of a laboratory investigation of the effects of a discontinuity on the thermal conductivity of a granitic rock.
PREVIOUS WORK
A number of investigators have measured thermal conductivities of intact rock samples. These investigators include, among others, Birch and Clark (1940), Ballard et al. (1950), Walsh and Decker (1966), and Roy et al. (1968). Although conductivities have been measured in large masses of rock that have discontinuities (Lundstrom and Stille, 1978), little has been done to assess the effect of specific discontinuities on the thermal conductivity of an intact rock. Therefore, it is not clear how discontinuities affect the thermal properties that must be used in temperature modeling of subsurface rocks that are fractured and/or jointed. Discontinuities must affect heat transfer across them. In 1925, Terzaghi provided a physical explanation of a friction process (Skempton, 1960) that serves as one of the models for heat transmission. Terzaghi reasoned that the actual area of contact was only a small fraction of the total area and that the normal load would cause yielding of the contacting asperities. The contact area would then be given by the normal load divided by the yield stress of the material. Terzaghi's analysis was later proposed independently by Bowden and Tabor (1950). The surfaces of contacts at a microscopic level are rough and composed of asperities with different spacings and heights. Greenwood (1967) has shown that the contact area, even under elastic conditions, is proportional to the load, if the statistical variations of the asperities are considered. Neglecting radiation and conduction through air near contacts, Nikic (1974) found that the solid conductance was proportional to pressure for either plastic or elastic deformatin of the asperities.