Makoto Terada

The changes of elastic constants of rocks during thermal cycling between 110K and 370K.

The dynamic moduli of three kinds of dry and saturated rocks were determined during thermal cycling between 110K and 370K at a slow heating rate (0.5K/min.). In the dry condition, the moduli of both Murata basalt and Ogino tuff increased linearly with decreasing temperature. The temperature coefficients of bulk and shear moduli were expected to be the same as those free from the effect of microcracks. On the other hand, the temperature coefficients of the bulk and shear moduli of Westerly granite were affected by both thermal cracking and the opening and closure of the pre-existing microcracks. They showed a sharp decrease when the granite was heated above 330K in the first cycle. A permanent decrease in the moduli was observed after both heating and cooling. During the second thermal cycling where few cracking presumably occurred, the coefficients of the moduli were almost positive. In the saturated condition, the moduli of the basalt, tuff, and granite increased rapidly when the rocks were cooled below room temperature. These changes coincided with those observed in thermal expansion down to 230K. Below 230K, the moduli, however, gradually increased with no change in thermal expansion. These observations suggest that the increase in the moduli is due to freezing of water within pores and microcracks between 270K and 230K, and that stiffening of absorbed water in the surface of pores and microcracks has an effect on the increase in the moduli below 230K.

Thermal expansion of saturated rocks subjected to cyclic temperature change between 110K and 300K.

Thermal expansions of several saturated rocks were measured under a cyclic temperature change between 110K and 300K at a slow rate. The following changes due to water saturation were observed in thermal expansion; suppression of thermal cracking, hysteresis in thermal expansion, and an increase within thermal expansion coefficient. These changes were explained by freezing of pore water within cracks and the difference in temperature between freezing and melting of pore water. When the ice forms in the crack pore of granitic rocks, it bonds the surfaces of each crack to impede crack extension. The residual strain and the temperature at which thermal cracking initiated, therefore, decreased in these rocks. The temperature difference between freezing and melting is probably caused by supercooled water and a capillary force acting on the pore water. The hysteresis therefore appeared in the thermal expansion of nongranitic rocks which showed no hysteresis in the dry condition. These rocks exhibited no residual strain in the wet condition as well as in the dry condition except for the tuff in which a large amount of residual strain was observed after the first cycle.