冷却法による高温岩石の熱応力破壊に関する研究

Abstract

Theoretical and experimental investigation s were carried out to make clear the fracturing mechanism by thermal stresses induced around a borehole in underground hot dry rock when cooling water was circulated through the borehole.Cubic rock specimens of a side of 30 cm which have borehole of 6 cm In diameter in the center were gradually heated up to a temperature set in an electric furnace, and then they were rapidly cooled by passing water through the borehole.The results of experiment showed that rock specimens were fractured mainly by the tangential thermal stress, σθ, arounc the borehole when the temperature difference between the surface of borehole and outside of specimen reacne around 50°C.Cracks caused by the radial stress, σr, an d the axial stress, σz, were sometimes found in Kofu andesite.The value of theoretical temperature difference required to make fractures was considered to apply for practical use, though it was somewhat different from the experimental value.By the mathematical calculation, it was estimated that fractures would occurred in the hot dry rock at the depth of 5000 m if the temperature around the borehole is reduced about 280°C from the original rock temperature.