ABSTRACT

The thermo-mechanical behavior of rock masses having numerous discontinuities is different from that of fresh rock. It is important to investigate its effects on discontinuities related to the crack occurrence and the propagation of precracks due to cooling down. Thermo-mechanical analyses are performed with a jointed rock mass model using PFC2D code to investigate the effect of discontinuities during cooling down.The thermal stresses induced by a temperature dropwere calculated to figure out the amount of induced stresses only, excluding initial stresses, in the model. The magnitude of induced thermal stresses becomes larger when the thermal expansion coefficient is increased. The induced thermal stresses become larger until fractures are generated as the temperature drop increases. But the magnitude of induced stresses has decreased steeply to below −50°C, where the fracture initiates, and a fair amount of fractures are generated in the model as the temperature drops more. The results found in this study are not entirely comparable with the observations obtained from real sites due to complicated geological and groundwater conditions. However, it is possible to estimate the fracture mechanisms for jointed rock masses under cryogenic conditions with PFC2D models.

1 INTRODUCTION

One of the important problems related to underground storage of cryogenic material is preventing the leakage of liquid and gas from the containment system to the rock mass caused by tensile stresses due to shrinkage of the rock mass around the caverns (Monsen & Barton, 2001). In general, it has been regarded thatwhen numerous discontinuities exist in rock masses, the extension of joints is much easier to achieve than occurrences of new cracks in fresh rock during cooling down. This means that fractures can be initiated or developed more easily under lower induced thermal stresses because the mechanical characteristics of the discontinuities of the fractured rock are weaker than those of the fresh rock. Groundwater generally flows through discontinuities in rock mass but it would stay in discontinuities locally if rock masses around caverns were badly drained during the construction of caverns. In this case, groundwater could be frozen during LNG storage and frost-heaving pressures could be created in the discontinuities. Therefore, it is important to figure out the effects of thermomechanical behavior on discontinuities related to the crack occurrence and the propagation of pre-cracks during cooling down because the thermo-mechanical behavior of jointed rock masseswould be different from that of fresh rock. In this study, thermo-mechanical analyses are performed with a jointed rock mass model using PFC 2D code.

2 BASIC PRINCIPLES OF CRACKS

In the cases of the crack occurrence and the propagation of pre-cracks in jointed rock masses, the presence of joints in rock mass before cooling down should be considered.

2.1 Crack occurrence due to cooling down

It is known that when rock is cooled slowly thermal cracks are generated due to differences in thermal expansion between components of rock, although the temperature gradient is not steep (Lee, 1993).

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