The influence of steam pressure on thermal spalling of sedimentary rock: Theory and experiments

Thermal spalling is the process by which surface material cracks and breaks off due to variation of temperature. The process is complex and the physical mechanisms that cause thermal spalling are not well understood. The theory and the results of experiments carried out to determine the effects of water and steam pressure on the phenomenon of thermal spalling are presented. It is shown that in theory the rise in pore pressure due to the presence of water as gaseous steam or as compressed liquid is insufficient to cause thermal spalling iii sedimentary rock. A new criterion for spalling due to steam saturation pressure which is based on the concept of critical stress is presented. The results from uniform heating experiments and from linear heating rate experiments have led to the conclusion that explosive spalling is caused by saturated steam alone, at local pressures in access of the unconfined tensile strength. The results of thermal shock experiments are more difficult to interpret because of the high compressive stresses at the heated surface. Spalling due to thermal shock is probably caused by a combination of steam pressure and fracturing associated with high compressive thermal stress. (C) 1998 Elsevier Science Ltd.