Permeability and P-wave velocity change in granitic rocks under freeze-thaw cycles

An extensive experimental investigation of microstructural changes in granites under freeze-thaw cycles using permeability and P-wave velocity measurements is described. Two types of natural granite rocks are considered and tested under dry and saturated conditions. The specimens were subjected to 200 heating-cooling cycles (-20 degrees C/+ 20 degrees C); each cycle had a duration of 24 h. The results indicate that the ageing process decreases the permeability and the P-wave velocity. The reduction in P-wave velocity is likely to be due to microcrack development (material damage). In dry samples, the microcracks result from the repeated differential contraction-dilatation of the mineral components. In water-saturated samples, there is an additional effect of freezing and thawing of water in the porous network. The decrease in permeability in the dry samples is due to partial closure of existing microcracks. In water-saturated samples, there was no increase in the permeability. A physically acceptable explanation is that new microcracks are not necessarily connected with those that already exist. Therefore the physicochemical process resulting from water-rock interactions also affects the permeability. This phenomenon reduces fluid flow in the material.