TEXTURAL AND MICROSTRUCTURAL INFLUENCES ON THE DURABILITY OF WAIKATO COAL MEASURES MUDROCKS

Data on durability, petrography and microstructure are presented for 2 7 specimens of Waikato Coal Measures mudrocks: fine-grained, carbonace ous rocks, which are dominated by kaolinite clays, and have low durabi lities. The poor durability is not caused by swelling clays. Rather, s laking is believed to result from the release of residual stresses wit hin the rock following weakening by water adsorption on to clay surfac es and a consequent loss in cohesiveness. This mechanism favours fragm entation into small, water-stable fragments, rather than dispersion of clays into the water. The amount of clay-sized material in the rocks provides the most direct control on the durability, but microstructura l features are also recognized as important: specimens with a disconti nuous matrix have a slightly lower durability than those with a contin uous matrix.