Some effects of weathering on joints in granitic rocks

Abstract

Standard engineering weathering classifications, most of which are based on weathered granite, typically describe the appearance and condition of the weathered material (e.g. whether or not it is friable), the condition of individual minerals (e.g. degree of pitting and micro-cracking and changes in mineral composition), and the degree of staining on joint surfaces or the distance that staining extends into rock from the joints. These classifications do not address changes in frequency, length, and appearance of joints in the rock mass as weathering progresses. This account of work on vertical or steeply dipping joints in granitic rocks shows that not only are there statistically significant differences in mean joint spacings and mean trace lengths with increased weathering, but that the physical appearances of joints also changes as weathering progresses. Mean joint spacing is wide in fresh rock, becomes closer in moderately weathered rock, and then becomes progressively wider from moderately weathered rock through highly and completely weathered rock. Mean joint lengths follow a similar but inverse pattern. Mean trace lengths become shorter from fresh to slightly weathered rock, lengthen in moderately weathered rock, then progressively become shorter from moderately weathered rock through highly and completely weathered rock. Furthermore, joint appearance changes with increased weathering. Joints in fresh rock are sharp-edged and typically straight. The edges begin to round in moderately weathered rock, and in highly weathered rock, joint traces become sinuous and discontinuous around mineral grains. In completely weathered rock, the only visible joints are filled with minerals or marked with iron staining. It is proposed that this evolution in joint pattern and appearance results from increased cracking along grain boundaries and thermal expansion of individual mineral grains, and within-grain micro-cracking as weathering proceeds. These factors allow individual mineral grains to “move” into the spaces between joint surfaces, thus obscuring individual joints and making them appear shorter. These apparent changes in joint properties could lead to incorrect rock mass classification and thus, to inappropriate engineering design and costly errors during construction.