Slaking behavior of clay-bearing rocks during a one-year exposure to natural climatic conditions

Slaking of clay-bearing rocks, upon exposure to natural climatic conditions, is a common problem at many engineering projects. Depending upon the regional climate, clay-bearing rocks slake in response to heating and cooling, wetting and drying, and freezing and thawing cycles. However, laboratory methods used for assessing the slaking behavior of clay-bearing rocks, including the standardized methods by the American Society for Testing and Materials (ASTM) and the International Society for Rock Mechanics (ISRM), involve only wetting and drying treatments. In this study, we investigated the slaking behavior of clay-bearing rocks by exposing them to natural climatic conditions for a period of one year. clay-bearing rocks, consisting of 5 claystones, 5 mudstones, 5 siltstones, and 5 shales, were selected for the study. Twelve replicate samples of each rock type were prepared so that each sample consisted of 10 pieces, each piece weighing 40–60 g, with a total weight of 450 to 550 g. The samples were placed in separate metallic pans and exposed to natural climatic conditions from September 2009 to September 2010. Each month, one replicate sample of each rock type was removed from natural exposure and its grain size distribution was determined. Disintegration ratio (DR), defined as the ratio of the area under the grain size distribution curve of the slaked material for a given sample to the total area encompassing all grain size distributions, was used to evaluate the extent of disintegration upon slaking. DR can range from 0 to 1, with 0 indicating 100% slaking and 1 indicating no slaking. After 3 months of exposure to natural climatic conditions, claystones, mudstones, siltstones, and shales exhibited DR values of 0.223, 0.302, 0.698, and 0.494, respectively. These results suggest that, over a given period of time, claystones slake the most and siltstones the least. Relating DR to clay content of the rocks studied reveals that rocks with higher clay contents slake more rapidly and extensively under natural climatic conditions than those with lower clay contents.