An Investigation of Disintegration Behavior of Mudrocks Based on Laboratory and Field Tests

Mudrocks are highly susceptible to disintegration under natural climatic conditions. Using grain size distributions of the slaked material, this research compares the disintegration behavior of twenty different mudrocks (5 claystones, 5 mudstones, 5 siltstones, 5 shales) subjected to the standard slake durability test in the laboratory with the disintegration behavior of replicate mudrock samples exposed to natural climatic conditions, by placing them on the roof of the McGilvrey Hall, for 1 to 12 months. Disintegration ratio, D_R, (area under the grain size distribution curve of a sample divided by the total area encompassing grain size distribution curves of all samples) was used to quantify the disintegration degree of lab and field samples. The research also examines the relationship between engineering properties, geological characteristics, and disintegration behavior of mudrocks and purposes a mudrock durability classification based on disintegration ratio.

Results show that the relationship between slake durability index and disintegration ratio is nonlinear. The relationship indicates that rocks with high slake durability index values can have low disintegration ratio values based on grain size distributions of the slaked material. For the mudrocks studied, average disintegration ratio values after the 2nd cycle of slake durability test are 0.324, 0.461, 0.846, and 0.705 for claystones, mudstones, siltstones, and shales, respectively. Claystones disintegrate rapidly into mostly small fragments whereas siltstones disintegrate slowly into relatively larger fragments. Disintegration ratio of laboratory samples shows a better correlation (R² ranging from 0.70 to 0.79) with disintegration ratio of roof samples than the slake durability index (R² ranging from 0.60-0.70), indicating that the disintegration ratio better describes the disintegration behavior than the slake durability index.

Statistical analysis shows that the geological and engineering properties that have the most influence on disintegration ratio of mudrocks are clay related properties including absorption, adsorption, clay content, amount of expandable clay minerals, and void ratio.

Based on the results, a durability classification is purposed which categorizes mudrocks into four classes: low (D_R = 0-0.20), medium (D_R = 0.20-0.65), medium high (D_R = 0.65-0.85), and high (D_R = 0.85-1.00).