Multivariate statistical approach to re-evaluate the slake durability index test (ASTM 4644–08)

The slake durability index test is designed to quantify durability of mainly weak rocks. The test is basically tumbling 10 pieces of rocks each weighing 40–60 g for a two ten minute cycles in a water submerged mesh drum (2 mm mesh size). Second cycle slake durability index values (Id2) are the percentage of the fraction of material retained in the drum after the second cycle test is completed. The difference in Id2 values between durable vs. non-durable mudrocks can sometimes be very small, due to the fact that fragments of material retained in the test drum can be larger than 2 mm making the slake durability test inadequate to quantify durability of mudrocks. Therefore, it is necessary to investigate if other mesh sizes can better quantify durability and discriminate between durable and non-durable mudrocks. For this purpose, the grain size distribution of the material retained in the mesh drum after the slake durability test was sieved by #2 (6.3 mm), #4 (4.75 mm), #5 (4.0 mm), and #10 (2 mm) sieves. Multivariate/bivariate statistics was used to investigate which grain size fraction better correlates with the degree of disintegration. To quantify the degree of disintegration of the retained material, the fragmentation index is introduced. Another parameter used as a descriptor of the degree of disintegration is the sieved slake durability index. The results showed that the % > 6.3 mm size fraction has the highest control on both dependent variables. Discriminant analysis was also performed to identify which sieve size best discriminates between durable and non-durable mudrocks such as between shales and claystones/mudstones. Similar to the result of the linear regression analysis, the discriminant analysis showed that the % > 6.3 mm size is more sensitive than the % > 2 mm size to discriminate between the durable and non-durable mudrocks. We recommend sieving the retained material with the 6.3 mm mesh and using a modified the slake durability index equation to better reflect the disintegration of mudrocks during the slake durability index test.