Durability characterisation of weathered sedimentary rocks using slake durability index and jar slake test

Abstract

Weathering causes the rocks to loose their original strength and the increment in slakability can trigger slope instability. Furthermore, weathering of heterogeneous sedimentary rock mass produces a non-systematic weathering profile that is dependent on the characteristics of the unweathered, interbedded sedimentary sequence. However, the increase in slakability limits sampling of rocks for physical and mechanical rock tests. Thus, slake durability test and jar slake test have been applied to characterise a sedimentary rock mass weathering profile. The advantages of these methods are in the simple test procedures which only requires small size of rock block samples and relatively low cost testing equipments. The study was conducted at a roadcut that consists of Kati Formation which had undergone slight to complete weathering. Results show that the durability decreases with increases of weathering grades. However, presence of iron staining and iron recementation improve the rock durability. Completely weathered sandstone has the mean second cycle slake durability index, Id2 of 0.3 % to 15.0 %, highly weathered sandstone is from 22.0 % to 67.0 %, slightly weathered sandstone is 87.0 % to 98.0 % depending on the presence of iron recementation and fresh sandstone is 91.0 %. For mudstone, mean Id2 for completely weathered is 2.0 % while highly weathered mudstone is 12.0 %. Moderately weathered mudstone has an average value of 68.0 %. Mean Id2 for slightly weathered shale, is 83.0 % and slightly weathered chert has the average Id2 of 98.0 %. Jar slake test has the advantage to differentiate between highly weathered and completely weathered sandstone and mudstone. Completely weathered rocks have higher slaking properties where the rock samples breakdown completely. For highly weathered rocks, the breakdown is into chips and fractures. Jar slake test is suitable only for weathered rocks with slake durability value, Id2 below 25 %.