A STUDY ON THE SLAKE DURABILITY INDEX OF DIFFERENT WEATHERED ROCKS IN SRI LANKA

Studies on the engineering properties of different rock types provide a prior knowledge on the quality of such rock types as well as potential locations of suitable sources for the industry. Slake Durability Index (SDI) quantifies the resistance of a rock to wetting and drying cycles, and is seen as a measure of the durability of the rock. The SDI varies according to the type of minerals in a rock, its’ origin and the degree of weathering. If rocks are using for embankments or other construction related applications the estimation of their deterioration over a period of time when subjected to water immersion is necessary. With the aim of that, fresh and differently weathered samples of different major metamorphic rock types were collected from different localities in Sri Lanka to study the variation of Slake Durability Index with the degree of weathering. Samples were tested in the geotechnical engineering laboratory according to the Standard Method ASTM D4644. Using the results, classification of SDI values by Gamble (1971)* was used to classify the durability of the weathered rocks in Sri Lanka. The results are given in the following Table.

Accordingly, all fresh rocks (FR) show very high durability. Except hornblende biotite gneiss slightly weathered (SW) rocks show high durability. Moderately weathered rocks (MW) show medium durability category and all highly weathered rocks (HW) fall to low durability. This indicates that the recognition of weathering grade of a rock is enough to use the Gamble’s durability classification table temporarily in the field.

Table. Results of slake durability index tests for some rock types in Sri Lanka.

Rock type (Metamorphic Rocks only)	Weathering Grade	SDI Range %	SDI Mean %	Durability (according to*)
Charnockite/ charnockitic gneiss	FR	100	100	Very High
	SW	98-97	97.5	High
	MW	94-89	91.5	Medium High
	HW	60-40	50	Low
Garnet sillimanite gneiss	FR	100	100	Very High
	SW	98-97	97.5	High
	MW	94-87	90.5	Medium High
	HW	60-50	55	Low
Hornblende biotite gneiss	FR	100	100	Very High
	SW	92-91	91.5	Medium High
	MW	82-87	84.5	Medium
	HW	50-55	52.5	Low
Granulite	FR	100	100	Very High
	SW	96-98	97	High
	MW	90-92	91	Medium High
	HW	35-40	37.5	Low

Reference* https://www.slideshare.net/Shoaibafridi/6-40835695