On the reliability of the strength retention ratio for estimating the strength of weathered rocks

• Rs may underestimate the strength of slightly weathered rocks.
• There is a relationship between degrees of weathering and Id2.
• Slightly weathered rocks can retain 80–90% of fresh rock strength.

This technical note examines the validity of the strength retention ratio (Rs) as a criterion for estimating the strength of weathered rocks. Although Rs has been widely used for classification purposes, it seems to significantly underestimate the strength of slightly weathered (SW) rocks. To better understand the limitations within which Rs can be used, a series of point load and slake durability tests were performed on weathered rocks of three types. The obtained correlations between the point load and slake durability indices were used as a basis to refine the current method of determining Rs. Using this new procedure, the available data from the literature were revisited, and Rs was re-calculated providing a better match between the quantitative and qualitative description of weathered rocks.