Behaviour of the Weak Rock Cut Slopes and Their Characterization Using the Results of the Slake Durability Test

Abstract.

Slopes excavated in argillaceous rocks often show degradation mechanisms and local falls which affect both safety and maintenance costs of the roads. However, such behaviour cannot be generalized. Some cut slopes remain unaffected for years while others degrade very rapidly. The degradation process occurs in both marine and continental geological formations, which involve a variety of lithologies and textures of a complexity that is not reflected in the simple terms used to designate them (i.e. claystones, siltstones, shales or marls). Previous investigations have shown that relationships between the materials properties and their long-term behaviour in cut slopes are not evident. In our research, we have used samples from 35 cut slopes distributed along several roads of the Catalan sector of the Ebro Basin in Spain. Both intact samples and samples aged by the action of drying-wetting and freeze-thaw cycles have been tested with the Slake Durability Test (SDT). The results have shown the importance of freeze-thaw cycles in the development of cracks and weakening of the sample matrix. Most durable materials show only a slight loss of weight in the SDT and a linear degradation trend when tested up to five cycles. Weakest samples degrade very fast, some of them may fall apart with only few cycles of the SDT while showing a non-linear trend. Aged samples are found to reproduce more adequately the long-term behaviour of the cut slopes. The ageing cycles increase the contrast between durable and weak rocks, thus allowing a much better prediction of their future behaviour. The results of the laboratory tests are in agreement with the observed degradation of the excavated slope, which have been classified taking into account the response to the degradation process.