Methodology for testing the soil water characteristic curves of soft rocks – example of Eocene marls

Soft rocks undergo weathering and deterioration over an engineering timescale. This process is closely related to the transitions between saturated and unsaturated states and suction variations. Therefore, to assess weathering in soft rocks the determination of the soil-water characteristic curve (SWCC) is essential. It is also a challenge, because the tested materials decompose from the parent rock into fine-grained material, requiring methods that account for both soil-like and rock-like behaviour. A methodology for determining the SWCC of soft rocks was developed using Eocene flysch marl from central Dalmatia (Croatia) as a case study. The research considers two extreme material states: intact rock (intact samples) and residual flysch soil (reconstituted samples), where multiple methods were combined to determine the corresponding SWCCs. For intact samples, the vapour equilibrium technique and the dew point method proved suitable, and for reconstituted samples, a combination of the axis translation technique (for suction up to approximately 500 kPa) and the dew point method. This study describes sample preparation, testing procedures, and method limitations, along with SWCC results highlighting the differences between intact and reconstituted samples. The findings contribute to understanding SWCC behaviour in soft rocks and assessments of weathering effects on slope stability.