Experimental study on water retention behaviour of Eocene flysch marls from the region of Dalmatia (Croatia)

Flysch marls are soft rocks highly susceptible to weathering under atmospheric conditions, primarily due to repeated wetting–drying cycles. As these cycles are closely linked to the unsaturated state, their influence on water retention behaviour is critical for understanding marl degradation. In this study, thirty marl and calcareous marl samples with calcium carbonate (CaCO₃) contents ranging from 39 to 83% were collected and tested for physical–mechanical properties and mineralogical composition. Three unsaturated test methods—the axis translation technique, dew point method, and vapour equilibrium technique—were applied to both intact and reconstituted (weathered/fully disintegrated) samples. Soil–water characteristic curves (SWCCs) were modelled using Brooks-Corey, van Genuchten, and Fredlund-Xing equations, with the Fredlund-Xing model giving the best performance based on its ability to capture high-suction behaviour. Intact marls with CaCO₃ < 65% showed higher saturated gravimetric water contents and lower air-entry suctions (≈215–1 500 kPa), whereas calcareous marls with CaCO₃ > 65% had higher air-entry values (≈5 100—40 000 kPa) and lacked a distinct residual zone. Reconstituted samples displayed an inverse trend, with low CaCO₃ marls having higher air-entry values. These findings highlight the impact of mineralogy on the water retention behaviour and weatherability of marls, influencing their physical and mechanical properties. The novelty of this research lies in applying unsaturated testing methods to intact marls and explicitly assessing mineralogical effects on SWCCs—an aspect not previously addressed. The findings advance understanding of marl weathering and provide a basis for more reliable evaluation of slope stability and durability of soft rock terrains.