Comparison of Experimental and Predictive Approaches for Determination of Water Retention Curves of Intact Samples of Quaternary Soils

Retention curves were measured for intact samples of Quaternary soils, frequently involved in rainfall-triggered landslides. A full characterization of the retention curves required combined use of several direct and indirect experimental techniques. Experimental data was compared with predictive methods based on grain size distribution and in situ relative density ratio, namely a simple capillary tube prediction model, and the modified Kovacs method which also takes into account adhesive forces. Predicted drying curves captured the trend of the experimental curves, but generally lay below the experimental curves. Retention curve hysteresis was well predicted, particularly with the “Universal Mualem” method. Quick desaturation of the soils for suction above air entry value qualitatively explains why steep natural slopes with intermediate Quaternary sediments remain stable, except during intense short-term rainfall. Deviation between predicted and experimental curves in the low-suction range will affect infiltration analyses. The low-suction range of the retention curve should therefore preferably be measured for studies of rainfall triggering of landslides.