A field study of stress-dependent soil–water characteristic curves and permeability of a saprolitic slope in Hong Kong

It is well recognised that field measurements of stress-dependent soil–water characteristic curves (SDSWCCs) and the permeability function (coefficient of permeability with respect to the water phase) are vital for assessing transient seepage, pore water pressure changes and the stability of unsaturated soil slopes. In this study, SDSWCCs and the permeability function were measured by using the instantaneous profile method on a saprolitic hillside in Tung Chung, Hong Kong. The ground profile comprises colluvium and completely decomposed tuff (CDT). A 3 m diameter circular plot was instrumented and subjected to two wetting–drying cycles. Throughout a 48-day monitoring period, variations of volumetric water content and matrix suction in the uppermost 3 m of soil were measured continuously using time-domain reflectometry moisture probes and jet-fill tensiometers, respectively. The field observations reveal that there is a clear trend of decreasing sizes of hysteretic loops of SDSWCCs with depth. In a comparison of the field- and laboratory-measured SDSWCCs, general agreement can be seen in the first wetting–drying cycle but not in the second cycle. The field-observed unsaturated permeability function is highly dependent on suction history (wetting–drying cycles). The field-measured permeability ranges from 4 × 10⁻⁷ m/s to 3 × 10⁻⁴ m/s in the colluvium, whereas the corresponding measured values vary from 4 × 10⁻⁷ m/s to 1 × 10⁻⁴ m/s in the CDT. Predictions of SWCC and permeability function using existing empirical approaches based on particle size distributions (PSDs) should be treated with caution. There is a clear qualitative and quantitative inconsistency between predictions of permeability function from PSD and field measurements.