Hysteretic water retention behaviour of unsaturated hydrophobised soils

Hydrophobised soils found in the superficial region of earthen infrastructure can affect the hydrological processes of these structures. Hysteretic water retention curve (WRC), which governs these processes, for hydrophobised soils has rarely been reported. Existing apparatus was unable to measure the WRC of unsaturated soil as they cannot control the condition of pore water pressure (u(w)) in excess of pore air pressure (u(a)) when the contact angle is larger than 90 degrees. This study created a new apparatus, which sandwiches a soil sample by a pair of high water-entry value (WEV) membrane and a high air-entry value (AEV) ceramic disk, to control the u(w) < u(a) and u(w) > u(a) conditions. Contact angle hysteresis and menisci evolution in the test materials during wet-dry cycles were measured to interpret the WRC. Results show that the WEV of soil was increased with decreasing chemical heterogeneity and particles size. When drying the hydrophobised soils, they could retain water before reaching a certain negative u(w). The WEV identified from the second wetting cycle was lower than that from the first cycle, whereas the AEV remained largely unchanged. The WEV and AEV decreased when the particle size of the hydrophobised materials was increased, resulting in a smaller hysteresis size.