Influence of Contact Angle on Soil–Water Characteristic Curve with Modified Capillary Rise Method

The contact angle quantifies physicochemical interactions at the liquid–solid interface and is therefore critical to many physical processes that involve the interaction of soil and water. In geotechnical engineering, such interactions are the basis for the formulation of the soil–water characteristic curve (SWCC). However, the role of the contact angle in SWCCs has not been adequately recognized. A comprehensive study is reported on applying the capillary rise method (CRM) to measure the contact angles of soils. Analytical solutions to two forms of the Lucas–Washburn equation are presented to provide the theoretical basis for applying the CRM to soils. The disadvantages of conventional CRM analyses are demonstrated with experiments. A modified CRM was proposed on the basis of an analytical solution to a more complicated form of the Lucas–Washburn equation. This modified CRM exhibited reliable performance on numerous specimens made of a subgrade soil and a silicon dioxide sand. Testing procedures were designed and strictly followed, and innovative apparatuses for the preparation, transport, and accommodation of soil specimens were fabricated to ensure repeatability. For the modified CRM, experimental results for virgin specimens demonstrated good repeatability, and for sieved soils, clear trends were observed in the variations of contact angle with respect to pore size. Contact angles much greater than zero were observed for all tested specimens; this finding contradicts the assumption of perfect wettability in previous SWCC studies. In addition, it was demonstrated that neglecting the variations of contact angles with respect to pore radius could result in significant errors in SWCC construction.