Development of Soil–Water Characteristic Curves through Control of Relative Humidity

The relationship between soil suction and soil water content in unsaturated soil mechanics is important but can be difficult to measure. The objective of the research described herein was to develop soil water characteristic curves (SWCC) from relative humidity values by incorporating the use of glycerin-water mixtures. Glycerin-water mixtures can be used to achieve different relative humidity levels by varying the concentration of glycerin within the mixture. The relative humidity created by each mixture was measured with a relative humidity sensor, and the relative humidity values and Kelvin’s equation were used to compute the total suction. Soil water characteristic curves were then developed using the relationship between the applied total suction and the measured gravimetric water content of a 15-bar high air entry stone and four soils, including: illite, red clay, kaolinite, and Donna fill. The soil SWCCs, obtained using glycerin-water mixtures, were compared with the SWCC developed using a chilled mirror hygrometer and the filter paper method. For the red clay and kaolinite, the water-glycerin method effectively developed the portion of the SWCC at low water content and high suction values ( R 2 values of 0.87 and 0.96, respectively). For the soils that behaved as cohesive materials, there was sufficient data to establish the SWCC using the relative humidity method for suction values greater than 1,000 kPa. At lower suction values, there was a lack of suction data, requiring data from other methods to complete the curves. For the soil that behaved as a cohesionless materials, the relative humidity method resulted in all data residing along the residual water content, making the development of the SWCC difficult. Based on the results, the inexpensive and simple relative humidity method may be used to develop SWCC for cohesive soils for the drying path at low water contents and high suction values.