A drying cake (DC) method is invented for measurement of the suction-stress characteristic curve (SSCC), the soil-water-retention curve (SWRC), and the hydraulic conductivity function (HCF) of soils under drying conditions. The DC method employs particle image velocimetry (PIV) technique to acquire digital still images of the radial displacement field of a disk-shaped soil specimen during drying, while the moisture content of the specimen is recorded using an electronic balance. A linear elasticity theory employing the suction-stress-based effective stress in both total and incremental forms is developed to calculate suction stress, suction, and hydraulic conductivity from the moisture-content-dependent displacement fields; thus permitting definition of the SSCC, SWRC, and HCF of a soil. Five different soils, representing a wide spectrum of soil variety from pure sand, to silt and organic silt, to non-swelling and swelling clays are used to illustrate and test the principle and theory, the validity, and applicability of the DC method. Established measurement techniques for the constitutive relationships of unsaturated soils, such as the Tempe cell, constant flow, transient water release and imbibition, and shear-strength relationships were used to validate the DC method. The results from the DC method were found to compare well with those techniques. Repeated DC tests confirm that the results from the test are unique. It is shown that the DC method is superior to other existing methods in: (1) providing simple and accurate data acquisition (involving taking sequential digital still images and monitoring specimen's moisture content by an electronic balance without use of suction or moisture probe), (2) facilitating fast testing time (in less than one week for the primary drying path of the SSCC, SWRC, and HCF), (3) permitting concurrent measurement of the SSCC, SWRC, and HCF by using one soil sample, and (4) its applicability to all types of soils under wide suction and moisture-content conditions.
