The soil-water characteristic curve (SWCC) has traditionally been represented using equations whose fitting parameters do not individually correspond to clearly defined soil properties or to features of the curve. As a result, unique sets of parameters are often nonexistent, and sensitivity analyses and statistical assessments of SWCC parameters become difficult. In order to overcome these difficulties, a new class of equations to represent unimodal and bimodal SWCCs is proposed. The chosen fitting parameters are the air-entry value, the residual suction, the residual degree of saturation, and a parameter that controls the sharpness of the curvatures. The physical meaning for the soil parameters is discussed for different soil types. A unique relation between each of the equation parameters and the individual features of SWCCs is assured. The proposed equations are fitted to data corresponding to a variety of soil types and a good fit is observed.

The hydraulic behavior of a residual gneiss soil compacted at conditions that produce collapsible characteristics was studied using a triaxial permeameter system that allows independent control of net stresses and matric suction. Wetting paths under differing but constant loads were utilized to reflect field conditions associated with a collapsing soil earth fill dam. The experimental results showed that different stress paths produce different amounts of volumetric collapse and distinct variations in the coefficient of permeability, which at relatively high matric suctions was mainly affected by the increase in degree of saturation. However, as the soil approached saturation, the coefficient of permeability became dependent on void ratio and net mean stress, with the saturated coefficient of permeability being reduced at larger amounts of collapse. The results also suggested that entrapped air in the soil voids may produce a significant reduction in the coefficient of permeability. A constitutive relatio...

Laboratory apparatuses for the analysis of infiltration and runoff enable studies under controlled environments and at reduced costs. Unfortunately, the design and construction of such systems are complex and face difficulties associated with the scale factor. This paper presents the design, construction, and evaluation of a portable rainfall and runoff simulator. The apparatus allows the evaluation of unsaturated soils with and without vegetation cover, under a wide range of simulation scenarios. The apparatus also enables the control of the intensity, size, and uniformity of simulated raindrops for variable surface slope, specimen thickness, and length conditions. The monitoring of the volumetric water content and matric suction and a rigorous computation of water balance are ensured. The obtained results indicate that the automated rainfall generator produces raindrops with Christiansen uniformity coefficients higher than 70%, and with an adequate distribution of raindrop sizes under a range of rainfall intensities between 86.0 and 220.0 mm h⁻¹. The ideal rainfall generator conditions were established for a relatively small area equal to or lower than 1.0 m² and considering rainfall events with return periods of 10 to 100 years.