Closing the Loop: Measurement of the Complete Soil Water Retention Curve Between Positive and Negative Pore Water Pressure.

The Soil Water Retention Curve (SWRC) provides critical information in the analysis of hydrological and mechanical systems involving unsaturated soils. The hydrological hysteretic behavior is commonly observed between the drying and wetting states, where the volumetric water content for wetting corresponding to zero suction (θ^w_s) is smaller than the saturated volumetric water content or the volumetric water content at zero suction for drying (θ^d_s). While there are different methods in the literature to obtain the drying and wetting paths of SWRC under negative pore water pressure conditions, there is very little information on the behavior of this curve in the wetting state at moisture contents larger than θ^w_s or when the pore water pressure is positive. However, it is becoming increasingly evident that many phenomena such as rainfall-induced landslides may occur within this positive pore water pressure range. A new method was developed to obtain the complete SWRC including data points in the positive pore water pressure regime; thus closing the soil water retention loop. The existing Transient Retention and Imbibition Method (TRIM) device was modified in order to obtain the complete loop for SWRC: (1) the drying state SWRC, (2) the wetting state SWRC, and (3) the wetting state SWRC when the pore water pressure is positive. This last segment of SWRC is measured by controlling the positive pore water pressure while monitoring the water content variation through a constant flow pump. Tests on silt and sand samples show that the new system can successfully and accurately obtain the complete SWRC.