Measuring the Soil Water-Retention Curve Under Positive and Negative Matric Suction Regimes

The soil water-retention curve (SWRC) is commonly theorized and measured in the positive matric suction or negative pore water pressure domain. However, all soils do not reach full saturation when matric suction decreases to zero. To date, few methods or theories have been developed to understand the SWRC in the negative matric suction domain, which could play important roles in field mechanical stability conditions, such as slopes under heavy rainfall or levees under rapidly rising water table conditions. A method employing both the transient water release and imbibition method (TRIM) and a constant flow method (CFM) is devised to measure a soil's complete loop of the SWRC under both wetting and drying, and positive and negative matric suction conditions. Although the TRIM is used to measure both drying and wetting paths of the SWRC in the positive matric suction domain, the CFM is used to quantify the soil water-retention behavior in the negative matric suction domain. The TRIM method has been previously validated and extensively tested. The novel feature is the cyclic application of the CFM in the negative matric suction domain. The head loss in the high air entry ceramic stone because of the application of the CFM is calibrated in the range of the applied flow rates. Similar sets of flow rates are used to validate the repeatability of the measured SWRC behavior in the negative matric suction domain. Three different soils, sandy, silty, and clayey soils, are used to demonstrate the applicability of the methodology for various soil types. It is shown that, for the sandy soil, a few kPa of negative matric suction are needed to fully saturate the specimen, whereas for the clayey soil, over 10 kPa of negative matric suction are needed to fully saturate the specimen.