Search

A transient water release and imbibitions (TWRI) method for measuring the soil water retention curve (SWRC) and hydraulic conductivity function (HCF) under drying and wetting states is presented. The intellectual merit of the TWRI method is its integration of physical and numerical experiments: it employs the simple and reliable measurement of transient water content by electronic balance to record the signature of transient unsaturated flow in soil sample, and it takes advantage of the robust inverse modeling capability to simulate the physical process. The TWRI method; therefore, has two integrated components: testing and modeling. The testing consists of water release upon two-steps increase in matric suction and water imbibitions upon one-step decrease in matric suction. Each process lasts sufficient time in order to obtain transient water content. The data is then used as an objective function in an inverse numerical modeling process to obtain the unsaturated hydrologic parameters that fully define the SWRC and HCF of the soil. A novel feature of the TWRI method is its capability to measure SWRC and HCF under wetting state. The apparatus can accommodate both undisturbed and remolded samples. The testing time required for completing a full drying and wetting loop is approximately one week for most soils. Validation of the technique has been performed both experimentally and numerically. Comparisons between data obtained with the TWRI method and the independent Tempe cell method verify the reliability, applicability, and accuracy of the TWRI method. Results from three different soils are presented to illustrate the procedure and performance of the TWRI method for different soils from fine sand to clayey silt. The TWRI method provides a fast, accurate, and simple testing tool for obtaining SWRC and HCF of various types of soils under both wetting and drying states with high range of matric suction several orders of magnitude above the air-entry pressure of the ceramic stone used in the experimental setup.

Modifications to a conventional laboratory testing system are described for direct shear testing of unsaturated soils at relatively low matric suction and net normal stress. Matric suction ranging from zero (saturated) to about 10 kPa is controlled using a hanging column assembly (ASTM D6836). Net normal stress ranging from about 0.3 to 10 kPa is controlled by directly applying dead loads to the specimen via a series of aluminum top caps machined to varying thicknesses. Precise control of suction and normal stress within these ranges makes the apparatus ideal for examining the shear strength behavior of unsaturated sands, which are characterized by relatively low air-entry pressures and for which the influences of matric suction on mechanical response can be subtle. Soil-water characteristic curves are concurrently obtained during the shear testing program by measuring transient and equilibrium pore water drainage under the imposed suction changes. Testing procedures and recommended protocols are described. Results from a series of tests using saturated and unsaturated specimens of poorly graded fine sand are presented to demonstrate application and performance of the system. Relationships between shear strength and matric suction are non-linear and exhibit peak shear strength at matric suction within the range of the air-entry suction. High friction angles measured for the portions of the failure envelope at low matric suction and normal stress may indicate the effects of dilation on the strength development.