Development of a Near-Surface SWRC Device (NSD) for Measuring Suction under Low Stress Environments

The soil-water retention curve (SWRC) is fundamental in understanding unsaturated soil behavior and is related to both the unsaturated shear strength and hydraulic conductivity. The SWRC shows that a saturated soil undergoing a drying process will see an increase in matric suction as the water content decreases and vice versa as soil moisture increases. Laboratory devices and techniques used to determine the SWRC operate using uniaxial translation and significant confining pressures to derive matric suction and inferred behavioral characteristics, specifically around the residual suction and air-entry values. Current laboratory data and devices may not adequately represent near-surface soils and their resulting behavior, especially soils at or near atmospheric (zero gage) confining pressures as demonstrated through unconfined self-supported sand sample testing. These tests indicate that failure occurs at approximately 72 % saturation, well below the SWRC air-entry value for the same soil. To correct this shortcoming, a newly designed near-surface SWRC laboratory device (NSD) is presented wherein unconfined self-supporting nonplastic specimens are subjected to three-dimensional vapor flow in a controlled temperature/humidity chamber. The interior specimen humidity and temperature were measured through novel internal microsensors located within the top and bottom third of the specimen, whereas the mass of the pore fluid was measured via an external scale to prevent thermal balancing errors. A parametric analysis of a poorly graded sand was undertaken to compare the NSD results to those obtained from traditional uniaxial translation techniques. The preliminary results from the NSD provide insight into the discrepancies between confined and unconfined unsaturated soil behavior, specifically with the contributions of suction to the granular structure of cohesionless material.