Modified Unconfined Compression Testing System to Characterize Stress–Strain Behavior of Unsaturated Soils at Low Confining Stresses

Abstract

In geotechnical and pavement engineering, many projects involve shallow, unsaturated soil with low confining stress. Suction-controlled triaxial tests usually are used to investigate stress–strain behavior (i.e., deformation and strength) in such soils. However, because they require significant modifications to the conventional apparatus used to test saturated soils and are time-consuming to perform, the tests cannot be justified for time-sensitive engineering projects. A modified unconfined compression testing system is presented to investigate the stress–strain behavior of unsaturated soils at low confining stresses. Negative air pressure (i.e., vacuum) provides the low confining pressure required for the tests, and high-suction tensiometers are used to monitor variations in the soil matric suction during testing. A photogrammetric method is used to reconstruct the three-dimensional model of unsaturated soil specimens, from which total and localized volume changes are calculated. Methods are proposed for unsaturated shear strength analysis. A series of tests was carried out to demonstrate the use of the modified unconfined compression testing system to evaluate the stress-strain behavior of unsaturated soil under different confining stresses. Results indicated that the newly developed modified unconfined compression testing system was a cost-effective method for the rapid evaluation of stress–strain behavior of unsaturated soils under low confining stresses.