Modified Direct Shear Apparatus for Unsaturated Sands at Low Suction and Stress

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.