STRENGTH OF HILLSLOPE MATERIALS IN A STEEP ZERO-ORDER CATCHMENT, ELLIOT STATE FOREST, OREGON COAST RANGE

We describe results from material strength tests performed on colluvial soil collected in bulk from a steep (35 - 40°) instrumented catchment in the Elliot State Forest, Oregon. Tensile and shear strengths were determined as functions of moisture content and soil suction, respectively. These relations were determined using testing apparatuses that were developed or modified to operate over the range of normal stress (0 - 300 kPa), soil moisture contents (0 - 100% saturation) and matric suction (0 - 300 kPa) suitable for stability assessment of shallow landslides, which are typically less than 3 m thick.

Soil tensile strength may have impacts on the stability of many steep slopes. It controls potential crack development in hillside materials and tensile strength test results provide an indication of the soil’s ability to sustain an external tensile stress without breaking. The tensile strength of soil as a function of moisture content was measured using an apparatus that consists of a specimen tube (diameter of 6.1cm) that can be put under tension by an automated loading system (Geojac) until the soil specimen fails.

Shear strength as a function of soil suction was measured using a direct shear apparatus modified for suction control. The soil specimen was placed on a high air-entry porous membrane in place of the more commonly used porous stone. Pore water pressure was maintained at a lower pressure than the air pressure by axis translation through the membrane, thus allowing matric suction to be controlled and maintained. The primary advantage of the membrane is that the time needed for pore pressures to reach equilibrium is reduced. We conducted a series of tests on samples with controlled porosities and water contents and a tensile strength characteristic function and failure envelopes with different degrees of vertical loading and matric suction were obtained for the hillside material. We then reconciled the measured tensile strength characteristic curve and shear strength dependency on matric suction using a unified effective stress theory.