DIRECT SHEAR APPARATUS FOR SUCTION-CONTROLLED TESTING OF LANDSLIDE-PRONE MATERIALS AT LOW STRESS LEVELS

Relations between soil shear strength and suction are needed to apply coupled hydro-mechanical models for shallow landslide prediction. These relations cannot be measured using conventional direct shear apparatus but rather require modifications for suction control and measurement. Axis translation techniques are commonly used to control suction in direct shear tests. However, these techniques are typically valid for suctions of 2 to 1500 kPa and are not well suited for testing coarse-grained materials such as regolith from landslide-prone hillsides. Testing of such materials requires an apparatus capable of manipulating both matric suction and net normal stress at relatively low magnitudes and over a relatively small range. Effective characterization of these materials requires precise control of suctions in the range of the air-entry value, typically less than 10 kPa for coarse-grained soils. Because the range in suction over which drainage occurs is small, the contribution of matric suction to intergranular stress is also small. Thus, control of normal stress on the order of a few kPa is also required.

We describe modifications to a conventional direct shear apparatus for testing granular soils over ranges of suction and normal stress appropriate for shallow landslide modeling. Matric suctions ranging from 0 to 10 kPa are controlled using a hanging column assembly; normal stresses ranging from 0.3 to 10 kPa are controlled by directly applying dead loads to the top of the specimen. In addition, the soil-water characteristic curves for either the wetting or drying directions can be obtained by measuring the volume of water expelled or imbibed as a response to a change in suction. We present results from a series of tests on poorly graded fine-grained glacial outwash sand collected from a steep coastal bluff in the Seattle, WA area. Nonlinearity of the failure envelope was evident, with φ=42.1^o for normal stresses less than 2 kPa, and φ=33.3^o for a higher normal stresses. This behavior depends on the magnitude of suction, with the largest friction angles occurring at suctions near the air-entry value. Definition of relations between soil suction and shear strength for landslide-prone materials are critical for the advancement of coupled hydro-mechanical frameworks used for predicting the occurrence of shallow landslides.