Automated Borehole Shear Tests (ABST) were performed in a uniform loess to explore the suitability of the test as a rapid in situ technique for characterizing suction stress characteristic curves (SSCC) of unsaturated silty soils. The recently developed ABST device enables automated control of a Borehole Shear Test through use of a data acquisition system, stepper motor and controller, pressure sensors, and an electro-pneumatic pressure regulator. To measure the unsaturated shear strength parameters of loess, ABSTs were performed in a hand-augured borehole at five depths ranging from 0.35 to 2.14 m. For each test depth, partially disturbed samples were retrieved and in situ measurements of volumetric moisture content were made by inserting a moisture probe directly into the soil below the bottom of the borehole. Laboratory tests were performed for measurement of the unsaturated shear strength and SSCC for comparison to field measurements. BST and direct shear test results exhibited close agreement in terms of friction angle, but the BST gave consistently lower values of apparent cohesion. However, the BST failure envelope is shown to be capable of producing suction stresses having the same order of magnitude as those obtained from the slower laboratory tests. With further development, the ABST combined with measurement of moisture content therefore shows promise as a technique for rapid in situ measurement of SSCCs for variably saturated soils.
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