Suction monitored apparatus to investigate shear strength of unsaturated granular material at low (total) stress

Rainfall-induced shallow landslides have failure surfaces typically at depths of 1-2 meters. Depending on the organic matter content of soil, the bulk soil density is in the range of 1,000-2,000 kg/m3, this means that the vertical total stresses at the failure surface fall in the range of 10-40 kPa. Conventional geotechnical laboratory apparatuses (direct shear box, triaxial cell) are not suited to accurately test specimens under such low total stresses. The Unconfined Compression (UC) test is a promising alternative for quick and simple testing at low total stress. However, conventional UC testing does not allow for a full characterisation of unsaturated shear strength because volumetric strain and suction are not monitored. Both factors are crucial to understand and model the shear strength behaviour of unsaturated granular soils. Here we develop a novel unconfined compression experimental set-up with volumetric strain monitoring via photo capturing and image processing and in situ suction monitoring using a METER Group T5x low-capacity tensiometer in constant water content tests. Silty sand specimens were tested using this novel apparatus at different strain rates to test whether suction propagates sufficiently fast to be captured by the tensiometer placed at one end of the specimen.