Monotonic Drained and Undrained Shear Strength Behaviour of Compacted Weathered Airfall Tephras

The use of weathered airfall tephra deposits for geotechnical applications such as backfilling in embankments or foundations requires investigation. As a part of an experimental laboratory investigation addressing this issue, this paper focuses on the monotonic shear strength evaluation under drained and undrained shearing conditions of three airfall tephra deposits—namely Kaharoa (white–grey and golden brown) and Maungataketake (black-grey) ashes belonging to New Zealand. The shear strength results include tephra samples compacted at 90% degree of compaction. It is found that the geotechnical characteristics of such tephra deposits generally varied with the mode of deposition, resulting particle sizing and mineralogy of the tephra deposits. For instance, the white–grey Kaharoa tephra (closer to the vent) is primarily a granular material showing a dilative response (i.e., increases in volume in drained conditions and generates negative pore pressure in undrained conditions) and higher friction angle (ϕe ≈ 38–43°) at the end of test; on the other hand, the golden Kaharoa and black-grey Maungataketake ashes are comparatively fine-grained materials (away from the vent and reworked) showing a contractive response (i.e., decrease in volume in drained conditions and generate positive pore pressure in undrained conditions) and possess lower friction angles (ϕe ≈ 34–36°). The obtained friction angles 34–43° of the tephras are comparable or even greater than those of hard grained sands (ϕ ≈ 30°) making them applicable for geotechnical applications. Under large strain levels, the specimens seemed to be approaching critical or steady state of deformation, with breakage of particles during shearing and progression towards less resistant soil structure allowing deformations at constant shear stresses.