A Laboratory Examination of the Undrained Cyclic Shear Behavior of Pyroclastic Sands

Pyroclastic sands derived from intermediary weathering of volcanic rocks and other volcanic deposits have a mineralogy dominated by minerals other than silica. This affects both particle crushability and roughness, which leads to differing intrinsic soil properties. Natural deposits of pyroclastic sands can be found in areas of active and paleo vulcanism, and a famous example is the Shirasu sand of the Japanese island of Kyushu. Specimens of pyroclastic pumice and scoria were milled to a consistent gradation and tested in undrained cyclic simple shear to study their undrained cyclic shear behavior. Mineralogy and elemental composition of the two pyroclastic sands is confirmed post-milling using X-ray fluorescence analysis (XRF). Experiments were conducted at a normal stress of 100 kPa, while applied cyclic shear stress ratios varied from 0.2 to 0.5. Using 3.75% double strain amplitude criteria for liquefaction, the number of cycles to liquefaction was observed for these two materials. Post-cyclic residual strength measurements were also performed. This experimental study furthers the growing evidence in the geotechnical literature that intrinsic soil properties such as critical state friction angle and Bolton’s Q and R parameters are fundamental drivers of undrained cyclic shear behaviors of liquefaction and cyclic softening.