The shear strength and dynamic shear stiffness of some New Zealand volcanic ash soils

A significant proportion of the surface deposits of the central and upper half of the North Island of New Zealand consists of volcanic ash soils. Given that New Zealand is situated within an active seismic region, the dynamic properties of these soils are an important geotechnical earthquake engineering consideration. A systematic investigation of three volcanic ash sites was performed to help characterise the dynamic properties of these soils. In situ down-hole and cross-hole seismic tests produced small strain shear modulus (G(max)) values ranging between 17 and 300 MPa, with excellent agreement between the two test methods. Bender element and free vibration torsion testing of specimens trimmed from high quality samples produced G(max) values which were 28% to 75% lower than those measured in the field. This disparity was considered to result from negative pore pressures potentially present in the field and irreversible changes in the structure of the soil that occurred during removal of the soil from its in situ stress environment. Normalised reduction curves (G/G(max) versus shear strain) produced results which were comparable to those presented by other researchers for cohesive soils, though the influence of the plasticity index on the form of the reduction curves was observed to be negligible. Investigation of the G(max)/s(u) relationship from field investigations of volcanic soils produced average values that ranged between 560 and 890.