Undrained cyclic shear behaviour of weathered tephra

Abstract

Weathered tephra is prevalent across volcanic islands like the North Island of New Zealand and is composed of volcanic airfall materials that have been subjected to various soil processes. Understanding their undrained response to cyclic loading is essential for geotechnical engineering applications in these regions because of frequently occurring local earthquakes. The authors describe for the first time the cyclic undrained behaviour of a weathered, clay-rich and highly sensitive tephra through triaxial tests. The weathered tephra experiences brittle failure and exhibits higher friction than sedimentary clays. Cyclic contour diagrams, covering the whole compressional and extensional range of stress conditions, are used to compare the cyclic shear strength of Pahoia tephra with those derived for sedimentary clays. It is found that weathered tephra: (a) is more resistant to small cyclic loading; (b) fails within a smaller range of cyclic shear stresses; and (c) exhibits a cyclic shear strength that peaks at zero average shear stress, in contrast to sedimentary clays where cyclic shear strength peaks at small compressive average stress.