Geochemical and Compaction-Breakage Characteristics of Eathered Air-Fall Tephras

The North Island of New Zealand is a region of high volcanic activity with eruptions occurring every few years. Past events suggest that future volcanic eruptions may produce considerable amounts of air-fall tephra or tephric soil deposits (both ash and granular), which raises issues of disposal and storage. The use of such deposits in geotechnical applications is a viable solution, but it requires a full understanding of their physical, geochemical, mineralogical and mechanical properties in their fresh and weathered state. That is, when tephra deposits undergo weathering, their grains become more crushable, compromising their mechanical response. Aimed at providing useful insights, in this study, selected heterogeneous weathered tephras from Rotorua and Auckland regions were characterized. Specifically, compaction-induced particle breakage was evaluated by means of standard Proctor compaction test and sieve analyses, while elemental and mineralogical properties were attained by X-ray Fluorescence Spectroscopy (RF) and X-ray Diffraction (RD) techniques. The test results confirmed that the higher the degree of weathering is, the higher the degree of particle breakage is. et, the degree of weathering and, thus the associated particle breakage, is linked to the quartz and feldspars contents. It was seen that reworked volcanic deposits, having high silica (quartz) content and/or low feldspars content due to their mode of deposition, were generally more weathered and showed higher particle breakage.