Compressibility and collapse potential of compacted slightly-weathered tephras from the North Island of New Zealand

Abstract

Volcanic ash air-fall or tephra deposits comprise nearly 31% of the North Island of New Zealand. For those belonging to slightly weathered or negligible to little cohesionless nature (silty sands to sandy silts), the compressibility and collapsibility features of compacted tephras have not been largely investigated. Correspondingly, the compressibility and collapse potential (CP) of compacted airfall tephras (at 90% and 100% degrees of compaction) were evaluated. At vertical stresses up to 200 kPa, the consolidation coefficient cv and permeability coefficient k were in the range of 10−5−10−8 m2/s and 10−6−10−8 m/s. Interestingly, for feldspar−silica type cohesionless tephras, it was possible to correlate the weathering degree and mineralogy to the compressibility. The results of the CP tests showed that the CP increased with the decrease in degree of compaction and increase in vertical stress; with values ranging from 0% to 5% indicating low-to-moderate collapsibility of the tephras upon properly compacted placement condition. The compacted tephras, as such, could be considered suitable structural fills for typical geotechnical applications owing to features such as low compressibility, low permeability and low-to-moderate CP.