Site characterisation and liquefaction potential of Blenheim gravelly sandy deposits

Abstract

Gravelly soils are a very common feature of the New Zealand’s geological setting. From a geotechnical viewpoint, however, such soils are considered problematic being difficult to characterise and because their liquefaction potential is largely unknown.

Contrary to the general belief that gravelly soils do not liquefy, case histories from at least 27 earthquakes worldwide have indicated that liquefaction can actually occur in gravelly soils (either natural deposits and manmade reclamations) causing severe damage to land and civil infrastructures. Three of such case histories are from New Zealand and include the 1929 Mw7.6 Murchison earthquake, the 2010 Mw7.1 Darfield earthquake, and the 2016 Mw7.8 Kaikoura earthquake. The latter case history refers not only to the well-known Wellington CentrePort’s gravelly reclamations; in fact, following the 2016 Mw7.8 Kaikoura earthquake, surface liquefaction features were observed also in Blenheim, at sites underlined by loose alluvial sandy gravel deposits at shallow depths.

The main objectives of this EQC project were (1) to identify and use reliable and cost-effective field techniques for properly characterise, from a geotechnical viewpoint, typical New Zealand alluvial gravelly soil deposits, and (2) assess their liquefaction potential using adequate liquefaction triggering procedures for gravelly soils. To this scope, three relatively close sites where gravelly soils liquefaction manifestation (i.e. soil ejecta and settlement) was observed or not in Blenheim during the 2016 Mw7.8 Kaikoura earthquake, the 2013 Mw6.6 Cook Strait/Seddon earthquake and the 2013 Mw6.6 Lake Grassmere earthquake were carefully selected and characterised.