WHAKATANE LIQUEFACTION CASE HISTORY FROM THE 1987 EDGECUMBE

WHAKATANE LIQUEFACTION CASE HISTORY FROM THE 1987 EDGECUMBE EARTHQUAKE: EXAMINATION OF AN EXTENSIVE CPT DATASET SUPPLEMENTED BY PALEO-LIQUEFACTION INVESTIGATIONS Sjoerd van Ballegooy 1, Sarah Bastin 2, Liam Wotherspoon 3, Rolando Orense 3, Michael Pender 3, Nick Mellsop 1, 3 1 Tonkin + Taylor, 105 Carlton Gore Rd, Newmarket, Auckland 2 Quake. Co. RE, University of Canterbury, Christchurch New Zealand 3 Civil and Environmental Engineering, University of Auckland, New Zealand

Project aims: • Compare predicted liquefaction using standard CPT and SPT based methods with that observed following the 1987 Edgecumbe earthquake • Evaluate reasons for the over prediction of liquefaction from existing geotechnical data, trenching, and detailed insitu geophysical VP and VS testing •

Model Inputs Liquefaction Observations PGA Model (Shaking intensity) • Conditions of Edgecume earthquake derived from observations, GMPE, and ground water models Groundwater Model (Depth to GW) Subsurface Data • Modelled to determine whether wider Whakatane area was predicted to liquefy during the Edgecumbe earthquake

Geomorphic mapping and trenching • Geomorphic mapping to identify variability across the area • Trenching conducted at five sites • Trenches at sites know to have liquefied during Edgecume earthquake revealed lateral spreading fissures • Trenches at sites predicted to liquefy contained no evidence of liquefaction • No pre-Edgecumbe liquefaction features revealed

Geomorphic mapping and trenching • Geomorphic mapping to identify variability across the area • Trenching conducted at five sites • Trenches at sites know to have liquefied during Edgecume earthquake revealed lateral spreading fissures • Trenches at sites predicted to liquefy contained no evidence of liquefaction • No pre-Edgecumbe liquefaction features revealed

Possible reasoning for over-prediction • High Shear resistance • High Hydraulic Conductivity Groundwater Characteristics from Crosshole (Vs. Vp) Testing • Not saturated to deeper than expected P-wave Velocity (m/s) Water table Saturated Depth (m) Sand Characteristics From Trenching

Project Outcomes • Whakatane • Characterize the liquefaction susceptibility in the region • Assist in the assessment of earthquake prone buildings within the CBD • Inform future land-use development decisions. • Add results to the international liquefaction case history database • Improve the understanding of the liquefaction performance of silty soils