Evaluation of sediment properties in the Lower Hutt and Porirua areas by means of cone and seismic cone penetration tests

Abstract

Cone and Seismic Cone Penetration Tests (CPT and SCPT) have been carried out at a total of 101 sites in the Porirua and Lower Hutt regions of New Zealand, initially (from 1975 to 1987) as government-funded public- good science, and later as part of a multi-disciplinary microzoning project under contract to Wellington Regional Council. The shear wave velocities and depths determined in the study were successfully used to predict small-signal resonant frequencies for two sediment-filled basins, and the inferred cohesions were used qualitatively to predict the importance of nonlinearities in limiting ground motion amplification. The major results were the identification and shear wave characterisation of a surface layer (depth 12 m, diameter 600 m) of cohesive flexible sediment in the Porirua basin and a successful prediction of the associated basin resonant frequency, identification and shear wave characterisation of a thick surface layer (depth 30 m, length 2 km, width 500 m) of cohesive flexible sediment in the Wainuiomata basin which explained an observed resonance, identification and shear wave characterisation of a wedge of flexible sediment, cohesive below and uncohesive above, in the lower part of the Hutt Valley, and identification of a pocket of flexible sediment in the Naenae area.