Distribution of geological materials in Lower Hutt and Porirua, New Zealand
Date
1992-12-31Authors
Dellow, G. D.
Read, S. A. L.
Begg, J. G.
Van Dissen, R. J.
Perrin, N. D.
Metadata
Show full item recordAbstract
Geological materials in the Lower Hutt, Eastbourne, Wainuiomata, and Porirua urban areas are mapped and described as part of a multi-disciplinary assessment of seismic ground shaking hazards. Emphasis is mainly on the flat-lying parts of these areas which are underlain by variable Quaternary-age sediments that overlie Permian-Mesozoic age 'greywacke' bedrock. Within the Quaternary-age sediments, the two material types recognised on strength characteristics are:
1) Soft sediments, typically composed of normally consolidated, fine-grained materials (sand, silt and clay), with typical standard penetration values (SPT) of <20 blows/300 mm; and
2) Loose to compact coarser-grained materials (sand, gravel), with SPT values of >20 blows/300 mm.
The total thickness and nature of Quaternary-age sediments in the study areas is described, with particular emphasis on the thickness and geotechnical properties of near-surface sediments. Such sediments are considered likely to have a significant influence on the an1plification and attenuation of ground shaking intensity during earthquakes.
In the Lower Hutt valley, near-surface soft sediments greater than 10 m thick have an areal extent of -16 km2. Such soft sediments underlie much of Petone and the Lower Hutt urban and city centres, and have a maximum known thickness of 27 m near the western end of the Petone foreshore. In the Wainuiomata area, near-surface soft sediments greater than 10 m thick have an areal extent of - 3 km2, and attain a maximum thickness of 32 m. In Porirua, near-surface soft sediments have a maximum thickness of 14 m, cover an area of -2km2, and underlie much of the city centre. In the Lower Hutt valley the total known thickness of Quaternary-age sediments, including near-surface soft sediments is 300 m. Quaternary-age sediments are very much thinner in other areas, with thicknesses of 60 m recorded in Wainuiomata, 10 m in Eastbourne, and 90 m in the Porirua area.