Much of New Zealand is composed of relatively soft sandstone, mudstone and limestone of Tertiary and early Quaternary age. These are traversed by main transportation routes and rivers of hydro-electric potential and contain reserves of lignite and coal. Problems associated with roading, typically cut slopes which fail by slabbing or slaking, and the long term stability of embankments, have led to the investigation of the engineering properties of these materials in the field and laboratory.

The initial aim of the investigations was to classify the soft mudstones (comprising siltstones and clayey siltstones) and sandstones in engineering terms. Relationships between physical properties, lithology and mineralogy have been investigated to determine those properties that are relevant to the performance of these rocks in roading.

Published systems of classification, principally those for shales in the U.S.A. (Strom et al 1978, Underwood 1967, Wood and Deo 1975) have generally been developed for the use of shales in embankments. The direct application of these classification systems for the soft rocks of New Zealand has not been successful. This paper gives the properties of soft rocks of Miocene to Early Quaternary age from the Central and Southern regions of the North Island.


In New Zealand, engineering geological usage of the term "soft rock" refers to the sedimentary clayey siltstone, siltstone and fine sandstone of Tertiary and early Quaternary age. In their unweathered state these rocks have strengths normally ranging from less than 1MPa to 5MPa and not exceeding 10MPa. This definition excludes non-sedimentary or weathered rocks of comparable strengths. The term shale is not considered appropriate for New Zealand soft rocks because of their lack of fissility and low strengths, with regard to the maximum of 500MPa that has been assigned to shales (Vutukuri et al. 1974).

Lithology and Age

The distribution of Tertiary age sedimentary rocks in New Zealand is given on Figure 1. These rocks were deposited during a marine transgression which commenced in late Cretaceous and early Tertiary times and culminated with the deposition of limestones during Oligocene times. Marine deposition continued during the subsequent regression, although a more complex pattern of sedimentation resulted from crustal downwarping and uplift. Deposition during Quaternary times was largely terrestrial, apart from that in the southern portion of the North Island (Suggate et al 1978).

Siltstone, clayey siltstone and fine sandstone are the dominant lithologies. Sequences of the rocks may vary in thickness from a fey: hundred metres to several thousand metres. In most places tedding is absent or only poorly developed, although locally thick sequences of alternating sandstones are_ siltstones occur.

(Figure in full paper)

The depth of burial has not been great generally ranging from 300 to 2000 metres. Geologically they are referred to as sediments or poorly consolidated rocks. Tectonic deformation has not been severe; dips of bedding generally do not exceed 20°. Jointing is rare and where present the spacing is greater than 1.5 to 2.0 metres.

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