Detailed engineering geological investigations of ground formed by greywacke sandstone and interbedded argillite have been carried out for the construction of the Wellington Urban Motorway. The rocks have been indurated, closely jointed and faulted, and subsequently weakened to depths of many metres by weathering.
Petrological studies of the sandstone show that the weathering breakdown occurs principally through development of very fine-grained secondary minerals in joints and microcracks and along inter-grain boundaries, and that the destruction of larger particles and of the microtexture of the rock occurs only in the most advanced stages of weathering.
Clay-mineral analyses suggest that in the area studied the amount of illite increases with weathering up to a certain level (about 7–8 m below the surface). Near the surface, however, halloysite is the main clay mineral and is abundant in completely weathered rocks. Most of the halloysite probably results from breakdown of plagioclase feldspar, but some may also come from alteration of illite.
These studies should lead to a better understanding of the physical properties of weathered greywacke sandstone, which has been the subject of extensive geotechnical testing in recent years.
Wellington City is underlain in many places by soil derived from intensely weathered greywacke sandstone, and major civil engineering works in recent years have necessitated.. detailed studies of the engineering properties of the weathered materials (Pender, 1971, 1980; Raisbeck, 1973; Martin and Millar, 1974). Petrographic and mineralogical investigations, initiated partly in relation to the work of Martin and Millar, were undertaken in an attempt to understand the weathering process. The present paper is a preliminary account of features observed in sequences of weathered sandstone encountered during construction of the northern approaches to the Terrace Tunnel of the Wellington urban motorway, notably at a cut slope in Shell Gully and in a drill-hole in Bolton Street (Fig. 1).
(Figure in full paper)
The bedrock of the Wellington district is indurated grey sandstone, interbedded in many places with finer-grained rock, argillite, consisting of siltstone and mudstone. Geologists and others have long used the name greywacke in New Zealand not only as a collective term to include the sandstone and the coeval rocks closely associated with it, but also in a specific sense, for the sandstone itself. To avoid possible confusion, the terms greywacke sandstone or greywacke-type sandstone have been commonly employed for the latter. Details of the distribution, petrography and chemistry of the sandstone and accompanying argillite and other rocks have been given by Reed (1957), Kingma (1967) and Grant-Taylor (1976). Over much of the Wellington district the sandstone is interbedded with argillite in pairs of graded beds, each pair (sandstone/ argillite) ranging in thickness up to about one metre. Massive beds of sandstone up to several hundred metres thick are found in a few places (Reed, 1957: 12; Grant-Taylor, 1974: 13). Massive beds of argillite are also rare. Jointing of the rocks is widespread and may locally be intense, more especially near major faults.
