Weathering of greywacke sandstone, Wellington, New Zealand

Detailed engineering geological investigations of ground formed by greywacke sandstone and interbedded argil!i te 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.