Porcellanite and the urban geology of Darwin, Northern Territory

Porcellanite is a siliceous duricrust which has developed within altered Cr etaceous mudrocks in the Darwin city area and is well exposed in coastal cl iffs to the north. Its distinctive geotechnical properties include low bulk density, variable strength that is highly dependent on moisture content, a nd relatively high (but inconsistent) durability. The porcellanite rock mas s is inhomogeneous and anisotropic: it is unrippable at the surface, but be comes weaker with depth. It is also highly permeable in places. even karst- like, due to solution cavities. These characteristics are common to all dur icrusts and result from processes of solution, replacement and redeposition by silica-laden groundwater. Such processes may have been intermittently a ctive through most of the Cenozoic and there is evidence that they continue to the present. The upper 2-4 m of the porcellanite profile is made up of a brittle, high-strength rock with a silica content approaching that of sil crete. The underlying altered and porous siltstone is much weaker and defor ms plastically under loading, due to a cellular microfabric composed largel y of opaline silica replacing clay minerals. Porcellanite has long been use d as the main building stone in Darwin and is now quarried for shoreline fi lling. Although some of it is of select fill or road sub-base quality, crus hed porcellanite contains an excess of plastic fines making it unsuitable f or basecourse. Porcellanite has been a failure as breakwater stone, despite producing blocks of adequate size.