ABSTRACT
In the Sydney area, the Narrabeen Strata are generally covered by the Hawkesbury Sandstone and the Wianamatta Shale. At the Mangrove Creek damsite, Narrabeen rocks form the valley sides and lie immediately beneath Hawkesbury Sandstone which forms cliff lines at the edge of the plateau. Hence these rocks correspond to the Newport Formation although this formation name was not used at Mangrove Creek. Methylene Blue Absorption tests were conducted at the Mangrove Creek damsite to provide an indication of the proportion and expansive nature of clay minerals within the rock matrix. The Mangrove Creek embankment was constructed from rockfill obtained from a quarry in the Narrabeen rocks. Settlement monitoring during construction showed that the rockfill achieved a mass modulus of about 100 MPa. It was found that the railway tunnel on the Main Northern Railway line some 3 km south of Woy Woy had been excavated mainly in sandstone of the Narrabeen Group, but partly in sandstone of Hawkesbury Sandstone.
