Port Macquarie water supply augmentation geological notes on proposed new Hastings River pumping station site

Abstract

One vertical and one inclined drillhole, sunk to investigate the foundation conditions at the site of a proposed pumping station, encountered rock conditions much as anticipated. A surface highly weathered zone is under- -lain by progressively less weathered zones. More extensive weathering affects appear to be related to the presence of inherently slickensided or (?) sheared - carbonaceous shale units interbedded within a mud stone/siltstone sequence. One major fault or crush zone was intersected, which indicated that specific support provision would be required if such a feature was encountered in any excavation. The rock has been subjected to low grade metamorphism and has had imparted to it an inherent cleavage which is an effective rock defect. Whilst these defects reduce the maximum bearing capacity of the foundation, they would make for easy blasting and mucking of spoil. Both pyrite and calcite are present in the "weathered rock zone and testing of the groundwater pH is recommended before concrete specifications are set.