Engineers Australia

Sydney Sandstone geomaterials – Broken, crushed and friable rock products

Greg McNally and John Whitehouse

Abstract

Sandstone in fragmented form, derived from the Hawkesbury Sandstone, the Banks Wall Sandstone and the Terrigal Formation, is – by geological default – an important and growing source of geomaterials in the Sydney area. Although of only moderate strength and durability, this sandstone breaks down to produce well-graded sand with a soft gravel fraction and low plasticity fines. Crushed sandstone is good to excellent as earthfill, adequate as rubble, but is unreliable as rockfill for dams. It performs well as slope protection stone in embankments and the more durable sandstone is sometimes suitable for marine breakwater stone. It has been used in the past for concrete aggregate, especially in ‘cyclopean masonry’ dams, and is still employed as aggregate to a very limited extent in low-strength backfill and bound sub-base. Crushed sandstone is, however, generally unacceptable for unbound pavement courses because of its high inherent clay content (up to 30% <75 um), water sensitivity and only moderate particle strength. Its dry UCS is typically 10-30 MPa, and only 30-80% of this value when wet. Two important and growing sources of sandstone geomaterials are tunnel spoil (about 2 Mtpa) and quarried friable sandstone (also about 2 Mtpa). The environmental impact of sandstone quarrying is generally positive, in that it substitutes for scarce sources of high value hard rock. Because sandstone is nearly ubiquitous around Sydney, quarries can be sited in areas of low scenic value, require no blasting and can be shaped to productive end-uses. However, sand washing generates 10-30% of clay tailings, which are deposited in slurry lagoons, some of which have collapsed in the past due to inadequate spillway capacity. Although the tailings are a potential source of kaolinite and brick clay, cost-effective methods of de-sanding and dewatering have yet to be developed.