Sydney's geology presents a unique challenge for deep excavations. Under the CBD, a thin layer of Quaternary alluvium overlies the Hawkesbury Sandstone, but along the Parramatta River and in the inner west, you'll find the Ashfield Shale with interbedded sandstone lenses. Our diaphragm wall design for Sydney follows AS 4678-2007 specifically for earth-retaining structures, and we always calibrate our numerical models against local SPT data from adjacent sites. We've seen too many designs fail because they used generic soil parameters instead of Sydney-specific stiffness profiles. Before we start, we typically run a [MASW survey](https://sydney.sondajespt.com/masw-vs30) to map the bedrock profile across the site.
In Sydney, the interface between the Hawkesbury Sandstone and the Ashfield Shale is where most diaphragm wall failures originate – the transition is often a slickensided bedding plane.