Welcome To Geotechnical Engineering In South-East Queensland “Soft Clay One Day, Hard Rock The Next”

South-East Queensland is loosely defined as the area from the NSW border in the south to Gympie in the north and inland to the Great Dividing Range. The main population centres include Brisbane, Ipswich, and the Gold and Sunshine Coasts. It is a geologically complex area and consequently presents the geotechnical newcomer with a wide range of geological environments and their associated challenges.

In South-East Queensland we are fortunate to be well supplied with detailed geological maps, notes and texts from the Queensland Division of the Geological Survey of Australia, and this is the best place to start in order to understand the local geotechnical conditions. The publications available not only describe the detailed superficial geology of the area, but also, through a series of notes and texts, provide a clear outline of the geological development, rock types and stratigraphy. A simple text, which provides a very thorough geological introduction for the newcomer, is “Rocks and Landscapes of Brisbane and Ipswich” by W Willmott and N Stevens (Geological Society of Australia, 1992). Similar texts are available for other areas through Southern and Eastern Queensland. The other essential geological information available for the area is the1:100,000 and 1:250,000 geological maps. A very good series of maps, now no longer available, but still held and jealously guarded by many practitioners, is the 1965 1:31,680 series. Maps, memoirs and books can be obtained from the Queensland Division of the Geological Society by contacting them on [email protected].

To complement the geological maps, there are also soils maps available. The Department of Natural Resources and Mining, Qld (DNR) published two Acid Sulfate Soils Maps at 1:100 000 scale, which show the distribution of potentially acid sulfate soils along the coast between Tweed Heads and Noosa. The DNR also publishes several soils, vegetation and terrain maps which can be of use. These can be obtained via their website at www.dnr.qld.gov.au.

In hard rock engineering jobs, such as deep excavations in rock, rock slope stability or road cuttings etc, the rock type generally dictates the investigation and design approach. The strong, massive and widely jointed Brisbane Tuffs lend themselves to deep excavations and steep slopes, with dominant jointing patterns dictating failure modes. Support measures such as bolting and anchoring are typically adopted when necessary. The highly foliated metasediments of the Neranleigh-Fernvale beds and the related Bunya Phyllite pose different failure mechanisms and support approaches. Foliation directions and the extent of weathering are critical, and minor block and wedge failures are much more prevalent. Rockfall mesh, flatter slope angles and surface protection are typical support approaches, in addition to bolts and anchors. The Tuff–Phyllite interface is of particular importance, and clay seams, carbonaceous material and paleo- soils have all been encountered at these locations. Further afield, the Mesozoic sedimentary beds, and also the younger Tertiary sedimentary and volcanic rocks cover a large area of the south-east. These require a different geotechnical approach, with generally deeper weathering profiles and larger variations in jointing, bedding and strength.

The stability of natural slopes throughout the south-east is of concern in many areas. Consequently, engineering works routinely require special attention. The steep slopes of the Main Range from the Gold Coast Hinterland, through to Toowoomba and the D’Aguilar ranges, and behind the Sunshine Coast, have always been prone to stability problems. The south-east’s sub-tropical rainfall, characterised by intense summer storms, contribute to instability in these areas. Other areas of inherent instability occur, typically but not always due to localised steep terrain and concentrated storm runoff. In the Brisbane suburb of Oxley for example, the weathering of the underlying rocks of the Oxley Group has resulted in an area that has experienced numerous large landslips, although the natural slopes are no steeper than many other stable areas within Brisbane.