The escarpment guide describes the geology under the Illawarra's steep suburbs: rock, clay and old landslip material. A geotechnical report is the document that turns that general picture into answers for one block.
What is a geotechnical report?
It is an engineer's assessment of the ground under a proposed building: what the soil and rock are, how stable the slope is, where water sits, and what footings, excavation and retaining the site can take. On escarpment land it often includes a formal assessment of landslide risk.
What does a geotechnical report cover?
- Boreholes or test pits, and a description of the soil and rock profile
- Depth to rock, and groundwater observations
- Slope stability and landslide risk, assessed against the Australian Geomechanics Society guidelines
- A site classification for footing design
- Recommendations on footings, excavation depth, retaining, batters and drainage
- Conditions the design and construction must follow, such as inspections during excavation
When does Wollongong require a geotechnical report?
Wollongong's Development Control Plan 2009 has a chapter for it: Chapter E12, Geotechnical Assessment of Slope Instability, which adopts the Australian Geomechanics Society's 2007 landslide risk management guidelines. Land within the escarpment precincts is also covered by Chapter B6, Development in the Illawarra Escarpment, which requires a geotechnical report with development applications there. If your block is steep, mapped, or near a known slip, assume the report will be needed and get it early.
What is site classification?
Every house site in Australia is classified for footing design. The classes are A (sand and rock), S, M and H (increasingly reactive clays), E (extremely reactive) and P (problem sites). Many escarpment blocks are class P because of slope, fill or landslip risk, which means the footings must be designed by an engineer rather than taken from standard tables.
Why should the report come before the design?
Because it can change everything the designer draws: where the house sits, how deep it can cut, whether the floors are on slab or piers, and how high the retaining walls can be. A design drawn first and checked later often has to be redrawn. The report also lets the builder price the footings, excavation and retaining with real information instead of allowances. The options it leads to are compared in suspended slab, piers or cut and fill, and the cost effects in sloping blocks: what drives the cost.
That is why it is worth working with builders who start every steep-site design from the geotechnical report: the price you sign is then based on the ground you actually have. If your block is also near old coal workings, check mine subsidence as well.
Sources
Checked on 25 October 2026.
- Wollongong City Council, DCP 2009 chapter list: E12 Geotechnical Assessment of Slope Instability, E14 Stormwater Management, E16 Bushfire Management, E19 Earthworks.
- Wollongong City Council, DCP 2009 Chapter B6, Development in the Illawarra Escarpment: precincts, geotechnical report requirement and access road limits.
- Australian Geomechanics Society, Practice Note Guidelines for Landslide Risk Management 2007: the guidelines adopted by Wollongong DCP E12.
- Australian Building Codes Board, NCC Part 3.2.4 Site classification: the A, S, M, H, E and P classes.
General information from a builder, not engineering or planning advice. Your geotechnical engineer, structural engineer and certifier decide what your site needs.