Steep site house in Wollongong: geotechnical, landslip, cut and fill

A new house on a steep block in Wollongong is assessed under Wollongong Local Environmental Plan 2009, where clause 7.6 requires consent for earthworks and lists what council must consider, and under Wollongong Development Control Plan 2009, where Chapter E12 Geotechnical Assessment of Slope Instability sets the rules for land known or suspected to be subject to slope instability. On the escarpment foothills and coastal slopes from Helensburgh and Stanwell Park through Thirroul, Austinmer, Mount Keira, Mount Pleasant and Figtree, the geotechnical report is usually the document that decides the DA, because council will not approve development where the risk to life or property exceeds the acceptable level in the Australian Geomechanics Society’s 2007 landslide risk management guidelines. For builders, the program risk is that the geotechnical investigation, the civil design and the cut and fill strategy all have to be settled before the architectural drawings are final.

Who this is for: builders pricing or programming a new house on a sloping Wollongong block, who need to know which reports council will require, in what order, and how cut and fill and landslip controls shape the design and the construction sequence.

Which controls apply to a steep residential site in Wollongong?

Three layers apply. Wollongong LEP 2009 sets permissibility (dwelling houses are permitted with consent in the residential and conservation living zones), the mapped height under clause 4.3 and floor space ratio under clause 4.4, and the local provisions in Part 7 that matter on slopes: clause 7.6 earthworks, clause 7.8 Illawarra Escarpment area conservation, clause 7.4 riparian lands and clause 5.21 flood planning where a watercourse crosses the site. Wollongong DCP 2009 then supplies the design controls: Chapter B1 Residential Development for setbacks, height and site planning, Chapter E12 for geotechnical assessment, Chapter E19 Earthworks (Land Reshaping Works) for the general limits on cut and fill, and Chapter B6 where the land is inside the Illawarra Escarpment area. Council’s planning rules page links the LEP and the DCP chapters.

The third layer is the National Construction Code. NCC 2022 Volume Two and the Housing Provisions Part 3 (site preparation) and Part 4 (footings and slabs) govern site classification, drainage and footing design, and on a landslip site the footing system will be designed by the structural engineer from the geotechnical report rather than from a standard slab detail.

What does clause 7.6 of Wollongong LEP 2009 require for earthworks?

Clause 7.6(2) states that development consent is required for earthworks unless the work is exempt development or council is satisfied the work is of a minor nature. On a house DA the earthworks are assessed as part of the application. Clause 7.6(3) lists the matters council must consider before granting consent: the likely disruption of or detrimental effect on existing drainage patterns and soil stability in the locality, the effect on the likely future use or redevelopment of the land, the quality of the fill or excavated soil, the effect on the amenity of adjoining properties, the source of fill and destination of excavated material, the likelihood of disturbing Aboriginal objects or relics, and the proximity to and potential impacts on watercourses, drinking water catchments and environmentally sensitive areas.

In practice this means a house DA on a steep site should include a cut and fill plan with existing and finished levels, retaining wall heights and locations, a sediment and erosion control plan and, where the fill is imported, a statement of its source and classification. The common mistake is to lodge architectural sections that show the finished house on a flattened pad without any levels, leaving council to guess the depth of cut. That guess usually becomes a request for information.

Chapter E19 of the DCP sets the general controls for land reshaping, including the depth of cut and fill that council will normally accept for a dwelling. Confirm the current figures against the chapter on council’s website before designing the pad, because the limits and the retaining wall setback rules have been amended and site-specific chapters can vary them.

When does Wollongong DCP 2009 Chapter E12 require a geotechnical report?

Chapter E12 applies to land within the Wollongong local government area that is known or suspected to be subject to slope instability, which council identifies through its mapping of known and suspected landslip areas and coastal geotechnical risk. Section 1 of the chapter explains that slope instability predominantly occurs as rockfall, landslide or debris flow on hillside land after prolonged or intense rainfall, and along the coast through wave action. Section 4.1 deals with identification of land that may be subject to slope instability, and section 5.1 sets out the development to which the policy applies.

The chapter adopts the Australian Geomechanics Society Practice Note Guidelines for Landslide Risk Management 2007 (AGS 2007), together with the AGS guidelines on susceptibility, hazard and risk zoning and the Australian GeoGuides for slope management, as reference documents. Section 3 defines acceptable risk: for loss of property it is low or very low on the AGS 2007 risk matrix; for loss of life it is one order of magnitude lower than the tolerable risk for the person most at risk. A site assessed at moderate risk is not necessarily lost, but section 5.3 requires the measures described in AGS 2007 to be adopted, and council will not approve a DA where the risk cannot be brought within the acceptable level.

Section 5.2 sets the requirements for the geotechnical report itself, which must be prepared by a geotechnical engineer or engineering geologist meeting the qualification, experience and professional indemnity insurance definitions in section 3 of the chapter. Section 5.4 (structural design) and section 5.5 (final certification) carry the requirements through construction, and the chapter’s Form M11 Geotechnical Declaration and Verification accompanies the DA. Builders should note that the final certification is a geotechnical sign-off of the works as built, not just an engineer’s structural certificate.

What should the geotechnical report and site analysis contain?

A geotechnical report for a steep Wollongong site that will satisfy Chapter E12 normally covers the following, and the site analysis required by Chapter B1 and, in the escarpment, Chapter B6 section 10.1 should be prepared from the same survey:

1. A detail and level survey with contours, existing structures, retaining walls, trees, drainage lines, seepage and any evidence of past movement (tension cracks, hummocky ground, leaning trees or poles).
2. Desktop review of the regional geology, council’s known and suspected landslip mapping, previous investigations in the locality and aerial imagery.
3. Subsurface investigation (test pits or boreholes) to establish the soil and rock profile, the depth to bedrock and the groundwater conditions across the building footprint and the cut faces.
4. A landslide risk assessment to AGS 2007 for the existing site and for the site with the proposed development, for both property and life, stating the risk level for each identified hazard.
5. Recommendations on footing type and founding depth, maximum unsupported cut heights, retaining wall design parameters, batter slopes, subsoil and surface drainage, and any restrictions on filling.
6. Construction phase requirements: staging of excavation, temporary support, inspection hold points and the certification the geotechnical engineer will provide at completion under section 5.5.
7. A completed Form M11 signed by the engineer and referenced to the DA drawings.

Council’s engineers read the risk assessment table first. A report that provides a site classification for footing design but no AGS 2007 risk assessment does not satisfy Chapter E12 and will be returned.

How do the escarpment and subdivision chapters change a house DA?

Where the land is inside the Illawarra Escarpment area mapped under clause 7.8 of the LEP, Chapter B6 Development in the Illawarra Escarpment applies in addition to Chapter B1. Section 9 of Chapter B6 states that a geotechnical report will be required for the majority of development within the escarpment, except where previous investigations have conclusively shown a site is not subject to slope instability, and that the report must be prepared to the geotechnical guidelines chapter in Part E. Clause 7.8(3) of the LEP prevents consent unless council is satisfied the development is located to minimise adverse impact on the natural features of the escarpment and incorporates conservation and rehabilitation measures on the land. Chapter B6 section 10.1 makes site analysis the first step in design in the escarpment, which is good practice on every steep site.

Where the block was recently created, Chapter B2 Residential Subdivisions may have fixed a building envelope on the plan of subdivision. Chapter B2 requires a building envelope, exclusive of the Chapter B1 setbacks, where a lot carries inherent constraints such as flooding or geotechnical constraints or significant vegetation, and a 15 metre by 10 metre envelope for battle-axe lots in R2. If the house is proposed outside that envelope, the DA has to justify it and the geotechnical report has to cover the new location.

How should a builder sequence a steep site DA in Wollongong?

The order that avoids redesign is: survey first, geotechnical investigation second, cut and fill strategy and civil design third, architecture fourth. On a landslip site the geotechnical engineer’s recommendations on maximum cut heights, retaining wall parameters and drainage will drive the floor levels, and a house designed before those are known is usually redesigned. Council’s assessment will involve its own geotechnical engineer reviewing the report, so allow time for that review and for any request for additional investigation.

During construction, Chapter E12 requires the geotechnical engineer to remain involved through inspection hold points, structural design coordination under section 5.4 and final certification under section 5.5. The common program mistake is to treat the geotechnical report as a lodgement document only, then discover at the construction certificate stage that the consent conditions require the geotechnical engineer to inspect every cut face before shotcrete or retaining walls are built. Price those inspections and the certification in the tender.

Stormwater is the other steep site item. Clause 7.6(3)(a) of the LEP puts drainage patterns first on council’s list, and on a hillside block the discharge point, the interception of upslope runoff and the subsoil drainage behind retaining walls all need to be designed together. Where there is no gravity discharge to the street, the civil design needs to resolve it before lodgement; council will not condition it away.

Definitions

Slope instability

Movement of soil or rock on sloping land, including rockfall, landslide and debris flow, as addressed by Chapter E12 of Wollongong DCP 2009.

AGS 2007

The Australian Geomechanics Society Practice Note Guidelines for Landslide Risk Management 2007, adopted by Chapter E12 as the reference for assessing and accepting landslide risk.

Acceptable risk

Under Chapter E12, a risk to property that is low or very low on the AGS 2007 matrix, and a risk to life one order of magnitude lower than the tolerable risk for the person most at risk.

Earthworks

Excavation or filling of land; clause 7.6 of Wollongong LEP 2009 requires consent for earthworks that are not exempt or minor.

Form M11

Wollongong City Council’s geotechnical declaration and verification form lodged with a DA on land subject to Chapter E12.

Illawarra Escarpment area

Land shown on the Illawarra Escarpment Map under clause 7.8 of Wollongong LEP 2009, where Chapter B6 of the DCP also applies.

URBA prepares Statements of Environmental Effects for houses and knock-down rebuilds on steep and escarpment sites across Wollongong and the Illawarra, coordinating the geotechnical, civil and architectural inputs so that clause 7.6 and Chapter E12 are addressed at lodgement. See our Statement of Environmental Effects service and our Wollongong town planning page; for stormwater and retaining wall civil design our sister company Contrive Consultants works alongside the geotechnical engineer. To get started, request a fee proposal.