Building on a steep Blue Mountains site almost always involves cut and fill earthworks, cutting into the slope on the high side and placing that material as fill on the low side to create a level building platform. This is how the majority of the Mountains’ residential housing stock was developed, and it’s still the standard approach for new construction, garages, driveways, pools and major landscaping.
But cut and fill doesn’t work without retaining walls. A cut face left unsupported will erode. A filled embankment unsupported will slip. Retaining walls are not incidental to cut and fill works, they are structurally integral to them.
Quick answer (BLUF)
For any significant cut and fill on a Blue Mountains residential lot, retaining walls on both the cut and fill sides are generally required. The cut-side wall retains the existing slope; the fill-side wall supports the newly placed fill. Engineering input is needed to size both walls correctly and to design the fill placement and compaction specification. BMCC consent is required where earthworks exceed Codes SEPP exempt thresholds, which is most significant cut and fill projects.
The mechanics of cut and fill
When a building platform is created on a sloping site:
Cut side: Existing slope is excavated away, creating a vertical or near-vertical face. Without support, this face will erode, how fast depends on soil type, rainfall and slope angle. A retaining wall on the cut side holds this excavated face in position.
Fill side: Excavated material (or imported fill) is placed beyond the original ground level to extend the level platform downslope. This fill material sits at an angle steeper than the natural angle of repose unless retained. A retaining wall on the fill side holds the fill in place.
Balance: In theory, cut and fill projects are balanced, the material excavated from the cut becomes the fill on the other side. In practice, some excavated material (particularly rock) is difficult to use as fill, and additional imported fill may be needed. Excavated soft material often needs to be removed if it is not suitable for compacted fill.
Why retaining wall design must be integrated with earthworks design
Retaining walls for cut and fill applications are often structurally different from standalone garden terracing walls:
- Fill-side walls are retaining recently placed material that may not be fully consolidated. The lateral pressure from fresh, lightly compacted fill is higher than from long-established undisturbed soil. Engineers typically apply a compaction and time buffer to fill-side wall design.
- Cut-side walls may be bearing against rock or against undisturbed soil, both of which present different design requirements. Where the cut encounters sandstone bedrock, the wall footing transitions from a conventional concrete footing to rock anchors.
- Surcharge loading from the building or structure above is a critical design input. The wall must resist not just the soil load but any additional loads from the building footprint.
An engineer needs to see the full site design, building location, levels, fill depths, surcharge loads, to size the retaining walls correctly. Retaining walls specified in isolation from the earthworks design are frequently over- or undersized.
BMCC consent for earthworks
Under BMCC’s DCP, earthworks beyond specified thresholds require development consent. These thresholds typically include:
- Cut depth exceeding 600mm, 1,000mm (site-specific thresholds apply)
- Fill depth exceeding 600mm
- Earthworks within 40 metres of a watercourse
- Total disturbed area exceeding specified limits
- Earthworks on slopes above specified gradients
- Any earthworks in heritage conservation areas beyond minimal disturbance
For residential development (house construction, pool, shed), the DA for the main structure typically includes the associated earthworks as a component. But earthworks carried out independently of a building DA, for example, site levelling in preparation for a future development, generally require their own consent.
Compaction requirements for fill-side retaining walls
Inadequately compacted fill is one of the most common causes of fill-side retaining wall failure. Fill that has not been properly compacted will settle under load, potentially unevenly, causing differential movement in the retaining wall.
Engineering specifications for compacted fill typically require:
- Fill placed in controlled layers (lifts) of 150-300mm
- Each lift compacted to at least 95% of maximum dry density (Standard Proctor)
- Compaction testing for significant fills
- Unsuitable material (organic soil, material with high moisture content) excluded from structural fill zones
These requirements are not onerous for a professional earthworks contractor but are difficult for untrained owner-builders to achieve reliably. If the fill beneath your retaining wall was not compacted to specification, the wall may experience settlement problems even if the wall itself is well-built.
Drainage for cut and fill retaining walls
Drainage for cut and fill retaining walls has two components:
- Behind the cut-side wall: Standard ag pipe and drainage gravel to intercept groundwater seeping from the cut face and discharge it safely away from the wall.
- Behind the fill-side wall: Drainage within the fill material to prevent water build-up in the fill zone. This is achieved with a drainage layer against the back of the fill-side wall and, for larger fills, sub-drainage through the fill body.
The discharge from both drainage systems must be directed to a safe outlet, typically a stormwater system or a point of discharge well clear of the slope below.
FAQs
Do I need separate DAs for the earthworks and the building?
Usually not, the earthworks are assessed as part of the building DA. However, if you want to carry out earthworks ahead of the building DA (to prepare the site), you may need to lodge a separate earthworks DA first. Confirm the required approach with BMCC before commencing.
Can I use excavated sandstone rock as fill?
Solid sandstone rock is not suitable as general fill because it doesn’t compact to a uniform density. However, clean, crushed sandstone material of appropriate grading can be used as structural fill if it meets the engineer’s specification. Contractor assessment of the excavated rock material is needed before deciding on reuse.
How deep can I cut before I need a geotechnical assessment?
There is no single threshold, the requirement depends on the cut depth, slope angle, proximity to structures, and soil type. In the Blue Mountains, where sandstone bedrock at shallow depth can create unexpected conditions, a geotechnical investigation is recommended for any cut deeper than 1.0m near structures or on slopes.
What happens to the spoil from cut that doesn’t go back as fill?
Excess spoil must be disposed of at a licensed facility. In NSW, disposal of uncontaminated excavated soil at a registered clean fill site is the standard pathway. Your contractor should identify the disposal site and confirm it is licensed to accept the material. Dumping spoil on public land, in bushland or in gullies is illegal.