Case Study: Forest Reach Residential Development – Managing Coal Washery Reject for a Safe, Stable and Compliant Community

Clients: The Forest Reach Residential Development

Overview

The Forest Reach Residential Development near Wollongong represents one of the most complex ground engineering challenges in the region: transforming a former Coal Washery Reject (CWR) emplacement—approximately 1.5 million cubic metres of industrial by-product—into an 85-hectare masterplanned community with around 800 residential lots, open space, and retirement living precincts.

Douglas Partners (Douglas) was engaged by Stockland to review decades of existing investigations, identify critical data gaps, and deliver a safe, practical, and regulator-acceptable earthworks and ground improvement strategy. Through meticulous analysis, advanced modelling, extensive fieldwork and laboratory testing—including the establishment of a dedicated combustibility testing laboratory—Douglas helped establish a clear and achievable pathway to development compliance and construction readiness.

This case study summarises the site history, challenges, methodologies and outcomes from Douglas’ multi-stage involvement in this coal-affected site.

1. Background: A Site Defined by Coal Washery Reject

What is Coal Washery Reject?

Coal Washery Reject is the coarse to fine by-product remaining after coal is washed and processed for market. These materials contain varying proportions of coal, carbonaceous shale, and non-combustible waste. Historically, reprocessing CWR into a saleable product was cost-prohibitive. As a result, large emplacements were created throughout the Illawarra, particularly prior to the introduction of regulatory levies.

At Forest Reach, approximately 2.5 million tonnes (1.5M m³) of CWR was placed over several decades beginning in the early 1970s. The material was heterogeneous and potentially combustible, with previous investigations reporting combustible contents between 20% and 70%, well above the Wollongong Development Control Plan (DCP) requirement of <30% average and <40% maximum combustible content.

A Complicated History

From 1961 to 2011, the site evolved through agriculture, dam construction, ongoing emplacement, and eventually capping. Numerous consultants had previously investigated the land—with varying methodologies, assumptions and data quality. Regulatory approval had been attempted before and rejected.

This created a technical and approval environment that required:

A fresh, independent review,
A robust, defensible ground model,
Clear strategies aligned with council, EPA and planning expectations,
And a practical way to treat millions of cubic metres of CWR to meet DCP combustibility limits.

2. Douglas Partners’ Engagement: Reframing the Problem

Stockland appointed Douglas Partners to undertake a comprehensive data review, identify missing information, and develop an evidence-based earthworks and risk-management strategy.

Key Questions Driving our Early Work

Who is the consent authority, and what information will they accept?
How do we navigate the previous development refusal?
How can we construct safely on an emplacement of variable, compressible, partially combustible material?
What treatment methods are feasible at this scale (≈1.5M m³ CWR + 900,000 m³ inert fill)?
How do we accurately assess and manage combustibility to meet statutory thresholds?

Our early review focused on:

Filling data gaps,
Reassessing settlement and ground behaviour,
Understanding variability in combustibility,
Exploring ground improvement options,
And strengthening the approval pathway through robust modelling and transparent communication with council.

Early Douglas Investigations

3. Settlement, Compressibility and Ground Behaviour

Given the industrial fill history and layering of natural soils beneath the emplacement, Douglas undertook a detailed program of:

New fieldwork and sampling,
Laboratory one-dimensional consolidation testing,
Additional Cone Penetration Testing (CPT),
Review and reinterpretation of legacy CPT data,
And multi-model settlement analysis.

Advanced Modelling Tools Used

Consolidation modelling,
T-Rex modelling,
Waddell & Wong creep modelling.

Key Considerations

The ground behaviour model needed to capture:

Unloading and reloading of natural soils,
Primary settlement and secondary creep,
Internal compression of new engineered fill,
And the impacts of blending methods and placement thicknesses.

These insights allowed us to refine feasible construction thicknesses, pad configurations, long-term performance expectations and future building tolerances.

4. Combustibility: A Critical Constraint for Development

Under the Wollongong DCP, CWR used as fill must comply with strict combustibility limits. Earlier consultants had conducted limited or inconsistent testing, and the existing dataset showed extreme variability.

Douglas’ response was to adopt a rigorous, statistically defensible approach:

13 new boreholes,
Near-continuous SPT sampling,
Laboratory combustibility testing by Bureau Veritas,
Integration of historical and new data (205 tests—approx. 1 per 12,000 tonnes of CWR).

Modelling Combustibility at Scale

We applied a 1.5-metre depth-slice statistical model, allowing the team to spatially map combustibility.

We recommended:

A design target of 27.5% combustible content, providing compliance margin,
And inert blend ratios designed to achieve site-wide conformity.

This modelling became the foundation for earthworks planning and council engagement—clarifying achievable compliance and demonstrating robust risk management.

5. Rock Depth Investigations

To optimise earthwork efficiency and reduce over-excavation, we undertook targeted rock depth investigations to profile the non-rippable surface beneath the emplacement.

Improving the accuracy of this model enabled:

More precise volume estimates,
Better cut-to-fill balancing,
And reduced cost and construction risk.

6. Large-Scale Earthworks and Blending Trials

Given the unprecedented scale—2.4 million cubic metres of blended fill, 4,000+ compaction tests, and 10,000 combustibility tests— Douglas Partners supported Stockland and the contractor through detailed earthworks trials.

Methods Tested

Dozer and compactor blending (conventional),
Rotary spader (tractor-mounted),
Vermeer surface miner,
Varying layer thicknesses and pad configurations.

These trials allowed the team to evaluate:

Blending uniformity,
Production rates,
Safety considerations,
And in-situ combustibility outcomes.

A 60-Week Earthworks Program

The program was designed around a maximum production rate of 12,000 m³/day, requiring streamlined testing logistics.

7. Establishing a Dedicated Combustibility Laboratory

Douglas developed inhouse procedures for the sampling and testing of the combustible content for blended CWR. These procedures were reviewed and approved by Council (i.e. the approval authority). Due to the volume and turnaround speed required, Douglas Partners’ Wollongong laboratory created a dedicated combustibility testing setup, including:

Jaw crushers,
Pulverisers,
Thermogravimetric analyser (TGA),
Strict sample-drying procedures (<40°C to avoid loss of combustibles).

Due to drying constraints, each combustibility test took up to 72 hours to complete from the time of sampling, and any non-conforming result triggered re-blending and re-testing to maintain compliance.

This laboratory capability became a cornerstone of project success and a demonstration of Douglas’ commitment to practical engineering solutions at scale.

8. Continuous Improvement Through Collaboration

Throughout construction, we provided:

Ongoing audits,
Construction phase advice,
Review of contractor methods,
Identification of operational improvements,
And updates to blending, compaction, and sampling strategies.

This iterative approach helped optimise:

Safety,
Productivity,
Compliance,
And long-term ground performance.

Our advice also extended to addressing challenges such as:

Non-uniform CWR pockets,
Moisture sensitivity,
Removal of non-conforming materials,
And enhancing contractor blending techniques.

9. Outcome: A Compliant, Build-Ready Platform for a New Community

Today, the Forest Reach site presents as a stable, engineered, DCP-compliant platform, ready to support residential development. The combination of advanced modelling, rigorous testing, high-volume earthworks management and practical engineering advice has enabled Stockland to unlock a site previously considered too risky or complex to develop.

Douglas’ involvement helped:

De-risk approval pathways,
Establish council confidence,
Optimise earthworks planning,
Achieve combustibility compliance,
And deliver a safe, predictable foundation for future infrastructure and homes.

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