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Detailed cell design documentation in Eton

Detailed cell design documentation in Eton

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SECTOR

Waste

SOLUTIONS

Geo-environmental engineering

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Colin Stapleton

Senior Principal

Overview

Senversa was commissioned by Mackay Regional Council to prepare detailed design documentation for the construction of Cell 4 at Hogans Pocket Landfill, located in Eton, Queensland. The project was initiated to ensure continuity of waste disposal services in the region, with Cell 4 providing approximately 4.6 years of airspace capacity based on current disposal rates.

Scope

The scope of consultancy work included the design of the Cell 4 basal structure, leachate collection and transfer systems, stormwater management infrastructure, and a new perimeter road. The design was informed by extensive site-specific investigations including topography, geology, hydrology, climate data, and historical geotechnical assessments. Regulatory compliance was ensured through alignment with Queensland DES landfill guidelines (2021 and 2023 drafts), Environmental Authority (EA) permit conditions, and development approval requirements.

Key design elements included:

A composite liner system comprising a 2.0 mm textured LLDPE geomembrane, geosynthetic clay liner (GCL), and a compacted clay-rich fill layer.
A leachate collection system with perforated HDPE pipes, a 300 mm drainage aggregate layer, and a sump with a 630 mm riser.
Groundwater separation of ≥2 m across the cell footprint, with seepage performance exceeding DES minimum standards.
Embankment geometry with internal and external slopes of 1V:3H and crest widths of 4–5 m.
Stormwater infrastructure designed for 10% and 1% AEP events, including culverts, drains, and erosion protection.
Safety-in-design features addressing hazardous areas, landfill gas risks, and interaction with heavy vehicles.

Senversa also undertook slope stability modelling, liner seepage assessments, and veneer stability evaluations to ensure long-term structural integrity and environmental protection. The design incorporated provisions for future landfill development and operational flexibility.

Challenge

Key challenges included managing complex site conditions such as shallow rock, historical undocumented earthworks, and variable groundwater levels. These were addressed through targeted geotechnical investigations and adaptive design strategies. The design achieved a minimum 2 m separation to groundwater, mitigating uplift risks and enhancing leachate attenuation.

Hydraulic challenges were resolved through detailed modelling of stormwater and leachate flows, ensuring infrastructure could accommodate peak events without overtopping or environmental harm. The leachate system was designed to handle up to 50,000 L/day, with redundancy built into the pump and pipeline configuration.

Safety considerations were central to the design, with high-risk activities such as excavation near existing services and exposure to landfill gas addressed through detailed construction specifications and risk mitigation plans. Hazardous areas were classified and signage, intrinsically safe equipment, and operational protocols were specified.

Outcome

The Cell 4 design successfully met Council’s objectives for continued waste disposal capacity, environmental compliance, and operational efficiency.

The design provides approximately 440,000 m³ of airspace and integrates with existing infrastructure, including leachate transfer pipelines and sediment basins.

The new perimeter road design extended operational access and reduced future reconstruction costs. Bulk earthworks were balanced to minimise material importation, and erosion protection measures were specified to maintain long-term stability.

Overall, the project delivered a robust, compliant, and future-ready design tailored to the unique conditions of Hogans Pocket Landfill.