Brownfield landfill: the battle for airspace is won on the ground
In landfilling, airspace is everything. For the waste management sector, ‘airspace’ — a landfill’s capacity to accept waste — equates to revenue.
Because everyone is trying to maximise the remaining capacity of existing landfills, attention is turning to opportunities to claim fresh airspace by siting new landfills on brownfield sites such as disused quarries and mines.
It makes sense to use brownfield sites for the public’s ever-increasing need for landfill. At many sites, pits already exist; however, these airspace opportunities come with risks, and it’s vital to manage these properly for development to proceed, while also protecting the health of our land and water, and the safety of workers and the public.
A brownfield site such as a mining void or quarry brings a set of geotechnical challenges that must be addressed before landfill use can be considered. Underground tunnels and shafts, remnant highwalls, stockpiles of challenging and potentially combustible materials, large volumes of water in former open pits, as well as stability, seepage and safety are just a few of the challenges that must be considered. Yet, each of these challenges can be overcome when innovative and safe designs are applied that take full account of a site’s unique geotechnical features and constraints.
Assess, address and communicate geotechnical risks
The first step in converting brownfield into airspace should be a geotechnical risk assessment that considers a site’s history and its geological and hydrogeological settings. Some initial clues about legacy risks may be collected from plans and documentation of the former site workings, but can you be certain of the accuracy of the records? While a desktop study will identify potential risks and gauge their relevance to the project, relevant issues must be investigated more thoroughly with intrusive or non-intrusive methods.
Once the risks are better understood, preliminary approaches can be designed to mitigate the risk of geotechnical failure and optimise airspace. Mitigation measures must be considered both for the design and operational stages.
During the development of the design, maintaining a risk register will track that risks are safely mitigated to the extent practical, and that the owner and operator understand the design intent, how risk mitigation has been achieved and whether residual risks remain.
Get your lining right
An emerging opportunity to get the best results from brownfield landfill is through innovation in lining systems. Choosing the most appropriate and cost-effective lining system involves balancing risk (environmental and geotechnical) against airspace. Finding the best solution depends on understanding how individual components interact with the subgrade, with each other and with the overlying waste material.
Choosing a lining system requires a clear understanding of a site’s geotechnical characteristics. It is also important to determine which lining option will deliver the best long-term performance as a barrier, for drainage, to manage seepage and the effects of groundwater, to minimise strain and to maximise stability. It is important that the desire to maximise airspace is not placed before the need for stability and safety.
Many effective lining systems are available, and the most commonly used now are composite systems that comprise a geomembrane with an underlying material of low permeability, such as compacted clay or geosynthetic clay liner. However, these landfill lining systems are not impermeable, and designers need to estimate the potential seepage to assess the risk to the surrounding environment.
Potential changes to groundwater levels (due to seasonal, regional or operational factors) must be factored into the choice of lining system as well. Changes in the groundwater table could result in significant construction delays and operational complications, as well as risk of seepage and additional leachate generation. Any development of airspace below the long-term predicted groundwater table requires very careful consideration.
Don’t analyse in isolation
Strain and stability will impact the choice of lining system as well. In former coalmines, the lining system may be placed under unacceptable strain if poorly backfilled materials subside or underground workings collapse. In the case of a former quarry or highwall with a near-vertical lining system, strain may be due to settlement of waste or the subgrade. Because the subgrade and lining system interact, their stabilities shouldn’t be analysed in isolation. Geotechnical modelling of the subgrade in conjunction with the liner system and waste loads may be necessary to decide whether the lining system or subgrade should be reinforced. Strain may be managed by introducing an intermediary material between the waste and barrier, which also needs to be appropriately analysed and designed.
In former mines and quarries, the stability of highwalls and landfill lining systems needs detailed analysis. Steep wall lining systems pose significant health and safety risks during construction and operation. Technology such as photogrammetry can be used to develop a digital terrain model to identify and measure key geotechnical features — such as faults, toppling and rock wedges — that can affect the constructability and integrity of the lining system.
In the end, the battle for airspace won’t be won out of a textbook, and it can’t be copied and pasted from a previous design. Every site is different and will raise different geotechnical risks and challenges. The best result will come from a customised approach that addresses site and local challenges, drawing on the input of a range of specialist expertise.
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