Regenerating former landfill sites

As the construction industry is seeing an increasing shift towards landfill redevelopment, Stephen Oakden, Director at BE Design – the multi-disciplinary architecture and engineering practice – outlines the biggest obstacles faced by developers when constructing on such sites and how to meet environmental challenges with practical and safe solutions.

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Are landfill sites really suitable for redevelopment?

In short – yes. Former landfill sites and brownfield land throughout the UK are being earmarked for development as demand for space increases.

Naturally, these sites present additional engineering and design challenges. But, when managed effectively, the development of these sites can be a catalyst for regeneration – while protecting existing greenbelt land in the area.

The safety of construction teams and end-users on landfill sites is the first challenge to be addressed, and developers should allow additional time to accurately assess the risks in detail before any projects can begin.

Sites should undergo rigorous environmental and safety testing, and greater care needs to be taken with awareness relating to the risks of the site and the necessary protective clothing required. Most disused landfill sites are capped and closed once filled, but one can never be certain what ‘hidden gems’ may be discovered once work begins on site.

There has been an instance where a former landfill site uncovered hypodermic needles, highlighting the importance of covering skin and wearing protective clothing at all times.

Two of the biggest issues when preparing a landfill site for development are dealing with the gases produced by decomposing material and consolidation.

Reducing the risks of toxic gases

Ground content will continue to decompose even after capping, releasing CO2 and methane gases into the air.

In large spaces, these gases will have very little impact but managing these risks through several tactics is still crucial.

Most landfill sites have a metre-thick, impervious capping layer which seals the site and helps to stop pollution, but it is essential to ensure a strong seal between the sealant material and the piling to minimise leaks, together with using piling techniques that do not cause pathways for contamination to enter into underlying soils or groundwater.

Under-building ventilation systems also help manage gas emissions. These vary in make-up, but all follow a process that encourages airflow out from underneath buildings, either by natural or mechanical means, to release gases safely.

This is often achieved by installing a layer of porous rock on the capped landfill surface, with horizontal piping to direct the air out. This method requires close measurement because, whilst it is safe to allow extracted gas to release straight into the atmosphere, if levels are deemed significant, flaring will need to be introduced to overcome odour problems and comply with environmental regulations.

Whilst this system has been successfully implemented in many properties, mechanical ventilation systems do require maintenance for many years, which should be considered as part of the wider cost implications for the owner and any future purchasers of the building.

Furthermore, service ducts and vents for cabling and utility services must be sealed in a truly permanent fashion, and it is highly recommended that audible gas detection devices should be installed as a final precaution.

Managing settlement

Consolidation – the sinking and settling of the land as waste breaks down – is another of the biggest problems affecting redevelopment on landfill sites.

As decomposition occurs, depressions can develop on the site surface, and fluid that trickles out of a landfill (known as the leachate) also carries contaminants and has the potential to spoil the ground water in the surrounding area.

Whilst a significant amount of the settlement takes place within five years of a landfill’s closure, it can continue for decades, albeit at a slower rate. Engineers must evaluate the land carefully before making recommendations to build on the site to ensure the area is safely stabilised. Structures will often be piled and suspended to ensure the firm ground beneath it is bearing the majority of the weight, rather than the landfill itself.

Care needs to be taken to ensure that differential settlement between the ridged piled structure and the ground-bearing external areas do not cause significant problems. This can be specifically pronounced with drainage which should be checked annually.

Implementing ground improvement techniques

Ground improvement techniques are continually being developed, employed and refined within the construction industry. Projects such as Island Road – a former landfill site with 6m of domestic landfill in Reading where BE Design has delivered a five-unit industrial scheme over two phases – evidence how innovation in sustainable engineering has improved logistics networks.

Dynamic compaction was used on the ground during the first phase of the Island Road development in 2017. This entails the repeated dropping of a heavy weight on the ground at regularly spaced intervals; the density of the soil was increased, and 230mm of consolidation was averaged on-site. The impact of the free-falling weight creates stress waves which can penetrate up to 10m – depending on the weight and the height from which it is dropped.

Rapid impact compaction was implemented during the second phase of development during 2018, consolidating loose-fill soils on the site to help minimise ongoing settlement. This technique uses a hydraulic pile-driving hammer to continually strike a plate on the surface of the ground. This constant contact drives energy into the ground, which mitigates concerns over flying debris and creates a uniform support for foundation footing.

These methods should be tested on site ahead of time to gauge how it will respond before major works begin. Both techniques should be carefully monitored and recorded; as if either is overused it can create a localised heaving of the soil which is detrimental to the soil strength.

The ground can also be tested after the use of this technology to evidence the level of improvement accomplished. Using a cone penetration pressure method, groundwater content and soil density can be measured in comparison to pre-ground improvement statistics.

This data is likely to prove invaluable in the future as more waste sites are likely to be earmarked for regeneration – driving the country’s use of brownfield land for construction.

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