Characteristics
The critical characteristic of landfills is that their contents are variable in both composition and physical state. Such uncertainty poses health, environmental and financial risks to development. Understanding the history of the landfilling and the way the land surface has changed over time is a helpful starting point in characterising and, if necessary, remediating old landfill sites on or adjacent to new developments.
Hazardous properties
Landfill hazards can be broken down into:
- contamination: arises from the range of chemicals present in the wide variety of buried waste materials
- geotechnical: hazards include poor load bearing and great potential for differential settlement, affecting buildings, subsurface utilities or roads and pavements
- hazardous gas: decomposing waste releases a changing mixture of hazardous gases that can poison, asphyxiate or explode
- leachate: both leachate and landfill gas can migrate laterally, affecting development sites adjacent to, or in the vicinity of, the landfill; liquid leachates, often of low pH (acid), can pollute groundwater or connected surface water bodies
Pathway
- Poor load-bearing properties can damage overlying buildings and structures.
- Direct contact of people with soil contamination and leachate or inhalation of hazardous gases.
- Collection of explosive gas in confined spaces and explosion following an initiation event.
Occurrence
In Gateshead, landfilling of former quarries was predominantly with colliery ash and general made ground. However, major landfilling of former quarries and valleys with domestic waste has occurred in the Windy Nook, Beggars Wood and Blaydon areas.
During extensive demolition of high-rise tower blocks and Victorian housing in the 1980s, there was a trend to use demolition waste as a fill material for the core of landscaped perimeter mounds of development sites or public open spaces, such as the 1990 National Garden Festival site (Dunston/Teams).
In Felling Riverside and Bill Quay, chemical waste has been spread over much of the area and used to backfill former clay quarries (e.g. Gateshead Stadium pitches site). The chemical industries also generated substantial amounts of waste material, some of which was disposed of locally.
Some of the landfills in Gateshead are unlined and uncapped; others are both lined and capped and include leachate and gas management systems.
Natural occurrences
Landfilled wastes are not natural deposits but can often be found in natural or artificial depressions or low-lying land.
Site investigation
Desk study and walkover
Normal review of historical mapping and regulatory records will identify land used for landfilling or land that has been infilled or raised, potentially by waste, and the date of disposal. This should lead to an understanding of the contaminants that might be presented and/or the likely geotechnical properties. It should also review any pollution control measures expected to be present.
The extent of landfilled areas is not always accurately known, even for sites that were regulated. Inferred extents from changes in topographic maps as well as satellite imagery can help delineate the extent of buried waste.
Major industrial facilities should be expected to have their own waste disposal locations that might not be recorded elsewhere. These may have been built over as works expanded. Such industrial wastes will reflect the facilities' industrial processes and will be relatively homogenous compared with municipal solid-waste landfills. The Environment Agency's historic landfill sites dataset defines the location of, and provides specific attributes for, known historic (closed) landfill sites, i.e. sites where there is no environmental permit.
Landfills can have uneven surfaces and host physical hazards if the waste is not adequately covered.
Areas of unexpectedly poor vegetation may indicate hazardous materials or the escape of hazardous gases, including methane.
Drains and water courses may contain aggressive chemicals and should be avoided because of their physical as well as chemical or biological hazards.
Intrusive site investigation
Site investigations need to establish the extent and depth of waste, its composition, gas generation potential and geotechnical properties.
Drilling through the base of a landfill should only be carried out if any leachate at the base of the waste can be contained and not allowed to drain deeper down the borehole.
Hazardous ground gases are generated by decomposing waste and can migrate laterally along geological horizons, backfilled utility trenches or pipework, if there are low permeability capping layers above the waste.
Foundations
Landfills generally have poor geotechnical properties and usually require special foundations, both to reach load-bearing ground and to withstand the often-aggressive chemistry in landfills. Landfills generating hazardous gases need active gas control.
Remediation
Pathway interruption
Developments, including occupants of buildings, are often protected by gas- or vapour-proof membranes intended to keep hazardous gas out of the building. Such membranes need to be made of material appropriate to the gas or vapour composition and installed properly.
Source removal
Excavation, material recovery and residue disposal elsewhere are increasingly adopted approaches to rendering landfill sites suitable for residential development.
Waste disposal
Landfills contain historically disposed waste materials. Any material generated during drilling or excavation would need to be disposed of in accordance with current legal requirements.
Regulatory aspects
The extent to which a historic landfill will have been regulated depends on both when it was receiving waste and the composition of that waste.
Glossary
Capping: the process of placing the final cover material over the waste.
Cell: landfills are constructed in phases (cells) that are separated by a berm (raised bank) to contain leachate within an area.
Closed landfill: a landfill that has reached its permitted waste capacity.
Leachate: the fluid percolating through waste material contained in landfills. Leachates are generated from liquids present in the waste and from outside water, including rainwater.
References
Environment Agency. 2020. Historic Landfill Sites.
Document contact
Dr Darren Beriro: darrenb@bgs.ac.uk