Air 'dam' system bars gas migration
The completion of a land reclamation project in Birmingham has involved the implementation of new technology to transform a previously contaminated site affected by high levels of gas emanating from nearby disused waste landfill sites to a standard fit for human habitationThe technology applied to the site is the Air Wall system, a new method for controlling gas migration. Developed by Prestige Air Technology and Earth Science Partnership, with assistance from the European Regional Development Fund, it involves installing a ring of vent wells around the gas source and a ring of air injection wells downstream. By balancing the air injection pressure with the air pressure in the ground, an air “dam” is formed. Slightly increasing the injection pressure forces gases to vent harmlessly through the inner ring of vent wells.
The system has just been installed at a 10 hectare site in the Castle Vale area of Birmingham, where high levels of methane gas had prevented redevelopment. Extensive desk studies, ground investigation and monitoring revealed the main source was a landfill east of the site. Other potential sources included small scale dumping over several decades and sewage sludge beds.
Installation of a bentonite slurry trench barrier and a gas vent trench had failed to control gas levels. Consultant Geotechnical Developments (UK) assessed a number of gas control methods and decided to explore the Air Wall system. Prestige Air carried out a series of pressure connection tests using gas monitoring wells to complement the ground information and to help establish migration pathways.
This showed the pathway of gas migration from the landfill was around the junction between the slurry wall and the vent trench and that construction of the previous protection measures had been compromised by the installation of large utility services. Between January and March 2001, a 300 metres long air wall was installed to control the area of leakage.
The first element was a pilot scheme to demonstrate viability. The main wall was built in three weeks and activated at the end of March. Prestige Air reports that independent monitoring before and after the Air Wall was installed proved that it prevented migration of landfill gas towards the site. Over seven months, monitoring of the site and the land between it and the landfill showed concentrations of landfill type gas steadily reduced to acceptable levels (less that 1% methane).
Geotechnical Developments estimated that methane and carbon dioxide levels would start to fall four to six weeks after commissioning of the Air Wall. At borehole 218, 5 metres outside the perimeter of the wall, methane levels dropped from 16% to below 1% within 10 weeks.
However, gas in borehole 119, 380 metres away, and just outside the main migration path of the gas, took 12 weeks to fall to below 1%. Borehole 117, in the centre of the gas path, 420 metres from the wall, initially had the highest concentration of methane - 25%. After six months it had fallen below 1%.
Independently commissioned vent stack monitoring carried out at the beginning of March this year showed gas levels are now at a level similar to free air. With methane levels maintained below 1%, the site has been “signed off” for housing redevelopment. Monitoring will continue throughout the project and then be done annually by the local authority.
Commenting on the completion of the project, Malcolm Martin, Managing Director of Prestige Air Technology, said: “Naturally, we’re delighted with the success of this first project, which vindicates the years of hard work and research we’ve put into our product. It was also a great pleasure to assist Castle Vale Housing Action Trust in bringing their vision for the site closer to fruition.
“More importantly, we’re now in a position to take this technology to other brownfield sites in the UK, and indeed abroad, making their successful redevelopment a reality, which is good news for the environment.”