Virtual Curtain - Recent Case Studies

SEL Environmental Ltd     Tel: 01254 589987     Email:

Virtual Curtain Gas Migration Barrier - Housing, Marsden Avenue, Warrington

The proposed development will comprise 104nr. low-rise residential dwellings, complete with private gardens and associated infrastructure (roads, parking areas, utility services etc.) Residential dwellings were constructed on piled foundations and floor slabs were suspended and passively ventilated. The site was operated historically as a landfill, licensed to accept inert industrial waste. The site was historically known as the ‘Westy Lagoon’. It is understood that the ‘lagoon’ was constructed in the 1940’s, with 3m high clay bunds constructed around the periphery of the land – resulting in the level change which is now evident. Infilling of the lagoon comprised tannery waste slurry (from the lime treatment of cattle hides), which was subsequently capped with hardcore. It is understood that filling of the lagoon continued until the 1970’s, with the landfill license being surrendered in 1978. However, construction waste may have been deposited onto the site following surrender of the landfill license, up until the late 1980’s.

EPG Ltd were appointed by Halebank Developments to design the gas mitigation measures for the development. Their gas migration solution was to install 3 sections of the virtual curtain to intercept the gas pathway and a grid of standalone pressure alleviation vents within the gassing source. These measures enabled reduced gas protection measures to each dwelling comprising a gas membrane barrier and a sub-floor void ventilated using airbricks. The Virtual Curtain gas migration barrier was installed by SEL Environmental Ltd and is summarised below:-

  • Virtual Curtain 1 - 113m long to 7m deep. Vent nodes to 7m deep at 2m spacing along east / northeast boundary with vent bollards at 25m centres.
  • Virtual Curtain 2 - 131m long to 3m deep. Vent nodes to 3m deep at 1m spacing along southern boundary with vent bollards at 25m centres.
  • Virtual Curtain 3 - 92m long to 3m deep. Vent nodes to 3m deep at 1.5m spacing along southeast boundary with vent bollards at 25m centres.
  • Virtual Curtain; Individual Nodes, 12nr to 7m deep. Vent nodes to 7m deep at 10m spacing along access road with vent bollards at each position.
ground gas, gas barrier, gas migration
brownfield, gas protection
gas migration barrier
vent bollard, gas migration, gas barrier
Virtual Curtain Gas Migration Barrier - Sports Facility, Waterside Drive, Walton on Thames

The development comprised the construction of three sports pitches (1 synthetic (3G), 2 turf), a running track with athletics facilities and sports pavilion. Other elements included as part of the development included a car park, picnic / children’s play area and dog walking / landscaped zone(s).

Willmott Dixon Construction Services Limited (WDC) has been employed by Elmbridge Borough Council (EBC) as Design and Build Contractor for the construction of the new recreational and leisure facilities at Waterside Drive, Walton-on-Thames. EPG Ltd undertook a Gas Risk Assessment for the proposed scheme on behalf of WDC. SEL Environmental Ltd installed the Virtual Curtain gas migration barrier and the pressure alleviation system.

The site is a former landfill and one of the constraints identified with the development relates to the potential for landfill gas to be present. From a landfill gas perspective, the main risk associated with this development is that the construction works could effectively ‘seal’ the site surface. To clarify the pre-development state of the site had no engineered capping allowing any landfill gas generated by the waste to diffuse to atmosphere. As part of the development much of the former landfill will be capped. This capping may include the placement of soil (e.g. to ‘break’ direct human health contamination pathways) or the placement of Class 6F2 material and concrete slabs (to manage geotechnical issues associated with settlement of the waste in relation to the development proposals). 

By effectively ‘capping’ the site with this makeup it limits the area available for landfill gas diffusion to atmosphere and could result in increased gas pressure within the ground.

The solution designed by EPG Ltd was to install gas protection measures to any new structure comprising a gas resistant barrier and underslab ventilation. To install a pressure alleviation system to alleviate gas pressures within the landfill waste deposits and a Virtual Curtain gas migration barrier to mitigate risks associated with the off-site migration of landfill gas toward existing properties. The benefit of using the Virtual Curtain system is that it generates very little waste, the nodes are vibrated into the ground and do not result in spoil generation. SEL Environmental Ltd installed the gas protection measures for the scheme as described above. The pressure alleviation and Virtual Curtain requirements are summarised below: -

  • Virtual Curtain; gas alleviation system comprising 225nr individual Nodes to 4m deep. 4387m of gas collection duct including biofilter media for odour control. 18nr vent stacks and 68nr ground level vents below all the hard surfaced areas.
  • Virtual Curtain – 219mm long to 5m deep. Vent nodes to 5m deep at 1.5m spacing with vent stacks at 15m centres along the northern boundary.
gas migration barrier
alternative to, gravel vent trench
passive venting, gas barrier
vent stack, closed landfill
Virtual Curtain Gas Migration Barrier – Closed Landfill Site, Dale Lane, Bromsgrove

The objective of this scheme was to reduce risk of ground gas attributed to Dale Lane Tip Migrating to nearby houses. Gas generated by the landfill had been shown to be present in potentially hazardous levels. As such the proposed solution was to install a Virtual Curtain gas migration barrier system to reduce gas concentrations near the site boundary and the local houses. Post construction of the M42 an area of land still owned by The Secretary of State for Transport has been continually monitored for landfill gas and contamination assessments carried out to check levels of methane (CH4) and carbon dioxide (CO2) as well as other gases and groundwater.

Over the years levels of gases have fluctuated but reached levels that are potentially harmful to nearby residents. The Highways Agency (HA), based on recommendations provided by Amey decided on a course of action to control the levels of gases. 

The HA asked Amey to advise on a passive cut off trench system that mitigates the risk from the landfill gases. The final design solution was developed by EPG Ltd, verified and checked by Amey. SEL Environmental Ltd installed the Virtual Curtain gas migration barrier under the supervision of Carnell Support Services Ltd who were the Main Contractor on site. Access to site was extremely limited and a temporary access route directly off the M42 had to be constructed through a noise bund.

Plant and material deliveries had to be achieved during night time hard shoulder closures of the adjacent section of the M42. As mentioned above SEL Environmental Ltd installed the Virtual Curtain gas migration barrier in accordance with the following arrangement: -

  • Virtual Curtain - 180m long to 5m deep. Vent nodes to 5m deep at 1m spacing with vent stacks at 15m centres. The barrier had to accommodate a high pressure gas main that passed through the site.
gas migration barrier, no-dig barrier
part 2a, gas migration barrier
vent stack, gas barrier
vent stack, closed landfill

Home               Overview               Background                Case Studies               Contact

© Copyright SEL Environmental Ltd, Canal House, Bonsall Street, Blackburn, BB2 4DD