Introduction / principles
The virtual curtain wall system comprises vent nodes connected together to form a venting system. The basis of this system is the utilisation of the forced dilution principle to create a zone of low pressure that attracts and dilutes ground borne gases to acceptable levels and provide an appropriate pathway break.
The virtual curtain wall is not an impermeable membrane and will not change the groundwater characteristics of a site. Groundwater will continue to act in the same manner avoiding any potential issues with flooding, water abstraction and settlement / heave. In respect of gas, it acts as both a pathway break and conduit for controlled and safe passive venting to atmosphere.
The virtual curtain is vented through purpose made stainless steel stacks or bollards which exert a ventury effect in the ground. This encourages gas migration to the virtual curtain, provides clean air for mixing / dilution (thus avoiding odours) and allows passive venting to atmosphere. Stacks and bollards can be designed in any format and be made to look like traditional street furniture. They can also be off-set from the virtual curtain proving even more flexibility for subsequent construction.
Virtual curtain wall systems are devised to mitigate ground gases such as:
- Methane (CH4)
- Carbon dioxide (CO2)
- Carbon monoxide (CO)
- Diesel range organics (DRO)
- Petroleum range organics (PRO)
- Volatile organic hydrocarbons (VOCs)
Design & Installation
The virtual curtain must be specifically designed for each site. Gas types, concentrations and flows are used to calculate the size and spacing of vent nodes that are driven into the ground using a method that minimises the amount of excavation required. The individual nodes are connected to a geo-cellular collector duct system which is wrapped in a membrane. The duct system is located within a shallow trench which is backfilled on completion of the wall. The design will confirm the capacity, number and spacing of the above ground venting infrastructure. The system is virtually maintenance free and has been widely accepted by regulators all over the UK.
VertaseFLI was commissioned to design and construct a virtual curtain wall to mitigate potential ground gas migration from a landfill into a residential development. Soils were contaminated and would have attracted a disposal premium if they were exported. The site was being subjected to long term groundwater monitoring as part of the local authorities Part 2A obligations and it was critical not to change the groundwater regime beneath the site. Finally, the client was most concerned about public perception and did not want to sterilise part of the site.
The virtual curtain wall was designed, installed and met all the aforementioned objectives. It was 690m long and 6.5m deep and avoided the generation of contaminated soils or disturbed the existing groundwater regime. It removed the risk and allowed residential development to proceed. Attractive, stainless steel bollards were specified by the client to minimise visual intrusion and were placed at the required spacing, although a number of bollards were off-set to allow construction of internal roads.