The Virtual Curtain Gas Migration System

The virtual curtain is a no-dig gas migration interception system and can form a major element of a remediation strategy to satisfy a Part 2A determination.

To date over 45,000m of virtual curtain vent nodes have been installed.

virtual curtain gas migration barrier

SEL virtual curtain vent nodes under installation. These provide a preferential pathway for the ground gas to escape.

Background

The use of in-ground barriers, particularly vent trenches, is a common method used to protect development from landfill gas migration. The virtual curtain is a system for installing passive venting barriers and can help overcome many of the disadvantages associated with conventional trench systems.

There is little design guidance available for such barriers, because the performance depends on a complex relationship between several highly variable parameters.

A simplified design methodolgy has been developed with Arup which allows the relative performance of different vent node spacings and ventilation methods to be assessed. This should help to ensure that gas vented to atmosphere from the system is at acceptable concentrations.

Key Elements

A typical installation comprises 3 parts:-

Highly efficient geocomposite vent nodes, which are driven into the ground at predetermined depths and centres. These are based on site specific factors some as ground type, depth of waste and location of potential pathways. The vent nodes are installed to intercept the pathway of ground gases and provide a new preferential pathway (route of least resistance) to atmosphere for these gases. Vent nodes can be installed up to 10m deep into the ground. The vent nodes introduce a zone of low pressure in to the surrounding ground which will attract ground gas to itself from both sides of the virtual curtain installation.

The vent nodes connect to a collection / dilution duct which is normally installed within the inert capping material below 500mm of cover.

The collection / dilution duct is installed in discrete lengths by introducing a discontinuity or break in the duct. At these break positions a conection is made to either vent stacks or vent bollards, to allow safe venting to atmosphere. The length of each section is determined by the gassing regime and the dilution requirement. Once within the virtual curtain system the ground gas will be diluted with fresh air to safe levels before dispersal to atmosphere via a vent bollard or vent stack. This is all achieved using passive ventilation technologies.

The SEL virtual curtain gas migration system can be installed very close to site boundaries

Design & Installation

The virtual curtain design can be adjusted to suit for each site. Gas types, source, 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.

Negligible Excavated Arisings

A virtual curtain installation will displace appx. 0.05m3 of soil per linear metre. This is usually from the overlying inert material and can be regraded to suit original levels which means there are no arisings requiring off-site disposal. Use of the virtual curtain gas migration barrier system minimises spoil and contact by installers with contaminated soils and can be installed in restricted spaces.

Leader trench is only 650mm deep. Vent nodes are driven fro the base of the trench and then collected to an upper gas collection duct laid in the base of the trench.

Upper collection duct is laid over the top of the vertical vent nodes. The collection duct is covered with as-dug material.

Maintenance

The system is virtually maintenance free and has been widely accepted by regulators all over the UK. 

Groundwater

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.

Ventilation

The virtual curtain is vented through purpose made hdpe steel stacks or bollards which have independant high and low-level vents. This creates a pressure differential and encourages gas migration to the virtual curtain and provides clean air for mixing / dilution and allows passive venting to atmosphere. The vent stacks and bollards can be be off-set from the virtual curtain proving even more flexibility for subsequent construction. Contaminants

Virtual curtain 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)

SEL can use vent stacks or vent bollards to vent the diluted