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Site first uploaded: September 2003 Site last updated:
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LANDFILL GAS INFORMATION
Landfill Gas Cut-Off Trenches
Landfill Gas Cut-Off Trenches can be a simple, and when designed correctly, also usually a reliable option for the remediation and prevention of landfill gas migration from unlined gassing landfills. The purpose of a Cut-Off Trench is to block the flow path for landfill gas to stop it passing out from the landfill in the ground, where it might:
damage vegetation and crop growth;
cause an explosion or asphyxiation risk.
What is a Landfill Gas Cut-Off Trench?
It usually comprises a trench which is backfilled with single size stone, to allow landfill gas to vent (flow) upwards and not continue horizontally through the ground. A non gas-permeable membrane (usually HDPE (high density polyethylene) or LLDPE (Linear Low Density Polyethylene) is usually placed on the slope of the trench remote from the landfill.
The depth of the trench and membrane must penetrate low enough into the ground to prevent the landfill gas flowing beneath the cut-off. It is usually extended downward to key into a low gas permeability soil or rock strata, or to 1 metre approximately below the lowest groundwater water level experienced at the location of the trench.
Landfill Gas Cut-Off Trenches are normally passively vented (no extraction pumping takes place).
Does it Need Maintenance?
Maintenance requirements will differ, but generally it is essential that the gravel at the top of the cut-off trench remains open to the atmosphere, and does not become covered over a period of time. This would inhibit natural "venting" and must be avoided. Ensuring that the top of the cut-off trench is achieved by regular inspection and maintenance to remove any material which falls or is washed onto the top of the cut-off trench.
Is the Top of a Cut-Off Trench Liable to be Hazardous?
If correctly designed by an expert, and accompanied by a rigorous site specific risk assessment, the trench will be designed on one of the following principles:
1. The top of the vent trench is left open so that free flow of air into the cut-off trench by natural convection will ensure that the air above the trench dissipates the landfill gas rapidly and that the gas concentration present (% age of the LEL (Lower Explosive Limit)) always stays well below a hazardous level at the surface. (This is the normal situation for landfills see the Waste ICoP at www.esauk.org )
For this reason it is usually important that the whole of the trench surface remains free from obstruction, for the free venting of these gases. However, it may not be necessary to zone this as a hazardous zone under DSEAR/ATEX Directive requirements.
2. The top of the vent trench is covered and vented at a pipe or flu such that the landfill gas below the vent will be at or above the LEL. In this case a hazardous (explosive zone) will exist and will be hazardous requiring special attention. See the Waste ICoP at www.esauk.org
When are Landfill Gas Cut-Off Trenches Used?
They are often installed around old (unlined) domestic waste landfills when these are capped, or subsequently in the event that landfill gas migration becomes evident.
Landfill Gas Cut-Off Trenches are not generally used, nor should they be necessary for landfills designed to current EU, and most national standards worldwide. Good practice dictates that landfills must be lined, and that lining must exhibit a suitably low gas permeability. Furthermore, all UK landfills must be provided with actively pumped LFG extraction systems, to further minimise gas migration risk.
Landfill Gas Migration Problem? If you need asistance with a landfill gas problem we can recommend experienced landfill gas engineers that have designed and successfully installed many landfill gas Cut-Off Trenches, just go to our Contact Us page. For more information, fill in the form, or call us at the telephone number provided.
Landfill Gas migration is a serious matter. Landfill Gas migration has caused death. Ensure that landfill gas migration issues are treated seriously and resolved by experts.
Construction of Cut-Off Trenches can also be hazardous. Ensure full Risk Assessments are carried out and the resulting measures to ensure safety are rigourously applied.
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