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Landfill Gas Cut-Off Trenches

Landfill Gas Cut-Off Trenches are a simple, and when designed correctly, also usually a reliable option for the remediation and prevention of landfill gas migration from 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 below the lowest groundwater water level experienced at the location of the trench.

Landfill Gas Cut-Off Trenches are normally passively vented.

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 rigourous site specific risk assessment, the 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 % age of the LEL (Lower Explosive Limit) always stays well below a hazardous level. 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.

When are Landfill Gas Cut-Off Trenches Used?

They are often installed around old domestic waste landfills when these are capped, or subsequently when landfill gas migration becomes evident.

Landfill Gas Cut-Off Trenches are not generally appropriate to the design of landfills to current EU, and most national standards worldwide standards. 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? Enviros Consulting's experienced landfill gas engineers have designed and successfully installed many landfill gas Cut-Off Trenches, go to our Contact 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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