Landfill Methane Vent (Cutoff) Trench Construction

This is a short video taken during construction of a cutoff trench (or vent trench) at a Georgia landfill.

More information on AEM's landfill services can be found at:
http://www.advancedenvironmentalmanagement.com/SolidWstSvcs.htm

Category:

Science & Technology

Tags:

• landfill
• gas
• lfg
• methane
• vent
• trench
• solid
• waste
• georgia
• environmental

[drumajo]

How deep was the landfill during it's active life? Surely deeper than 20 feet, so how is the waste past the 20 feet trench vented out?

[AEMGA]

@drumajo: the waste was not much over 20 feet deep. It's a small facility.

There is no guarantee that gas will not migrate beneath the trench. However, the reduced pressure within the trench would cause upward flow gradients beneath.

That being said, I try to get my vent trenches as close as possible to the groundwater table, to prevent/minimize the potential for migration beneath the the trench. That wasn't feasible at this site, but the trench has been effective.

[dumi11]

I have read some articles regarding waste gas latteral migration, but I can not imagine, that biogas can go underground?! Waste gas usuali goes up and lateral, but in the ground? How is this possible? The methane is lighter than air.

[AEMGA]

A typical landfill is required to put a low-permeability cover over the waste unit. This limits the vertical migration of gas and results in a build up of pressure in the waste unit. The gas has to go somewhere and often, laterally is the path of least resistance. If you happen to have a more permeable layer (such as sand) at the right (or wrong) depth, the gas will fly through that layer preferentially.

It really comes down to the path of least resistance.

[BukeyeB]

What was the depth of the trench and how did you determine the depth? What was the depth of stone? (I presume #57 limestone?) Also, if you don't mind, did you erect any piping? or just the gravel trench?

Looking to do same at former SW landfill (pre-76)that vertical waste placement is unknwon.

Thanks.

[AEMGA]

Gravel (#57 Granite Stone) is placed up to appx 4 feet below grade initially. Then I install a header pipe (slotted 4-inch PVC) with a tee every 50-100 feet on top of the gravel (horizontally). Attach solid PVC risers at each tee to extend about five feet above the surface elevation. Cover header with another foot of gravel, and backfill with soil.

Do NOT use limestone. Landfill gas can turn limestone into concrete. Consider using recycled concrete/brick also.

[AEMGA]

The trench is approximately 20 feet deep - the maximum depth of typical excavators. I like the bottom of my cut-off trenches to be within 5 feet of the groundwater but costs increase dramatically if more than 20 feet. One thing I do on some sites (including this one) is lower the surface grade prior to trenching.

[AEMGA]

The trench is constructed between the waste disposal unit and the property boundary - outside the garbage. The purpose is to intercept and vent methane migrating underground from the garbage before it can go off-site and possibly enter a structure.

Thanks for the comment!

D

[slimydick23]

I was under the impression that if you dig that deep into a landfill you'd see some evidence of garbage.