Final Cover System and Landfill Gas Piping – Design Considerations

June 24, 2020

SCS Advice from the Field

Landfills are complex systems with many pipes for liquids and landfill gas running in many different directions. Some of these pipes are at the bottom of the landfill, such as leachate collections pipes, leachate toe drain pipes, pressure release pipes, etc. Other pipes are near the final cover system, either below or above, and closely interact with the final cover geosynthetics. Many of these are for control of landfill gas or leachate seeps at the landfill surface. Pipes may include vertical gas wells, horizontal gas wells, condensate sumps, condensate force main, compressed air lines to gas well pumps and condensate sumps, seep control sumps, electric conduits to condensate sumps and seep control sumps, leachate recirculation force main, stormwater downchutes, etc.

When pipe locations are near the final cover geosynthetics, below or above, or penetrating the final cover, design plans should show details of how the pipes or associated components interact with the final cover components. Lack of sufficient information may cause difficulties years later when scheduling the construction of the final cover. Most often, it becomes evident that many of the pipes constructed years earlier are too short for extending through the final cover.

Another aspect of piping and their interaction with the final cover is conflicts among different pipes, more specifically conflicts among gas pipes and liquid carrying pipes, in and near the final cover system. Liquid carrying pipes may include stormwater downchutes, rainwater toe drain pipes, and leachate toe drain pipes. Stormwater downchutes are usually large diameter pipes extending from the top of the landfill to the perimeter stormwater system. Rainwater toe drain pipes – pipes that receive water from the final cover geocomposite drainage layer, and leachate toe drain pipes – to collect leachate seeps below the final cover geomembrane, are co-located at terraces on slopes and the toe of the slope near the perimeter berm.

A few design considerations can be useful as guidelines during the preparation of design sets to address the relative position of these pipes and the final cover geosynthetics or to avoid conflict among pipes.

  • Include the final cover layers in the gas design details where gas wells installations exist near the landfill’s final surface.
  • If flow control valves locations are below the final cover near the perimeter of the landfill, design a vertical casing around the valve tall enough that booting the future final cover to the vertical casing is possible.
  • Condensate sumps and associated stub outs (such as condensate force main, compressed air lines, or electric conduits) installations should be tall enough to accommodate construction of the final cover system around the condensate sump with sufficient space to boot the final cover geomembrane to the exterior walls of the condensate sump.
  • Leave pipes exiting the liner boundary at the perimeter of the landfill at least 1 foot above the anchor trench shoulder. This allows the installation of a geomembrane boot on the pipe at the point of penetration through the final cover geomembrane.
  • Flow control valves located near the landfill perimeter and within the lined area should be in consideration with the future location of a rainwater toe drain system at the toe of the slope.
  • Gas pipes located above the final cover geomembrane and crossing terraces or access roads may create conflict with the rainwater toe drain at the terrace or adjacent to the road.
  • Large gas headers located across the slope above the final cover geomembrane may cause conflict with stormwater downchutes.
  • Large gas pipes on top of the final cover geomembrane crossing a tack-on swale may cause conflict with the flow line of the tack-on swale.

The complexity of landfills varies from site to site, and issues related to conflicts among gas and liquids pipes, and pipes and final cover geosynthetics vary depending on the geometry and other landfill features involved at each location. The best way to resolve conflicts before construction is to have a coordinated effort among parties involved in the design to discuss and find solutions to every conflict at the design stage.


 

About the Author:  Ali Khatami, Ph.D., PE, LEP, CGC, is a Project Director and a Vice President of SCS Engineers. He is also our National Expert for Landfill Design and Construction Quality Assurance. He has nearly 40 years of research and professional experience in mechanical, structural, and civil engineering.

Learn more at Landfill Engineering

 

 

 

 

 

 

Posted by Diane Samuels at 6:03 am
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