final cover geomembrane

December 28, 2020

State regulatory agencies normally require landfill slopes reaching final grades to close within a certain period. This requirement leads to closing landfill slopes in phases, normally referred to as partial closure. Generally, partial closures start from the bottom of the landfill slope up to a certain elevation, with geosynthetics in the final cover temporarily anchored along the partial closure’s sides and upper boundary. Engineers propose different designs for securing the lower boundary of partial closures at the bottom of the landfill slope. Some engineers propose an anchor trench outside the bottom lining system anchor trench to secure the final cover geosynthetics. Others specify welding the cover geomembrane to the bottom lining system geomembrane.

Anchor Trench for Final Cover Geosynthetics at the Bottom of the Slope

Experience with anchor trenches at the bottom of the landfill slope for the final cover geosynthetics has not been positive because of these issues:

• Landfill gas may escape through the opening between the bottom lining system anchor trench and the final cover anchor trench.
• Leachate seeps below the final cover geomembrane that reaches the bottom of the landfill slope may penetrate the landfill perimeter berm through the opening between the two anchor trenches.
• High concentrations of landfill gas may be detected along the landfill perimeter berm at the location of the two anchor trenches during surface emissions monitoring.
• If high leachate levels are developing inside the landfill cell, landfill leachate may escape through the opening between the two anchor trenches.

Welding of Final Cover Geomembrane to the Bottom Lining System Geomembrane

To eliminate the issues above, engineers weld the final cover geomembrane to the bottom lining system geomembrane for cases when there is a bottom lining system below the waste. The welding completely seals the landfill interior space from the outside environment and keeps regulated materials, such as waste, leachate, and gas, within the sealed system. Of course, the engineer should design proper means to address these behind the sealed system; designs may include:

• A leachate toe drain system below the final cover geomembrane at the bottom of the landfill slope to collect and convey leachate seep liquids to the leachate collection system at the bottom of the landfill.
• A suitable landfill gas collection system below the final cover geomembrane, at the lower boundary of the landfill slope, collects gases accumulating in the area.
  • This is an important consideration because the closest gas collection well may be over 250 ft. away, up on the slope.
• A rainwater toe drain system above the final cover geomembrane, at the bottom of the landfill slope, collects and drains the water in the final cover geocomposite.

Leachate Toe Drain System (LTDS)

Leachate toe drain system is a concept originally developed by SCS and incorporated into landfill final cover designs over the past 20 years. Unfortunately, many solid waste engineers are unaware of the need for LTDS, so their designs lack this important feature. LTDS saves a tremendous amount of repair money in the long run by avoiding complications for landfill operators.

Rainwater Toe Drain System (RTDS)

A rainwater toe drain system removes water that moves laterally within the final cover geocomposite toward the slope’s bottom. The RTDS includes a perforated HDPE pipe encased in gravel and wrapped in geotextile. Also, install the RTDS on terraces along the depression on the interior side of the terrace. Along the landfill slope’s bottom, position the RTDS behind a HDPE flap welded to the final cover geomembrane. The RTDS is sloping with high and low points along the RTDS alignment. Lateral drain pipes located at low points remove water from the RTDS to the perimeter ditches.

Other designs involving extending the geocomposite to daylight at the slope surface cause problem such as those listed below:

• Excessive vegetation impacts the opening of the geocomposite at the outlet edge.
• Soil erosion from higher-ups clogs the opening of the geocomposite at the outlet edge.
• Algae grow at the opening of the geocomposite at the outlet edge.
• Gradual discharge of water from geocomposite softens the perimeter berm soils in the vicinity of the outlet edge.
• Water percolates into the landfill perimeter berm and causes stability issues; and
• A slippery surface develops along the outlet edge on top of the landfill perimeter berm, creating a health and safety issue for landfill personnel.

Similar issues can also occur at the outlet of such systems on landfill terraces, making the RTDS a superior design.

Landfill owners who are aware of the associated features mandate their inclusion to ensure their landfill final covers’ long-term superior performance.

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, Construction Quality Assurance, and Elevated Temperature Landfills. He has over 40 years of research and professional experience in mechanical, structural, and civil engineering.