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.
Experience with anchor trenches at the bottom of the landfill slope for the final cover geosynthetics has not been positive because of these issues:
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:
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.
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:
Similar issues can also occur at the outlet of such systems on landfill terraces, making the RTDS a superior design.
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.
It is a general misconception that leachate seeps stop or disappear when slopes receive the final cover. In fact, it is only true if the source of leachate is located directly below the cover, but in most cases, the leachate originates from another location. Continuing seeps eventually reach the bottom of the slope, where two scenarios can happen depending on how the final cover geomembrane is secured at the landfill’s perimeter.
In the first scenario, where the geomembrane is anchored in an anchor trench, liquids will gradually flow underneath the cover geomembrane in the anchor trench and enter the perimeter berm structure. Leachate entering the berm structure softens the berm’s structural fill adversely impacting its shear strength. Additionally, leachate gradually seeps through the berm structure and enters natural formations below the berm and possibly into the groundwater. The operator is alerted when monitoring shows a localized structural failure or a groundwater impact in a nearby groundwater monitoring well.
In the second scenario, where the final cover geomembrane is welded to the bottom lining system geomembrane, leachate seeping out of the slope reaching the toe of the slope accumulates at the toe because it has nowhere to go. Accumulation of leachate behind the final cover geomembrane forces water to gradually move laterally along the landfill perimeter berm behind the final cover geomembrane damaging a larger area behind the final cover. Vertically, more of the area above the toe of the slope becomes engaged by the accumulating leachate. The two obvious consequences are the softening of the soil layer below the final cover geomembrane at the toe of the slope and the water-bedding effect of the area near the toe of the slope.
In the first scenario, the operator has to handle a non-compliance issue, either a failure in the slope or impacts to groundwater. In the second case, the leachate remains contained, but the operator has to address the issue by opening the final cover and removing leachate accumulated behind the final cover geomembrane. The geomembrane opening is closed, and final cover soils are restored after liquids are removed. Both are costly and complicated solutions. Moreover, the problem does not end after completion of the repair because the source of leachate seep is not eliminated.
Landfill operators can require their engineers to design a leachate toe drain system located at the toe of the slope and connected to the leachate collection system at the bottom of the landfill before the final cover geomembrane is installed. The leachate toe drain system is the only way to collect and route leachate to a location at the bottom of the landfill constructed for removal of leachate.
If you are closing a portion of your landfill slope and you find no leachate toe drain system in the construction plans, you can ask for a system to be added to the design plans before the commencement of the construction project.
SCS has significant experience with various types of leachate toe drain system constructed at different locations under various conditions. If you like to know more about the design of leachate toe drain systems or if you are looking for an experienced engineer for the design of your next final cover contact SCS.
Author: Dr. Ali Khatami