Landfill Cap Design Stability: What You Need to Know | AGRU America

Stability in Landfill Cap Design

Why do landfill slopes fail? Some are more extreme than others, but they all share commonalities that are important to identify. Here, we identify proper landfill cap design and examine the mechanics behind landfill slope collapse, the consequences of landfill slope failure, and how geosynthetics can be used to prevent failure.

Dissecting the mechanics behind landfill slope collapse

Slope collapse can be attributed to loss of shear strength in the surface materials (“veneer” slope failure), excess loading (typically associated with “global” failure), or erosion. In other words, a slope collapse is a result of a lack of slope stability. Global failure is a function of waste mechanical properties and configuration. This failure type is avoided with proper site design and typically not related to veneer interface shear strength.

For veneer failures, the focus of this article, most landfill slopes will not collapse, but those that do are often caused by shear failure as the soil or surface material approaches saturation. Therefore, under most conditions, building effective slopes relies on improving shear strength and reducing the ability for water to saturate the surface material. Shear strength can be improved with vegetation or other kinds of covers like artificial turf. Surface drainage using channels, catch pits, and sand traps are effective at controlling water flow, while subsurface drainage like implementing a drain liner beneath the soil can help prevent soil saturation.

The consequences of failure

An inadequate slope design will eventually fail and the consequences for such a failure can be catastrophic for the region surrounding a landfill. A landfill slope collapse can damage the protective liners that separate contaminates from the groundwater, spreading pollution. The ecological damage caused by the contaminated leachate can pollute the environment and spread odor or other air quality problem.

The necessary remediation and reconstruction can be quite expensive to those responsible and the collapse will damage the reputations. For these reasons, Federal and state officials have implemented strict regulations governing slope performance, requiring a 1.5 safety factor for static stability. Accepted designs for seismic stability typically require less than 12 inches of system deformation to minimize damage to cover components.

Using geosynthetics as a tool to help improve slope stability in landfill cap design

Proper landfill cap design is as important as properly maintaining one. Municipal solid waste landfills are subject to specific final cover design requirements outlined in regulation 40 CFR 258.60(a) and 258.60(b) – aka “SubTitle D.” According to these rules, the final cover system should be designed to minimize infiltration and erosion by meeting several criteria (3).

First, the cover system should have a permeability less than or equal to the permeability of any bottom liner system or natural subsoils present, or a permeability no greater then 1 x 10-5 cm/s, whichever is less. This is achieved by use of a prescriptive infiltration layer that contains a minimum 18 in. of earthen material. The prescriptive standard for erosion control of the final cover is a soil layer that contains a minimum 6 in. of earthen material that is capable of sustaining native plant growth.

For containment, polyethylene (PE) liners are highly effective thanks to their flexibility, strength, degree of impermeability, and resistance to chemicals and corrosive compounds. Products like AGRU America’s HDPE/LLDPE MicroSpike or Super Gripnet meet or exceed GRI GM13/17 test values, providing a highly reliable solution that satisfies accepted material specification for landfill closures nationwide.

AGRU America utilizes the flat-die calendared extrusion process, which yields superior asperity consistency and liner thickness. AGRU Super Gripnet liner, when combined with AGRUTEX geotextile, form the Integrated Drainage System (IDS) that incorporates an advanced drainage structure within the geomembrane component.