Geogrid Strengthening of an Existing Coal Ash Landfill Cap

A coal ash landfill, located in the northeastern United States, was constructed within an abandoned quarry and capped with a smooth PVC geomembrane liner system with no internal drainage. Movement of the cap following heavy rainfall events was observed on several occasions, resulting in the PVC liner being exposed at several areas. The results of investigations and analyses indicated that the cap system was unstable under wet conditions and had an unacceptable factor of safety during dry conditions. The selected remediation option included cap strengthening with a high strength geogrid placed above the existing PVC liner and installation of a subsurface drainage system. One of the greatest design challenges was how to install the geogrids at the corners of the landfill so that they resist the various three dimensional forces at these locations. This was overcome by developing a unique overlapping layout of the geogrids so that they could resist the forces in various directions. Additionally, a subsurface drainage system was designed that included a network of perforated pipes that intercepted the water infiltrating through the cap and discharging it at the base of the landfill; thereby, improving the drainage conditions above the liner.