Horizontal landfill expansions - often designed in a "piggy-back" fashion adjacent to disposal areas to allow new refuse to be placed over capped and closed slopes - have become a traditional way to maximize airspace. Waste Management of Pennsylvania, Morrisville, however, decided on an unconventional twist to this process: a horizontal expansion of a double-lined landfill equipped with a side slope riser leachate pump system.
Because of its complexities, horizontal expansions have not been considered an option at landfills using leachate pump systems. For example, in the case of Tullytown (Pa.) Resource Recovery Facility southern expansion of 120 acres, the critical element was constructing new disposal cells adjacent to the existing site, and, in the process, connecting liner and leachate collection systems - a project that had never been attempted previously at a Waste Management facility.
Tullytown's disposal areas used a pumped leachate extraction system installed into side slope riser pipes that rested on the geosynthetic liner system. Without the piggyback expansion, Tullytown would lose a significant volume of airspace.
Before expansion could begin, the pumped system had to be removed because there was no practical way to maintain the pumps and electrical system under the new waste. The pumped system would be replaced by a dual-containment, gravity leachate system, which would allow waste from a new landfill cell to be placed directly onto the adjacent waste slope area of the existing cell.
Once the pumped system was removed, the new systems needed to be connected to the existing cells' leachate collection systems and routed below the future double-lined disposal area. This design allows for continuous liquid collection and transmission from the existing landfill cells.
Connecting the gravity system to the existing cells' leachate collection systems called for excavation of the final cover soils and refuse up to 50 foot depths - exposing the existing primary and secondary liners and main leachate collection pipes. To manage the leachate until the final pipe connections were made, temporary high-density polyethylene (HDPE) flaps were installed.
To complicate the project, soft river dredge material had to be removed to accommodate the gravity leachate pipes. These dredge deposits - some 25 feet thick- were replaced with compacted structural fill to serve as the new disposal area's foundation soils.
Once the dredge was removed, leachate pipes were embedded in compacted pea gravel. Line and grade for the dual containment piping was maintained by continual construction surveying and laser control.
Due to the trench's proximity to the existing landfill liner system, the general contractor, OHM Remediation Services, Trenton, N.J., devised a support system to keep the trench stable during open excavation. A cantilevered wall design (bottom end fixed into the soil) was selected. Sixty-feet-long steel sheets were driven 10 feet underground and placed along 800 lineal feet of the excavation.
A sheetpile cofferdam was installed along 100 feet of the corridor to create a temporary, watertight enclosure. This cofferdam allowed for installation of the dual containment manholes while minimizing groundwater flow into the excavation area.
The support system's design not only maintained trench stability, but also allowed the gravity pipes to be placed only 30 feet from the existing anchor trench.
Instead of excavating through the perimeter soil berm, OHM bored through the berm horizontally to connect the existing landfill collection sumps with the main pipe corridor.
This horizontal boring method, in conjunction with the cantilevered sheet piling trench shoring, provided a cost-effective, safe excavation method.
In addition to saving time and money, these alternate construction methods maintained existing liner system integrity.
Liner Durability The existing liner system had to be reconstructed during expansion, and Waste Management capitalized on the opportunity to study the long-term durability of the landfill's geosynthetics.
When a portion of the liner was removed to make the leachate piping connection, samples of the geomembrane were sent for laboratory testing at the Geosynthetic Research Institute at Drexel University, Philadelphia.
The evaluation concluded that the HDPE's physical properties suffered no degradation even though the liner had been exposed to leachate, methane and static and dynamic stresses for approximately eight years.
The data also indicated that the HDPE had not aged to any measurable degree since its installation in 1988. Thus, Tullytown's selection of HDPE as the synthetic barrier was technically sound.