Construction: Landfill Gas Vent Trench and Cut Off, Prince William County, VA

During routine monitoring, landfill gas was registered in perimeter gas probes on the Prince William County, VA landfill. The County Public Works Department wanted immediate action to mitigate the gas migration following perimeter probe testing.

SCS developed an approach that allows for a passive venting system constructed at a lower cost to the County, but still have the option to connect to the active landfill gas system should the gas probes continue to register readings. SCS constructed a 625-foot long, 3 feet wide trench to a maximum depth of 20 foot deep installed along the perimeter of the landfill. The system utilized a 6” perforated HDPE pipe installed at the bottom of the trench that allowed for air injection and a 6-inch perforated pipe installed 5 ft from the surface that allows gas to vent through above grade vent pipes. The trench is designed with options to vent passively vs actively without reconstructing the trench, which provides options based on the gas sample results.

Outcome and Benefit
Phase 2 of the approach, cutting off passive venting and applying a positive vacuum on the trench was implemented after detecting a continued presence of landfill gas in the perimeter probes. Having designed the system to operate passively or under vacuum saved time and money for the County. The system is currently controlling the migration of gas – keeping the County in compliance.