Langan Engineering provided the environmental and geotechnical engineering and consulting required to develop a closed landfill in the San Francisco Bay Area, California. This mega-project requires compliance with numerous local, regional and state regulatory agencies to re-purpose over 200 acres of landfill to a new mixed-use development that will include retail, commercial and residential land use. Redeveloping landfills is particularly challenging not only because of the issues associated with cleanup, but also because of the environmental and geotechnical issues of building on refuse. Landfill gas (LFG) containing elevated levels of methane pose a significant risk of fire, explosion and odor. Other components of LFG, such as sulfur-based corrosives like hydrogen sulfide and volatile organic compounds (VOCs), also pose significant exposure risks to human health, safety and the environment. With methane levels as high as 50% by volume (10 times the lower explosive limit), the ability to design a methane collection system that will protect human health and the environment for the life of the development is a must when obtaining regulatory approvals. In order to design an effective LFG collection system, Langan conducted a month long pilot study and subsequent 3D pneumatic air-flow computational modeling to determine key design parameters of the required LFG collection and mitigation system.
Data collected during the pilot test was used as input to a 3D computational pneumatic model in the SVAIR™GE software module. The 3D pneumatic model was used to simulate the LFG flow field in the refuse and determine the correlation between applied vacuum and LFG extraction flow rate at the test well and the resultant subsurface pneumatic parameters (i.e., subsurface vacuum propagation, LFG flow rate, LFG velocity vectors, and pore volume exchanges) at varying distances from the test well. Pneumatic modeling allowed for the incorporation of varying site conditions (i.e., the site's future architectural, structural, and geotechnical features) and predicted the effects of refuse settlement, seismic activity, proposed development constraints (i.e., piles and other structural elements), and fluctuating LFG generation rates and barometric pressures on the proposed LFG system's operation. The proposed system's key design parameters included methane generation rates, radius of influence (ROI) of the LFG extraction wells, number and spacing of LFG extraction wells, and the required LFG extraction flow rates and vacuums. The model allows the prediction of system performance at any spatial point in the landfill at any given time to evaluate whether the system will be protective of human health and the environment for the life of the proposed development. This state-of-the-art modeling, as far as Langan and SoilVision Systems Ltd. are aware, has never been applied on the scale and to the level of detail being performed for this project.
Numerous regulatory agencies reviewed the conceptual LFG collection system design that relied heavily on the analytical outputs of the SVAIR™GE computational pneumatic model. The ability to model and demonstrate results observed during the pilot test activities and subsequently translate those results into a demonstration of the anticipated effectiveness of the proposed full-scale LFG collection system was crucial for the acceptance of the conceptual design. 3D pneumatic modeling is an extremely effective tool which can be utilized for a number of applications and has the ability to provide not only design refinement and certainty, but also significant cost savings.
Learn more about SVAIR™GE software.