Formatted Title
Hybrid Landfill Gas Mitigation System Implementation
Background/Objectives
Redevelopment of closed landfills provides an opportunity for converting underutilized land into usable urban spaces. Landfill redevelopment is particularly challenging because of the environmental and geotechnical issues associated with building on refuse, such as landfill gas (LFG) generation and refuse settlement. Methane, the primary component of LFG, is an odorless gas that is flammable at concentrations ranging from approximately 5% by volume (lower explosive limit [LEL]) to 15% by volume (upper explosive limit [UEL]). Typically, landfills aged from 20 to 50 years post-closure can potentially generate methane gas concentrations ranging from 30% to 60% by volume, well above the LEL, posing a significant risk of fire and explosion. In addition, as the refuse continues to degrade, differential settlement may occur, posing significant structural and geotechnical concerns over a long time span, if not managed properly.
Langan has been retained to design a hybrid LFG mitigation system to control the potential intrusion of LFG into a world-class cancer institute, hospital, and associated parking garage to be built on top of a former landfill in Miami, Florida. The former landfill was closed prior to modern regulations and has no LFG collection system or cap. Methane concentrations in the refuse have been measured up to 60% by volume.
Approach/Activities
The major concerns associated with the new building construction on top of the former landfill are described below.
- Methane gas will be trapped under the confining building slab layer and can accumulate to levels above the LEL, if not vented effectively. Additionally, there is the potential for methane intrusion into the building, posing a potential explosion risk.
- Differential settlement is expected over the lifespan of the structures. This could lead to sagging of the vapor barrier or cracking of the mitigation system piping, potentially limiting the effectiveness of the mitigation system.
- Because the LFG mitigation system will be operating at a medical center, extra precautions needed to be taken during the design process and future system operation to protect patient health and safety.
To address these landfill redevelopment challenges, Langan designed a robust methane mitigation system with the following components and considerations:
- Removal of refuse directly below the main medical center structure - The removal of refuse from the medical center footprint and the implementation of deep dynamic compaction (DDC) reduces the risk of settlement over the lifespan of the building. In addition, flexible HDPE piping, pipe hangers, and GeoVentTM were used in the design to prevent potential breakage in the LFG collection piping.
- Hybrid Methane Mitigation System – The system components include a vapor barrier, a sub-slab vapor collection manifold system consisting of GeoVentTM and HDPE piping in a highly permeable ventilation layer, and roof-level vertical vent risers that convey LFG from the sub-slab ventilation layer to the exterior of the building. Because of the variable slab thicknesses and utility/piping infrastructure, required to support hospital equipment, passive venting alone was not an option. The hybrid system allows for automatic interchange of passive or active venting of LFG accumulated below the building slab based on in-line methane concentrations, or time elapsed since the most recent active venting.
- Methane Monitoring System – The methane monitoring system consists of indoor and in-line methane sensors connected to a central control panel with alarm set points and interlocks. Considering input from the architects, discreet indoor air sensors that resemble smoke detectors were selected and placed at locations free of potential patient interaction or potential hospital equipment interference.
Results/Lessons Learned
As part of the design phase, Langan coordinated extensively with the project architects, contractors, and coordination team, and utilized a Building Information Modeling (BIM) to evaluate the piping layout. Construction of the methane mitigation system, under Langan supervision, began in Fall 2023. Langan expects to observe significant sub-slab methane concentrations and primarily active venting for the first few months post-construction. Active venting is expected to decrease over time due to increased pore volume exchanges. As the landfill ages, less methane will be generated. As such, the hybrid mitigation system is expected to rely primarily on passive venting after several years of operation.