Methane Mitigation Systems for Buildings
A methane mitigation plan is a comprehensive design that eliminates the hazards of methane soil gas intrusion into buildings. For instance, new structures above historical oil fields, landfills or soil contamination areas require a mitigation system to ensure the safety of occupants. Without proper methane mitigation, toxic vapors can migrate through foundations and affect indoor air quality. Consequently, these processes cause a lethal accumulation of methane gas indoors. And as a result, explosive and asphyxiation hazards endanger the occupants. Updated March 29, 2020.
Reason Behind Methane Mitigation
In the City of Los Angeles, these plans are a requirement for projects within Methane Zones or Methane Buffer Zones. Similarly, other regions of California, Colorado, and Texas also require mitigation systems in special hazard districts. Typically, these zones (or districts) are a result of regional petroleum fields, oil drilling sites, and natural tar surfacing, as well as landfill areas. As a result, the hazardous gas accumulates inside buildings and can concentrate to combustible levels overtime. In fact, explosions and fires have occurred before, resulting from subsurface methane gas.
Methane Soil Gas
Methane (CH4) is the largest chemical compound of natural gas, in terms of volume. Other components are ethane (C2H6), propane (C3H8), isobutane (C4H10) and various other hydrocarbons. The term “natural gas” means combustible hydrocarbon gas originating from natural biological and geological processes. The formation of natural gas stems from the microbial and thermal decomposition of subsurface organic material.
Bio-Genic Gas vs Petro-Genic Gas
The combustible gases resulting from microbial decomposition is “bio-genic gas.” This type of gas is most identifiable in landfill areas, swamp fields, marshes, and wetlands. On the other hand, the explosive gas which comes from the thermal decomposition of organic matter underground is “petrogenic gas.” This is most common within oil fields and tar pit areas. Regardless of origin, both of these gases exist in the form of higher-pressure pockets underground. Most subsurface natural gas pockets warranting building vapor intrusion mitigation systems, are the result of organic thermo decomposition and petrogenic gas.
Methane gas is highly flammable. Moreover, it’s odorless and colorless. Thus, it’s presence within confining spaces isn’t easily detectable by humans. The subsurface soil gas does move through soil. Theoretically, the gas takes the path of least resistance, and can build-up within pockets, at high pressure. Moreover, the flammable petrogenic gas has the tendency to rise to the surface and build-up within the lowest occupied spaces of commercial and residential developments.
There is an optimal range of concentrations for methane gas to ignite. This is known as the methane gas combustibility index. The minimum concentration is the “Lower Explosive Limit” or “LEL.” And the maximum concentration is the “Upper Explosive Limit” or “UEL.” As a result, methane mitigation systems must continuously monitor the interior and subsurface concentrations of natural gas and immediately alleviate the risk of an explosion.
Methane Zones & Districts
Urban development areas within proximity to oil fields, tar pits, swamplands, and landfills are known to have an abundance of petrogenic and biogenic gases underground. And the presence of these underground combustive gases pose a threat to the safety of commercial and residential building occupants. As a result, most government agencies have zoned such areas, as special soil gas hazard areas. Consequently, soil gas surveys, soil testing and vapor intrusion mitigation standards apply when building.
Huntington Beach & Los Angeles Mitigation Standards
The City of Los Angeles publishes a map of high-risk “Methane Zones” and “Methane Buffer Zones.” For properties in a Methane Zone or Buffer Zone, there is a requirement to comply with Los Angeles Department of Building and Safety (LADBS). Site-specific design parameters for the system are based on the data from a methane soil gas test. There are various LADBS mitigation design levels. Accordingly, the mitigation system requirements increase with each level. As a result, the plans can include specialized ventilation systems that are either “Passive” or “Active.”
Generally, the design parameters of a methane mitigation system are a reflection of the results of a methane soil gas test. Using modern drilling methods, geologists follow a strict set of standards to install soil gas probes at various depths below the ground surface. Each soil gas probe includes a special vapor implant which then applies to collect a representative vapor sample from the pore spaces of soil underground. And an assessment requires numerous samples from various depths and locations.
Los Angeles Building Code
A methane test provisionally screens the subsurface for methane soil gas, per the standards of the LADBS. In fact, the soil gas survey process provides design input data for a methane mitigation system engineer. There are five primary methane design levels in the City of Los Angeles. And the testing and design parameters are in general accordance with Division 71 of the Los Angeles Building Code. Most other building departments, fire departments, and environmental agencies across the nation implement similar codes, which base on the LADBS standards.
Methane Mitigation Construction
In any case, the methane mitigation construction process must fully comply with local building code requirements. In this area of work, most jurisdictions have standards that are similar to the City of Los Angeles codes. Nonetheless, the designer of any methane mitigation plan needs to be in constant communication with the construction team, as well as the methane deputy inspectors.
Design Methane Concentration
The design methane concentration is the highest methane measurement observed at any soil gas probe during the methane test. A design methane concentration is the primary factor in determining the mitigation level. For instance, a property with methane concentrations as high as 16,000 parts per million (units of concentration), falls under the “Level 5” category. As such, LA City Level 5 design parameters become a requirement for future development at that site.
Design Methane Pressure
The design methane pressure value is the highest earth-pressure measurement from any soil gas probe in a methane test. The design methane pressure also plays a significant role in the determination of a site-specific methane mitigation level. For example, a Level 3 site with pressure readings above 2.0 inches of water (units of pressure) requires additional methane mitigation components, comparing to that of a site less than 1.0 inches of water.
System Levels 1 through 5
Typical methane mitigation standards comprise a “Passive Mitigation System,” an “Active Mitigation System,” and “Additional Miscellaneous Components.” In fact, most agencies provide tables and charts which demonstrate methane test results against site-specific mitigation requirements. Moreover, industry standards are available for engineers to reference when preparing a site-specific methane mitigation plan.
Passive Vapor Mitigation Systems
A complete passive mitigation system includes a series of perforated horizontal ventilation pipes within a network of gravel blankets and an impervious membrane, as well as vertical ventilation risers. Passive ventilation systems are complex engineering projects which intend to block and remove hazardous gas from underneath buildings. A passive system relies on the natural rising characteristics of hydrocarbons and volatile organic compounds (in the vapor phase), in order to capture the accumulations underground. Furthermore, the system is strategically set-up to naturally direct the gas upwards and around the structure, exhausting it into the atmosphere.
Sub-Slab & Vertical Riser Ventilation Piping
At a minimum, a Passive Mitigation System includes a network of horizontal ventilation pipes underneath the building foundation. Engineers custom-design each system to operate efficiently, and optimally. Part of the plan includes specifications on piping frequencies, piping diameters, perforation widths, gravel types, thickness, and more. Gravel and sand blankets are also overlain by a special VOC-and-hydrocarbon-impervious membrane. And a series of vertical ventilation risers connecting with the horizontal pipes allow the captured methane to migrate around the building, as opposed to into the building.
VOC & Hydrocarbon Impervious Vapor Barrier
Another crucial component of any methane mitigation system is the special vapor barrier that stops the soil gas from invading the lowest occupied space. Underlying the slab (and also the surrounding the walls of a subterranean foundation) is a gas barrier that is physically and chemically manufactured to be impervious to volatile organic compounds and hydrocarbon gases. The designing and installation of this barrier is highly critical and requires special training and certification to do so. In fact, each methane barrier installation requires on-site inspection and approval by the regulatory building department, fire department, and manufacturer. The ramification of any error in this process can be catastrophic.
Active Vapor Mitigation Systems
An active mitigation system includes earth-pressure sensors below the impervious membrane of a structure, as well as a mechanical extraction system, an indoor ventilation system and a gas detection/alarm system. Active methane mitigation systems comprise electrical and mechanical designs, for optimal methane gas extraction. Typically, the systems operate using vacuum blowers, pumps, fans, sensors, and a control panel. Moreover, HVAC systems are synchronized to accelerate indoor ventilation. Additionally, an active system includes methane gas sensors and an alarm system. These systems can warn occupants if dangerous levels of methane occur in living spaces.
Subsurface Ventilation & Soil Vapor Extraction
An Active Mitigation System includes a network of sub-slab and ambient air methane gas sensors that are hooked up to a main detection system and control panel. Additionally, sub-slab pipes are mechanically enhanced for the proposes of ventilating the subsurface of a building within a methane zone. In the same way, and a mechanical ambient air ventilation system exists to reduce combustibility indoors. In a nutshell, when concentrations of methane gas approach the lower explosive limit, the sub-slab and ambient air ventilation systems instantly activate to mitigate the hazard of combustion.
Alarm System & Sensors
A control panel and operating system processes the data from the sensors and accordingly activates the ventilation units. Nonetheless, the control panel will periodically self-activate the ventilation system in order to maintain a proper air exchange rate indoors and at the sub-slab. To illustrate, the control panel is the brains of the mitigation system and facilitates a communication platform amongst all of its components. Moreover, the control panel system triggers an alarm system to warn occupants about peaking levels of methane soil gas, or system malfunctions. Alarm systems are engineered to be noticeable by all occupants of a building and are audible, as well as visual.
Miscellaneous Design/Construction Components
Miscellaneous components include trench dams, as well as the use of special conduits and cable-seal fittings. Trench dams block the intrusion of methane gas through subsurface utility line trenches. For instance, sewer laterals or water mains. Additionally, the use of specialized conduit and cable-seals prevent the seepage of hazardous gas from within utility lines.
Dewatering and Waterproofing in Methane Mitigation
Groundwater levels fluctuate throughout time. And variations are based on local environmental impacts, climatology and nearby human activity. When the bottom of subterranean structure supersedes the depths of groundwater, the methane mitigation system and overall lowest occupied space of a building are at risk of flooding and failure. As a result, the infiltration of groundwater, as well as hazardous methane soil gas, compromise the structure. Moreover, the hydro-static pressures of surrounding groundwater can also compromise the structural integrity of the foundation. Thus, design adjustments need to be made.
Lowering the Water Table
When an impervious membrane and sub-slab ventilation piping network are within proximate depths to historically highest groundwater levels, there becomes a strict requirement for a dewatering system. A dewatering system lowers the groundwater table to a safer elevation with respect to a subterranean space and methane mitigation system. Additionally, special waterproofing applications prevent groundwater from passing through a methane vapor barrier. Ultimately, dewatering and waterproofing systems are designed in accordance with methane mitigation systems when necessary.
Geo Forward Professional Services
The geologists and engineers at Geo Forward are experienced with the latest design standards and building codes. Geo Forward specializes in designing these systems for commercial and residential projects. Moreover, the Geo Forward team is an expert in barrier applications for subterranean parking garages, underground vaults, basements and more.
For more information about methane mitigation plans, call (888) 930-6604 to speak with an expert.
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