Environmental Engineering and Contracting

Arroyo Geoscience Inc., Geotechnical Engineering, Glendale, CA
Arroyo Geoscience Inc., Geotechnical Engineering, Glendale, CA

Office: 818-967-5977

A-HAZ Engineering Contractor Lic. No. 1062092

Methane Mitigation System

Methane Mitigation System Design (Phase II)

Every methane mitigation system needs a sound, thoughtful design that underpins the whole plan and ensures reliable, proper functioning.

Methane gas mitigation is a critical component of any construction project or development within Methane Buffer Zones (or simply, ‘Methane Zones’). These special zones exist because of the development site’s proximity to hazards like gas wells or petroleum fields, where subsurface methane gas is ever-present and dangerous.

This Los Angeles methane zone map gives you an idea of what we’re dealing with when it comes to sites with methane mitigation requirements in Southern California.

Why is Methane Gas Mitigation So Important?

The migration of methane gas into buildings is no joke – there’s a reason why these methane zones in Los Angeles exist. 

If a building doesn’t have the proper toxic vapor intrusion mitigation in place, contaminated air can make its way into your office, your living room, or your gym. 

In addition to being quite bad for you to breathe, if active or passive vapor mitigation systems aren’t in place, the amount of methane will reach combustible concentrations. Yes, that means major fires and explosions are possible (and even likely) if developers don’t take this crucial proactive step.

What methane Mitigation System Design Options Are Available?

Luckily, methane mitigation contractors in Los Angeles like the experienced crew at Arroyo Geo have several different fundamental designs to tackle any type of project.

  • Passive Methane Extraction – as the name implies, these types of designs help minimize methane in a passive way, without any fancy gadgets. Since subsurface methane wants to float upwards, we can use that property to design a system that captures it before it can accumulate to dangerous levels.
  • Active Methane Extraction – these designs rely on powerful mechanical fans to push methane gas away from the structure area. Through induced pressure gradients, active designs like the ones created by Arroyo Geo can be surprisingly effective for a lower cost than you expect.
  • Methane Ventilation Systems – the expert HVAC team at Arroyo Geo can install a ventilation system in your substructure to keep the air breathable and the methane accumulation to an absolute minimum.
  • Trench Dams / Backfills – through a carefully designed combination of materials, Arroyo Geo can create a backfill alternative that has incredibly low permeability without breaking the bank. 
  • Methane Sensors / Alarms – for an extra added layer of security and peace of mind, methane sensors can monitor subsurface gas around the clock — and trigger an alarm if the concentration becomes hazardous.

Methane Mitigation System Design

After the site design level is determined, the next step is for the design engineer to design the appropriate mitigation system for the building. In the city of LA, properties within the Methane Zone will still need a passive mitigation system even if no methane is detected during site testing. The reason for this is two-fold: on the one hand, site testing only involves making between 2 to 4 monitoring wells on a site, and subsurface “pockets” of methane gas concentrations that may be present between monitoring well locations can be missed. 

Moreover, properties in the Methane Zone (as opposed to Methane Buffer Zone) are usually located in areas of former landfills, oil fields, dairy farms, and other surface features that are generally known to generate methane. So the requirement for a passive system to be installed is generally considered a safeguard.

Los Angeles Methane Mitigation Standards & Common Scenarios

In sites in which methane gas is detected above the screening threshold, this will require the installation of both a passive system and an active mitigation system. 

Passive Methane Mitigation System Design

The passive system, which is commonly installed for methane as well as other soil gases, generally involves the installation of a subslab ventilation system, consisting of 4” perforated pipe encased in gravel, and the installation of either plastic vapor barrier, or for larger buildings, a spray-applied membrane that dries to the design thickness.

The subslab ventilation system is connected to risers which vent any collected sub-slab gas away from the building.

Active Methane Mitigation System Design

The active system consists of the components of the passive system as described above, as well as electronic components including alarms, extraction systems, sensors and probes above and below the membrane, and/or mechanical ventilation systems.

The exact components are designed by the engineer based on the needs of the proposed building.

Modified Methane Mitigation Design Based on Los Angeles County Code Requirements

In some cases, a modified design is done, which may include some components of a system but not all, typically used in the case of existing buildings with slabs in which a subslab system cannot be installed.

Los Angeles County Building Code - Methane Requirements

Properties in the LA County jurisdiction have slightly different requirements. Cities with past and current oil fields, such as Huntington Beach or Brea, typically have their own requirements for mitigation of methane gas. Arroyo’s design engineers have detailed knowledge of the requirements of local jurisdictions, and design the system accordingly.

The LADBS website has initial screening requirements, but that’s just the starting point. The city of Los Angeles has five primary methane design levels, in compliance with Division 71 of the LA building code. Depending on the concentration of methane present (as well as the pressure level), your development will require one of those 5 standards of mitigation to account for the real level of accumulation on-site.  More information on DPW website.

ArroyoGeoscience Professional Methane Mitigation Design Services

Our team of expert geologists and methane mitigation experts know how to craft the most effective, budget-conscious mitigation plan for any development.  Call us at 818-326-3200 or fill out the form below today to get more information about a methane mitigation plan that’s perfect for your needs.


Other Steps In The process..

Previous: Phase 1 - Testing

Mitigation System Testing

Next: Phase 3 - Installation

Methane Mitigation design details