The Science of Making Belmont Safe

RIVERSIDE COUNTY — Under a hot, humid March sky, down the road from a cow shed, another subdivision is on the rise at the end of a featureless cul-de-sac. But there’s one complication. These four houses under construction are located atop ground that contains methane gas — a natural byproduct of decaying biomass, in this case, cow manure from a dairy. Methane disperses harmlessly in open areas, but it can accumulate to explosive concentrations in enclosed spaces. Unless something is done, methane could eventually collect in these homes and become a fire hazard. Light a match in a room full of methane — and poof.

A similar challenge confronts the Los Angeles Unified School District at the half-finished Belmont Learning Complex. Belmont, like these houses and like thousands of other Southern California properties, sits on property that emits methane gas. At Belmont, the gas comes from the oil field below, a common situation in Los Angeles. But methane also is a problem for contractors building on wetlands, land that used to be swampy or former agricultural sites, especially dairies.

Francesco Fonseca knows exactly what to do. He’s handled this problem many times before for GeoKenetics Inc. "We’re making a burrito," he explains. More precisely, he’s building a system that will draw in methane from under the house and then vent it harmlessly into the open air (see "How to Make a Methane Burrito").

Fonseca and two coworkers have dug a series of trenches and lined them with a synthetic, woven fabric. The trenches snake under where the living room, the bathroom and the garage will be.

Plastic pipes slotted with vents trace the same path, inside the trenches. These pipes connect to a vertical joint that will soon lead an exhaust pipe above roof level. Gravel from a backhoe is then shoveled in by the workers. At this point the system more resembles a taco than a burrito. But then the ends of the synthetic fabric are folded over, and fastened together with duct tape. There’s your methane burrito.

Methane, which is lighter than air, follows the path of least resistance. So before it can find a crack in the floor slab, it will encounter the open vents in the pipes and begin its journey to the atmosphere.

Day and night cyclical variations in barometric pressure, says GeoKenetics principal engineer Glenn Tofani, will ensure that methane is drawn out and fresh air brought back into the pipes. This "passive" way of evacuating methane is by far the most common method. But it can be made "active" by installing a suction fan.

But there’s more. A week later, Tofani is on site to supervise as workers spray on — over the filled-in trenches — a membrane called Liquid Boot, which dries quickly to a thin white coating. The job of the Liquid Boot is to act as a barrier keeping methane from the house. If all works as planned, the Liquid Boot coating will, in effect, route the methane toward the vented pipes.

For this job, Tofani has decided to add another barrier too — a layer of plastic called high-density polyethelyne.

Installing safety systems to protect against methane has become a cottage industry in Los Angeles in the wake of a 1985 methane explosion that injured about two dozen people in the Fairfax District. The technology was originally developed for the petroleum industry and landfills, which can emit methane and other gases. Barrier-and-venting systems are underneath, among thousands of places, just about everything new built in the Fairfax District as well as L.A.’s Central Library, a Wells Fargo bank on Beverly Boulevard, housing tracts in Orange County and a Marriott hotel in El Segundo. Whole hillside developments in Brea are planned on former oil fields.

At Belmont, there’s an additional challenge. The ground also contains spot concentrations of hydrogen sulfide, which is toxic. Though not as pervasive as methane, hydrogen sulfide too can be found in parts of the L.A. basin that are fully developed with homes, schools and businesses. It can be dealt with through machinery that separates hydrogen sulfide from methane, so that the hydrogen sulfide can be collected in a tank.

Such a system is in place in Newport Beach at Hoag Hospital, which has higher concentrations of underground hydrogen sulfide than the Belmont site. Hoag uses its "cleaned" methane to heat the building.

The safety system at Hoag incorporates a network of wells and trench collectors that pump soil gases to a central unit, where air scrubbers remove the toxic hydrogen sulfide. The hospital also has backup measures: A pipe-and-barrier system underneath the building slab serves as extra guard against gas leaking through the building’s foundation. Inside the hospital, there are air pumps that pull in fresh air at a rate faster than the gas can come through the foundation — should gas sensors detect seeping methane.

A similar layering of fail-safe systems would most likely be the plan at Belmont. For instance, contractors could install a plastic lining under the buildings, and below this, a venting system to pull out soil gases. Hydrogen sulfide could be collected in a tank, as at Hoag. Methane alarms could be installed below the barrier, above the barrier, or both, as could an alarm signaling a malfunction in the venting system. The setup sounds complex to a layman, but it’s relatively simple, established technology, according to engineers interviewed by the Weekly.