Source: www.TurfMagazine.com
Repairing the damage of the Hogan Butte landslide
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Right after New Year’s Day celebrations, on January 2, 2009, residents in the Hogan Butte area of Gresham, Ore., were startled out of their sleep to what sounded like an airplane bearing down upon their neighborhood, about to crash. The sound was actually a landslide.
“This huge mass of cold soil, boulders, chunks of wood, bits of rocks, anything and everything that was within its path started roaring down this draw right between two houses,” says Stephen Caruana of the landslide that awoke the Gresham residents. “It charged down through there, and then continued on after about a mile or so until it all dissipated.”
Caruana is the environmental planning and permit leader for Kleinfelder in Beaverton, Ore., a company Gresham officials called upon to deal with creating a plan to stabilize the area in the aftermath of the landslide.
Kleinfelder is a multidisciplinary engineering and environmental firm headquartered in San Diego, Calif. Services include environmental planning and permitting, natural resource studies, completing environmental impact statements and wetland delineations. The company has 2,200 employees throughout the country, with most centered in the western U.S.
Kleinfelder’s work has a strong geological and geotechnical component to it, says Caruana. The firm’s work has encompassed a number of different types of projects, including postfire restoration, dam removal and landslide remediation. The firm has grown through acquisitions, such as the recent acquisition of a civil engineering architectural firm, and as such has broadened the scope of services it offers clients.
For many public entities, a landslide is a nightmare in the making. Wet weather, expanding soils and steep slopes can dovetail into a recipe for disaster, bringing about an unexpected disruption of soil stability, threatening critical infrastructure, shutting down roads, and leading to more and even larger slope failures, as well as the potential for loss of life.
Ongoing maintenance to repair recurrent shallow landslides can be costly and can affect property insurance rates.
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Mitigating the effects of the landslide illustrates the strengths of the Kleinfelder operation. The mitigation cannot merely be a “bandage” approach to a problem, but a temporary repair upon which is built a permanent, stable solution that is designed to prevent such a problem from occurring again.
Following the landslide, Kleinfelder collaborated with Gresham officials in conducting an analysis and repair of the landslide and stream repair. The project involved determining the geological and geotechnical causes of the landslide failure, and then creating the consequent design and implementation of the corrective measures.
Kleinfelder produced a design that called for the removal of the deposited and unconsolidated landslide spoils; reshaping 1,500 feet of stream bank; stabilizing the soil; protecting valuable native trees; and the placement of proven best management practices of wattles, erosion control blankets and seedings.
Although it took everyone by surprise, the landslide had actually been months in the making, Caruana explains.
“When you think about the climate of the Northwest, you typically think of mild and rain,” Caruana points out. “Typically, the rain starts in October or November and a slope gets saturated. What sometimes happens here, which leads to the worst sort of conditions and potential for landslides and hazards, is if we get a big arctic air mass that’s cold enough and strong enough and pushes to the west and not the east, the ground gets frozen.
“That’s not typical here, as opposed to the Midwest, where the ground is frozen all winter long,” he adds.
What had happened at the Gresham site, which had an elevation of 980 feet, is that a foot of snow had accumulated.
“Then we got a warm storm from the Southwest; they call that a Pineapple Express,” says Caruana. “It comes in with lots of moisture and dumps that on the already existing snow there, and because the ground is still frozen, it tends to come off in a big rush. The ground was fully saturated, and because of the inherent nature of the soil, it let loose with a huge mass of material, which came roaring down.”
One major concern for Gresham officials was that the landslide started in an area located near a reservoir, a water tower and other infrastructure that could have sustained damage. Residents were concerned that their property was not stable.
When Kleinfelder employees assessed the area where the landslide originated, they discovered a number of unconsolidated masses with the potential to remobilize in a subsequent heavy rainfall, Caruana says.
There were also some concerns about stabilizing the area without compromising established vegetation that served a beneficial purpose.
“There were valuable, large, old-growth Douglas Fir trees,” he says. “There was a desire not to just go in there and bulldoze everything. We developed a solution to stabilize the area so it wouldn’t slide any further.”
Designs called for the flat areas to be stabilized to prevent them from sliding again, and for the streambed to be restored by cleaning it and stabilizing it.
“We used just about every potential erosion control practice that should be used, including erosion control blankets,” says Caruana. “We mulched and seeded the area with native grasses, then the city will follow this spring with tree planting.”
A key factor to the project’s success was Kleinfelder employees’ ability to immediately recognize the geological cause of the landslide, Caruana says. “Landslides can fall into many categories,” he explains. “They can be deep-seated slides that continue to go because vegetation has been cleared off and be super saturated, which was the case here. The root cause was identified and it was likely to continue, so we had to determine how far back up the hill did it needed to be stabilized.
“We may have looked at it strictly from a geotechnical point of view, but with my previous experience, and from a point of view in erosion control, I knew it was appropriate to separate out the underlying geological causes from a superficial surface solution that could also work,” Caruana says, adding that it was important to deal with the root cause of the land failure and to also stabilize everything downstream that was not a cause of the landslide, but was affected by it.
It also helped to work with people who were open to various solutions to the problem, says Caruana, crediting Gresham officials and a watershed manager with a background in bioengineering with enabling Kleinfelder to apply the most appropriate approach to the situation.
“We approached it as a design/build and took it as a more integrated approach to the solution of the program,” he adds.
Perhaps one of the most important consequences of the approach is the peace of mind Kleinfelder’s work brought to property owners who had encountered a frightening experience as a result of the landslide.
“You design such a project to solve a problem to alleviate that fear in a person, because a landslide can sound like a freight train coming down the road,” Caruana says.
Carol Brzozowski is a member of the Society of Environmental Journalists and has written extensively about environmental issues for numerous trade journals for more than a decade. She resides in Coral Springs, Fla.