Challenging Landslide Restoration

Thursday, January 2, 2014

Challenging Landslide Restoration

By Steve Goldberg

an excerpt from Of Erosion, Soils, and Seeds

Published in the Jan/Feb 2014 issue of Erosion Control Magazine

On the outskirts of Los Angeles, between Santa Clarita and Gorman, a major landslide occurred several years ago, shutting down several lanes of the southbound portion of Interstate 5.

"That area is known for sliding, slumping issues because of a high water table," says Christopher Stevenson of Caltrans (the California Department of Transportation). "The fracturing, underlying soil has a very high clay content. It swells and is a very difficult soil material."

After the landslide, emergency repairs were needed, as Interstate 5 is one of the main arteries in and out of Los Angeles. More than a million cubic yards of material were removed. "To save the state the high cost of hauling all that material away, it was deposited in Violin Canyon below," says Stevenson. "That was phase one, which was mainly geotech engineering work. They benched the mountainside, putting in several plateaus in what is called a 'buttress slope,' which holds the underlying material behind it."

The landslide occurred on US Forest Service land, which is under a 99-year lease with Caltrans. The site includes about 1.5 acres of wetland.

"We're on the hook for 3:1 mitigation," he says. "This means that we had to restore impacts to the wetlands and waters of the US. Generally, the first order of restoration, if possible, is to do that onsite, at the location of the impact. In this case, because of the issue of the high water table and an area with a high probability of further landslides-and the potential of water causing that-we were not able to do any onsite wetland restoration.

"What was really more the problematic issue to deal with was this soil that we exposed hadn't seen the light of day for several million years, since we scraped off the top 80 to 90 feet of soil. We thereby exposed this very friable, fracturing soil and then had to figure out how to restore the upland portions of that area," he says.

"In phase two of the restoration work, we had a lot of failure due to the fact that this soil was so friable and just eroded away so easily. It was expansive soil with heavy clay, so things did not grow so well. We had to figure out how we were going to get something to grow in this difficult soil.

"Two test spots were run, each about an eighth of an acre, one on a 2:1 slope, and one on a 4:1 slope. Those test plots incorporated a method called 'compost and incorporate.' We brought in topsoil and compost and ripped the soil to a depth of at least 18 inches to incorporate the compost material. This helped with deep saturation and percolation, so instead of water running off and eroding, it would soak and go into the soil.

"After seeing that this was quite successful, we incorporated three different types of erosion control measures and restoration planting. There was the compost and incorporate, in which we had to rip the soil down 18 inches and incorporate a 3 to 4 inch layer of compost. Another method was a compost blanket. In this case, the soil is just roughed up a little, with a layer of compost placed on top. The third method was standard hydroseeding, with no pretreatment to the soil."

The seed mix, supplied by S&S Seeds of Carpinteria, CA, was a nonstandard seed mix, composed of all native seeds.

"It was specified because of the sensitivity of the particular location in Forest Service land," notes Stevenson. "We worked in coordination with landscape and environmental planning biologists who came up with specific seed mixes for sensitive sites."

There was a delay reaching this final revegetation stage. An earlier series of plantings hadn't gone well, and two rainy seasons led to problems with erosion. Then there was roadway work, which took about two years to complete. The contractor had to remove much of the expansive soil from under the roadway because it was causing buckling and warping of the road.

So it wasn't until spring 2012 that this latest revegetation effort was undertaken. Approximately 12 acres were involved, roughly evenly divided among the three different revegetation methods-compost and incorporate, compost blanket, and hydroseeding.

Stevenson explains that one of the many challenges on the project involved determining proper irrigation for the site.

"In the first phase, we tried drip irrigation, but that did not work well. This time the contractor chose overhead irrigation, which is still a bit of a challenge and difficult, because it's a very hot and dry and windy location. Getting water to land where you want it to land is difficult."

RainBird units were used initially for overhead irrigation. "They're really made for more temporary installations," says Stevenson, noting that the goal for any Caltrans highway restoration project is that ultimately, natural rainfall will be sufficient for irrigation needs.

"They typically plan an establishment period. On average, it's usually only about 120 working days. In this case, because of the difficulty of the site and wanting to make sure that we get things well established before we pull the plug, this one had a two-year plant establishment timetable."

A number of other erosion control measures were also employed.

"We used fiber rolls," says Stevenson. "They also installed a concrete box channel on part of the slope. This unfortunately didn't do so well. There is also a rock riprap channel to handle the flow of surface water off the roadway. We planted willow and mulefat within the rock crevices. We're trying to get that established in hopes of meeting the requirements of the Army Corps [of Engineers] and [the California Department of Fish and Wildlife] for wetlands and water restoration and mitigation."

The project has continued to present ongoing challenges. The site experienced a high plant mortality rate for the first year of restoration. Those areas were reseeded, relying more on hydroseeding this time and much less planted material.

"There were really only two methods for the reseeding-it was either hydroseeding or using potted plant materials, depending on the slope and the angle of the slope," he says.

One of the original goals had been to restore the coastal sage scrub habitat. "But due to the fact that we really changed the soil profile completely, we're just happy to see native species establishing there," Stevenson says. "We now have a very high alkaline soil, so we're not seeing the typical plants that were, for example, growing next door. I don't think we're going to see typical coastal sage habitat being restored. But we are seeing some native plants being established. Over time, Mother Nature may just come in and change this."

Because of the difficulties encountered with this project, Stevenson stressed the importance of problem-solving utilizing a diverse group of individuals.

"When you have a site like this, you really need to have a team approach, working with people with different approaches and different insights. We've had an expert with experience working on restoration with difficult soils, and other experts with different viewpoints and understandings of these kind of complications. It really helps us to figure this out and get it done."

 To read the full article click here.