Sculpted Soil Nail Walls Blend In on Highway 92

Its creators hope no one will even notice the results of their two-year effort. But if you find yourself driving along California’s Highway 92 on the San Francisco Peninsula, it’s worth watching for. Nine segments of sculpted soil nail wall–over 160,000 ft.² in all–along a 2.3-mi. portion of road make this project the largest of its kind in the United States.

As increased traffic mandated widening Highway 92, the California Department of Transportation (Caltrans) sought a method to reinforce the steep roadside embankments along this mountainous stretch. “There were other ptions, but this was the most economical,” says the wall’s designer, Hooshmand Nikoui, branch chief of geotechnical engineering for Caltrans in Oakland, CA. He estimates that a traditional tieback wall would have cost twice as much as the soil nail wall.

Caltrans has used two types of retaining walls along Highway 92. The relatively short slopes below the roadway have soldier pile wall with wood lagging, with no tiebacks needed because the walls are generally no higher than 13 ft. During road widening, however, Caltrans chose the passive-reinforcement soil nail walls for the hills above and on both sides of the road, many of them nearly vertical.

Disturbing the terrain as little as possible was another consideration. Unlike other types of retaining walls that require excavating and backfilling, the soil nail wall allowed minimal disturbance. “The other option was to cut the hill,” says Nikoui. “That would remove a lot of very environmentally sensitive area.”

Building in Segments

Soil nail walls are typically used to reinforce existing embankments. First used in France in the early 1960s, this type of wall allows steeper slopes to be excavated and then reinforced, permitting efficient land use in a limited space. Soil nail walls are built in stages from the top down. The soil nails–high-grade rebar–penetrate at an angle into the existing soil to a depth of 10-40 ft., depending on soil conditions. The entire reinforced section of earth has the coherence and stability to act as a retaining wall for the soil behind it.

Each segment of the Highway 92 project started with a bench cut at the top of the wall, about 6 ft. high and 15-20 ft. deep. A drilling rig drilled holes into the face of the wall 4-6 ft. apart. The nails were centered in the middle of each hole and grouted in place. When the grout dried, the nails were tested and had to exceed 200% of minimum pullout.

Next, a rolled geocomposite drainage board was attached to the slope. Welded wire mesh was placed over the face and fastened to the nails, then an initial coat of shotcrete–the structure coat–was applied. The structure coat was about 4 in. thick on the Highway 92 project but can be as much as a foot thick, depending on soil conditions. Bearing plates slid over the still-protruding nails and were screwed down while the shotcrete was still wet.

Once the shotcrete dried, the nails were again tested for pullout, and a second coat of welded wire mesh was attached. On Highway 92, Boulderscape Inc. of Capistrano Beach, CA, sculpted the wall to match the surrounding natural rock. The company creates rock formations for residential and commercial projects, including many exhibits at the San Diego Zoo. Before applying the second and final coat of shotcrete, Boulderscape technicians painted multicolored rock formation lines onto the face of the wall.

“They determine where we’ll have outcroppings, where we’ll have bedding joints, and the strata,” explains Steve Jimenez of Boulderscape. “That tells the architect or engineer where the rock is going to be formed on the wall. It gives them a skeleton. It also lets our nozzlemen know where to place more or less shotcrete.” Working from a manlift, a two-person crew applied a layer of shotcrete that ranged from 4 to 15 in. thick, depending on the desired effect on different parts of the wall. “We follow right behind them with our sculptors and sculpt the rock while the shotcrete is wet,” he describes.

The height of the wall segments varies, depending on the geometry of the hill. As the gradient changes, Nikoui explains, “when you widen the road the same distance, the top of the wall is going to be higher or lower.” The variation also gives the wall a more natural appearance. A concrete drainage swale runs along the top of the wall to catch and remove rainfall and runoff from the top of the hill as quickly as possible. The geocomposite layer behind the face of the wall also drains water quickly to drainage areas at the bottom of the wall to prevent trapped water and the buildup of hydrostatic pressure.

Imitating Nature

Before beginning a project, Boulderscape photographs natural rock formations in the area and plans the colors, the horizontal and vertical cracking, and other features of the sculpted wall to blend in as closely as possible. The near-vertical segments of the Highway 92 project presented an ideal medium for the company. “We get most of our best sculpting on walls that are actually vertical rather than battered back,” notes Jimenez. “We get a bit more rock sticking out of it, and it looks a lot better when it’s uneven.”

For a natural look from the beginning, the sculptors stain the finished wall, although natural coloration also occurs, caused by mineral deposits, soil, atmospheric staining – walls near the ocean receive salt deposition, for instance – and vegetation. The stains Boulderscape uses have undergone accelerated weather testing and will last 50 years, although Jimenez says they start to fade after a decade or so. “But it’s picked up by natural staining, so the wall always has color, and it will change with time.”

How many people driving on Highway 92 will realize the work that went into the wall? “Hopefully, nobody,” replies Jimenez. “Our goal is to sculpt a rock formation so much like nature that when motorists drive by they never know that man ever touched the area.”

The public’s first reaction to the project, says Nikoui, was to complain about traffic jams caused by the roadwork, even though crews avoided working during rush hour. “But then it was built, and they couldn’t believe it. They couldn’t believe Caltrans did such a thing–manually carved a wall that looks like sandstone and is aesthetically beautiful.”