It hardly ever rains in California, but when it does it pours, in this case setting loose a 30,000-cubic-yard mass of clayey silt that threatened houses built on top of the slide and at the bottom.

By Penelope B. Grenoble

The winter of 2004–05 brought near-record rainfall to southern California. Over 34 inches of precipitation fell at the Los Angeles civic center, nearly three times the annual average for this semi-arid region, making this the second wettest season since record-keeping started. Much of the precipitation hit the ground in the first two months of 2005: 7.25 inches in January and 8.73 inches in February, which accounted for half the year’s total and set saturated slopes sliding.

The bad weather complicated what would otherwise have been a classic case of slope stabilization in the Orange County suburb of Mission Viejo near the foothills of the Santa Ana Mountains. The fix was further complicated by the slide’s location, surrounded by houses in a forty-year-old subdivision, which constricted access and limited equipment maneuverability, and the fact that although the land that slid was private property, the slide had the potential to affect public resources, and valuable time was lost in discussions about who would be responsible for the repair.

At issue was a 67-foot-high, 250-foot-wide, 20,000–30,000 cubic-yard mass of clayey silt moving downslope at the rate of 2 to 4 inches a day, endangering at least one house at the top of the slide and six houses at its toe. The slope was graded in 1967 and included construction of a 25-foot-wide by 5-foot-deep buttress key along the toe of a natural hillside followed by placement of up to 30 feet of compacted fill along the top of the hill. The graded slope was constructed at a ratio of 1.75:1 (2:1 is the current standard).

The houses at the bottom of the slope and on the cul de sac above it were built on roughly 40-foot-wide lots. Each of the down-slope houses had 20-foot-deep backyards that backed up directly to the hillside, and unlike newer subdivisions where open slopes are typically owned as community property and maintained by homeowner associations, residents who lived in the path of the Mission Viejo slide were responsible for maintaining the hillside behind their houses, from their backyards to the top of the hill. Hillside vegetation consisted of irrigated ornamental trees and non-native ground cover. Water runoff was facilitated by a series of surface V drains. Underground drains directed subsurface water into the street.

Although the slope in question had not demonstrated any previous failure, geologists at Petra Geotechnical, Inc. in Costa Mesa, CA established that it had been constructed over a preexisting layer of clay, 0.25 to 0.5-inches thick, which ran 10–15 feet beneath the houses at the bottom. Petra’s experts further concluded that excessive moisture from this winter’s heavy storms, together with long-term landscape irrigation, increased hydrostatic pressure and triggered the slide. The company’s engineers and geologists recommended a classic fix that involved taking the weight off the top of the slide and installing a gravity buttress at the bottom to check movement. But quick resolution was thwarted while homeowners and city representatives engaged in discussions about how to finance the temporary stabilization, the cost of which under less catastrophic circumstances would have fallen to the homeowners. Meanwhile the rain continued to fall and the slide continued to move.

January 21, 2005. Petra Geotechnical, an 18-year geological consultant to the city of Mission Viejo, is called in on what at first appears to be a routine case of homeowner distress. The magnitude of the situation is immediately established, however, and Associate Geologist Dave Seymour quickly consults the original grading records for the subdivision, establishing that the slide has occurred in a designed fill-over-cut area. Petra geologists and engineers recommend borings to determine subsurface conditions, but the city attorney blocks the action since the slide is located on private property.

Six residents whose homes are threatened by the slide hire Petra Geotechnical, which immediately proceeds to drill three 24-inch-diameter bucket auger borings, one at the top of the slide, one at the bottom, and one on the lowest terrace drain. Borings are also drilled in the street in front of the six houses. Coincidentally, both Petra and the city engineer informally monitor the slide’s movement on a daily basis using buildings and retaining walls as monitoring points.

Laborers spent days covering the hillside with Visqueen plastic.

“This was a deep-seated, slow-moving slide that had failed upon a preexisting clay layer that apparently the original geotechnical consultant had not identified,” says Seymour. “The extra water pressure from both behind the slope and underneath the clay layer as a result of the above-average rainfall built up sufficient hydrostatic pressure to cause the clay layer, which dips away from the hillside, to give way. The mass that failed was the fill-over-cut portion of the slope. By design the original engineer had loaded the top of the slope, which added to the situation.”

January 25, 2005. Aware of the consequences of continued saturation of the slide mass, and still acting on their own, residents are advised by Petra Geotechnical to cover the sliding hillside to keep water from accentuating fissures that have developed on the slide face and to forestall further movement. The homeowners attempt to do the job themselves. So far, 7.44 inches of rain have fallen.

January 26, 2005. The city of Mission Viejo steps in; the homeowners grant LT Engineering, Inc. of Laguna Niguel, CA right of entry to grub the hillside and cover the slide with plastic. Project Manager Len Brongo sends in a crew of 20 laborers to hand remove vegetation. Ten laborers spend two days covering the slope with 10 mm Visqueen plastic anchored by 15-pound sandbags. Another 0.01 inches of rain falls.

“It was more of a slump than a slide,” says Brongo. “We were able to get the slope completely covered, but it kept moving. Plus, it was windy and everyday we were up there adjusting the plastic and sandbags. By that time, the fissures were 30 feet deep and 20 feet wide. The water pooled, the slide pulled on the plastic, and the sandbags fell into the fissures, which created traps for the laborers. We felt we had to do something, but there was nothing we could really do because it was unsafe. Finally, I pulled everyone off the slope.”

“Those were depressing days,” says LT Engineering crew supervisor Chris Alves. “It was raining and the wind was blowing and everybody was frustrated. We tried to build a tent over the fissures, but you can’t bag over an open area. One of our laborers was up there when the slide fell a foot. When he came out he looked like he was in shock.”

The two days spent covering the slide were followed by almost two weeks of good weather, but the damage had been done. The weight of the slide pushed the ground up beneath one of the six houses at the toe of the slide and residents were evacuated. A swimming pool two houses down was drained and filled in to keep it from uplifting, and one house on the cul de sac at the top of the slide was yellow tagged (meaning residents could visit, but not sleep there). Ground fell away from one unanchored section of a swimming pool located next door.

January 28, 2005. Another 0.11 inches of rain falls, bringing the monthly total to 7.25 inches.

February 6, 2005. Another 0.07 inches of precipitation falls in the area.

February 10, 2005. Another 0.08 inches of rain falls. Petra Geotechnical develops a plan for temporary stabilization of the slide that calls for removal of approximately 15 feet off the top and adding approximately 10 feet at the bottom to create a small gravity buttress.

“Ideally it’s a balancing act,” says Seymour. “You’re trying to reduce the amount of load or driving force at the top and putting a resistance mass at the bottom. We were looking at a 1:1 cut from the top going down about 15 feet to a horizontal bench that would be established in the range of 20–30 feet deep. We saw this as the most effective way to substantially slow down the slide in the shortest period of time. It was also the most practical approach given the six houses located at the toe of the slide, which made access a key issue.”

“The movement was visual,” says Brongo. “You could see the ground raising up. You could see retaining walls and sidewalks moving. The idea was to unload the slope and get the dirt down to the toe. No problem. Then it started raining.”

Worried about the potential effect of the slide on the street in front of the endangered houses, the city authorizes Johnson-Frank & Associates, Inc., a land surveying company based in Anaheim, CA, to monitor the pavement below the slide for movement.

“It was difficult to tell where the slide actually quit,” says Johnson-Frank president Roger Frank. “We set up a system where we had four prime control points. Two of them were far enough away that we estimated we were completely out of the slide area. One was just north of the suspected movement area and one up the hill, east of the endangered house at the top. The area was probably three or four acres overall.

“We set up four static GPS units [Trimble 5700s and 4000 ssis] on these four prime points and measured in from four continuous operating reference stations [CORS] that belong to the Southern California Integrated GPS Network [SCIGN], a group of universities and public agencies studying earthquakes in conjunction with the USGS. The closest CORS is four miles away, another one is five miles away and another seven miles away. Together they essentially bracketed the job. The CORS units are permanently mounted; they record 24 hours a day and are downloaded once a day. Our approach was to download data from these CORS stations from the day before, put it together with the data we collected with our portable GPS units, and using software, compute the positions of our four prime points to see if they changed over the course of days and weeks. From this we determined that the slide was isolated on that hillside and the area around it wasn’t moving.

“Then using as our benchmark the ‘verified stable’ local control point that was down the hill and the farthest away from the slide, we ran precise levels through a group of monitoring points we set in the curbs on the street below the obvious slide area. We did this using a Leica DNA3 leveling system. The rods are made of bar codes and have INVAR metal tape on the face, which means they have a very low coefficient of expansion. This system gives us the most precise leveling available, allowing us to reliably position the monitoring points within ±0.0002 feet (about one sixty-fourth of an inch) on a recurring basis, which is important because the more accurately you are able to measure, the quicker you’ll see change over time. If the street starts to move, we want to know it right away. In addition, these same points in the curbs were positioned horizontally on a recurring basis using conventional total station surveying equipment in a networked manner to produce reliably repeatable results within ±0.004 feet (about one thirty-second of an inch).”

February 11, 2005. Another 1.36 inches of rain and a week and a half of heavy rainfall sets in.

February 12, 2005. 0.49 inches of rain.

February 17, 2005. Another 1.4 inches of precipitation.

February 18, 2005. 1.76 inches of precipitation falls.

February 19, 2005. 1.29 inches of rain.

February 20, 2005. Another 1.29 inches on this day.

February 21, 2005. Nearly 1.70 inches of rain, which begins to taper off after this day.

February 22, 2005. Another 0.73 inches of rain.

February 23, 2005. Yet another 0.65 inches of precipitation.

February 24, 2005. Projections are for a three-day window of good weather, which is sufficient, Brongo figures, to allow him to get his equipment in, unload the slide, and build a buttress at the bottom.

“When we took the plastic off,” says Brongo, “we discovered the entire slide had shifted. The fissure at the top had settled. If you stood on top, it looked like a massive earthquake had struck. The ground just opened up straight down.” But despite the rain and the wind whipping at the plastic, when crews removed the cover, they found that except for oversaturated soil at the toe (which Dave Seymour concludes was the result of superficial drainage and not seepage from the slide), the soil was so dry that dozer operators had to rip before they could begin grading. “Working on wet soil wasn’t the issue,” says Brongo. “The big issue was getting the opportunity to get in there and unload the slide. We had to have a dry enough window forecasted to have time to take the plastic off and get the dirt relocated the way the geologists wanted.”

Swimming pools were drained and filled in to keep from uplifting.

But to build the buttress, the wet soil at the bottom had to go. An excavator (John Deere 200) was brought in to feed the muddy soil into a central location from which a rubber-tired loader (Caterpillar backhoe), moving back and forth between two of the threatened houses, delivered it to trucks waiting in the street. About 1,200 yards of dirt were removed from the site.

To unload the slide and build the buttress Brongo brought in Dennis Barler, owner of Barler Equipment in Orange, CA, who specializes in slope repair and stabilization, and Barler bought in two John Deere 550 dozers, one of which was equipped with a slope board. Barler says the 550s were what he had available, and he thinks he could have done the job as easily with anything up to a John Deere 650 or a CAT D 4. The first thing was to fill in the fissures.

“The fissures were vertical to the direction of the slide,” says Barler. “The first thing we needed to do was bench a road up. I started on one side and sent my son up the other side and we met in the center.” Barler says experience prepared him for the challenge of filling a 30-foot-deep trench on a 1¾:1 slope. “Filling in the fissures was like filling a trench with a backhoe. You go head on to the crack and dozer the sides in and fill as you go. After we got the area leveled off, the dirt was still above the top of the dozer canopy on one side, but that didn’t bother us because the big vertical was going roughly downhill.”

Once the fissures were closed,” says Brongo, “it began to look more like a normal grading job.”

A Mustang MTL20 track-loader is delivered to a job site.

“At the top of the hill, the vertical was 30 feet down,” says Barler, “so at 1:1 that would be 30 feet back, and you would have undercut the house at the top. So, we just cut the slope so the edge wouldn’t be breaking off as much in order to take a little pressure off. When you’re on a steep slope like that, one of the biggest dangers is getting too far over. You’ve got to know when to stop and back up. If I had gone forward another six inches when I was cutting that vertical down, I could have gotten stuck and rolled over. A couple of times I got in that position, but I stopped in time to where I was able to back out of it.

“One of the biggest dangers on this job were the tree stumps. The slope was full of stumps from when it had been brushed three and a half weeks before. When you’re on a slope that’s 1.5:1 with a small dozer, a stump will cause the dozer to slide and twist, which could make you roll. We dug the stumps out with the dozers as we went along, but on a slope that steep with the houses at the bottom, we had to be very careful not to let one of those stumps get away from us and send it into someone’s living room.

“It was kind of hard getting started on the vertical to cut it back. We had to push dirt high enough so I could get high enough to reach up. At one point I had to have my son push me upslope so I could get on top to cut back a little farther. He backed down and I dozed off the knob at the top of the slide, trying not to go into the backyard of the house that was endangered up there, then headed down at an angle. I had enough dirt in front of me that I didn’t feel I was going to go end-over-end.”

Brongo’s crew worked three and a half days, from Thursday  morning through Sunday afternoon, including overtime on Friday. “By the end of the day Friday, we still hadn’t unloaded the slide,” says Brongo. “We didn’t have the weight off the slope, and I knew we couldn’t leave there Friday night and think the hill was going to be there in the morning. So, we got a meeting together, called in lights, and worked until 11. By Sunday afternoon, the grading was done.

“We graded it so the slope will have a positive surface flow and any water that hits it will go where we want it to. The drainage goes off either side of the slope into the backyard surface drains we dug at 2-feet wide and 1-foot deep to maintain a positive flow to the street. Each house has one. Everything is above ground, all mechanical. The slope was not compacted but the Barlers tracked the dozers over the fill as they were bringing it up.”

“The idea,” says Seymour, “was to follow the original slope configuration and build up a 1:1 slope at the bottom, about 10 feet high to place the fill. But the slope at the bottom became a little bit steeper. Given the time that had elapsed from the time we designed the fix until they could get in there to do it, the main scarp of the landslide had settled vertically 0.5:1 in some locations, 1:1 in other locations below the scarp. So what we ended up with was going up about 15 feet from the bottom of the slope at 1:1 to the horizontal bench, which turned out to be about 15 feet wide. Individual houses have a 10 to 15-foot buffer in their backyards.”

Next up was another layer of plastic to protect the temporary stabilization until the city and the homeowners decided on a permanent fix. Brongo secured the same 10 mm Visqueen with 2,500 sandbags anchored with 36-inch wooden stakes.

After the grading was accomplished, and prior to covering the slope with plastic, the city called for a topographical survey of the slope that would show the configuration following grading and to use in conjunction with Petra Geotechnical in continuing studies of the situation. Johnson-Frank used a Leica HDS3000 terrestrial laser scanner to scan the slope and create a digital model of the slope and adjacent areas, including the homes at the bottom and the swimming pool at the top. A conventional contour map, as well as sections at various locations, were produced from the digital model for further study. At this same time, Johnson-Frank installed some 28 pipes into the face of the newly configured slope, each with a reflective target attached as part of a system to monitor movement following temporary stabilization.

“After the pipes were installed,” says Frank, “we used our total station system, measuring from those same points in the curbs at the bottom of the slide area which we had measured half a dozen times to tiny tolerances, and we use these as our base points to shoot the pipes up on the hill.

“Typically,” says City Engineer Rich Schlesinger, “a surveyor would have to go up there with a prism and set it on top of the point. This way they just set up in the street and shoot up to the 30 points. It’s a lot cheaper and more efficient”—and as Frank observes, given that surveyors would be moving around on a steep hill covered in plastic, also safer. Currently Johnson-Frank is monitoring the slide twice a week and has determined that the slope is still moving about one-eight inch a day, which both Seymour and Schlesinger figure is due to settlement. Once the city is satisfied the slide has stabilized, it will allow residents back into their homes.

“It was a classic case,” says Seymour, “a classic geological sort of failure. If the city had owned the land, they would probably have been in there in a day or two.”                                         

Writer Penelope B. Grenoble specializes in environmental topics.

GEC - March/April 2008

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