In October 2010, ground was broken on a 57-acre site for Carlsbad High School, located just north of San Diego at the eastern edge of Carlsbad, CA. The $104 million project is being hailed as one of California’s most environmentally advanced and energy-efficient high school campuses. But situated as it is at the headwaters of one of the lagoons in the area, there were some important environmental issues that needed to be addressed, delaying the start of construction.
“The site was classified as an Environmentally Sensitive Habitat Area,” explains Mike Alberson, senior environmental manager with Barnhart Balfour Beatty, one of the largest builders of schools in the US. “If you look at the state water-quality control board, they have a map that identifies all of these sensitive areas, and we happen to fall into that area. So we were automatically classified in California’s highest stormwater risk category.
“There are three risk levels-one being the lowest, two being mid-level, and three being the highest. California projects categorized as risk levels two and three must perform sample monitoring for turbidity and pH when it rains a half-inch or more. We also were required to check suspended sediments as a part of that monitoring. If the stormwater effluents exceed certain limits for turbidity or pH, which actually happened, the project must then monitor on a more continuous basis.
Alberson describes the topography on which his crew had to work. “There are very steep hills throughout the school site. On the north side is located a very environmentally sensitive habitat corridor, which ties in to one of the local reservoirs above it, about a quarter-mile away. That area is designated as a park area and a habitat area. We are adjacent to it, on the south side.
“There is also a stream flowing within that habitat area, and we also have another drainage area located on our south side, all of which tie in together west of the project. This is why this project is considered a risk level three.
“We had to grade the hills down to make it buildable, which involved moving about a million yards of dirt. Moving that much dirt can cause a lot of different issues. We knew of an existing fault on the property. By removing all that dirt, we actually caused a major slump or landslide about 3 feet wide, with about a 9- to 10-foot vertical drop. It was a major effort to regrade and recover the dirt that moved. The slopes were very steep, 2:1 or more, and some of them are very long.
“To restabilize this, we knew from the get-go that we were going to have to do everything we could with structural and erosion controls in order to hold that slope. It wouldn’t work to just do a bonded fiber matrix or something similar, so we wound up designing more controls into it.
“When I originally started working on this project, two years ago, we designed a bunch of stuff into it, and it later went forward to the engineers. The engineers incorporated our designs into the final BMPs for the project, and what we wound up using were coconut mats on the steep slopes along with Ertec ProWattle spaced up and down the slope, or terraced as flow spreaders across the contours of the slopes.”
ProWattle, from Ertec Environmental Systems, is designed to protect slopes from erosion and site perimeters from sediment movement during construction. As opposed to concentrating water flow, it is designed to spread water flow while filtering particles. It is made from recycled materials and is reusable and recyclable.
“There are always other options,” Alberson notes. “We could have gone with the standard wattle and other kinds of mats, or even spraying. But we wanted the highest-performing, most durable combination of products. Durability was important because we had to keep the slope stable until vegetation returned. In this climate, that could take up to three years. None of the other products would have lasted.”
Weather is always a factor to contend with, and this project has been no exception.
“In November and December, we received some really heavy rains, to the point where we met the requirements for an exemption for a 10-year/25-year storm. In that area, we received about 2.6 inches of rain in one 24-hour period, and because of it, we had lakes everywhere. We wound up creating ponding areas within one of our drainage areas-we literally had a Cat out there working in the rain, pushing dirt up, trying to compact everything.
“We had tremendous flows that were trying to leave the site, and we were trying to hold them back. We did a pretty good job. We did have some discharge; fortunately it was an exempted situation because it was over a five-year 24-hour storm.”
Alberson explains that at this point the pads are in and grading is done. “We’ll soon start going vertical on it.”
Revegetation is planned, but can’t be taken care of just yet. “We can’t even do that until late in the project. That’s why we went with the heavier matting. We won’t be able to do any of that because actually the slopes will move. The way the buildings are going in, we’ll wind up doing backfill and bringing the height of the slopes down. It will change the whole topography of the site once we’re done.”
Rockfall Protection on the Columbia
The Columbia River, second longest in North America, serves as the border between the states of Oregon and Washington. The spectacular Columbia River Gorge is a river canyon cutting through the Cascade Mountains, and is some 80 miles long and up to 4,000 feet deep. The north canyon walls lie in Washington State, while the south canyon walls are part of Oregon. The topography is such that the Columbia River Gorge has one of the largest concentrations of waterfalls in the world.
Tim Shevlin, regional manager for the northwest US and western Canada for Geobrugg North America, says, “The cliffs of the gorge create rockfall problems. In between some of the rock outcrops, there are areas of shallow soil that need to be stabilized too. That is the standard application for an anchor TECCO system from Geobrugg.”
He notes that over several decades, various rockfall protection strategies have been tried in the gorge, some of which are visually unappealing. In 1987, the Columbia River Gorge Commission was established to implement policies and programs to protect and enhance the gorge’s scenic, natural, cultural, and recreational resources. “They have a large say on what is done in the gorge area to ensure it is consistent with resource protection,” says Shevlin. “Anchored TECCO systems stabilize slopes while allowing vegetation to regrow, providing both safety and an aesthetically appealing mitigation strategy.”
To protect the 3,800 daily travelers from falling rock, soil, and other debris along State Route 14, the Washington State Department of Transportation authorized a rockfall prevention program along two portions of the narrow, winding road that closely follows the river below.
Chris Ingram of Hi-Tech Rockfall Construction worked on a portion of the project. “This was a project in the Columbia River Gorge, where we had a variety of stabilization techniques,” he says. “We did slope scaling to remove the loose rock first. We mitigated three different areas; one area utilized the pinned TECCO product.
“Usually the pin products are used where there is a big soil area where you want to hold the soil in place. We remove loose rocks on top, and we drill rock anchors on a pattern anywhere from 10 to 20 feet deep, with 8-foot centers all across the area. We then lay the mesh over the top of those rock anchors. Where each rock anchor is, we dig what is called a dell, which is a hole. When we put the mesh over the anchor, we put a plate on it, and as we torque the plate down into that hole, it stretches that high-tensile mesh and holds the soil in place.”
“One of the reasons the Washington State Department of Transportation specifically specified that TECCO mesh be used was because of its ability to allow slope revegetation,” Shevlin says. “The advantage is, number one, the high-tensile strength of the TECCO mesh, so that you’re able to increase your nail spacing and therefore decrease your cost. Secondly, since it’s an open mesh, the slopes can revegetate. Grass grows through, and the mesh is covered up so that you don’t see the stabilization method. Sometimes people will just spray shotcrete alone on a slope to stabilize erosion. But shotcrete can crack, and if it’s not attached with nails and some mesh below it, it has the potential to erode. That’s the reason the Washington State Department of Transportation specified an anchor TECCO system specifically, and that’s the reason why, I believe, the Gorge Commission approved it. This system was chosen for its aesthetic properties, plus the high-strength mesh, making TECCO ideal for these slopes.”
He explains that the TECCO system is composed of three major elements: the nails that go in the ground, the high-tensile-strength TECCO mesh, and a spike plate that goes over the nail and holds the mesh to the ground. “There are some other system components, and one of them is an erosion control blanket. We don’t necessarily specify that, but we always recommend, in a situation where you expect erosion, to put some sort of erosion blanket down. In this case, they used jute mat or coconut mat.”
Ingram says, “Typically when we use this pin system, there’s an erosion control blanket that goes down first, underneath the TECCO mesh. You put the TECCO mesh on, then you seed over the top of it in order to enhance the ground cover, aiding in slope stabilization.”
Shevlin adds, “The landscape architect came up with a seed mix, and they applied, by hand; the mixture of seed and mulch that was spread out over the top of the erosion control blanket.”
The project presented a number of challenges.
“One design challenge was the irregular shape of the area to be stabilized,” says Shevlin. “It was a fairly steep slope, and there were actually five different areas of all different shapes-one was shaped like an hourglass, another was rather round. Typically, a slope that is stabilized with a TECCO system is square-ish, or rectangular. So the contractor had to adapt the mesh to the slope shape, which TECCO mesh is able to accommodate.
“When anchoring the system, we always want to put the anchors in naturally low spots on the slope, or you can dig out a small divot around the nailhead. One of the installation steps is to have the spike plate go below the ground surface. That actively tensions the mesh and holds the mesh tight. That’s one of the design concepts-to have the mesh strung tightly across the slope. An engineer will come up with a nail spacing, and the contractor goes out and lays the nails out on the slope according to that spacing, but that may not necessarily fall on the natural low spot. But what makes TECCO ideal is that nails can be put anywhere within that panel and you’re still going to have proper transfer of forces.
“On this project, there were some areas of fairly deep holes due to the way the soil graded into the rock slopes. The contractor had the ability to put extra nail in at these specific low spots and achieve the proper tension. With other systems, you’re somewhat controlled by the size of the panel or the rope structure in the panel.”
The depth of the nail insertion can vary, according to Shevlin.
“It all depends on the site conditions. The nail spacing and the depth of the nail are going to be controlled essentially by two things-the shear strength of the soil and the geometry of the slopes.
“The more severe the slope is, the more earth pressure is going to push on the mesh. Once you have more pressure on the mesh, you have to drill your nails deeper.
“Also, if you have a weak soil, you’ll have longer nails than if you have a strong soil. In this case, I think they assumed the soil was approximately 2 meters deep, and the design requirement was for the nails to be approximately 3 feet into solid rock. So assuming you have 6 feet of soil, you have to go at least 3 feet past that. On this site, there were areas where there was that exact situation-6 feet of soil. But there were areas where it was only 3 feet thick, and certain areas where it was deeper, perhaps 10 to 15 feet of soil. So there was variable nail depth determined onsite by the contractor and the design engineer.”
Traffic control during construction wasn’t a major problem, but there were periods of between two and four hours in which State Route 14 had to be completely closed. On the opposite Oregon side, there is a four-lane highway, but the Washington side has only a narrow two-lane road. The TECCO portion of the project was completed in approximately two weeks, and the entire project concluded in September 2010. The revegetation effort has begun to show results, and growth has started coming through the mesh.
In addition to the TECCO system, cable net drapes were also installed, and significant blasting was required. According to the Washington State Department of Transportation, more than 31,000 cubic yards of rock and soil were blasted or carved away, “enough to bury a football field 21 feet deep.” As the DOT states, “The payoff is a safer drive on SR14 in the gorge.”
Greening a Little Corner of Chicago
In May 2011, a large local electric utility put in new transmission lines in the southeast portion of Chicago, running from a steel manufacturing plant over to 103rd Street and cutting across a number of major thoroughfares.
“It ran along a railroad right of way,” says John Donahue, president of Emerald Site Services, the soil and erosion control contractor for the project. “They put in a huge caisson, a big concrete footing underneath the transmission tower, on a slope in the railroad right of way by a bridge overpass, adjacent to Cottage Grove Avenue.
“Because they did not have a lot of room, they put this on the side of a railroad embankment slope. Some areas had a 1:1 slope, some were 2:1, finally tapering down to a 3:1 slope. It died down at the bottom of the slope about 4 feet away from the edge of Cottage Grove, which is a very busy street.
“The soils there were kind of rocky and sandy, so it was a real nasty little mess they had. With a little rain, there would be a lot of erosion dumped on the street. We talked to the engineer and suggested putting in a Presto Geoweb system, a reinforcement to stabilize the slopes, and they went ahead and designed it, and that’s what we put in.
“We used two different-sized Geoweb spans on the slope-the GW20V and the GW30V. We regraded the slopes with a Gradall excavator with an extended boom bucket, installed the Geoweb in those tough areas, then added topsoil, seed, and blanket. We put a polymer on as well so that the thing would hold in place really well.
“At the top, we installed a diversion berm and swale, because everything from the railroad came down this way to the low point. After we laid the Geoweb, we built the diversion berm that cut across and cut out as much water as we could, to bring that down away from the slope as well.”
The treated area was small, only about 1,000 square feet, and was completed in three days. “It wasn’t a huge job, just a nasty one,” explains Donahue.
There were some alternative options considered before the Geoweb system was installed.
“They were talking about putting up gabion walls adjacent to the curb of the street and building those up into more of a flat slope, but the Chicago Department of Transportation [CDOT] didn’t like that idea. They were also talking about extending the concrete flare wall that came down from the abutment of the overpass, and they thought that that was going to be too expensive, and it also got so close to the street that it presented potential safety issues, with cars careening off the curb and smashing into the wall, so they dropped that idea.
“We came up with the slope stabilization idea that would actually be green. It’s a fairly industrial area, and a lot of the area is really ugly-a lot of trash and rock and debris-so this was a better solution to green it up a little bit.”
The total project was completed for a budget-friendly $27,000, which included all of the initial earthwork and importing of soils to make a more workable slope.
Donahue explains that traffic interruption was avoided in a unique manner. “We used a Gradall track machine with an extender boom on the top. They can usually reach about 14 feet down, but we put an extension boom on it so it could reach down 50 feet. So we basically worked down from the top of the slope. The guys on the bottom worked from the bottom with ladders up against the Geoweb. So we avoided tying up traffic; otherwise, you would have had to close off all the lanes.”
Revegetation was accomplished with an infill of deep grip prairie mix, so that it would eventually reach 3 to 4 feet deep into the ground. However, heavy rainfall had to be dealt with before the project could be concluded.
“We had a lot of gully washers. It rained in May and most of June, so we had to deal with a lot of weather,” he says. “We had no spring-it was just cold and wet. We installed erosion control measures and the diversion swale on top so we could cut off the water. Once we graded the slope, we polymered it to stop any erosion that would have wrecked what we did initially.”
Other challenges arose as well. “The steepness of the slope and the tightness of the work site were problematic,” Donahue notes. “At the top of the slope we did have to deal with traffic control items. The railroad, like most railroads, was very particular about anyone being near their tracks. It is a very heavily used track, so we had to stay within a tight little zone out of their right of way, with flaggers and barricades to keep people out of their buffer area.”
In the end, Donahue was pleased with the Geoweb application. “It worked very well. For an application like this, where you want to do something for a fairly reasonable cost, and where you have a difficult slope that sometimes people don’t know what to do with, it’s a great application. As long as you can toe this material in at the bottom and anchor it at the top and get some growth on it, it’s a very good solution.”
He adds, “CDOT is very happy, because what they were looking at before was 3 to 4 feet of junk, and they were unhappy with all this material that was getting dumped on the side of the road. That was a dangerous thing for them. So everybody is glad that we were able to stabilize it and clean up the mess.”
Software for Stability Analysis
In addition to the various BMPs noted above, software is also available to assist in slope stability analysis.
As an example, SLOPE/W 2007 from Geo-Slope International can model a wide variety of scenarios, including natural earth and rock slopes, sloping excavations, earth embankments, earth reinforcement such as soil nails and geofabrics, and anchored retaining structures.
A number of variables can be configured, including slip surface shapes, pore-water pressure conditions, soil properties, analysis methods, and loading conditions.
SLOPE/W is also able to calculate a stability factor by computing both total shear resistance and mobilized shear stress along the entire slip surface. The software then computes a local stability factor for each slice. SLOPE/W computes the probability of failure in addition to the conventional factor of safety.
