New Angle(s) on Sediment Control

June 13, 2013

When the Tennessee Department of Transportation decided to improve a 4-mile stretch of Interstate 65, project manager Todd Cheek of Mid-TN Erosion was called in to assist with sediment control issues.

As he describes the project, “It is the grading, drainage, and construction of a concrete I-beam bridge, a concrete box bridge, and concrete paving, incorporating 14 noise walls and 11 retaining walls. Basically, it is taking three lanes of concrete paving out to five lanes, both north- and southbound.”

Several BMPs were used on this project, according to Cheek. “We’re using silt fence with backing, silt fence without backing, 20-inch straw wattles, and the 12-inch Filtrexx SiltSoxx as check dams,” he notes.

Where portions of the project presented difficult access, Cheek used the straw wattles. “There is right-of-way fencing all the way on both sides of the project,” he explains. “On the northbound side, we used a lot of wattles against the fencing, just because there was no way to get the silt fence in there.

“And where the specs call for a rock check dam, we used the SiltSoxx. So wherever it calls for a rock check dam, we’ll take three 12-inch Filtrexx SiltSoxx, stacked two on the bottom and one on the top, and stake them down.

“We also utilized silt fence with backing, typically when we’re around a stream, protecting the stream from runoff. Most of the fence without backing is along the right-of-way fence, just trying to keep sediment out of people’s yards and out of the ditches.”

There was quite a bit of material used on the job. For temporary silt fence with backing, a total of 4,466 feet of product was installed. Considerably more-26,962 feet-of temporary silt fence without backing was also used. In addition, there were 401 check dams and 36 “enhanced” check dams.

Cheek explains that for a regular rock check dam, riprap is used. An enhanced rock check dam includes riprap, Type 3 geotextile covering the upstream side, and No. 57 stones (a coarse aggregate) on top of that fabric.

Where the SiltSoxx are used in place of the rock, a regular check dam incorporates three SiltSoxx, two on the bottom and one on top. An enhanced check dam would instead involve three SiltSoxx on the bottom, two in the middle, and one on top. On this project, however, the SiltSoxx were used only for the regular check dams.

The three-year project is currently ongoing. It began in late October 2012 and is scheduled to be completed on or before October 15, 2015. The TDOT inspector visits the project at least twice a week. So far, he’s been quite pleased with the sediment control part of the job, particularly as there has been a fair amount of rainfall.

“Filtrexx is excellent,” Cheek says. “It’s filtering the water clean. We’ve got them long enough so the water doesn’t go around them, it’s going through them. TDOT, so far, has been very happy with what we’ve done.”

The SiltSoxx are expected to hold up for the duration of the project, as are both types of silt fence being used. The straw wattles have a shorter expected useful life. “They don’t hold up as well,” says Cheek. “They’ll probably last about a year and a half or two years. But by then, we’ll have stabilization, and we’ll just pull those out.”

Zero Tolerance in the High Desert
Lancaster, CA, is in the high desert, located about 70 miles north of Los Angeles. But despite being a desert community, city officials are insistent about keeping sediment and dirt out of the local streets and sewer system.

This attitude was evident with the recent construction of a large concrete tilt-up building in town. Doug Sadler, owner of BMP Contractors in Moreno Valley, CA, was tasked with the job of ensuring that dirt on the construction site remains on the construction site.

“Most of our projects are kind of unique,” he says, “and this one was as well, not from an erosion control aspect as much as a trackout issue. The city was adamant about us providing ideas for preventing trackout.”

Sadler turned to Contractors Services, which supplies trackout devices and with whom he has worked for about five years. “They come and check in with me because I’m the end user, and we come up with different ideas. Bill and Roger over there are innovators-they’re always thinking of the next step, and then I put it in place.

“So we work together on different things. What they’ve been trying to do is put different trackout systems in, and their trackout plates are the best in the market. I have hundreds and hundreds of plates out per year, and theirs are the ones that stand up the longest.

“They’re spaced out strategically, so they really work. A lot of plates don’t work-you just drive over them and you don’t accomplish anything. It’s just something to throw out there.”

The deployment of the plates was key. “On this job, we can’t have any dirt on the sidewalk or on the street. Even though we have a street sweeper out there, we had to provide an excellent system. So we added six plates instead of our typical three, and we staggered them. We have real silky material that really sticks to the tires.

“We dig a hole and go down 6 to 12 inches,” he continues. “You’re looking at a 20-foot by 50-foot size that is pretty typical these days of any construction entrance. So it’s a 20-foot by 50-foot block, about 6 to 12 inches deep. We line the bottom with Mirafi filter fabric, and then we’ll go ahead and place 3-inch minus rock. With our entrance of 100 feet, we have rock throughout the whole entrance, and we’ll stagger the plates. So we’ll have some rock, then we’ll have two trackout plates, some rock, and two more trackout plates.

“The rock will spread the tire treads out, due to the rocks’ oblong shape. This gets all the dirt loose, and as you hit the rumble track, it shakes the dirt completely off. We want to make sure we get this completely off, so we’re doing that three times in this one entrance.”

The smaller rock size was also critical. “Some installers use a 6-inch rock. It doesn’t work; it creates more problems than anything. On another project, someone put that in their specs, and it was just horrible. Cars get stuck, trucks get stuck, it damages equipment-it just doesn’t work,” he says.

“In contrast, the 3-inch minus rock interlocks itself and creates a nice porous surface. When it does rain, water will hit that and hit our filter fabric and go right into the ground. It will not cause a ponding effect, where it fills up with water and we’re tracking mud out on the street.”

Sadler notes that until crews put the concrete driveway in and pour all the concrete for the parking area, they’ll still have to have the trackout entrance in place to continue to provide proper sediment control.

He explains that the trackout system is expected to last for the duration of the project. “They’re big steel grates, and they will sit in place. The only time we move them is if we have a big rainstorm and we have contamination to the rock. That can happen if water has built up so far that it brought mud and sediment into the rock and it is not providing perfect trackout control the way we like it.

“The only maintenance we have to do is to send out guys to sweep; they’ll just sweep those corrugated tracks at the entrance. Unless we get a big rainstorm and there’s a lot of mud tracked in, we don’t usually have to replace the rock.

“It’s a good system, and it’s economical. Before, people were just using rock. But just using rock doesn’t work. With the first rainstorm, you could lose a thousand dollars’ worth of rock right off the bat, and then you’ll have to replace that, for as many rainstorms as you have. But providing the filter fabric allows the rock to sit in place, and the water will permeate that filter fabric and penetrate the ground.”

Interestingly, one of the bigger problems Sadler faced was from other workers on the job. “A lot of the construction workers don’t know the extent of the rules. They love to drive around the trackout system, so we actually lined the area with some silt fence, so we pretty much forced everybody onto this thing and made them drive through it. So part of the challenge was keeping these guys driving through it, maintaining it, and really depositing no dirt onto the street.”

Inspectors have been out at least once a week. “It’s in a high-traffic area,” Sadler says. “They want to maintain their streets, because the city is regulated by the state, and they’re going to come out and make sure that we’re doing our job, and the contractor is doing his job. The city will get in trouble with the state if the state comes out and inspects and sees things in shambles.

“The superintendent has been doing a great job about the maintenance out there, sweeping out those plates.”

Wattle World
Bob Hanson of Gator Guard says his initial inspiration came back in 2004. He had been using straw wattles on a number of jobs and saw that they were repeatedly getting torn up by his equipment and other contractors’ equipment as well. In addition, they regularly seemed to be undermined by water flowing underneath them.

“We looked all around at straw wattles,” he says, “and everywhere that we saw these straw wattles on a slope, they were having these underflow washouts.”

Unhappy with what was available, Hanson began making his own wattles. “In developing our wattle, we decided we needed an apron on it, about 8 inches wide, that goes on the top side, the uphill side of the wattle, when it’s installed. You just flatten out the dirt-you don’t need to trench it in, although you can if you want to.

“Then you stake it down every 3 feet, and put an inch of dirt over the apron. That seals it to the ground, so when the water comes down and sits on top of that apron, it doesn’t have that underflow washout like you get with straw wattles.”

Eventually, Hanson had his wattles tested at San Diego State University. This evaluation demonstrated that standard wattles suffered approximately 20 times as much dirt loss as his system, for the same storm equivalent.

Particular about the definition of a wattle, Hanson notes, “A wattle is a ponding device. Most people think that silt fence and straw wattles and synthetic wattles like ours are filtering devices. But in reality, they are not. They are ponding devices. They pond the water behind the wattle or the silt fence, so that there is time for all the fine particles and the coarse sands to settle out of the water. Typically, on a job site, most of your wattle or silt fence is going to just stop the water and channel it downgrade until it comes to the end corner of the project.”

Some time ago, Hanson became concerned about the foam inside his wattles coming out, so he began using larger fill material, assuming that the bigger the foam, the harder it would be for it to escape.

“We started out with little finger-sized pieces of foam in there, because that’s what we got from our local packaging plant,” he says. “It seemed to work well. As time went on, we found that these things sometimes got ripped open, and all these little pieces of foam would blow across the ground.

“Over time, we made the pieces of foam bigger, about thumb-sized, and then wrist-sized, and then finally fist-sized. These pieces of foam are recycled scrap foam from packaging plants. It’s a material we can use that fills the log or wattle to 6 inches in diameter. You can drive over it with a bulldozer, excavator, truck, whatever, without destroying it, which is what happens with straw wattles or silt fence.”

As a variation on the product, he says, “we started making our extreme wattles, and eventually our extreme foam logs. They are made out of a 1-inch-thick sheet foam that comes in 4-by-8 sheets. We cut the sheet in half, slit it, roll it up, and wrap it into basically a foam log insert that goes inside the same geotextile. These can be coiled up into a larger unit.

“The reason we went to those is that we had customers come to us and tell us that they want a product that, if it gets ripped in half, there’s not going to be any pieces of foam coming out.”

Hanson mentions one of several test sites he has maintained for his standard wattles, the most prominent dating to 2008. “It’s called Phantom Creek. It was a two-block-long housing development that had some slope to both the uphill and downhill sides of the street. The developer had a landscaper come in and put in silt fence on the whole thing. But within six months, the silt fence became just strings on sticks.

“The landscaper had used really cheap silt fence on 2-by-2 stakes, and apparently it didn’t have any UV resistance to it at all. I found the developer and told him we needed a place to test our wattles. I said I would put these in at basically cost and keep them up over time. So we ripped out the silt fence and installed about 1,000 feet of our wattles. The silt fence had been put in incorrectly. Instead of trenching it in, the landscaper just stacked dirt on top of the silt fence, which a lot of people do, but it’s the cheap and dirty way to do it.

“It took us three times longer to rip out that silt fence and clean up the mess where the dirt had been piled up as it took for us to put in the new wattles.”

He has been monitoring the site for the last five years. “We were able to get good experience for how well they are working. Every time it rained hard, I would run over there. We had some pretty good runoff events. In one event, from the houses up above, a flood came down and filled up to the top of the wattles along a pretty good distance, then ran over the top and across the sidewalk. So there was dirt on the sidewalk.

“So we asked, “˜Would this have been better with a silt fence here?’ But in reality, a silt fence that had been there as long as the wattles had would have been destroyed anyway. The fabric gets worked in the wind, it gets holes in it. Then as soon as you get water built up behind it, it will find the weakest point and it will blow out, and suck out the mud from behind it as it goes through.

“We looked at it, and thought, for situations like this, if you think there is going to be a problem, a better thing to do is just put in a second wattle upslope from the first one to catch the runoff. It will fill that one before it comes down and fills the second one. This creates extra protection for areas that you know are going to channel on you.

“On this particular project, the wattles lasted for four years. This past summer, we took them out and replaced them, because some of them were deteriorating along the top, and you could see some of the foam.”

Hanson notes that Gator Guard is switching to a new monofilament fabric for its wattles, one that is thicker, stronger, and more UV resistant. He anticipates a five-year life for this new product.

There may be another, possibly unexpected benefit to the Gator Guard synthetic wattles. “One of the reasons the dealers really like our wattles,” he notes, “is that they don’t get any mice in them. People who have straw wattles in their warehouses end up with a lot of mice because straw wattles still have grain in them, as well as being a cozy place to live. The mice will chew their way in there.

“I understand also that in some places, Arizona particularly, there have been rattlesnakes getting into straw wattles, because it’s a nice cozy place to go. So the people handling them have to be careful not to pull one up and find a rattlesnake inside.”

Turbidity Curtains 80 Feet Deep, 3,000 Feet Wide
Bass Lake, CA, is located about an hour east of Fresno, virtually in the shadow of Yosemite National Park. Steve Ackart, president of Title Marine Construction, explains that Bass Lake “has an old dam that was completed in 1910. It’s an earthen dam, and they’re doing a seismic retrofit for it.

“We’re dredging out at the toe of the dam along the whole face-dredging out the soft dirt and getting the hard rock. We’re then introducing new riprap rock instead.”

On the downstream side-where there is no lake-the work is easier, he notes. “But on the lake side, as you start dredging in the water, you get all the sediment and the plume in the water. We need to contain that; the lake has fish in it and all kinds of marine life. There are fishermen there, too, and they didn’t want to stop them from fishing on the lake. So we put two 3,000-foot parallel curtains around the whole construction site. We were in charge of installing the curtain, maintaining it for a year, and removing it.”

Installation of these curtains, reaching some 80 feet deep, began in February 2012 and was completed just under a year later. Parker Systems, based in Chesapeake, VA, supplied the Type II turbidity curtains.

“It’s floating on the surface, with pontoons, and the net just hangs. On the bottom of it, there is a chain hemmed into it, so it’s weighted. You put anchors perpendicular to the curtain, maybe every 100 feet, and you run the anchors all the way to the floatation part. That holds it in place. With anchors every 100 feet, we had about 60 anchor systems.” Determining how many anchors are needed, he says, depends on the current and how much wind is anticipated.

“They are semi-permeable, allowing very small items to pass through,” says Ackart. “They had third-party water-control people out and put sensors in the lake, and they monitor to make sure we’re not putting in any more than we said we would.

“The Type II curtain floats on the water, and it has reefing lines spaced regularly, with chains on the bottom, so you can regulate the depth by coming up on those lines, like blinds on your windows,” he explains.

“We have the chain just touching the bottom, so if the lake went up a couple feet, then on the bottom of the curtain we’d be a couple feet off the bottom of the lake. In that case, we’d have to let those lines down. You start at one end and just go all the way to the other end. Conversely, when the lake went down, now the chain would be hitting bottom, and we’d have to come up on it.”

No maintenance is required unless a curtain is torn, as by boaters getting too close to it. “The other issue is if the lake fluctuates, if your chain is at bottom or just off bottom, there are all sorts of trees and debris down there it can get snagged on it,” he says.

“As the lake comes up, you can tell if it’s snagged, because the pontoons start getting pulled below the water surface. A tree stump or root is holding that chain, and we’d have to dive down to clear it.”

Ackart notes that despite the general lack of maintenance required, crews were onsite frequently to be able to quickly handle any issues that did arise. “We did two days a week minimum, every Monday and Tuesday for a year. Sometimes we’d stay all week, if there were a lot of things going on with it.”

He adds that there was no single type of sediment that the turbidity curtains were meant to minimize. “When they’re dredging, they’re clam-bucketing and using excavators, so whatever was on that lake bottom gets suspended as soon as they put their bucket down and grab it out of the water. It just makes a big plume.

“Once they finish dredging-and that took about a year-you have to wait until all the sediment inside the curtain settles back down to the bottom before you can pull it out. We had to wait about two months for that.”

The suspended sediment isn’t removed from the site. “The sediment will all just migrate back down to bottom,” explains Ackart, “and when the inside of the curtain has the same water quality as the outside, then we can remove the curtain.”