Putting the Brakes on Silt – Part 2

April 5, 2015

“For years, a crew would have to come out to a job site and install SiltSoxx in the field with big blower trucks,” says Zock. “Some people didn’t like that, and sometimes it was hard to get scheduled. That’s all changed.”

Filtrexx still has professionally certified teams all over the United States doing advanced applications for streambank restoration jobs, wetland recharging, steep slope stabilization, spill clean up, and stormwater filtration, he adds.

A new way that SiltSoxx are being used is as a pyramid to create a type of filtration berm. “A classic sediment trap is a small pond or depression with a big dirt berm around it,” Zock says. “Where water would flow into pipes or channels to put the water in that little basin, typically the dewatering structure of such a device is a riser pipe with a trash rack, anti-vortex plate, or a rock check dam surrounding a riser pipe with holes in it, so there is a single point-source release outside or somewhere downstream of this sediment trap.

“Historically, that put you in a situation of having to manage [the location] downstream where the single-point-source release can cause more erosion.”

When a pyramid of filter socks is constructed, he explains, “The dewatering structure, instead of a single-point-source release from a traditional trap, is a level spread release through a filtration berm. The water is cleaner, the water gets level spread release, we don’t have a buildup of energy to cause downstream erosion, and it’s easier to take out.

The technique is in use at the Indiana County (Pennsylvania) Jimmy Stewart Airport, where 10 such pyramids were installed a few years ago for a runway expansion project near a sensitive wetland area.

“It’s a very popular application,” Zock points out. “You pyramid it up to create various heights so that you can hold the amount of water you want and dewater that trap at the correct rate.”

To manage the stormwater, the traps were placed close to the sensitive area. “The danger is if you have that single pipe as a single-point source being released that close to a wetland, you could really mess up that wetland if you get erosion below the trap,” Zock says. “The answer was literally going very close to the wetland and creating the pyramids of berms and traps. The water on the trap side was murky brown, water coming through the filter berm was fairly clean, and the water being released through the trap was great.”

Zock says the SiltSoxx are more affordable now than ever before. Five years ago, someone using compost socks had to spend extra money by getting new products, hiring installers, and using newly purchased equipment, he says.

“Today, not so much,” he adds. “A typical 8-inch sock in most markets in the United States would sell on a pallet for a retail price of about $2.25 a foot and is installed for probably $2.50 to $2.75 a foot. Most people will tell you $2 to $2.25 a foot to buy the commodity is pretty affordable.”

In contrast, he says, “In most places in Pennsylvania, being the project manager for developers, we would typically pay from $7.50 to $10 a linear foot for super silt fence to be installed on jobs. We can go out and put 24-inch compost filter sock at about the same price at $8 to $8.50 per foot and it’s much more robust. It won’t blow over or push over. It’s a big fat log that sits on the ground and handles a tremendous amount of lateral force when you push against it.”

Another factor to consider with affordability is the removal at the end of the job, says Zock. “I can tell you from that experience I typically need to get a backhoe or a skid-steer or a couple of guys to get out there to rip the super silt fence out at the end of a job.

With compost filter sock, I send a laborer out there with a utility knife. He splits up the sock, pulls out the compost, we pull out the jersey tubing, and I save a bunch of money at the removal at the end.”

Zock has seen sediment built up against compost filter socks and has rolled the sock away from it, scooped out the sediment, and rolled the sock back in place.

“Even with the larger sock, it’s easy to remove,” he adds. “It’s very hard to remove sediment that’s built up against typical silt fence and then expect that silt fence to survive another major sediment push.”

Zock also finds the compost sock to be versatile. “Because it lays on the ground flat, if you need to drive a machine through the area where there is sock, you can cut the sock, flop it open, drive the machine through and flop the sock back,” he says. “It’s very easy to work around on the job site. Although not typical, you could pick it up and move it to various places if you needed to.”

His company often keeps 10-foot sections of the sock stockpiled. “If the sock needs to be repaired or reinforced with another piece, it’s easy to manage and repair and it’s easy to reinforce an area where you might need some more protection.”

In choosing an appropriate product for erosion and sediment control, Zock says at the end of the day, it comes down to feasibility.

“Compost filter socks have been around for 10 years,” he says. “It’s improving every day. I think it is going to be the emerging BMP in the next five years.” development in the United States.”

Craig Higgs is president of Bridgetown Farms in Centreville, MD. His company services the Delmarva Peninsula, encompassing Delaware, Maryland, and Virginia. The company’s services include providing sediment and erosion control as a bond release service for land developers. When residential or commercial developers are at the end of a project and ready to go, Higgs’s company does the onsite work necessary to get the bond released. The developers pay him a deposit up front and the remainder once the bond is released.

“We work with the inspectors to get the work done,” Higgs says. “The developers have piles of soil that they’re digging into. If they sit unattended for 14 days or more, then they have to be stabilized with silt fence or straw, or seeded. All of the different municipalities or local agencies have different regulations for that, as well as different enforcements. Some are known to enforce very strictly, and some are known not to enforce.”

About the Author

Carol Brzozowski

Carol Brzozowski specializes in topics related to resource management and technology.