Hard and soft armor in combinations
Contractors have a multitude of tools to use in their never-ending battle to keep soil and water apart on construction sites, especially when the sites are on slopes.
Sometimes soft armor, such as vegetation, erosion control blankets, turf reinforcement mats (TRMs), or coir logs, will control the erosion and stabilize the slopes. Sometimes contractors have to bring in the hard armor. These structural best management practices (BMPs) include such heavyweights as retaining walls, gabions, concrete structures for shoreline protection, and concrete block mats. Each has its place, and as the following projects show, each can be an essential component of a remarkable project.
Pond “A” Rehabilitation
When Hurricane Jeanne blew through Sarasota County, FL, on the Gulf of Mexico, in 2004, the wind and waves tore into the banks of High-Hat Ranch Pond “A,” an effluent reuse storage pond owned and operated by the City of Sarasota Public Works Department. The storm eroded the banks of the manmade lake to the top of the perimeter berm as well as the wave break dikes that had been built to stop erosive forces.
“The pond was built back in the ’80s,” says Jimmy Byrd, who managed the rehabilitation project for the general contractor, QGS Development of Wimauma-Lithia, FL. The water district that serves the county sells the reclaimed water for landscaping and agriculture. It’s clear and visibly free of suspended materials, less expensive than potable water, and not subject to water-use restrictions.
QGS Development, which specializes in site development and golf courses and also has a turf division, used an articulated block matting system, open-cell Cable Concrete mats from International Erosion Control Systems Inc. in Ontario, Canada, to restore the banks. These mats can be manufactured in the plant in the required configuration and then assembled onsite, which cuts down on the number of trucks needed to transport them to the site. The mats conform to the terrain and even to changes in terrain due to freezing and thawing. In addition, vehicles can drive over them.
The pond is in the countryside, where the land is mostly flat and the soil sandy. It covers 110 acres and has a perimeter berm approximately 13,000 feet long. For the duration of the project, from November 2010 to April 2011, existing water in the pond was pumped down to allow the repairs.
“We went in and the areas that were washed, we repaired,” Byrd says. QGS then compacted and regraded the slopes to 3:1 and lay down Winfab 160NW, a 6-ounce nonwoven geotextile by US Fabrics in Cincinnati, OH. Crews installed approximately 250,000 square feet of CC-55 open-cell Cable Concrete mats on top of the geotextile.
Because the mats were manufactured in the plant and assembled onsite, 330 fewer truckloads were needed than would have been otherwise, says Jeff Peterson, the representative from distributor R.H. Moore & Associates. Peterson worked closely with the design engineer from AECOM USA Inc. on the size, weight, and construction of the block to be used.
“We’ve been offering this service to contractors for years,” Peterson says. “We’ve found that scheduling production is made much easier when we are located right onsite. We can communicate daily with the installing contractor.”
The mats conform well to the terrain because the blocks are pyramidal in shape, so they articulate between 20 and 60 degrees, depending on their size. In addition, the blocks are connected by cables, so they’re flexible. The cables form loops on all the edges of the mats.
QGS toed in the mats at the bottom of the pond at elevation 30 feet. The mats cover 18 feet of slope, to the exposed top of the revetment at elevation 36 feet. Crews joined the adjacent mats by clamping the loops together with stainless steel clips for maximum stability. They had some record days of installing 110 mats in a day, Peterson says.
“There was no hurry,” says Byrd. “We just had some good people.”
Another advantage of the blocks is that they have up to 70% open area for water permeability and vegetation growth. QGS will vegetate the mats above the maximum water line at elevation 34 feet with sod and will monitor it until the sod is established.
“It was a good project,” Byrd says. “Any time you’re working under a load, you have to be careful of a lot of things, but it’s just construction work.”
Original article can be viewed here: Structural Solutions