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.
The Dallas Waves
Thanks to the power of creative engineering, there’s year-round whitewater kayaking on the river that runs through the city of Dallas, TX, which lies on virtually flat terrain.
The Dallas Waves is part of the Trinity River Project, a public works project whose original goal was to improve the city’s protection from flooding. According to the Trinity River Corridor Master Plan, two levees are being extended and two others are being raised. In addition, a series of wetlands within the Trinity basin is being constructed to divert excess water away from the river. These efforts will protect the city from an 800-year flood event.
“Part of the project was to improve the public’s perception of the river,” says Tom Johnston, client relations manager at One Engineering Group LLC, a structural and civil engineering firm in Dallas, which took over the design of the Dallas Waves. Although wastewater effluent still flows into the Trinity, it is treated and released in clean form.
“The Dallas Waves is designed to appear as an entirely natural feature,” says Glenn Campbell, P.E., the One Engineering Group engineer who designed the structural components of the Waves. “It was designed to function as whitewater even if the river is at low flow. It appears to be on the way to becoming a huge hit. Whitewater enthusiasts really love it.”
Campbell chose gabions from Terra Aqua Gabions in Arkansas for the two wave structures as well as to stabilize the side slopes of the river between the two wave structures.
“First and foremost, gabions are cost effective,” he says. “Second, there were some technical issues with slope stability, and gabions are much more flexible than concrete to present solutions to those issues. Third, they are ideally suited to use in wet conditions.”
The Dallas Waves is adjacent to Moore Park in the southeastern part of the city. It was created by two dam-like structures, 140 feet apart, which extend 150 feet each all the way across the Trinity River. Both structures are located at a point where the river narrows, immediately downstream of the Dallas Area Rapid Transit “Light-Rail” line. The portion of each structure that creates the standing wave for the kayaking sport comprises roughly two-thirds of each structure. The remaining third is a kayak bypass channel. The cross-sectional shape of each structure is triangular, approximately 36 feet wide and 18 feet tall, with gabions on top and clay and concrete below. Each kayak bypass channel is 70 feet in length.
“When the river passes over the structures, there’s a drop,” says Campbell. “The drop creates the whitewater, called a “˜standing wave.’ Even at low flow, it’s superb as a whitewater feature.”
One Engineering was hired by the general contractor, Ark Contracting Services LLC of Kennedale, TX, to value-engineer the structural components of the project, Johnston says. “Through the bidding process and more detailed analysis by Ark Contracting and the engineering and parks and recreation departments of the city of Dallas, it was determined that the design by another company wasn’t cost effective. We used our knowledge of gabions and anchors to bring the cost of the project from $7 million to the $3.5 million range.”
Work on the Dallas Waves structures themselves began in August 2010 and was completed the last week of December 2010.
Ark Contracting excavated the diversion channel on the north side of river, which ran parallel to the river around the project site, and stockpiled the excavated clay to fill the channel back in when the project was completed. Crews built two cofferdams, one upstream of the project to divert the water into the diversion channel and one downstream of the project. The diverted river water re-entered the river downstream of the downstream cofferdam.
Ark Contracting brought in new material to build the cofferdams. The face of each cofferdam exposed to the river was clad with sheet piling and clay to seal the work area between them from water infiltration. The cofferdams were also designed to function as access roadways during the construction of the project. The contractor pumped out the water between the cofferdams, built up the 150-foot-wide river bottom to make it level, and built additional ramps down into the construction zone in the river.
“Building the cofferdams high enough to hold the river back for practical periods of time, knowing that the river was going to experience peak flows, was challenging,” Campbell says, “but maintaining the cofferdams was the most challenging part of the project.”
The river flooded six times during the project. Once, water rose more than 10 feet above the top of the cofferdams. Dry periods, during which the contractor would work, lasted two weeks to a month. When the river flooded, the contractor would move its equipment out of the river, wait for the rain to stop, pump the stormwater back out of the work area between the cofferdams, and start working again.
“You can’t do that with formed concrete,” says Campbell. “Gabions are much more adaptable to harsh building conditions and make it much simpler to remobilize and get back to the job site. If we hadn’t used gabions, the contractor would have lost a lot of the structures.”
Stabilizing the sides of the river, which has a slope of 2:1, was another challenge. “Each side of river had a slope stability problem,” Johnston says. “The sides of the river between the two wave structures were unstable in that they had a propensity to cave in and experience landslides.
“Glenn designed anchored gabion structures to extend along each embankment of the river between the two wave structures to pin back the sides of the river,” he continues. “Secondly, the gabion walls serve to prevent erosion between the two wave structures. And as a third benefit, they provide a place for kayakers to get in and out of the river.”
The gabion slope stability structures line both sides of the river, from the river bottom up to the shoulder of the embankment, for a total height of 15 feet. They are composed of 3- by 3- by 12-foot cells and are stacked lengthwise for a total length of about 140 feet along each side of the river. Each gabion row is offset 18 inches back from the gabion row below it to form a wall face with a vertical batter of 2V:1H. Lengthwise, they’re connected with integral concrete beams located inside every other row of gabions. Vertically, they’re anchored into the limestone river bottom by high-strength all-thread steel rock anchor tiebacks on the south side of the river where the limestone is located at the river bottom, and by concrete pier foundations and tiebacks on the north side of the river where the limestone layer is much deeper.
The two dam structures that create the standing waves were the last phase of the project.
“The geometry of the structures was determined by Schrickel, Rollins and Associates Inc. of Arlington, TX,” Johnston says. “The original plans were done with concrete structures. We used gabions in exactly the same shape that their hydraulic engineering design called for. The hydraulics determined the overall cross-sectional shape of the gabions.”
Ark Contracting brought in clay fill for the core of the structures and built a concrete cutoff wall through the top of the structures so water could not penetrate them. Once the clay core and concrete cutoff wall were finished, crews placed Mirafi filter fabric over the clay core and installed gabion mattresses on top of that.
Gabion mattresses can be hundreds of feet long and are very adaptable to every shape, size, and slope, says Campbell. For the two wave structures, both 12-inch-thick and 18-inch-thick mattresses, as well as regular 3- by 3-foot gabion baskets, were used to cover the 150- by 70-foot structures. After the gabion mattresses were installed, the final layer of 18-inch-thick grouted limestone riprap was constructed on top of the mattresses. A 5-inch-thick layer of broom-finished concrete was then placed over the actual 50- by 14-foot crest of the wave structure itself. “This design has never been done before, but the gabion installation and assembly was easiest part of the job,” Campbell says.
Once the structures were completed, the contractor removed the cofferdams, let water flow back into the river, and filled the diversion channel with the excavated clay. A canoe/kayak launch area was constructed immediately upstream of the wave structures with 3-foot-by-3-foot gabions, gabion mattresses, and a concrete topping for a ramp down into the river.
The city will vegetate the area with grass and shrubs to blend in with Moore Park.
“We’re very proud of this,” Johnston says. “It’s unique. The project was going to be scrapped because it was too expensive. One Engineering Group’s and Ark Contracting’s collaborative knowledge of how to design and build gabions made the project possible.”
Canal Bank Stabilization, Highlands County, FL
Slope stabilization is usually fairly straightforward work, but when most of it is done underwater, and murky water at that, it’s a different story.
“Our divers work completely with their hands,” says David Lickliter, senior project manager for the contractor Underwater Engineering Services Inc. (UESI) in Fort Pierce, FL, which is restoring the slopes of the C-41A Canal in south Florida. “They can only see about one foot in front of them. Our engineers designed tools for them to be able to use without seeing.”
In Florida’s Greater Everglades Ecosystem, the topography is flat and low. A spillway at the south end of Lake Istokpoga maintains optimum water levels in the lake and discharges water into three canals, including the C-41A or Slough Canal. Water from the C-41A Canal flows into the Kissimmee River, and from there into Lake Okeechobee.
The canal is a crucial feature in the area’s water control system. It’s also a popular waterway for boating, especially fishing and sightseeing. Levees approximately 20 feet high line the canal. A system of dikes approximately the same height and locks regulates water levels. At lower elevations, pumps move the water when necessary.
“There were some pretty powerful erosive forces on the canal prior to our work,” Lickliter says. The banks were scoured by increased flow and wind and rain erosion. Between 2004 and 2010, hurricanes, some of them back to back, were strong enough to force open the locks and badly erode the banks.
UESI installed silt fence and turbidity barriers by Aer-Flo Inc. of Bradenton, FL, before construction began. The turbidity barriers were chosen mainly because of their ability to prevent the wake activity of boats from eroding the work and adding more turbidity to the canal. UESI is using Pyramat, a high-performance turf reinforcement mat from Propex Geosynthetics, to stabilize the restored banks.
The canal is open and in use while the slopes are being restored to their original design grade of 2.5:1. UESI, which has construction, commercial diving, and nuclear power divisions, began work in January 2011 and is completing it in sections. The entire project, a 4.69-mile segment of the canal, will be completed in 390 days.
UESI installed contractors grade silt fence by Mutual Industries in Pennsylvania on the ground in low-lying areas. This silt fence comes in a pre-pocketed design so that each stake is completely surrounded by silt fence material, which eliminates wind tear. Crews installed a total of 6,000 feet of Type II floating turbidity barriers from a boat, using both the boat and long stick track hoes to maneuver them into place. The barriers surround their work area and are also in some areas on the canal that need protection from stormwater runoff during rain events. Divers anchored the barriers with small Danforth anchors at the end of every 50-foot segment.
The turbidity barriers have a flotation boom, a heavy galvanized steel chain along the bottom that provides ballast, and an impervious vinyl-polyester fabric curtain that extends between them underwater. They’re designed for lakes, streams, and intercoastal and tidal areas where velocities reach up to 5 feet per second. They’re easy to maintain unless the flow becomes too high, Lickliter says. They are supplied by R.H. Moore & Associates.
There was only one major concern during the installation.
“We’re in South Florida,” Lickliter says. “There’s always natural wildlife to be careful of-like alligators.” We’ve taken extra safety precautions for our divers by hiring a trapper to remove any nuisance alligators that pose a safety risk.”
Once UESI installed the BMPs, crews began working each section by clearing and grubbing the slopes. They backfilled with fill from nearby existing levees that were above the required height, then compacted and did a final grading of the banks to the original 2.5:1 slope.
Pyramat was placed on the canal banks for the entire length of the project, from the toe of the slope to the top of the bench. According to Propex, Pyramat is composed of polypropylene monofilament yarns woven into a uniform configuration of pyramid-like projections and is specially designed to lock soil and seeds in place on steep slopes and vegetated waterways. It also has high UV resistance.
The average height of the slope was approximately 32 feet, but about two-thirds of it was underwater, Lickliter says.
“Our surface-supplied divers were installing the TRM about 10 to 12 feet below the water surface. We had to come up with a lot of unique installation processes. Being a dive company, we’re able to adapt pretty quick.” The company’s project planning team designed special tools to place the mats and anchors. These tools have applications for future projects, he says, although they’ll have to be modified for each new project.
UESI drove 3-foot percussion anchors through the TRM, then used a setting tool to set the anchors and disks to hold the cloth to the soil. Crews are vegetating the slope above the waterline with sod.
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.”
