In the past 50 years, there has been a huge push to preserve and protect our natural environment-lakes, rivers, plants, and wildlife. Waterways have been listed as requiring special care and plants and wildlife tagged as endangered. Construction of any kind-building, housing development, roadway, or landfill-around sensitive water bodies or near the habitat of endangered wildlife must follow stringent regulations.

Protection and preservation of the natural beauty of our world is crucial. But one of the most important collections of products to accomplish that task comes from the synthetic world. The products are developed in the lab and produced in factories. They are not made of natural materials, but of polymers.

The use of geosynthetics has exploded in the erosion control industry because their characteristics can be designed and engineered to do just exactly what is needed. These products, from geomembranes to geotextiles to geogrids of all types, can be used to accomplish projects that would be extremely difficult or even impossible with all-natural products. Their strength, flexibility, and endurance give them a place in the protection of the natural world that can’t be denied. Even projects that do not affect impacted waterways or endangered species have profited from geosynthetics.

Geosynthetics are also a boon to the companies working on the projects. Designs that would be impossible with only dirt or stone can be accomplished with the addition of geogrids, geomesh, or other geosynthetics. The use of these products can bring substantial savings in cost and time.

Companies around the country and the world are specifying and using geosynthetic materials for a wide variety of projects. Learning from what others have already done may spur ideas for your next projects.

First-Time Winners
Mark Marsack, engineer for Modern Corp. of Model City, NY, has worked in the field of landfill construction for 25 years. But a landfill project near Lewisville, NY, was his first using a mechanically stabilized earth (MSE) berm. His company, along with David Lennox of Daigler Engineering, designed and built the MSE using Strata geogrid products.

The MSE berm was required because a high-pressure gas line passed right through the landfill site. The original plan was to move the gas line, but that would have significantly increased the costs and the time required to complete the project. Landfill capacity was critical enough to consider any plan that would preserve it. A traditionally built earthen wall would mean that up to one-third of the area would be lost to the wall. So the project was designed with a 3:1 MSE berm near the gas line, to allow the landfill to have the same capacity with a smaller footprint.

The project began in 2012, and about one-third of the MSE berm, 1,000 feet, was completed during the summer construction season. The total project will be a landfill of approximately 230 acres. More construction is planned for the summer of 2013, but the landfill will be operating next to the berm that is already built.

During project design and planning, the engineers knew they would need some kind of geogrid materials for reinforcement of the berm. They chose Stratagrid, eventually using three different products with the tensile strengths needed. The front of the wall was stabilized with Stratagrid wire-faced structures filled with different sizes of rock.

Stratagrid, manufactured by Strata Systems Inc. of Cumming, GA, is composed of rectangular apertures made of polyester. The rectangular shape forms a biaxial grid, providing two directions of tensile strength. The woven polyester yarn and a UV-stabilized saturation coating make the product chemically impervious to differing soil conditions. It comes in strengths from 871 to 3,412 pounds per foot. The product comes in rolls that can be cut by hand to the size and shape needed.

The construction of the MSE berm required soil sources. One was found onsite and another at a nearby mine. Both had to be constantly tested for moisture content and shear strength. Once soil began to be placed, the underlying soil was wetter and so it had to be allowed to dry to the proper moisture content to provide a friction angle of 28 degrees. The soil consists of silty clay, so it compacted well but held moisture. Fortunately, according to Marsack, the weather remained dry during the construction process and did not slow the work significantly.

Lennox notes that the rock-filled baskets used for the front edge of the MSE berm were more of a challenge than expected. The largest rocks, 4 to 8 inches in diameter, were difficult to place and required a lot of hand-placing to get them sited and secured properly. Marsack agrees, adding that this part of the project took place in the heat of the summer and took longer than expected. The next consisted of rock 2 inches in diameter, topped by baskets with rocks of half-inch size. The rock-filled baskets provide stability and sediment-filtering for the front face of the wall.

GPS-equipped bulldozers owned by Modern prepared a level base for the MSE berm. The baskets were placed, and then the Stratagrid was stretched back into the berm area. Support struts were added, and the soil was compacted on top of each layer. Lennox says the installers quickly learned that they needed to keep the Stratagrid perfectly flat as the soil was added. Stratagrid 500 and 350, with the highest tensile strengths, were used for the primary reinforcement. Higher up, Stratagrid 150 was installed.

Another challenge faced by the installers was the transition from 3:1 wall to .5:1 vegetated slope. Lennox notes that the different lengths of geogrid and the ability to adjust each lift improved the process. “We’ll know better how to do it next time.” He says Strata Systems was helpful at bid time and with installation instructions.

Marsack points out a lesson learned: Don’t underestimate the labor effort required. He notes that they were fortunate to have equipment operators who also provided labor. Otherwise, staged construction would have resulted in longer waiting times. Both Marsack and Lennox agree that they will use Stratagrid for the completion of this MSE berm and for other projects. It will widen the possibilities for projects that they bid in the future.

Caltrans Highway
La Media Road required widening to handle increased heavy truck traffic heading to and from the US-Mexico border crossing. As part of improvements to California State Road 905, a test of roadway construction was planned in 2010. For the northbound lane, a thicker asphalt concrete and aggregate base section was used with no geogrid application. The southbound lane was constructed using Tensar Corp.’s product, TriAx geogrid, and a thinner section of both asphalt concrete and aggregate base. Comparisons were initially conducted during construction and will continue to be carried out throughout the lifetime of the pavement.

Garrett Fountain, vice president and principal geotechnical engineer for Southern California Soil and Testing, says there were three reasons for choosing TriAx geogrid. First, it reduced construction costs; using TriAx saved about $50,000 per lane mile. Second, it increased speed of construction; with the average of 12 cubic yards per load of aggregate base truck, about 200 fewer loads were needed per mile. Third, TriAx will enhance the life of the pavement. Research shows that the modulus of the TriAx-reinforced section ranged from 27% to 52% in the 17-inch aggregate base compared to the 29-inch unreinforced section.

Two challenges had to be overcome by the planning and construction of this roadbed. One, the road had to be built to carry large amounts of heavy truck traffic. Two, the existing road was a nonuniform thickness, going from thinner to thicker sections in a very short span.

The underlying soil is a mix of clayey sand and clayey sand with gravel. The unreinforced pavement consists of 7.2 inches of asphalt concrete and 29.4 inches of Class 2 AB.

For the reinforced lane, the TriAx geogrid was supplied through Pacific South for Tensar International Corp., whose headquarters are located in Alpharetta, GA. TriAx is manufactured from punched sheets of polypropylene and has triangular apertures that join to form hexagons. This shape and the high rib strength give the product stiffness through 360 degrees and provide stability to the road components. Biaxial grids provide stiffness in two directions, but TriAx adds a third dimension to give tensile stiffness in all layers of the aggregate. The triangular shape builds a stronger junction in the geogrid, improving the strength and stiffness.

The TriAx design spreads radial stress more efficiently, even over soft soil. This increases the lifetime of the subsurface and so decreases road maintenance costs.

The design also contributes to aggregate containment, lending more support and improved structural performance. Aggregate particles interact within the apertures giving a stiffer composite layer. TriAx geogrid is impervious to chemicals and resistant to weathering, including UV degradation.

For this roadbed, the layers are as follows:

  • 6 inches of asphalt concrete
  • 6 inches of Class 2 aggregate base
  • Tensar TX5 geogrid
  • 11 inches of Class 4 aggregate subbase
  • Tensar TX5 Geogrid
  • the subgrade

The unreinforced side of the road has 7.2 inches of asphalt concrete and 29.4 inches of Class 2 aggregate base.

The TriAx was rolled out by a two-man crew. The strength of the geogrid allowed aggregate trucks to drive over the geogrid and drop their load of aggregate base where needed.

Using TriAx saved about 10% in job costs, mostly because less material was needed. Time was also saved, as fewer truckloads were required. The road section has been shown to exceed the performance and endurance of the unreinforced side.

The project provides the opportunity to collect data and make comparisons of an unreinforced road and that of one reinforced with TriAx geogrid.

Stormwater Management in Canada
A Shell gas station in Abbotsford, BC, needed underground stormwater detention. The product chosen was the Atlantis Storm Water Management System from Layfield Environmental Systems, Edmonton, AB.

Challenges with the project included the busy work site and the ongoing construction of the Shell station. Also, high volume storage was needed in a small space. The tank was installed under the parking lot.

The Atlantis Rain Tank was assembled at the King Hoe Excavating’s yard and trucked to the work site. A nonwoven geotextile was wrapped around the tank to prevent fill material from entering it. A PVC geomembrane was installed beneath and up the sides of the tank to capture all water until it could be released. Geogrid was placed for ground reinforcement.

The project was completed in April 2010 and so far, the components are working exactly as planned.

Protecting a Pennsylvania Wetland
State Road 309 near Schnecksville, PA, was scheduled for road reconfiguration and widening. A growing population forced the need for a wider, higher-volume road. A big challenge for the project was the protection of a nearby wetland. The construction of a bridge over the wetland was considered, but was rejected because of the higher cost and potential impact to the wetland. The solution chosen instead was an MSE wall.

The project was awarded by the owner, the Pennsylvania Department of Transportation, and that agency also engineered the project. The contractor was J.D. Eckman of Atglen, PA.

A larger culvert was installed, and then construction of the MSE wall began. Galvanized wire baskets 18 inches by 18 inches by 10 feet were placed in lifts to provide face stability and vegetation. A turf reinforcement mat covered the face, and Miragrid XT geogrids of different strengths were installed for primary reinforcement. Mirafi 1100NPA was used for secondary reinforcement.

TenCate Miragrid products are uniaxial geogrids manufactured from woven polyester and coated with a polymer. They are flexible and durable with long-term design strength. The design makes the Miragrid XT geogrids lightweight and easy to install.

Miramesh is a biaxial mesh reinforcement made of polypropylene fibers. It can be used to wrap a wall face or as secondary reinforcement. The green color of the mesh provides appearance of grass until vegetation can be established. Miramesh is UV stable and supports vegetation growth.

Miramesh and Miragrid, along with other geosynthetic products, are manufactured by TenCate Geosynthetics North America, whose headquarters are in Pendergrass, GA.

For this project, the following supplies were used:

  • 11 rolls of Miragrid 7XT
  • 35 rolls of Miragrid 8XT
  • 421 rolls of Miragrid 10XT
  • 126 rolls of Miramesh geosynthetic woven mesh

The MSE wall built for this road reconfiguration is 1,050 feet long and in places reaches 50 feet in height. The reconstructed road runs on top of the wall. The wall face and the set back at each level now have vegetation, giving even more strength to the wall and providing more protection for the wetland down below. The TenCate products integrated smoothly to produce a project that met the requirements needed and at a reasonable cost. The state DOT and the contractor were pleased with the project. What they had considered to be the most difficult part of the project turned out one of the easiest sections.

Florida Success
Geosynthetics are available in forms other than geogrids. Presto Geosystems Geoweb can also provide road stability, drainage, and decreased maintenance costs.

A dirt road running through Eglin Air Force Base in Florida was displaying significant sediment loss during rain events. The sediment was flowing into a nearby creek, impacting an endangered fish species, the Okaloosa Darter. In addition, the road had washed out in spots and was almost impassable.

The road was rebuilt using the Geoweb Load Support System. The dimensions are 30 feet wide by 4,000 feet long. The Geoweb system starts with strips made from high-density polyethylene with UV protection. The strips are fashioned into cell units to hold the aggregate or soil. Cell size varies from 8.8 inches by 10.2 inches to 11.3 inches by 12.6 inches. Cell depth can be 4, 6, or 8 inches.

The roadbed was prepared and leveled and a geoweb fabric installed. Then the Geoweb was put into place and anchored with stakes. The cells were infilled with rock, providing natural runoff protection by holding the water and letting it percolate into the soil.

The stiffness of the Geoweb provides a stable road foundation and controls settlement of the gravel surface. Perforations in the cell wall also allow drainage between different road areas, especially valuable oversaturated soils.

A final layer of gravel was placed over the Geoweb installation, leaving a smooth road that will not wash out and that will filter stormwater to decrease sediment load into
the creek.

School Athletic Field Needs Stability
A slope next to the athletic fields of Saint Mark Catholic School in Huntersville, NC, had a big problem with surface erosion. Failure of the slope threatened the athletic fields, and the desire was for a stabilized slope that would be green and attractive.

The product chosen to accomplish this goal was Fortrac 3D, manufactured by Huesker Inc. of Charlotte, NC. Fortrac 3D is a three-dimensional geosynthetic mat made from polyester. It has low creep tendencies, elongating only 12%, compared to up to 65% for some products. A polymeric coating gives UV protection. Fortrac 3D can be used as a turf reinforcement mat or as soil reinforcement parallel to a slope. The three-dimensional design provides integration with the underlying soil and decreases soil slippage, providing erosion control even before vegetation can be established. The Fortrac 3D 30 chosen for this project has a tensile strength of 30 kN/m. It also has an open structure that promotes vegetation growth, leading to even more slope stability and erosion control.

The engineering firm for the Saint Marks project was ECS Carolinas, and the contractor was Uretek ICR, Mid-Atlantic. One challenge was to have the project completed within the owner’s timetable. The choice of Fortrac 3D and Platipus anchors made for a rapid completion.

The project began with removal of all vegetation and incompatible soil. The slope was graded to 2:1, and a 12-inch berm was added at the top. Catch basins and slope drains were installed to divert stormwater runoff from cascading down the slope. Onsite soil was used for repair and smoothing of the slope. Once the slope was ready, it was seeded.

The Fortrac 3D turf reinforcement mat was installed on the slope and pegged down with the Platipus driven-anchor system. The material was anchored at the top of the slope and allowed to roll down. Platipus soil anchors were placed to hold the Fortrac 3D.

A four-man crew worked 2.5 days to install approximately 2,500 square yards of the turf reinforcement mat. The installation went smoothly, and the owner says he would use these products again. A few weeks after the project was completed, the slope showed full revegetation, and the threat to the school’s athletic fields was contained.

Fortrac 3D is available in seven strengths, ranging from 30 kN/m to 120 kN/m. The rolls are sized at 4.5 meters by 100 meters.

Geosynthetic materials have become flexible, valuable tools for people in the erosion control industry. With so many different types of products-geotextiles, geomembranes, turf reinforcement mats, and all kinds and strengths of geogrids-there is a product for any project. As their use increases, new products will be introduced, and this sector of erosion control products will expand.

The ability of these products to be integrated together with different strengths, types, and uses adds to their value. Geosynthetics have a place in preserving and protecting the natural world. It’s the best of both natural and synthetic.

About the Author

Roberta Baxter

Author Roberta Baxter specializes in science and technology topics.