While the concept of soil reinforcement has existed at least as far back as the Egyptians, the use of synthetic materials to bolster soil gained popularity in the years when combating erosion became a priority.
The rapid growth of the polymer chemical industry in the second half of the 20th century has made possible the manufacture of products that can withstand the forces of nature, both hydraulic and biodegradation.
Gaining momentum in the 1960s and becoming widespread nationally and internationally by the 1980s, geosynthetics research and development continues to progress and to drive a burgeoning marketplace in concept and practice.
The use of geosynthetics and synthetic-natural composites for erosion control is likely to increase in the future as environmental regulations and enforcement become ever more stringent. A relatively new focus in the industry is sustainability, as evidenced by the Geosynthetic Institute’s request for papers on the subject for its recent conference.
A geosynthetic has been defined by the American Society for Testing and Materials (ASTM) Committee D35 on Geosynthetics as “a planar product manufactured from polymeric material used with soil, rock, earth, or other geotechnical engineering related material as an integral part of a man-made project, structure, or system.” The geosynthetics routinely used are geotextiles, geogrids, geomembranes, erosion control blankets and mats, geosynthetic clay liners, geocomposite drainage materials, and geonets.
The largest category of geosynthetics appears to be the geomembranes, which are considered relatively, if not completely, impermeable. With more than 30 applications, these are often used to line catch basins and settling ponds.
Geotextiles, the second largest category of geosynthetics, receive their label from their fabric-like makeup, which is strengthened by synthetic fibers making them suitable for applications that require porosity.
Geogrids, with their large open spaces, are often used for reinforcement in constructing retaining walls. They are extremely durable.
Geonets or geospacers are designed to assist water flow through a drainage area, and geosynthetic clay liners are particularly useful to prevent fluid infiltration in landfills.
Geosynthetics are used in all facets of construction and maintenance, with their principal functions being filtration, drainage, separation, reinforcement, and barrier protection.
While the variety of geosynthetics seems to be limited only by the imagination and entrepreneurial spirit of their manufacturers, those who install them might prefer a more cohesive approach, where everyone is “on the same page.”
While research and standardization have greatly improved in the last three decades, several issues remain, according to those who use the products. The variety of products makes it essential to choose the correct one for the application, and sometimes it is necessary to combine products for the best result. Specifications may not always match practice, and it is often the end user who has the practical knowledge and experience to request the best product for a particular site.
Those who wish to become acquainted with the latest developments in the field of geosynthetics have many options both for education and involvement. The International Geosynthetics Society (IGS) was founded in Paris in 1983 by a group of geotechnical engineers and textile specialists. From all parts of the world, the society brings together individual and corporate members who are involved in the design, manufacture, sale, use, or testing of geotextiles, geomembranes, and related products and associated technologies, or who teach or conduct research about such products.
The Geosynthetic Materials Association, representing nearly 80 member companies, is a central resource for information on geosynthetics. Its activities center on five areas: engineering support, business development, education, government relations, and industry recognition. Membership in the Geosynthetics Materials Association is open to any IFAI member interested in this niche marketplace.
The Geosynthetic Institute hosted its 24th conference, convening in Dallas, TX, in March 2011. Embedded in the ASCE GeoInstitute’s three-day conference, titled GeoFrontiers 2011, the GRI-24 conference focused on “Enhancing Sustainability Using Geosynthetics.”
Segmental Retaining Walls: Different Solutions
Randy Grego is general manager for B.C. Hardscapes, an installer of modular walls and brick pavers in Kansas City, MO. In the fall of 2010, the company completed an eight-month project for Briarcliff apartments in Kansas City. For other work done for Briarcliff Village, the company was recognized by the Interlocking Concrete Pavement Institute (ICPI) along with the Brick Industry Association and National Concrete Masonry Association for its segmental retaining walls in the 10,000- to 50,000-square-foot category.
The project involved leveling the side of a bluff, which dropped off from the road above it. “We built a 27,000-square-foot retaining wall on the bottom side and backfilled behind it,” Grego says. “Originally, a polyester geogrid was to be used on the project, but when the spec changed to a crushed recycled backfill, the cement content would have damaged this type of geogrid.”
B.C. Hardscapes brought in the Mesa Retaining Wall System and Tensar Uniaxial (UX) Geogrids from Tensar International. The UX Geogrids are manufactured using a punched-and-drawn method with high-density polyethylene (HDPE) resins that resist impact loads from installation of the backfill. The HDPE resin also provides a highly inert material that is resistant to degradation in high-pH environments. These geogrids carry large tensile loads applied in one direction (along the roll), and their open aperture structure interlocks with natural fill materials.
Mesa segmental concrete blocks, designed specifically to interact with Tensar Geogrids, create a positive, mechanical connection and have a compressive strength that exceeds NCMA standards (greater than 4,000 psi).
Grego says it is becoming popular to use recycled concrete for fill. “It’s part of the green movement.” In this case, 20,000 tons of concrete was brought to the site in dump trucks, but there was little access for the dump trucks to get behind the retaining wall, Grego says. “We set up a conveyor station and used the conveyer to get the rock behind the retaining wall. The rock would go out and over the conveyer belt and then fall approximately 30 feet down to the reinforced zone of the wall. Then we spread the backfill using Bobcats.”
Grego says Tensar’s UX Geogrid proved much more durable than the previously planned polyester grids. “It’s a little more expensive, but using the recycled rocks, we actually saved money in the long run.”
Temperatures, which ranged from cold in the spring when the project began to very hot by fall, had little effect on the project, Grego says. But rain brought complications. “We had a tremendous amount of rain,” he recalls. “There was erosion behind the wall, and we had to clean out silt on a regular basis. But the wall held up well; it looks good.”
The advantage of the Mesa Block and the Tensar Geogrid combination is its positive connection, Grego says. “The grid connects to the block. With a polyester grid, the connection is created by friction between the blocks.”
The retaining walls were designed by Shafer, Kline, and Warren, and the blocks were produced by Midwest Block and Brick.
Nick Jansson, P.E., LEED-AP, of Soil Retention Products Inc. in Carlsbad, CA, says his company has been using Mirafi geogrids from TenCate almost exclusively for the more than 20 years it has been in business.
“We do 25 to 50 projects a year, and we’ve gone 22 years without a failure,” Jansson says, adding that the company specializes in large segmental block retaining walls in the 10,000- to 50,000-square-foot range. In 2010, the construction company was an HNA Hardscape Project Award winner, placing first in the 10,000- to 50,000-square-foot category for segmental retaining walls, as judged by the ICPI along with the Brick Industry Association and National Concrete Masonry Association.
The winning project, The Shops at Sycamore Creek in Corona, CA, was a 36,000-square-foot segmental retaining wall with a 34-foot maximum height and 1,400-foot length. Requiring 15 days to complete (a company record for a project this size), the wall used 59,000 square yards of Miragrid 10XT and 9,000 square yards of Miragrid 20XT.
Because the company specializes in large wall construction for commercial properties, Jansson prefers the ease of Mirafi’s larger rolls of fabric. “We can use one 12-foot-wide roll versus ten 4- to 6-foot-wide homeowner rolls. In large-wall construction, that allows us to lay out more at one time.”
But the main advantage of Mirafi, according to Jansson, is its construction. “I’d like to educate readers on the difference between extruded plastic and polyester grids,” he says. “With extruded plastic, you have to pin down extensively. When Mirafi’s polyester grids are pulled taut and anchored with fill dirt, you can drive over it and it doesn’t move. It doesn’t require staking. Other companies talk about “˜creep,’ but this is a controversial term in the industry. If a wall is built correctly, it shouldn’t creep. We don’t talk about it because it never happens. We’ve built 8 million square feet of wall without a failure. We’ve never seen a wall fail because the geogrid broke in the failure plane. If a failure occurs, it is due to a poor connection in the wall face. If a wall is designed for the correct geogrid strength, that strength should be at the failure plane. If failure occurs, it is related to the face and the connection of the block and geogrid. If things are not compacted enough, it is because the system doesn’t allow for it. We consider the block-geogrid plus the connection as a system. They are not separate. Some say that the block is just a facing, but that is not accurate. They must be integrated as a system.”
A frequent challenge for wall builders is that of maximizing usable land in hilly terrain. When Pacific Life Insurance Co. chose to locate its Life Insurance Division of 1,000 employees to a new location in Aliso Viejo, CA, the nine-story glass building was situated high along a freeway. Adequate parking would require a five-story structure. But with a 60-foot-high grade separation between the office building and the parking garage base, the engineers were calling for a 50-foot-high retaining wall.
There were manifold challenges with the site, including tight wall curves, drastic height changes along the wall length, wall heights exceeding 40 feet with 2:1 slopes in a seismic area, a portion of the wall facing the road needing to be vegetated, and a need to accommodate a pedestrian and vehicular bridge foundation on vertical piers.
As contractor, Soil Retention Systems chose the SRS Verdura system, a geogrid reinforced SRW with plantable face. Construction involved placing the backfill with heavy earthmoving scrapers before placing the Verdura 60, a high-strength, 132-pound, 18-inch-deep block. As each course of blocks was stacked, the lip of each block interlocked with the two blocks below it, allowing the scrapers to run up behind the wall facing. Imported fill was stockpiled in front of and above the zone to increase scraper production. Miragrid 24XT and 10XT high-strength polyester geogrids were connected to the Verdura blocks using the continuous Schedule 80 PVC pipe connection. The Miragrid layers were overlapped directly on top of each other without soil cover in between the areas of tight curves and corners. The total wall area of 15,000 square feet was complete in nine days, 21 days ahead of schedule.
Geosynthetics Bolster Failing Shoreline for the Yankees
James Griffin is vice president of Bio Mass Tech Inc., a Tampa Bay company founded by his family in 1994 to meet the growing need for specially trained contractors to cope with sensitive environmental projects. The company has since become an industry leader in the erosion and shoreline industry.
In 2008, Bio Mass worked for two months to restore the shorelines of three ponds at the George M. Steinbrenner Field, opened in 1996 as spring home of the New York Yankees and full-time home of the Tampa Yankees (Single-A affiliate).
“Over the years, the banks had sloughed off and the ponds had migrated, jeopardizing the adjacent fences and interfering with attempts to landscape,” Griffin says. “Some of the fences were falling into the ponds.”
The first step in the project involved lowering the water levels in the ponds, which range in size from 1 to 3 acres. “Using large hydraulic pumps, we transferred millions of gallons of water from pond to pond and imported clean fill until we re-established the original lines and grades of the banks,” Griffin says.
To prevent further erosion, crews installed Geoweb from Presto Products Co. and filled it with #57 stone. “We have since started using Mod 4 granite for slope stabilization,” Griffin says. “The Mod 4 is a heavier stone; this additional weight is better suited for soil stabilization.”
“The Geoweb system with rock fill is far preferable to the previous approach of adding soil infill and then planting vegetation on it,” Griffin says. “Many of the local ponds have water levels that fluctuate greatly; this fluctuation kills most of the vegetation along the shoreline. Once the vegetation dies off, it leaves the unprotected soil infill vulnerable to eroding out of the Geoweb cells.”
To install the product, it was necessary first to grade the area, then to install an underlying filter fabric before staking and fastening the Geoweb panels into place and filling them with stone. “The filter fabric under the Geoweb allows hydraulic release and prevents migration of soils,” Griffin says. “Without it, the water will pull and move the soils through the stones in the Geoweb and cause the project to fail. For 10 cents a square foot, the filter fabric is a good insurance policy.”
The challenge on this site was gaining access, Griffin says. “Because of all the landscaping and the fences, we had to import dirt and create an access road inside the perimeters of the ponds and then work our way back to a 4:1 slope.”
Griffin says he believes that Geoweb will provide a “permanent fix” for the slopes. “It is durable and low maintenance and should last indefinitely. In contrast, riprap, while a good repair for some locations, has disadvantages. It can work its way loose, is unattractive to some people, poses a danger to foot traffic, and its large voids provide a haven for rodents and reptiles.”
Restoring a California Roadway
Troy Simning is project manager for Ghilotti Bros. Construction Co., a third-generation family business based in San Raphael, CA. Last year, the company completed a two-month project to rehabilitate a 1.7-mile portion of McGary Road in Fairfield, CA. The road segment had been closed to traffic for several years as a result of landslide activity and lack of road maintenance, according to an agenda submitted in 2009 to Fairfield’s Board of Supervisors. The completed roadway was to have paved shoulders for a bicycle lane, closing a key gap in the bicycle route between Fairfield and Vallejo.
Several years ago, a landslide in the area damaged the road. As the result of the damage and a fatal bicycle accident, Fairfield closed the road and ceased maintenance, causing further deterioration.
The proposed agenda for road repair stressed McGary’s importance as a frontage road, particularly in the instance of accidents affecting traffic on Highway 80, which runs above it.
“The road was in bad shape,” Simning says. “There was a concrete road underneath the asphalt that was severely cracked, and it needed slide repair in two spots. The largest of these was 150 feet long by 35 feet wide with a depth of 35 feet.” The first step was to remove the top layer of asphalt, followed by the concrete roadway and the soils beneath it. “It was weak soil, sandy-clay with more sand than clay,” Simning says.
Access to the damage proved problematic because the slip-outs prevented driving equipment onto the site. “We had to go in at either end, since we couldn’t drive through the hole where the slide was,” Simning says. “In addition, there were existing fiber-optic lines in the slide, which we had to support with a cable and keep out of the way at the top.” To reinforce the roadway and still keep the onsite soil for budget purposes, the crew used 39 layers of Stratagrid manufactured by Strata Systems Inc. “We put down 19 primary layers of Stratagrid SG200 on the full width of the slide with one and a half inches of dirt on top and secondary Microgrid every other layer to stabilize the slope face,” Simning says. “At two layers a day, it took a four-man crew two weeks to complete the installation.”
While those who spend every day on the job continue to experiment and refine their techniques, laboratories and regulators study policy and politics in order to satisfy an ever-growing market for new solutions in a changing environment.
