Geosynthetics to the Rescue
It seems if there’s a stretch of bare land that’s eroding, there’s some geosynthetic out there that’s designed to control it, no matter what it is. This is a feat of manufacturing specialization and engineering prowess, but when presented with such a panoply of options, it can sometimes be hard to see the most important applications.
Geosynthetics are used to improve drainage by keeping particulates out of spaces that are designated for water to flow through. They can also improve filtration, by providing a textile option for tasks that would otherwise necessitate an impermeable surface.
But they are not just for water, of course; they help keep soil in place as well. A layer under the ground can separate soil strata from mixing, while a layer over the ground can reduce or eliminate soil erosion. They can even be interspersed with soil for reinforcement, so much so that the reinforced soil can form the basis of a retaining wall.
Often, geosynthetics come up in the planning process as a cost-saving measure. To the uninitiated, that phrase may bring to mind penny-pinching and cutting corners, but in the large scale and the long term it translates into economic viability. Over the past 60 years, there’s no telling how many projects would have proved too costly without these miracle materials.
Simply put, there are some jobs that just couldn’t be done without geosynthetics. For such projects, a geotextile or geogrid can make the impossible possible, and can turn a client’s vision into a reality. I hunted down a few of these jobs, and spoke with the contractors who made them happen.
The first case is from Texas, where a company had run into trouble and decided to pay for a contractor’s help. “We were called in because a swimming pool company wanted to build on the side of a large slope,” said Jerry Sanders, owner and president of Sanco Design, Inc., based out of Plano, Texas. A home had just been built at the slope’s top, and its owners wanted a pool, but the back of their land sloped down into a lake.
The pool company had been right to look for help, because they faced some major problems. “Upon arriving at the site, I noted that the material did not appear to be compacted correctly into place, and that there was water seeping out at different points along the bank,” he said. On closer inspection, his initial assessment had proved correct; the soil was poorly compacted, and full of debris.
When the home was constructed, the company responsible had brought in fill material and pushed it off the side of the bank. They repeated this process with more and more fill, counting on the weight of the machines to work the soil in and compact it sufficiently. Unfortunately, that wasn’t enough. “To do it properly, they should have benched it into that native soil, and it should have been compacted in eight- to ten-inch lifts,” Sanders said.
Sanco Design needed to restructure the slope, but between the lack of compaction and the geography of the location, access would be limited. Sanders’ plan was to remove the soil, stockpile it further down the hill, and put it back in a structurally stable fashion. They would have to use what was onsite almost as soon as they’d removed it.
Had the soil been dry, that might have been enough by itself, but they had to drain the soil as well. “We used a geoweb along the base of that installation to serve as a drainage mat,” he said, “because we had a tremendous amount of water coming off of a thin sandstone rock ledge, which was 40 feet below the back of the home.”
The drainage mat served as their main collector, from which they extended drains down to the lake. Once that was in place, they started constructing a reinforced earth bank. “Each time we’d get to another section where there was water coming out of the bank, we’d use composite drains to intercept that water, and channel it back down into our drainage mat installation,” said Sanders.
A composite drain is also known as a wick drain, and it consists of a polysynthetic material wrapped in a filter fabric. It looks something like an egg crate, and while its width may vary, most are generally about one-inch thick. They serve to wick water out of the soil and transport it to a collection point. In this case, it allowed Sanders to keep the lake wet and the slope dry.
The face was then wrapped with a geogrid textile and was planted with ground cover materials and seed. The 1:1 slope was now stable, and the swimming pool company was then able to install a large swimming pool, complete with a waterfall. Water stays above the slope and below the slope, but it doesn’t stay in the slope.
Pringles Park outside of Jackson, Tennessee, faced an altogether different problem. You may not have heard of the Jackson Generals, the minor-league team which calls the park home, but you’ve probably heard of their major league affiliate, the Seattle Mariners. The Generals wanted to give their landscape a sorely-needed aesthetic upgrade, and they looked to a local company, Envirogreen Inc., to do it.
Christopher Todd, vice president of the Humboldt, Tennessee-based company, had his work cut out for him. “One of the problems was a very ugly, large drainage ditch that ran across their property,” he said. The ditch was overgrown with kudzu, an invasive vine common to the South. Worse still, it was highly visible, running along Interstate 40 right in front of the stadium.
Not only did the ditch need to be reshaped to have a more uniform slope so it could drain properly, but it had to look good, too. This ruled out the usual fix. “Normally, they would put riprap over geotextile fabric to stabilize the channel and the slopes,” said Todd. Riprap and geosynthetics might have solved the drainage issue, but the clients didn’t like how it looked, so that was off the table.
“In this situation, they wanted to go with something that was softer, that would be a little easier to maintain, and look better over a longer period of time,” he said. In the end, they chose to employ a Turf Reinforcement Mat (TRM) to fit their needs.
When the grading and the shaping had been finished, Envirogreen laid down rolls of TRM over the area, anchoring it in place with 12-inch round top pins. Once it was in place, they covered the TRM with a layer of Bermuda sod, and then anchored that with 6-inch turf staples.
Not only did the sod match the rest of the landscape, making the ditch appear seamless from the Interstate, it also solved the kudzu problem. “Because of the mowing frequency, they’ve been able to keep the kudzu out,” said Todd.
The traditional riprap solution would have restricted the clients to fighting the kudzu using strictly chemical means. Herbicides can be expensive, and repeated applications can result in the targeted plant building up a resistance. Using a TRM allowed the clients to add the channel into their existing mowing regimen, a cheaper technique that left their options open.
All the way across the country, another slope was in danger of collapsing, and Thunder Mountain Enterprises, Inc. in Sacramento, California, had been brought in to fix it. Though this slope was smaller than the one in Texas, it needed a lot of help, as it was already supporting a structure right at its top.
“The bank was holding up a tennis court, and if it had failed, that would have damaged the lights, the fence and the court itself,” said David Smiley, company president.
Not only did the slope need to be reinforced, but it needed better drainage and some erosion-control measures while it healed. Fortunately, Smiley was able to draw up a plan that addressed these three problems in turn, and geosynthetics proved integral to solving each one.
It started with a series of Percussion Earth Anchors, or PEAs, driven into the bank. They look a little like grappling hooks on steel cables, and function much the same. “You drive a PEA past where the failure zone is, so you’re into material that’s very solid, usually native material,” Smiley said. Once driven in, the anchor is like a molly bolt that’s been pushed in past a bad section of drywall.
PEAs were driven eight feet into the soil, spaced four feet apart, and then the geosynthetics came into play. A facing system attaches to the stainless steel tendons of the PEAs, and the load from the facing system pulls on the anchor, which rotates to a flat facing so that it locks in place. “We can then apply anywhere from 500 pounds to several thousand pounds of load on that anchor, against the facing system,” said Smiley.
In this instance, they used a geogrid, because a softer geotextile wouldn’t have been strong enough. Together, the geogrid and the PEAs put 2,000 pounds of load against the slope, pinning the unstable slope into the native soil. With that component done, the other parts of the job could begin.
Next, they rolled a highstrength TRM across the slope. Although this did add some additional strength to the structure, that was not its primary purpose. “It had a tight enough, three-dimensional matrix that could hold soil, and then we were able to plant through that,” he said. The TRM would prevent the slope from eroding through the grid, and provide a basis for revegetation.
Then the drainage element had to be fitted. “Once we stabilized the slope, we built a concrete stem wall at the toe of the slope, and between the slope and the stem wall, we put in a drainage layer so water couldn’t get trapped in there,” said Smiley.
The drainage layer was a French drain, consisting of rock wrapped in a six-ounce non-woven geotextile. The spaces between the rocks would provide plenty of room for water to funnel to a pipe running along the back of the wall, and the geotextile would prevent soil and fine particular matter from clogging up those spaces.
Without the geosynthetics that made each part of this plan work, holding up this slope would have been much more costly. “I would have had to do a conventional, build-up slope repair, which would have taken a lot more time and effort,” Smiley said. He estimates that an alternative measure would have cost twice as much to do, and couldn’t have been completed in the rain, like this job was.
When you work with a tool every day, it can be easy to get blasé about the advantages that it affords. Just think about how often people complain when the networked supercomputer with gigabytes of memory that lives in their pocket isn’t working fast enough. After all, you only think about your hammer when it breaks.
With that in mind, it can be a good idea, every now and then, to take a break and really consider the materials you use. Not only does it help stave off the mental stagnation of routine, but considering life without geosynthetics could help you better explain to others why they are such superior products. You might even find that the process sparks some fresh ideas about your business. That kind of creative thinking is what changes the course of businesses and lives. Isn’t that worth a thought?