The rolling landscape of Scott County, MN, is rural but not particularly remote. “That area is farm country, and the Minnesota River goes through the entire area. On the top of the bluffs it’s farmland, but at the river, the elevation drops about 200 feet in 800 feet,” says Paul Nelson, environmental services program manager for Scott County. The scene is typical for the Minnesota River Valley; as the river winds through the 30-square-mile Blakeley Township, he says, “Parts of it are up on old glacial deposits, and some of it in ravines where glacial rivers cut through drop 200 to 300 feet in a mile or less. There are lots of eroding ravines.”
According to Jake Balk, Scott County’s Highway Division program manager, “There’s a 15% grade on the roads—the ravines are steeper than that. Water flows through these ravines at 20 to 30 feet per second during storms.” And the local geology primes the soil for massive erosion. “There’s 10 feet of clay on top of everything, and under that is clean sand. So once the water breaks through the clay it gets into that sand and really starts to wash,” says Balk. Highlighting the pernicious erosive potential in the area, says Nelson, there is essentially no bedrock substrate present to tie into to enhance stability of the surface soil.
Contemplating a Threat
Erosion at Blakeley Trail ravine was threatening infrastructure along County Highway 61, while sediments released during the erosive process contributed to degradation of the Minnesota River. Scott County’s Environmental Services Department, along with the Scott County Highway Division, set out to find a remedy and undertook a stabilization project to shore up the slope of the ravine.
The project had multiple goals, ranging from protecting County Highway 61 running along the ravine, as well as other infrastructure from slides, to reducing the volume of sediments flowing into the Minnesota River where the creek emptied. The river had already been listed as impaired for high turbidity.
With plans for a retention basin at the top of the ravine to slow the water down, stabilization of the slopes, and a series of check dams at the bottom, construction began on the Blakeley Trail Ravine Stabilization project. In the spring of 2014, while work was underway, disaster struck in the form of unprecedented rainstorms. News accounts report flooding and damage across the entire midwestern United States during the mid-June storm, but Nelson says the Twin Cities region, including Scott County, seem to have borne the brunt of the storm’s fury.
Balk agrees: “We got 14 inches of rain in 16 hours.” Runoff from agricultural lands up on the bluffs, he says, carried huge volumes of sediments and dumped them on the lower-lying landscapes, including the roadways and communities like Blakeley Township.
Rebuilt slope in Scott County
Averting Catastrophe
Ravines all over the county began to collapse, including the project at Blakeley Ravine Trail. That project, at the time stripped of vegetative cover for anticipated construction, was nearly wiped out. But worse than that, slides had taken out all three road access routes to Blakeley Township itself, potentially leaving the farm community of fewer than 100 residents marooned in the face of rising waters.
“We had to move quick,” says Balk. A hurried evacuation in the rain, along a single lane of what was left of the collapsing road, shuttled the residents out of harm’s way. Balk says that in all, the storm resulted in at least 110 different landslides. In its aftermath, he notes, “We moved 80,000 cubic yards of clay, mud, sand, and gravel from roadways.” Attention quickly veered from the multiple potential benefits of the Blakeley Ravine Trail repair project to the singular goal of getting emergency repairs done to restore the lifeline and livelihoods of Blakeley Township residents cut off from their homes by landslides on County Road 1.
Barr Engineering was tasked with the daunting repair job. Steve Klein, vice president and senior civil engineer with Barr, had a long track record of success with Profile Products’ Futerra turf reinforcement mats (TRMs). When Profile introduced the GreenArmor System in 2007, Klein began specifying it whenever a situation needed quick germination and extra holding power. With “vertical fill planned for the site to a depth of 50 to 100 feet,” says Klein, the mat selected would need to be able to withstand any intrinsic settlement and movement that might occur on the newly reconstructed slope. In addition, the initial problem that resulted in the landslides remained. “The landscape was steep and we had the potential for high-velocity flows due to concentrated runoff coming off of the bluff into the tributaries.”
Klein says he considers Futerra TRMs to be “the most stout” of the TRM class, but at the same time, he says, “They still allow good growth and offer the advantage that tackifier can be applied directly.” The GreenArmor System consists of a Futerra TRM infilled with hydraulic mulch, Flexterra High Performance-Flexible Growth Medium (HP-FGM). The combination offers a technologically advanced solution with quick installation to protect high-discharge waterways.
However, Klein notes that one of the keys to success in any TRM application is careful installation. He visited the Blakeley Township site during construction to confirm that crews and contractors had used the recommended techniques. “When protecting an area for concentrated flows, where you have a swale going down a hill, the first row of that material needs to go right up the flow line of that swale, and then subsequent rows of that material are overlapped in a shingled fashion on top of that.” He says he has on occasion been called in on projects where TRMs have failed to help the owners figure out what went wrong. He observes that in these cases, “Inevitably, the contractor started from the uphill side and started laying the TRM material down shingled in the reverse order of the way it would normally be shingled, so it didn’t act as shingled product. As a result, water got under it and allowed it to scour beneath it, and it ultimately failed.”
TRMs and Flexterra hold the slope in place.
Klein also recommends a double-seeding technique, with one application of seed going down before the mats are run out and a second seeding on top of the mat, applied with the hydromulch. “Seed is a very inexpensive component” of a project, he says, and therefore a little extra seeding is an economical way to obtain extra holding power for a TRM application. “Double seeding works extremely well,” he says.
According to Balk, repairs to County Road 1 required 9,000 tons of riprap at the bottom of the channel along with Profile Products’ TRM, sheet piling, and subsurface drains to keep the water moving away from the slope. “We used a turf reinforcement mat with Flexterra, which has worked fantastic to date. We got five inches of rain last week and we haven’t seen any damage. The water’s staying on top of the Flexterra, and the vegetation looks great,” Balk said several months after the installation.
After completing the road repairs, allowing township residents to return to their regular lives, crews returned to revive the stabilization project on Blakeley Ravine Trail.
Klein and Adam Popenhagen, Profile’s market development manager, designed a GreenArmor System consisting of 20,000 square yards of Futerra R45 High Performance-Turf Reinforcement Mat (HP-TRM) infilled with Flexterra HP-FGM. J&L Larson, a contractor from Lakeville, MN, completed the installation in fall 2014 with a quick-germinating dormant seed mix provided by Ramy Turf Products of Mankato, MN.
Thanks to a mild winter and the durability of the GreenArmor System, spring vegetation quickly emerged and turf restoration is happening quickly. Scott County officials and Barr Engineering could not be happier with the results to date.
Reviewing the results at Blakeley Ravine Trail, Balk says, “It looks fantastic. The foliage is two-and-a-half feet tall. There’s no undermining underneath the mat. I think the key is that about every 75 feet we tucked an 18-inch overlap into the ground so if any water started eroding under the mat, it can only make it 75 feet before it would hit a wall of TRM and have to come back up to the surface. So if water were to get under the TRM for any reason, it couldn’t erode the dirt for very long before it would be forced back on top of the mat.”
Holding the Line Against Erosion
A 500-mile-long natural gas pipeline runs from Bennington, OK, to an interconnection point just outside of Butler, AL. Along the way, the Midcontinental Express Pipeline burrows through a wide variety of landscapes. Although it was installed in 2009, by 2013 the owners had returned to a certain stretch of the pipeline multiple times to address erosion and unstable soil conditions that threatened to expose the infrastructure and pose the risk of damage to the pipeline itself.
Bobby Thomas, president of Reel Neet Erosion Control (RNEC), says an approximately 1-acre parcel in the middle of a hilly region near Quitman, MS, had become a thorn in the side of pipeline management. “They had a failing slope there and they had been back two or three times to repair it to prevent sliding.” He says it wasn’t surprising that the hills in that area would pose some challenges. Familiar with the region, he observes, “Slopes fail naturally in the area all the time.” He adds that the slope in question “may have been failing before the pipeline infrastructure was installed,” but he says there “might have also been an underground spring somewhere on the slope” that could have contributed to the soil saturation and instability. After several attempts by the company to shore up the slope proved not to be fruitful, says Thomas, the pipeline owners invited several firms, including RNEC, that had performed work on separate projects for the company to propose a comprehensive, permanent fix for the problem area. Thomas saw the problem as not so much to reinforce the soil covering the parcel, but to reinforce the slope itself from the bottom up, building a stable soil structure where none had existed previously.
Slope failure threatened the roadway.
Digging in to Find Solutions
RNEC hired a geotechnical engineer to do soil bores and a full soil analysis of the slope. Thomas proposed rebuilding the slope from the base up and holding it in place with strategically placed soil anchors and erosion control matting. He chose a solution involving Bonar’s Enkamat R45 HPTRM used in conjunction with Platipus percussive driven earth anchors to effectively (and permanently) modify the geotechnical properties of the failing slope to provide stability for the design life of the original project. The immediate project’s objective was to remove the risk of damage to the protective soil layer over the pipe to prevent exposure and subsequent damage to the pipe that could result in a catastrophic event, potentially risking loss of property, loss of use of the pipeline, or possible injury or death.
RNEC’s proposal was chosen and carried out in the fall of 2014. The design included 12- and 15-foot-deep alloy anchors with stainless steel tendons and all non-corrosive components. The anchors require preloading and locking into place at strategic placement and depth to effectively intercept the anticipated failure plane within the slope. Thomas recommended wick drains as well, to prevent destabilizing soil saturation and to drain off water from any underground springs on the site.
Getting to the bottom of things, Thomas dug into the project.
He observed that the pipe had been installed in the conventional manner 10–15 feet deep. “When they put the pipe in, they cut the hill out and installed the pipe and covered it over with fill.” As he examined the slope further, says Thomas, it appeared the soil “had not been compacted as well as it could have been” during the initial installation.
“We found trees in there left over from when they were clearing for the original installation.” That debris left in place, along with the inherent instability of the slope, had complicated matters, and he wanted to make sure there were no buried brush piles that would create unstable voids under the soil. After a complete analysis of the site, RNEC excavated the debris that had tumbled with the slides toward the creek banks and returned the soil upslope to restore the contours of the earth that protected the pipeline.
According to Thomas, the project also took on a time-sensitive aspect. This part of Mississippi is popular with hunters and outdoor adventurers, and Thomas says the landowner for the right of way wanted to be certain the repairs would be completed before the start of the deer hunting season in early November. With the landowner’s wishes in mind, Thomas felt that in a forested habitat populated with turkeys and deer and other wild animals, Enkamat represented an ideal solution to minimizing dangers to wildlife while controlling erosion. “The animals can’t get hung up on it because so much vegetation grows on it that it basically becomes part of the ground.”
Working in the Wild
In October when the construction began, the big challenge was the physical logistics of working on the remote and rugged site. Although Thomas says working on the steep slopes elicited complaints of achy feet from crewmembers, he notes, “There’s nothing you can do about that. But you have to watch what you’re doing. ” Furthermore, he says, the steep incline made the going just as treacherous for machines as it was for staff.
“It was a complicated job for slope control. That thing was so steep we had to cable off our equipment in order to even be on it. We had to put a 350 excavator on the top of the slope and get a cable to anchor onto the other equipment that we had working on the slope.”
Thomas began by cutting the slope out and putting all of the dirt that had slid to the bottom back in place. “We could drive the equipment down and then we had to work our way back up. We compacted the dirt with a roller hooked to a cable on a track hoe.” After laying down the Enkamat, Thomas brought in a mini-excavator to drive along the side of the slope, cabled off to the track hoe to drive on top of the mat for hammering in the anchors and wick drains.
The installers persevered through the month of October, laying 3,800 square yards of Enkamat R45, which was held in place with anchors driven into the subsoil to a depth of 12–15 feet. After installing the Enkamat, anchors, and drains, crews hydroseeded the restored slope with Flexterra. Seeding a combination of lovegrass, Bermuda grass, clover, and wheat straw as a temporary grass, they completed installation in advance of the landowner’s November deer hunting season deadline.
Anchored in and Tied Down
Thomas says the anchors were a unique and effective key to the project’s success. “The anchors are great. It’s a very good idea in this area. We don’t use anchors that often, but the Enkamat works great with them. It distributes the load over a wide span while tying into good earth at a distance from the failure plane.”
The high-tensile-strength mat “spreads the point load across the face of the slope,” says Andy Constantine of Bonar Inc. “If a particular anchor failed, the other anchors would easily pick up the load. You absolutely have to have a tensile member to spread the load. With Enkamat, the strength is attained right away. Other mats stretch quite a bit before they reach full tensile strength; they need to stretch around 30% before they reach full strength.” Enkamat, by contrast, “reaches full strength stretched as little as 8%.”
Looking back on the Quitman project, Thomas says, “Now we’re going on two years and we’ve had no failures. We’ve got a good stand of grass on it now, and that’s what they wanted—something to get in there and get it done. That way they don’t have to come back every year or two to repair it.”
Pipeline slope repair with anchors
Turf Reinforcement for Stormwater Capacity
Recently, residents of Texas may be feeling that they have suffered more than their share from major floods. Although each flood has its own stories, flooding in Texas is not without history. Pecan Creek, a tributary of the Trinity River’s Elm Fork, flows south through the center of the city of Gainesville, draining a 15.5-square-mile watershed. Periodic flooding has historically cut off one side of the city from the other. And these floods have been costly. The city of 16,000 residents has experienced annualized flood damages in the range of $783,300. Major floods occurred in 1981, 1989, 1990, and 1993. City manager Ron Sullivan says it isn’t difficult to see why flooding had become such a big issue for the city. “The city has flooded for years. We have several different little creeks, and all those little tributaries come in and guess where they all drain into—Pecan Creek.”
Attempts to tame Pecan Creek to alleviate the threat of flooding stretch back to the Works Projects Administration (WPA) era of the Great Depression, when workers, employed by the Civilian Conservation Corps, channelized Pecan Creek into a flagstone-lined streambed as one of a great number of major infrastructure improvement projects of the 1930s initiated by the WPA. However, the attempt to contain the creek within a designated channel did not alleviate its tendency to flood during heavy rains. The banks, even when later streamlined with concrete, could not contain heavy storm flows, and the trend of flooding continued to trouble the city.
In 1996, the city of Gainesville, in partnership with the US Army Corps of Engineers (USACE), began planning a project to permanently resolve flooding issues associated with the creek. But before it was implemented, the history of floods in Gainesville turned tragic. In 2007, Pecan Creek overflowed during a massive storm to engulf most of the city. The flood resulted in three fatalities.
A Wider, Tougher Creek With TRM
In the aftermath of that flood, says Sullivan, the district’s congressional representative helped to successfully push for a fast-track designation for the Pecan Creek Local Flood Damage Reduction project.
The existing creek capacity, in a trapezoidal channel 15 feet wide at the bottom, was insufficient to pass major storm flows. In fact, channel capacity was only 25% of the 100-year peak runoff and, according to the USACE, damages occurred even from two-year storms.
According to the USACE, the $15,000,000 joint project with the city of Gainesville would triple flow capacity by widening the channel bottom to 30 feet along a 7,860-foot stretch of Pecan Creek. Grass-lined banks were modified to less-steep 3.5:1 side slopes. Naturally occurring rock layers were used for the channel bottom and some side slopes. Ecological mitigation was accomplished within the project on 22 acres. The natural look and property acquisitions dovetail with city master plans for a recreational greenbelt in the heart of the city. The city’s participation in the project included moving utilities and building several new bridges to accommodate the widened stream.
Closeup of anchor
The creek-widening project called for a permanent TRM channel liner offering creek bottom and side slope protection. Maximum design conditions were 15 feet per second flow at 12 pounds per square foot shear stress.
In May 2012, installation of American Excelsior Recyclex TRM-V began, funded by the USACE Continuing Authorities Program. The Recyclex TRM-V was laid running horizontally on creek bottom and slopes with shingle-like overlaps because of the site’s geometry. Visits were made to the site as installation progressed, and discussions with Oak Grove Landscape’s site superintendent, Bart Weston, made installation go smoothly according to plan requirements. The installation also included the application of gabion walls and riprap.
The USACE completed the project in 2013.
After installation, the site was irrigated for a short period from a water truck and temporary piping. The first summer following installation was very dry, so the Recyclex TRM-V was left to protect the creek bottom and side slopes alone because very little vegetation became established during the drought conditions.
In May 2015, storms poured a near-record 18.6 inches of rain, nearly half the annual average precipitation of 36.9 inches per year, on the Gainesville area, creating a flow in the creek that was estimated at 10 feet per second, while the depth ran as high as 10 vertical feet. The storms moved trees and tree branches down creek that had been left offsite from construction. According to American Excelsior, the heavy debris flow did not hang on or tear out any Recyclex TRM-V.
Sullivan says some sections of gabion walls were shifted by the onslaught of this storm flow, and much of the riprap was washed out by the force of the current. The Recyclex TRM-V stayed attached to the bottom and side slopes without any failure.
He anticipates working with FEMA and the USACE to return to the creek to repair the damage inflicted by the storm, but says he is pleased that the modifications to increase the stream’s capacity prevented what could have been a catastrophic flood. “We were happy about how the project functioned after completion. It was the right project at the right time. I’m glad it was completed; we saved a lot of property and probably saved lives.”
Pecan Creek has now withstood several heavy rainfall events and has great vegetation growth in the Recyclex TRM-V mat.
Sullivan says he only wishes the project could have been extended further to the south of the city to protect other areas where the water starts overtopping the banks. “But we understand the federal government has regulations and cost-benefit analyses, and this was the area that met that requirement for cost benefit. We’re extremely happy that we didn’t flood this time.”