Highway 61 in southeastern Minnesota is one of the most picturesque roadways in the Midwest.
Part of the national Great River Road, Highway 61 leads drivers through a landscape bordered by steep, forested bluffs on one side and panoramic, scenic views of the Mississippi River valley on the other.
On Sept. 22, 2010, an autumn rainstorm dumped 6-10 in. of water on the area, causing more than $60 million in damage to the local roads and highways. Fixing the damage quickly and limiting traffic congestion required looking at unconventional methods and techniques.
Chris Dulian, soils engineer, Minnesota Department of Transportation (Mn/DOT), explained, “Along Highway 61 a railroad ran parallel to the highway. There were very steep, wooded slopes with guardrails at the top. The toughest part of the project is that we had limited room to work.”
Engineers with Mn/DOT hired Soil Nail Launcher, Grand Junction, Colo., to repair flood damage on the steep slopes bordering Highway 61.
Developed by the British military to launch chemical weapons and later adapted to civilian use, the Soil Nail Launcher is a compressed-air cannon that can fire a 1.5-in.-diam., 20-ft-long steel or fiberglass tube at up to 250 miles per hour.
“We use the cannon to shoot nails into the ground to provide both tension and sheer contributions to the soil to prevent the slope from failing. By shooting them in we can do that rapidly and relatively inexpensively,” noted Colby Barrett, president, Soil Nail Launcher.
Nailing down a solution
The 2-ton Soil Nail Launcher is usually mounted on a modified tracked excavator, long-reach excavator or crane basket frame. In this configuration the fully articulated launcher can be used in environments with tight turning radii, overhead wires, guardrails and other limitations.
Dulian noted, “A lot of these slopes are between 40-100 ft high with a railroad at the bottom. The slopes were wooded so we’d have to clear everything out before we could do anything traditionally. With the [launcher], we could reach over the guardrail and install the soil nails using a backhoe and put them into the slope. From a construction point of view, it’s less disruptive. It’s quicker and we could work around the guardrail without having to remove it.”
Intuitively, the first question that comes to mind is how the hollow steel or fiberglass poles do not crumple, bend or break when they hit the hard surface of the soil.
Barrett said it’s a difference between statics and dynamics: “The tubes we use are 1?8 in. wall thickness and 1½ in. in diameter. If you were trying to hammer one in from behind and pound it into the ground, the tube would break, bend or deform before it had any penetration into the ground. When objects are moving quite rapidly there are some weird effects. People hear about a piece of straw that gets embedded into a telephone pole in a tornado. That straw is moving 300-400 miles an hour, however fast the tornado winds get up to. By traveling that fast, the straw can go into a much harder object.”
The same principle applies to the Soil Nail Launcher. Launched with the high-pressure air cannon, the poles generate a shock wave that causes the soil particles to deform or “jump away” from the nail tip. At insertion, the soil particles collapse back onto the nail.
Initially, Soil Nail Launcher used solid steel bars. Barrett explained that the thought process was the more steel in the ground the better. Now, after further tests and applications, the company uses hollow tubes.
“A hollow nail that weighs less goes faster and gets better penetration than a solid pole,” he said. “The hollow tubes are also perforated on the first 5 ft of each end of the 20-ft-long nails. We want drainage from the slide. It’s common that water is the driving force in some of these slides. By allowing water a pathway out, the water can find the holes and drain.”
To add strength, the company can pressure-grout each of the nails. Grout travels down the hollow tube, out the holes and into the soil. Steel rods or epoxy-coated rebar also can be inserted inside the tubes while the grout is still wet. The additional materials make the nail stiffer, stronger and give it a longer life span.
Results at 220 mph
Although used successfully on another Mn/DOT project in 2007, Soil Nail Launcher demonstrated the technology again in 2010.
Dulian explained, “We looked at the case studies they provided and called the references they provided to see if it was performing as expected. Everything seemed to check out.”
The actual demonstration was more convincing than the videos provided.
“They shot a nail in and did a pullout test for us. The 20-ft nail penetrated the soil to 18 ft. Then they hooked a chain on the nail and tried to pull it out with a backhoe and demonstrated that you couldn’t pull it out. That eased our fears,” Dulian reported.
She continued, “The nail is going in at 220 mph and the soil actually jumps away from the nail on impact of the nail going in and then rebounds back into the nail when it stops. That creates the cohesion bound you are looking for.”
Gary Lovelace, construction engineer for Mn/DOT, said, “There is a kind of ‘gee-whiz’ thing for an engineer. But the science and engineering behind the pattern of the nails setup and the consolidation of the soil around the nails builds a more solid block of earth with the nails reinforcing the soil.”
One major advantage considered by Mn/DOT is that once installed, the soil nails work immediately, saving time on excavation or other site-stabilization methods.
Although Soil Nail Launcher was founded in 2000 and has been used in applications across the U.S., Barrett still addressed skepticism about the technology. He noted two main points of concern.
“First, engineers like the tried-and-true methods that are supported by all the codes and textbooks and technology that as been around 50-60 or 100 years,” he said. “Those have been around for a long time and people have good experiences with them. It’s easier to take one of these methods off the shelf, whereas new technologies require a certain ‘leap of faith.’”
He continued, “The second impediment with some designers is that you can’t exactly control the depth of the installation. We’ve never seen perfectly homogenous soil. I don’t think that exists in nature. The first nail you shoot may go in 19 ft and the second nail 18 ft. You have these minor variations in the penetrations. We work within tolerances but that’s something designers may sometimes have issues with.”
The company addresses those issues by working closely with engineers on project plans and designs and also offers a warranty on slide repairs.
“If a few years down the road it was a worse failure than we expected we actually would come back and repair that. That’s helped out both in teaching about the technology and overcoming some of the skepticism,” said Barrett. “Standing behind the repairs has also helped convince some of the skeptics. By and large most of the engineers and people we work with are most interested in solving a problem than following a rule book. So, with these two methods, we can make sure our focus is on solving the problem rather than working through all the minor administrative issues.”
Fixing the damage
Mn/DOT used the Soil Nail Launcher method on 11 locations from Dakota, Minn., to Lake City, Minn., that were damaged by the September 2010 floods. The repair method was used successfully in the same area after a flood in 2007. Lovelace noted that follow-up inspections of the sites repaired with Soil Nail Launcher nails showed no further slope erosion.
Once on-site, nail-launcher operators worked with Mn/DOT engineers to create a site plan.
“We see what the site looks like and the kind of failures they are seeing so we can get an idea of how many nails and how deep they need to go,” said Barrett.
Once the plan was approved the operator laid out a pattern for the insertion of the nails and began the process. The operator controlled the depth of penetration.
By adjusting the pressure, the operator also was able to push the nail in farther or reduce the pressure if the nail was going in too far.
Dulian said, “The nails were penetrating the slip failure plane at the top of the embankment. We were trying to get through that line of the slip plane failure area to reinforce that block back on to the embankment.”
The repair sites along the steep slopes of Highway 61 ranged in size from 25 lineal feet to 160 lineal feet. The 2010 repair contract called for insertion of more than 750 nails across all 11 locations. Of that total 380 were inserted after drilling through shallow bedrock. Grout was used to reinforce the nail’s connection to the slope. The remainder used the compressed air-driven method.
Dulian reported that when fired the nails were inserted 20 ft deep into the slopes.
During the repairs, roads were limited to single lanes. Mn/DOT engineers noted that other repair methods might have caused road closures and detours that would have frustrated drivers who travel Highway 61 every day.
“We will use this technology in the future,” Lovelace claimed. “We used it in 2007 and watched those sites and are pleased with the results. Now it certainly seems to be doing the job.”
Barrett noted,” People are starting to understand the technology and get behind it. Mn/DOT has been an early adopter, especially on the emergency projects.”
