Ground Proves Unsettling for Maintaining Grade

As a utility contractor, you try to prepare for the worst. But sometimes, spending dozens of hours planning with some of the industry’s most experienced engineers and horizontal directional drilling (HDD) operators can’t prevent the challenges that Mother Earth presents.

Such was the case for Charles Aars, superintendent of MCM General Contractors Inc., Gillette, Wyo., when he and his crews tackled the installation of a 42-in. steel casing containing a 30-in. sewer carrier line under highly traveled Interstate 90. Not only did they have to battle the wet and mushy ground conditions of a natural drainage area, but they also were tackling the largest-diameter bore in company history.

“It was a really flat bore. It was only .015 percent grade, so there was really no room for error,” Aars says. “I did foresee some obstacles with the groundwater. It was really wet, unstable ground conditions.”

MCM was hired by Hot Iron Inc., also based in Gillette, Wyo., to install the new trunk line that would service the City of Gillette. Most sewer jobs taking place in the area consist of installing much smaller pipe — between 8 and 16 in. in diameter, Aars says. Given the massive size of this line, it left little doubt that it would be an extremely difficult sewer installation job. Aars says that’s why Hot Iron handpicked MCM for the project.

“We have a long working relationship with Hot Iron, and we’ve done a lot of high-end projects for them,” he says. “We’ve completed some real tough jobs, and they knew that we could step in and probably get it done.”

Creative Thinking to the Rescue

When it comes to tackling boring projects, MCM has several Vermeer Navigator horizontal directional drills in its arsenal, including a D80x100 Series II, a D36x50 Series II, a D24x40 Series II and two D7x11 Series II machines. For a job this large, Aars says, “We used the Vermeer D80x100 Series II for the pullback force and the efficiency of the machine.”

The first few days of the job were spent planning and prepping the area. Aars says his crews performed all of the hauling and welding of the steel pipe, while he focused on finalizing the engineering design for the bore path. Two 80-ton cranes were brought in and stationed on the jobsite to lift and place the large 84,000-lb steel casing. Luckily, because the pipe was being installed 5 ft beside the existing sewer line, locating and drilling around existing utilities didn’t pose much of a problem.

But getting hit by a couple of thunderstorms didn’t help in firming up the soil in the drainage area. “We had four pumps running nonstop for about seven days,” Aars says. “I had a crew working around the clock, filling up generators and checking pumps just to make sure our holes weren’t filling with water and creating unsafe operating conditions.”

Using a standard Vermeer sonde housing with a spade bit drill head, Aars’ experienced HDD operator was able to easily complete the 320-ft long bore at the specified .015 percent grade with the D80x100. Once the bore path under the interstate was completed, crews performed three pre-reams before attempting to install the 42-in. casing. The Vermeer HDD machine’s 80,000 lbs of pullback force aided in successfully pulling the pipe through. Aars says they also used a custom-built, 56-in. backreamer and a slurry fluid mix to prevent soil particles from sticking to the pipe.

“We started the pilot hole about 9 a.m., and we had product on both sides by 9 p.m. that same night,” he says.

After Aars and his crew had successfully drilled the bore on grade and pulled back the product, it seemed the hardest part was over. But, unfortunately, one of the biggest challenges MCM crews faced was after the pipe had been installed, it began settling in the saturated ground.

“After the first weekend, it settled about 6 in.,” Aars says. “We brought some cranes in, picked up one end of the pipe, put a lot of rip-rap rock underneath it and basted it up. We finally got it to quit sinking, but it still ended up 6 to 7 in. lower.”

To solve the problem, Aars and his eight crewmembers pushed out the 30-in. carrier pipe, which is surrounded by factory skids that help center it inside the 42-in. steel casing. MCM crews then built their own skids that were shorter than the pipe’s factory skids, thus accommodating for the 6 to 7 in. that the pipe had settled.

“We took the factory ones, cut all of the legs off and built our own legs, so that we could force it to the top of the [casing] pipe on one side and to the bottom of the pipe on the other to make grade,” he says. “There was a lot of engineering and a lot of time spent doing that, but the end product turned out being perfect.”

When all is said and done, completing a job of this size successfully is really all any contractor can ask for.
“There were a lot of roadblocks and we just kept a level head about it and came up with solutions,” Aars says.

Experience Equals Success

Aars will tell you that experience is worth its weight in gold in the sewer industry. Installing on grade is difficult enough, not to mention when you’re faced with obstacles such as those encountered by MCM on this project.

Over the years, Aars has learned not only to surround himself with experienced and talented crewmembers, but he’s also learned the importance of adequate planning. “One of the biggest tips I would give other contractors is before you start a sewer job, go out and look at the jobsite after it’s been engineered,” he says. “I always go out and kind of re-engineer the job in the field.”

Taking this strategy helps Aars visualize the best installation approach and it also helps to identify potential challenges.

On this sewer installation project, Aars spent about 50 hours planning it. Working with the engineering firm PCA Engineering Inc., he says he was able to give his input and create a bore plan that met the specified requirements, while taking an approach his team was comfortable with.

“We’re able to submit our suggested changes to the engineers and say, ‘This is the way that you have it written down, but this is the way we would prefer to do it.’ They’ll sit down with me and will go, ‘OK, you have enough experience doing it. If it’s going to work, we’ll trust you,’” Aars says.

For contractors just getting their foot in the industry, patience is an important virtue when installing sewer lines on grade, and Aars says it’s a skill that an HDD operator tends to acquire over time.

“These jobs are a lot different than just going out and drilling across a road,” he says. “It’s having to drill 2 to 5 ft and then stopping to check the grade.”

Aars encourages new utility contractors to perform several steep grade bores to aid in training and gaining experience. In doing so, they should use HDD to drill the steep bore, then hydrovac down to grade and shoot the drill stem. Aars says this will ensure they’re on grade.

“After you get a few under your belt, then you’ll start to trust the electronics a little more, and it’ll get more natural to you,” he says.

Tara Deering-Hansen is a technical writer in Des Moines, Iowa.

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