Innovative Thinking in Maine HDD Project

A 220-ft bore may not sound like the most intriguing or difficult of directional drilling projects, but for Enterprise Trenchless Technologies Inc. (ETTI), this was no ordinary bore.

For such a short distance, this project came with a host of challenges for the ETTI and Sargent Corp. crews — including the depth they were drilling to preventing lake water from leaking into the entry pit — that necessitated the contractors to think outside the box to successfully complete the project.

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“This was a very unique project,” said ETTI president Scott Kelly. “I’ve never seen anything like it.”
ETTI was called on as a subcontractor to Sargent Corp., which was awarded the project by the Brewer Water Department to install 600 ft of 24-in. HDPE pipe in Hatcase Pond, located in Brewer, Maine. ETTI was brought in to install a small portion of the pipe using horizontal directional drilling (HDD).

Hatcase Pond is not your average water source. Hatcase Pond is a high-quality, pristine lake that serves as the principal water source for the City of Brewer and is one of a handful of public water sources in Maine that does not require a filtration plant due to its incredible water quality. The existing intake ran from the pump station approximately 150 ft into the pond. Sargent was to replace that intake with 24-in. HDPE, plus adding a 400-ft extension that would take the pipe to a deeper part of the pond. HDD was selected to install the first 220 ft of the new intake, with the entire project taking place underwater.

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“There have been a lot of shore approach jobs, but they always pull back above water level. In this case, we couldn’t pull back above water level as the pumping station was right at the exit pit, preventing us from pulling the pipe back to grade,” Kelly said.

Kelly said one of the challenges the crews faced was that the entry pit was 18 ft deep, which meant it was 18 ft below water level. “When we drilled out into the lake bottom and hooked up to the 24-in. pipe and pulled it back to shore, water was going to run through the borehole and fill the pit,” he said.
Sargent and ETTI planned safeguards to prevent water from leaking into the pit. ETTI set up its Ditch Witch 8020 drill rig about 150 ft from the entry pit in order to drill at the proper depth and pitch to pass through the entry pit. Steel sheet pile was constructed all the way around the pit as a steel frame, serving as a protector from the lake water. ETTI needed to drill the 10-in. drill pipe through that sheet pile, minimizing the amount of water that would come into the pit during drilling.

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Kelly said that Sargent crews used blow torches to cut holes through the sheet pile to allow the drill rod and drill bit to pass through the pit, mitigating where the water would enter.

“When we were coming through the backside of that steel, we would bump the steel pile to mark our spot and the hole would be cut to allow our drill bit into the box,” Kelly said. “The same thing was done for the other side of the sheet pile to exit the pit. We were trying to keep the hole as small as possible because we knew once we punched out, that water was going to flow into the box but would only come through the holes made in the steel plating.”

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As the pilot bore was being drilled, crews fused a 575-ft string of 24-in. HDPE, using a McElroy TracStar 900 self-propelled fusion machine. But because HDPE pipe is naturally buoyant, the pipe was not going to sink to the bottom when pulled into the water. To weigh the pipe down, crews attached pre-cast concrete split-ring collars every 15 ft to the pipe. However, because the pipe was empty when pulled back, it still didn’t sink so crews ended up filling it with water to submerge it to the lake bottom in order for the 32-in. backreamer to be hooked up.

As for preventing water from seeping into the entry pit once the 24-in. HDPE was pulled back, innovative thinking was again put to use. Sargent found a specially designed grout that expands up to 50 times upon its reaction with water. The 24-in. pipe had several grout tubes attached 50 ft from the reamer, which upon exit into the receiving pit were filled with the expanding grout which would then seal off the borehole and control the water flow into the receiving pit.

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“I don’t know if that’s ever been done before [in an HDD project],” said Kelly. “If it has, I never knew about it. What it did was seal off that lake water from the entry pit long enough for us to make our disconnections of the drillstring and reamer and for Sargent to make the connections to bring the pipe up to grade and into the pumping station… It was a significant gamble from the start but it worked exactly as planned with a 100 percent seal off from the pond water pressure.”

Another critical aspect to the success of the project was the use of construction divers, as all connections and disconnections occurred underwater. Commercial Divers Plus was hired to handle the underwater work.

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Kelly noted that he had used divers on past projects but not to this extent. Here, divers had to change out all the tooling and make all the connections in 24 ft of water — work that is typically done on shore. “The divers had to be fully trained on how to use our specialty hydraulic tools and breakout wrenches, etc., underwater,” Kelly said, noting that a temporary dock was constructed out to the exit location. “We lowered the tooling down to them and they made all the connections for us and removed the drill bit and hooked up the reamer and swivel.” He noted that the divers did all of this in dark waters with virtually zero visibility.

And don’t forget about how pristine Hatcase Pond is — ETTI and Sargent couldn’t, especially while they were pumping all that drilling fluid into the borehole. To secure the area from the limited impact of the drilling fluid, a full turbidity curtain was installed around the exit hole to capture any of the returns from the drilling process.

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Kelly credits all the pre-planning that ETTI and Sargent did for this project as to why they were successful. “We probably spent as much time in planning the project as we did actually doing it, which turned out to be a saving grace,” he said. “This was a pretty unique job.”

Sharon M. Bueno is managing editor of Trenchless Technology.

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