New Water Pipe Installed Under Lake Bed Made Easier Using Directional Drilling

Contractor uses a mix of horizontal directional drilling and underwater excavation to install water distribution pipe across Alabama lake.

New Water Pipe Installed Under Lake Bed Made Easier Using Directional Drilling

The receiving site of the directional drill bore included two barges that had to fit a mud recycling system, long-reach excavators and a crane. The receiving pit was in the middle of Wheeler Lake in Alabama.

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You start on one side of an obstacle and shoot underneath to the other side: That’s how horizontal directional drilling typically works. However, when the bore needs to end in the middle of a lake, things get tricky.

Garney Construction, a national contractor headquartered in Kansas City, Missouri, was brought in to connect the Decatur, Alabama, water system to the Limestone County Water & Sewer Authority in Athens, Alabama. However, to get from one side to the other, there was a 1.5-mile-wide reservoir in between. The reservoir consisted of the Tennessee River and a portion of Wheeler Lake and located just a few thousands feet upstream of the busiest port along the river.

In order to connect the two, Garney Construction would need to cross 8,800 feet of water. It was too much for a single HDD shot, so the company combined 3,000 linear feet of drilling with 5,800 feet of subaqueous excavation for the 36-inch transmission line. The two ends would be tied together after the HDD portion broke through the bottom of Wheeler Lake.

“Drilling out into the bottom of the lake, that’s highly unusual and actually I don’t know that it’s ever been done before,” says Jeff Seal, Garney Construction’s regional operations manager and lead on this project. “This is the only job I’ve ever seen where we have drilled out with a horizontal directional drill underneath the river, into the bottom of the lake, and then tied onto it underwater. That is very unusual.”

The project, which was started in April 2016, was bid out for 570 days, but Garney Construction was able to complete it 41 days ahead of schedule, overcoming several challenges along the way.

ACCOMMODATING PROJECTED GROWTH

This project was part of a 2007 hydraulic model assessment and long-term strategic plan by the Limestone County Water & Sewer Authority. Limestone County had a projected growth rate of about 1,000 new customers and 300,000 gpd each year, plus the possibility of a new industrial megasite that would require infrastructure expansions.

Water demand before the project was 4 mgd — today, it’s closer to 6 mgd — with projections exceeding 10 mgd inside of 20 years. The county has two main water sources: a treatment facility and purchased water from the neighboring city of Decatur.

Enhancing the connection with Decatur allowed for a capacity increase of approximately 11 mgd and was more cost-effective than the other proposed options, such as forging new connections with other towns or constructing a new facility to draw from the Tennessee River.

Along with the new transmission line, the Limestone County Water & Sewer Authority negotiated a 30-year purchase agreement with Decatur.

PLANNING THE PROJECT

Getting the project to the starting point was the first challenge officials met. The permitting process included nearly a dozen agencies and required nearly three years to acquire.

Once the route was finalized, decisions regarding pipeline materials had to be made. For the HDD portion of the project, a 36-inch butt-welded steel pipe with a wall thickness of 0.75 inches and minimum yield strength of 42,000 psi was determined to be the best.

The project area within Wheeler Lake, where 5,800 linear feet of pipe would be connected to the HDD portion and laid across the lakebed, also had design factors to consider. Due to the fragile ecosystem, the permitting agencies determined that during installation the excavated materials must be side cast and then spread over the top of the pipe once the pipe was installed. No backfill material was to be brought in, and no excess material was to be removed.

To condense the overall project duration and meet the timeline, the conventional opencut pipe installation and the HDD portion were done in conjunction. That also minimized the installation time for the subaqueous pipe once the pullback for the directional drill was completed.

SETTING UP FOR THE DRILL

The drilling and pullback for the HDD was subcontracted to TransAmerican Underground, and one of the first challenges they met was where to place the drill and the equipment for the pullbacks. One side of the drill was in the middle of a lake, while the primary drill rig was set up on the landside with a footprint of 125 feet by 125 feet.

Space was limited as it was in the parking lot of a midrise building — a retirement home — in a congested area of Decatur. Crews could only work from 7 a.m. to 5 p.m. “We actually had a lot of curious people. A lot of them would come out and sit there and basically watch us all day,” Seal says.

Within the area was the 1.1 million pound directional drill (American Augers) with a power unit, the driller’s operator cab, drill pipe trailer, trackhoe with pipe tong, two mud systems, a mud storage area and the entry pit.

On the receiving end of the drill, in the lake, a 50-foot-wide-by-80-foot-deep platform was built using modular barges. The floating work area accommodated an excavator with hydraulic pipe tong, mud recycling system, and drill pipe.

“It’s definitely unusual,” Seal says. “This isn’t the first time that we’ve done underwater excavation, and it certainly won’t be the last, but it is not something you see very often. It’s very specialized, and there’s only a handful of companies and people that do that kind of work.”

LIMESTONE COUNTY INDEED

Crews found out quickly how the county earned its name as they ran into hard limestone measuring between 5,000 to 12,000 psi.

“We had a hard time getting the drill to bite into the rock to start drilling into the rock because it just wanted to skip forward across the top,” Seal says. “In that area of the country, the rock and dirt interface is black and white. It goes from dirt to rock, and the rock, they say it’s like a tabletop.”

In order to get past that barrier, crews put a 16-inch casing down to the rock face as a track for the 12-inch pilot drill head. With that in place, the pilot bore was able to continue swiftly, but the heavily stratified geology made blowouts a concern.

“We were going through a lot of different layers because we had to go so deep to get underneath the river,” Seal says. “It was also very hard limestone; they don’t call it Limestone County for nothing. It’s probably some of the hardest limestone in the Southeast.”

A gyro steering tool guidance system was used to maintain line and grade during the drilling of the pilot hole because of it going under a waterway. The gyro gave the operator real-time location of the drilling head and allowed the operator to keep the drill on the proposed path.

For the 37.5-inch O.D. pipe, a 54-inch hole was drilled. To accomplish this, a total of four passes were done: an initial 12-inch pilot hole followed by three reaming passes of 30, 42 and 54 inches. It was one of the longer runs for that size Garney Construction has ever done.

Due to the abrasion of the rock, it took two reaming heads per each size to complete the 3,000-foot drill.

A UNIQUE PULLBACK

The pullback of the 3,000 feet of pipe was completed in a single, continuous 11-hour pullback. It was the one time an exception was given on the time restrictions for working. Crews planned in advance and gave the retirement community warning before the 11-hour session. “It was probably like an 18- or 20-hour day, because there’s stuff you’ve got to do before you actually start pulling and then stuff you have to do once it is pulled in,” Seal says. “That takes time.”

In order to counteract the buoyancy of the pipe during the pullback, potable water was pumped into the pipe once the pulling head reached the bottom of the drill hole. To pump water to the pull head, a 4-inch HDPE pipe was run from a fire hydrant on land back to the section of pipe floating in the water, which was capped with a blind flange.

Another 4-inch HDPE pipe was also installed inside of the steel casing through the flange to the front of the segment of pipe near the pulling head. The water needed to be pumped to the pulling head to add weight to the belly of the pipe to prevent it from floating to the top of the hole and creating friction.

The maximum force the directional drill used during the pullback was 325,000 pounds.

WORKING IN WATER

Garney Construction used the modular barges that were used for HDD for the subaqueous installation.

“We had an excavator that has a super-long boom and a super-long stick on it so that it could reach down into the water deep,” Seal says. “When we pulled up the material, it could reach out and put it to the side, far enough away where it doesn’t come back in your hole.”

The CAT 349 long-reach excavator had a Topcon GPS grade control system installed on it, allowing crews to put in the exact shape, grade, and elevation of the ditch, taking the guesswork out of the equation.

“When you’re digging underwater, you can’t see what you’re digging, but the GPS system on the excavator will show you exactly where your bucket is, where the bottom of the ground is, and where you need to dig,” Seal says. “So that’s how we dug the ditch.”

CONNECTING THE TWO PIPES

With a little ingenuity on Garney Construction’s part, the challenge of connecting the two ends had a surprisingly simple solution. On the end of the pipe that was directional drilled, crews installed a ball-and-socket joint with a 50-foot pipe attached.

“The ball joint solved a lot of those challenges with lining up the pipe segments,” Seal says. “That was a solution that we came up with. We did consult with the engineer, who designed the job, from a constructability standpoint, and as they were designing this, we had given them our suggestions on how it was possible to build this.”

The ball joint enabled crews to line up with the trench and Wheeler Lake pipe segment. The EBAA Iron Sales FLEX-900 ball joint allowed for 15 degrees of deflection. There was a 6-degree vertical deflection at the connection point.

After that, it was just getting the subaqueous pipe laid.

The last challenge was getting the pipe into place at the bottom of Wheeler Lake. With this being a potable waterline, they wanted to limit contamination; so sinking the pipe with lake water was off the table.

Instead, crews welded together 1,000- to 1,500-foot lengths of pipe on land, put flanges on the ends and bolted on flat plates with 4-inch waterline tie-ins. They floated the long sections into place and pumped in potable water. After the pipes had been sunk and attached to the system, precast concrete weights were put on top to keep it from floating up once the system water was flushed. The weights were set every 50 feet along the pipeline, spaced in between the welded joints.

AN EARLY FINISH

The line was successfully pressure tested to 150 psi and then backfilled with the native excavated material, consisting of mostly sand and small pea gravel.

The job was finished 41 days ahead of schedule.

“Drilling out into the bottom of the lake, instead of drilling from land to land, trying to put all the equipment that’s normally set up to be on land out on a barge floating in the middle of a lake, was just unique,” Seal says. “That was the first time that we’d ever done it, and I don’t know that it’s ever been done otherwise.”



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