HOT SPRINGS -- Later this summer, a contractor will be pushing a 60-inch-diameter tunnel one-half mile through Blakely Mountain, the longest tunnel of its kind in North America.

"It's the longest tunnel for this size, 60-inch," Ted Foltz, an operations manager at Michels Corp., said last week.

The city of Hot Springs tasked Michels with connecting a 17-mile-long raw waterline to Lake Ouachita as part of a $107 million project that will increase the regional water system's daily capacity by more than two-thirds. Blakely Mountain stands in the way.

Sending water through the mountain will allow the city to forgo the cost of pumping it over the top and is critical to the city's plan of gravity flowing raw water to a new treatment plant. The tunnel is part of the $19.27 million contract the Hot Springs Board of Directors awarded the Wisconsin company last year to design and build the lake tap and intake.

The Herrenknecht micro tunnel boring machine, or MTBM, Michels showed off at City Hall on Tuesday will be the leading edge of the 5-foot-wide, 2,600-foot-long passage.

Foltz said it will take 150 to 170 days for the MTBM's rotating mining head to churn through the mountain and reach the lake. It will leave a 56-inch steel pipe in its wake, the conduit between the lake and raw waterline that will gravity flow water to a new treatment plant 17 miles to the south.

The 56-inch pipe will be welded to the back of the MTBM's power unit. A clamp grasping the outside of the pipe will attach to a thruster. The thruster's two hydraulic cylinders will exert force on the clamp, pushing the pipe and MTBM through the mountain.

"It's the longest tunnel in North America using a micro tunnel thrusting system," Foltz said. "It's a direct pipe method. Unlike directional drilling, where you drill and you bore and then you pull the product pipe in, while we're mining, we're installing the product pipe."

The machine Michels brought to Hot Springs was built specifically for the Blakely Mountain job. It arrived in Wisconsin last year from Germany. Foltz said the machine's power unit can deliver up to 60,000 pounds of torque to the head, which rotates 10 times per minute. Bristling with cutter teeth, the head can cut through a variety of geology.

Core samples pulled from the entire length of the tunnel alignment in 2019 mostly comprised black shale and sandstone.

"There's a variation of rock formations here," Foltz said. "This head is what we call a little bit of a mixed head. It's designed to be able to cut all of the rock here."

Excavated material passes through openings in the rotating head to a chamber, where it's crushed into smaller pieces and pumped back out of the pipe and to the surface through a slurry line. The MTBM's 5-foot-wide interior allows cutter teeth to be replaced without having to bring the pipe and machine to the surface.

"On a smaller machine you couldn't go inside and change these cutter teeth," Foltz said. "You'd have to pull the whole machine out. This allows us to come inside the machine while it's still underground and change those cutter discs."

Through June, Michels had invoiced the city for $2.87 million, according to a report the city finance department provided. More than $60 million of the $106 million construction fund has been committed to the water supply project, which had change orders totaling $335,359 through the end of last month.

The water rate structure that took effect in 2018 is paying down the $109 million bond issue the city floated in 2020 to finance the project. The minimum monthly charge for residential customers inside the city rose from $5 to $13 from 2018 to 2021. Customers outside the city saw their minimum rate increase from $7.50 to $19.50 over the four-year period. Annual 3% increases took effect at the start of the year for both customer bases.

Print Headline: Mountain tunnel key to plan

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