HDPE replacing redwood pipes in renovation of 1910 electric plant

The city of Spearfish, S.D., will replace deteriorating 107-year-old redwood pipes that carry water to its hydroelectric plant with a modern product: high density polyethylene pipes.

The plant originally was built in 1910 to power the Homestake Mine in nearby Lead, which once was owned by business magnate George Hearst. He and some partners bought the claim for $70,000 during the Black Hills Gold Rush in 1877.

Located a few miles from the shanties, saloons and brothels of Deadwood, the mine became the second-largest gold producer in the U.S., yielding 39.8 million ounces of ore before it ceased operations in 2002.

For power, water was — and still is — diverted from the fast-moving Spearfish Creek and conveyed underground through a 5.5-mile tunnel system to a confinement area called a forebay.

From there, the water is carried by a pair of redwood pipes, which will be updated to HDPE, to a series of stand pipes that act as an elevation buffer. At that point, the water drops down a steel pipe to the hydro facility, where it spins twin turbines that activate a generator, which produces electricity.

The water exits the plant back into Spearfish Creek, which cuts through the city of Spearfish on the way to its confluence with the Redwater River just north of town.

Spearfish officials bought the hydroelectric plant in 2004, obtained a federal permit to operate it and have enjoyed a stable source of electricity, which is sold to the local utility, and revenue, which totaled $788,400 in 2016.

"The driving goal for our city to acquire this 100-plus-year-old facility wasn't to create electricity. It was for the purpose of keeping water in the tunnel and through the community," Public Works Administrator Cheryl Johnson said in a phone interview.

The tunnel system moves the creek water around an area of the stream bed with such unpredictable hydrology it has been dubbed a "loss zone."

"In drought years and other low-flow situations, the flow of the creek literally drops underground and travels in the aquifer," Johnson said. "We knew if the plant didn't continue to operate, there would have been times of the year when we had no flow of our creek through our community. We wanted to maintain the volume of water that flows through Spearfish. It's very much a feature of our community."

From the plant, the creek flows through a city park and fish hatchery to truck farmers with irrigation rights. Unusual in that it freezes from the bottom up, the creek also allows for year-round fishing for locals and tourists.

To maintain the plant and creek flow, the city needed to replace the two 48-inch redwood pipes, which have leaked and been patched over the last century. While redwood came with the plant purchase, milling it to the needed diameter is becoming a lost craft.

"We were able to contract with an older gentleman who used to work for Homestake Mine," Johnson said. "He was the only living guy around who knew how to mill it."

To modernize the operation, the city budgeted $1.2 million and sought bids for 1,200 feet of cast iron and HDPE replacement pipe with 48-inch diameters. The contract was awarded to a construction firm that submitted the lowest qualifying bid of $699,500 to do the job with a single piece of heat-fused PE pipe.

"It really came down to the bid price," Johnson said. "The HDPE was significantly lower. In fact, some contractors didn't even bid cast iron. They only submitted for the HDPE."

Before the replacement work is done in the fall, some sections of the redwood pipe will be salvaged and put on display in the city, which was founded in 1876 and originally grew as a place where truck farmers supplied food to the mining camps during the Black Hills gold rush.

The Homestake Mine and its hydroelectric plant were at the forefront of engineering advances of the late 1800s and early 1900s.

"One of our generators that was steam-driven and used for construction of the tunnel and hydro plant was built by [Thomas] Edison himself," Johnson said.

However, much of the tunnel, which was later lined with concrete, was dug by hand with crews starting in five locations. Engineers and surveyors tracked the progress daily and the five points all lined up within 1/8th of an inch of each other.

"That kind of engineering without computers and modern equipment is incredible," Johnson said.

When the hydroelectric plant was completed in April 1912, it improved the working conditions of the miners with ventilated air and electric hoist systems to move them, equipment and ore up and down shafts.

Decades later, in 1964, the plant and mine provided the right working conditions for a team of researchers that included future Nobel Prize recipient Raymond Davis Jr. The team set up a physics station in the mine 4,850 feet below the surface to study solar neutrinos, which are nearly massless and travel at almost the speed of light.