St. Mary Working Group tours Fresno Dam project

After a regular meeting Wednesday, members of the St. Mary Rehabilitation Working Group took a tour of Fresno Dam and the construction efforts aimed at shoring up the structure.

Built in 1939 on the Milk River in north-central Montana, the 84-year-old embankment dam has experienced 7 to 10 feet of settlement since construction and the U.S. Bureau of Reclamation has deemed it necessary to shore up and improve the structure to prevent any failure.

BOR representatives have said that there is no immediate threat of failure, however, between considerable settlement and cracks forming in the structure such a risk will eventually develop and they want to get ahead of that problem.

"The Safety of Dams project will add a sand and gravel filter and toe drain system, with an embankment fill overlay and a vertical sand filter trench to correct settlement-related safety deficiencies," said BOR Project Manager Steve Darlinton in a press release about the project earlier this year.

During the tour Wednesday, Darlinton took members of the working group through the job site and explained the process they and their contractor Northwestern Construction are using to improve the structure.

Darlinton said the project is a three to four year undertaking, requiring a great deal of material and labor, and for now they are focused on draining as much groundwater on site as possible.

He said the sandstone making up much of the dam is very effective at keeping water back, but over time it does develop cracks and they need to make sure all the water these cracks let through is being extracted before they dig too far into the structure.

He said they plan to excavate a considerable way down under the dam, but to do that safely they need to draw out the ground water so the structure remains strong enough to hold while they do their work.

He said they have dug 120 dewatering wells, most of which are producing 5 to 8 gallons per minute, with some pushing 20 gallons per minute.

Back when the structure was built, he said, this same job would take 800 wells, so advances in technology since then have made a considerable difference, but it is still a huge job.

Fresno Dam construction was completed in 1939 as an irrigation storage facility under the greater Milk River Project. In addition to providing irrigation water to the Milk River Project, Fresno Dam provides incidental benefits for flood control and recreation.

Darlinton said Hill County Electric is providing the considerable amount of power necessary to keep these wells going, and they have set up backup power in the event that anything goes wrong.

He said if, after they've finished their excavation, there was a power failure the groundwater would return and weaken the structure to the point that it would possibly washout.

That is why, he said, they have installed backup power and other safety measures to prevent that from happening. They have also run tests to evaluate how fast groundwater would return in the event of some kind of power failure.

Darlinton also said the project will require more than 80,000 cubic yards of material, some of which is being provided by Havre Sand and Gravel, and some of which they will need to take from a nearby hill, another significant task.

In the end, they will have extended the crest of the dam by 30 feet, he said.