Engineers stand by dam assessment

After further inspection of the Flower

Creek Dam, the City of Libby still must replace the dam.

Core samples of the dam were taken last

fall after a visual inspection recommended further testing. The

inspection was necessary to renew the city’s operating permit with

the Montana Department of Natural Resources and Conservation.

The core samples revealed that the dam

was deteriorating. The city’s engineering firm, Morrison-Maierle,

recommended replacing the dam, in part because the dam might not

hold up to a seismic event.

As required by the DNRC, URS

Corporation conducted a peer review of Morrison-Maierle’s

analysis.

URS disagreed with the first analysis

calling the conclusion that the dam could not withstand a seismic

event premature. Structural evaluations by URS also found that the

dam, under normal loading conditions, does not appear to be

threatened at this time.

Last week Morrison-Maierle released its

response.

“We’re standing by our initial report,”

said Ryan Jones of Morrison-Maierle.

URS also recommended further testing of

the dam.

Morrison-Maierle disagreed. The

presence of extensive cracking observed along the construction

joints and through the concrete preclude a realistic expectation of

safe dam service over the long-term, the firm wrote in its

response.

“We’re not in imminent danger, but I

can’t tell you that the dam won’t fail,” Jones told the City

Council Monday.

The DNRC will not require the city to

replace the dam, Jones said, but it might not renew the city’s

permit if action isn’t taken.

Mayor Doug Roll said the dam needs

replacing.

“The dam is in tough shape,” he said.

“Our most important job is public health and safety.”

The Flower Creek Dam, built in 1946, is

located about three miles south of Libby. The dam holds back a

reservoir on Flower Creek and is the city’s sole source of

water.

Core samples taken from the dam

indicate that in some areas the dam is too weak to withstand 1,000

pounds per square inch of pressure. Typical concrete cores

demonstrate strength of 3,000-4,000 psi. A portion of the dam that

was replaced in 1966 measured over 6,000 psi. This makes it

difficult to determine the dam’s overall strength because the

concrete varies widely.

The City Council still has to decide

whether to repair or replace the dam.

Morrison-Maierle recommends replacement

because of potential cost-savings.

The firm estimates that placing a

reinforced concrete veneer on the dam would cost about $1.5

million. This would only last about 25 years, however, and dam

replacement later would be an additional $6 million. Those costs

would be combined with testing and engineering for both

reinforcement and replacement.

The City Council was scheduled to hold

a special meeting on the dam Thursday after presstime.