Borough continues toward dam stabilization project

A planned project to stabilize Wrangell’s upper water reservoir dam is contingent on finding out just how stable the dam is. Core drilling is necessary to determine the materials and density of the dam which is built of a log-crib core covered with earth.

But before the drilling can be done, a stabilization assessment plan had to be submitted to the state for approval.

The core drilling “is part of the upper dam stabilization analysis project we are pursuing with (engineering firm) Shannon & Wilson,” said Amber Al-Haddad, borough capital facilities director. “The effort of this project is the first phase of engineering work … of the design and construction to repair the dam and address the instabilities expected to be caused by continued deterioration of the log-crib core and the known seismic instability of the dam.”

Shannon & Wilson will develop an alternatives analysis after core samples are taken to help borough staff consider different options when it comes to dam stabilization, Al-Haddad said. The project “would help prevent destabilization of the dam embankment, reduce seismic safety concerns and maintain adequate water supply delivery,” she said.

Modeling efforts led to concerns over soil strengths of the foundation of the dam, which led to the need for core drilling. Al-Haddad said the effort to date has cost $165,178.

Stabilization of the dam is necessary to pursue a reservoir bypass project, which is needed in case work is required on the lower reservoir, making it necessary to draw water directly from the upper reservoir, said Tom Wetor, public works director.

“Right now, we can’t draw off the upper reservoir. Water only goes over a spillway” into the lower reservoir, Wetor said. “Instead of having a siphon that’s susceptible to freeze, thaw and ice as it moves up and down on the reservoir in the springtime, we’ll have something permanent, built underground that’s unlikely to be affected by the elements.”

Wetor said having the bypass system in place would allow the community’s water treatment plant to draw off of either reservoir while maintenance work is done on the other. “Our dams are in rough shape; our dams need some work.”

Al-Haddad said the 95%-level cost estimate to complete the bypass project is $3.129 million, up from the $2.6 million estimated last year when the project was submitted for federal funding of 80%, or $2.08 million. A 20% local match is required for the federal grant.

In addition, a plan to replace all of the existing ductile iron pipe from the reservoirs with high-density polyethylene would add $1.431 million to the overall cost beyond the available federal funding.

Covering the 20% local match on the original $2.6 million estimate, plus the cost escalation in the past year, plus new piping, could add up to a cost of $2.48 million for the borough to cover.

Design work for the new water treatment facility is also underway. Wetor said that’s about 65% complete.

“Wrangell’s been working on this for quite a number of years, it’s been a long time in the making,” he said. “We’re seeing more movement in the last year than we’ve seen since this project started.”

 

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