Wrangell’s water reservoir dams need fixing, and the borough assembly last week approved spending $100,000 to help determine the best options.
The state dam safety engineer at the Department of Natural Resources has identified Wrangell’s upper and lower dams as Class I (high) hazard potential with recognized deficiencies that require rehabilitation, said the borough’s Capital Facilities Director Amber Al-Haddad.
A stability study performed for the U.S. Forest Service in 1993 and a 2006 seismic study for the borough both indicated that while stable under static conditions, a seismic event, such as a strong earthquake, could cause failure because the dams do not meet design standards, Al-Haddad said.
The borough has Shannon & Wilson, a geotechnical and environmental consulting company, under contract to conduct an analysis to weigh the options and determine an approach to repair and improve the dam’s stability to meet U.S. Army Corps of Engineers guidelines.
The analysis is expected to place the project in “shovel-ready” status as critical drinking water infrastructure, according to report prepared by Al-Haddad for the borough’s Dec. 14 assembly meeting, where the members approved the $100,000 contract.
Generally, a project deemed “shovel-ready” would be eligible for federal funding, but “the federal government does not seem to have a well-defined definition for shovel-ready,” Al-Haddad said.
Stafford Glashan, senior engineer at Shannon & Wilson’s Palmer office, said the work will be conducted using staff from Anchorage and Palmer, and drilling subcontractor Discovery Drilling, from Juneau.
The two primary concerns identified in the earlier studies of the upper dam are soft support soils, causing foundation instability, and the deteriorating timber crib, causing internal instability, according to Al-Haddad’s report to the assembly.
The review covers only the upper dam.
Shannon & Wilson began with evaluating methods for stabilizing the upper dam, Al-Haddad wrote. They built a seepage model earlier this year to use as the base analysis for the stability model.
“We’ve been involved with the borough through a number of projects, since 1985. We’ve been working with the city and borough on the upper dam for decades. We’ve got a long history of doing projects at the dam,” Glashan said. The seepage evaluation was part of an overall project to evaluate stability in the event of seismic activity.
Al-Haddad explained, “The subsurface soils investigation work that we just approved is an amendment to an existing contract we have with them.”
Starting with the profiles the Corps developed in 2006, the contractor ran into problems with the model in that they could not mimic the measured groundwater levels. With further investigation, Shannon & Wilson determined the rockfill on the downhill side of the dam and failures of the silt at the base are the problem, “which led to concern with soil strengths at the downstream embankment and dam foundation materials, which are the controlling features in stabilizing the dam.”
The engineers have recommended drilling into the soils to look for weaknesses and help determine the best options to strengthen the structure.
“We are currently conducting an alternatives evaluation for the stabilization of the upper dam,” Glashan wrote in a letter detailing the firm’s scope of work to the borough. “The static and seismic stability calculations conducted as part of the alternatives evaluation indicates that the strength of the downstream embankment and foundation materials is the controlling feature in stability of the dam.”
“This time of year isn’t good for any core work. We like to get long days in,” Glashan said. The work schedule will be driven by the availability of the subcontractor, Discovery Drilling, and the borough’s directions.
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