Following the recent discovery of a void under the spillway of the Plane 9 dam, the Altoona Water Authority must undertake a full dam evaluation, which could result in a rehabilitation project that would mean the reservoir behind the dam won’t be providing water for the Plane 9 treatment plant for perhaps two years, according to authority officials.

The plant can continue operating, however, as the authority is relying fully on Hollidaysburg Borough’s Muleshoe Reservoir to supply the plant, which normally gets half its water from Muleshoe and half from the Plane 9 reservoir.

The authority is awaiting delivery of a “compliance order” from the state Department of Environmental Protection’s Division of Dam Safety that will require a complete assessment of the dam, said authority General Manager Mark Perry.

The assessment will dictate what needs to be done to bring the dam up to current standards, according to officials.

The authority has applied for a grant to help pay for the assessment, which will include engineering work, according to Perry.

The odds look good for getting the grant, although it will require a 35% match, according to Perry.

The situation “has the attention” of the Dam Safety division, Perry said.

Meanwhile, authority workers under direction from consulting engineer Gwin, Dobson & Foreman were pumping grout into the spillway void, based on a plan devised by Gwin, officials said.

About 5.5 cubic yards of grout had been injected to the point where the void was 90% filled, according to staff engineer Jim Potopa.

The grouting is a stopgap measure, according to Potopa, responding to a question from board member Barb Kooman.

It is a “risk-reduction” first stage of the dam work, Potopa said later.

It’s fortunate that the Muleshoe water is available, so the plant can continue to operate, according to Perry.

A transition to membrane filtration for the treatment plant, however, is likely to be done in conjunction with whatever work turns out to be needed for the dam, Perry said.

The assessment of the 117-year-old dam will be “top to bottom,” said Mark Glenn of Gwin.

There will be geotechnical and hydraulic aspects as well as a look at spillway capacity, Glenn said.

Preliminary estimates peg the cost of the assessment at about $500,000, according to Perry.

It’s “highly unlikely” the assessment will lead to abandonment of the dam, given that the reservoir behind it is a valued water source, Perry said.

Mirror Staff Writer William Kibler is at 814-949-7038.