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ABUTMENT DESIGN SELECTED 5-6-15 LMS

The Salmon City Council has given Northwest Engineering Services directions to proceed with designing Mechanically Stabilized Earth abutments for the Island Park Bridge.

Project lead engineer for Northwest, Jeanne Bailey, presented three design options at the council’s May 6 meeting.

The first option was the familiar concrete abutment consisting of concrete walls and stem walls with footings at the bottom which is how the existing Island Park Bridge abutments were constructed. She said that the cost of construction depends on availability of materials and the going cost of concrete. Estimated cost is $110 thousand to $160 thousand. She said it is a labor intensive process and environmental concerns have recently been raised by government agencies regarding fish kill effects of concrete in the water. It is now considered a form of pollution due to lye and other chemicals in the concrete leaching into the water.

Retired engineer Councilman Jim Baker did not consider that a valid concern based on his experience. Bailey and her associate engineer Dan Sharp both said whether it is a truly valid concern or not, government agencies believe it causes fish mortality therefore it would be a bonus, approval wise, to avoid concrete abutments. They said it would also slow the National Environmental Policy Act (NEPA) process.

The second option presented was Mechanically Stabilized Earth (MSE) abutments. Bailey described it as a new type of construction that uses gabion baskets, straps attached to the earth or geo tech which is a kind of fabric. ‘Gabion’ is defined as a bird cage type container made of wire mesh and filled with rock and other types of fill. Sharp said the result is an abutment that amounts to a big fat wide foundation of rocks, dirt and mats contained in heavy wire mesh baskets. The baskets can be faced with any number of facades including rocks from the area.

Bailey said that is Northwest’s recommended option based on ease of construction, a price of from $50 thousand to $100 thousand and the fact it is environmentally preferred.

The third option was a Super Sill Precast Stay Form that consists of a preformed steel frame put on the ground as a foundation and filled with concrete. The abutment is then built on top. It costs more, at from $70 thousand to $120 thousand, and would be the fastest to construct. It would also lengthen the bridge by 20 feet which is one reason for its expense.

The MSE and Super Sill could both be built outside the high water mark and therefore require fewer environmental permits.

All things considered the council voted unanimously to proceed with the MSE design.

The engineers said the MSE technique has only been used on three bridges in the state but is being widely used throughout the country, is loved by federal agencies and is being promoted by the Federal Highway Administration


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