|BRIDGE ONE STEP CLOSER|
BRIDGE DESIGN DELIVERED 6-3-15 LMS
Dan Sharp, from Northwest Engineering of Salmon, presented the Salmon City Council with the final package of Island Park bridge designs prepared by the company. He told the June 3 meeting that the only things lacking are comments from the city and the various agencies involved in the bridge construction project. The city can make changes to the final concept as it sees fit.
Sharp said the set of plans are delivered as buildable and a bridge can be procured based on the plan dimensions. He said the design plans will be used in moving into the final permitting process and that Northwest Engineering has met the deadlines as given.
He went on to describe all the sections and specifications the document presented contains including the drawings, general construction requirements for the state of Idaho, survey notes, minimum requirements for standard erosion control, which will be up to the contractor to follow, and a Geotechnical Report which shows survey points of interest and identifies where the two test pit locations are located, what was found in the test pits plus more general notes on the test pits.
Sharp said the Erosion Sediment Control Plan shows straw waddle locations to prevent sediment from getting into the river and is meant to be a starting point for the contractor. He said it is a working document on which work with the Environmental Protection Agency and the Department of Environmental Quality can be based.
There is a detailed Excavation Plan for the overall excavation which includes using the existing abutments as a cofferdam. A cofferdam is a temporary structure used on rivers and lakes to prevent water from entering the site where construction is taking place. Once the high water level abutment and rip rap is installed the old abutments will be removed.
Sharp showed a bridge plan Profile of the new bridge location and abutments and explained the installation procedure. The new bridge will be two feet higher than the old one so ramps will have to be constructed.
Other documents included a Road Profile which shows the amount of fill that will be required, abutment construction details, straw waddling details and a final sheet showing an installation guide for the wire mats. The final bridge surface will be chosen by the city.
Sharp then answered council concerns about scouring by saying the reason the old bridge supports were eaten away by the river was that the bridge was placed in a narrowed channel which caused increased velocity flows around the abutments during high water years. He said the type of abutments for the new bridge are specifically designed as a scour or erosion mitigation tool and is a favorite design of highway departments for holding materials in place at the bend of a river. The rip rap would create even more protection. He added that of course general maintenance requires abutment rip rap checks after any high water flow year. Sharp said they won’t be able to see all the previous scour factors until the old abutments are actually pulled out of the way.
Due to the inherent unknowns, change orders are almost a certainty as the project proceeds and in this case would have to be permitted by all the agencies involved. Community Development Coordinator Mary Cerise will alert agencies to change order possibilities and start those discussions now. She said the fact that the project is fixing a really bad, unsafe situation puts the city in a more favorable position. Sharp added the city’s attitude of cooperation with the federal agencies is the right one to have.
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