Idaho Power, Upper Malad Bridge Replacement

This project included the design of the new 100-foot single-span bridge over the Malad River near Hagerman, Idaho. The existing Upper Malad Bridge was composed of two spans, and did not allow for access of heavy haul vehicles to IPC’s Upper Malad River plant. To assist with these unique requirements, the bridge allows for the last 10 feet of guardrail at the end of each span to be removable to enable wider and longer trailers to maneuver onto the bridge. The bridge was designed as a simple-span, eliminating the unwanted center pier in the middle of the pristine river, and reduced environmental impacts to endangered snails as well as hydraulic impacts.

The bridge superstructure consisted of concrete prestressed girders set on partial integral abutments, chosen to minimize maintenance, blend in with rocky canyon surrounding environment, and allow for the superstructure movement. Piles for the abutment were not recommended due to the nature of the undermining soils. An alternate foundation with large cast-in-place footings were provided, and a semi-integral abutment was incorporated to provide the flexibility needed of the superstructure. The upstream hydraulics of the river creates a steep and fast flow, and in the past 50 years the existing bridge has been overtopped on a number of occasions. A hydraulics study was completed and the new bridge was widened and raised to reduce overtopping potential. Grouted riprap and sheet piling were also designed to protect footings and abutments against scour and undermining.

The design of the bridge was fast-tracked to meet IPCO’s aggressive seven-month design schedule. The project included concept, type, size, and location reports; geotechnical, hydraulic and hydrologic studies; site layout; and structural design of bridge and retaining walls.