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Galena Summit Slide, Idaho Highway 75

An active landslide exists on the southern slope of Galena Summit that does, and will continue to, impact Idaho Highway 75 if left uncorrected.

This landslide is 300-400 feet wide and 300-400 feet long. It has damaged the highway various times in the past resulting in costly repairs, shifting of the roadway alignment and other impacts to the public. The risk of not completing slope stabilization may result in a large slide, slide enlargement, and large impact to the highway. A large slide could result in a highway closure indefinitely. With the use of federal funds, the Idaho Transportation Department will undertake a project aimed at stabilizing the landslide affecting ID-75 at approximately milepost 156.7.

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Selected Repair Option

ITD intends to lessen the impact of the landslide to ID-75 by constructing a hillside buttress support system. The project will place a rock/soil buttress at the toe of the slide area and reinforce the hillside supporting the highway. ITD will complete roadway improvements as needed and replace damaged sections of the existing Sawtooth National Forest Service trail system in the project area.

The “Toe Berm” will buttress the slide against the opposing hillside. The 80,000-100,000 cubic yards of rock fill needed can be obtained from local sources and this approach both meets project goals and is cost-effective.

Project Objectives

In addition to impacts on ID-75, if left uncorrected the slide would continue to impact the Sawtooth Park Highway/Old US-93, and various other trails within the slide zone. Project objectives include:

– Long-term stability and integrity of roadway
– Restore proper highway alignment to reduce curve around scarp
– Prevent movement of slide scarp into pavement
– Make repair compatible with environment and existing features

Project Impacts

– Traffic Delays – traffic signals, flagging, pilot cars (with 30-60 minute wait times)

– Possible nighttime closures to all but emergency traffic

– Closures during periods of blasting

– Recreational Access – seasonal trail closures and bicycle traffic closure in work zone

– Closures of turnouts and overlooks over Galena Summit

– Landscape – modifications and relocation of the trail in the toe berm area, removal of Camp Creek through-cut north of summit

– Adverse Effect on three historic properties eligible for National Register Historic Places within project area–Galena Toll Road, Sawtooth Park Highway/Old Highway 93, and Highway 75

– Staging areas may include Horse Creek stockpile site (milepost 153.8)

– Duration of work expected to be 12 to 18 months and encompass the summer and fall seasons with no work occurring during winter or spring

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