Snake River Canyon, Wyoming, USA: The Wyoming Department of Transportation has been reconstructing twenty-four miles of a 50-year old highway, widening its two lanes and adding shoulders to accommodate increased traffic and modern safety standards.
Most of the route is in a narrow corridor between the Snake River and steep mountainous terrain, where numerous landslides have caused significant damage to the roadway. One of the narrowest sections of the route is near the "Elbow," where the river makes a 90 degree bend.
The Problem: Here, the road sits on a marginally stable embankment. In recent years, the Snake River eroded the toe of the embankment, creating a slow-moving landslide known as the Elbow Slide.
Normally, it would be possible move the road away from the river by cutting into the backslope. However, in this case, investigators determined that cutting into the backslope would activate a large, ancient landslide above the roadway. They recommended that the roadway be moved away from the backslope and closer to the river.
Thus engineers were faced with the necessity of stabilizing the original embankment and then creating a structure that would support the new, widened roadway. Further investigation showed that the original embankment material would not support a structure, unless it were improved, but removal and replacement of the fill was not possible because at least one lane of the road had to remain open.
The Solution: Hayward Baker, Inc proposed that the existing embankment be stabilized and reinforced by two soil-nail walls. This would provide an adequate foundation for a Mechanically Stabilized Earth wall that would support the widened roadway. The MSE wall would consist of select backfill and modular block facing.
The lower wall is 725 feet long and contains approximately 308 forty-foot soil nails. The facing for the lower wall is 8.5 inch thick shotcrete, reinforced with wire mesh. Geocomposite sheet drains were also incorporated. The photo at right shows some instrumented soil nails ready for installation.
The upper wall, shown at left, is 743 feet long and contains 418 thirty-three foot soil nails. The facing, which is hidden by the MSE wall, consists of three layers of heavy welded wire fabric over a woven geotextile.
The 12,000 square foot MSE wall is 25 feet high and consists of Keystone compact blocks and reinforced, select backfill.
Instrumentation: The soil-nail wall project was designated an "Experimental Feature" by the Federal Highway Administration, and an instrumentation and monitoring program was undertaken. The objectives of the study are to evaluate the performance and cost-effectiveness of the soil nail walls as a way to control landslide movement and roadway damage, and also to compare actual loading of the soil nails to designed loads.
Instrumentation includes inclinometers and strain gauges. The inclinometers monitor movements behind the lower soil-nail wall and the MSE wall. Readings are taken monthly. The photo at right shows the ladder-like, metallic reinforcement and inclinometer casing.
The strain gauges are mounted on soil nails to monitor the mechanisms of load transfer from the ground to the structural components of the wall. There are four instrumented soil nails in the upper wall, each with five strain gauges, and three instrumented soil nails in the lower wall, each with four strain gauges. The strain gauges are monitored by a data logger that is powered by solar panel. Stored data are downloaded to a laptop for analysis.
Interim results show minimal slope movements since construction of the soil nail walls. Tensile forces mobilized in the nails are also within the values assumed for the design.
This story was condensed from a paper entitled: "Soil Nail and MSE Wall for Stabilization of the Elbow Fill Slide, Snake River Canyon, Wyoming" by John P Turner and Wayne G Jensen of the University of Wyoming, John R Wolosick of Hayward Baker, and Mark Falk of the Wyoming Department of Transportation. The paper also provides a useful list of references. Our thanks to these authors.