Summit County, Colorado
The Summit County Board of Commissioners approved plans for the Xcel's Ptarmigan Substation located in Summit County near Silverthorne's Angler Mountain Ranch subdivision. The substation serves Silverthorne and surrounding areas and provides a higher level of electric service reliability than previously provided.
To accommodate the substation components, approximately 2.5 acres had to be graded and fenced. CTL | Thompson performed a geotechnical investigation to determine soil properties that allowed the determination of existing slope stability and the effect proposed construction would have on slope stability. After geotechnical investigation and initial analysis, CTL provided design consultation for the construction of an earth retention system that would allow deep excavation for substation components and accommodate strict aesthetic requirements.
CTL evaluated several types of earth retention systems for the project which resulted in selection of about 25,000 square feet of permanent soil nail wall with grout columns to be constructed. The soil nail wall selection not only provided more than sufficient resistance to the existing hillside but also allowed the use of percussion drilling techniques which provided a known construction schedule limiting potential of unknown time to penetrate cobbles and boulders exposed in earlier investigation work.
Further, the selected earth retention system would allow multiple construction crews such as excavator, drilling, and shotcrete to work simultaneously at the site in a shorten construction season that usually exists in the mountain environments. In total, 541 nails were installed and a total nail length of 17,785 linear feet (about 3.3 miles) was drilled.
CTL also recommended the use of vertical elements such as grout columns incorporated behind the soil nail wall face for this project. Benefits of grout columns include pre-splitting of boulders encountered in earlier investigation, a defined interface for the excavator to follow increasing productivity, temporary support between soil nail rows, less vertical and horizontal displacement, and predictable shotcrete overrun.