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Mesa Verde National Park, Spruce Tree House Alcove Local Arch AnalysisAnya Benda, Lee Petersen, Ryan Peterson Itasca Consulting Group, Inc., Minneapolis, Minnesota USA
Mesa Verde National Park, located in southwestern Colorado, is the site of Spruce Tree House, which is the third-largest and best-preserved cliff dwelling in the Park. The Spruce Tree House cliff dwelling was constructed about 800 years ago in a naturally formed alcove about 216 ft wide by 89 ft deep. The alcove is located across a deep, narrow canyon from the Park museum and is a very popular destination. The nose of the alcove contains a thin arch delineated by a persistent curved crack (called the Richardson crack), which extends about 270 ft north from above the south end of the alcove. The crack is up to several feet wide, and the arch cross-section is roughly 10 ft thick by 20 ft high. A 1960s investigation led to a stabilization program that included rockbolts and a thorough cleaning and grouting of the crack. Recently, rockfalls from the arch and adjacent rock surfaces demonstrated the need for a new assessment of arch stability and stabilization.
A 3DEC model was developed including assumptions about the geometry of the crack that defines the arch. The model was calibrated, meaning sandstone strength properties were adjusted so that the arch was just stable with the existing 1960s rockbolts discounted. Three rockbolt configurations were modeled: reticulated and unreticulated patterns with 79 25-ft long rockbolts installed in three rows, and a reticulated pattern with 58 18-ft long rockbolts in three rows. The reticulated 58-rockbolt pattern is the recommended remediation. The recommended rockbolt type is a Williams R1H high grade, hollow-core, pretensioned rockbolt with a yield capacity of 100,000 lbs, tensioned to 38,000 lbs.
This paper was published and presented at the 66th Annual Geotechnical Engineering Conference, held at the University of Minnesota, Minneapolis, Minnesota on February 22 - 24, 2018.
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