Spruce Tree House Arch Stabilization, Mesa Verde National Park

Date:

2018

Industry Area:

Civil

Client:

U.S. National Park Service

ITASCA Office:

Minneapolis

3DEC

Project Background

Mesa Verde National Park, located in southwestern Colorado, is the site of Spruce Tree House Alcove, the third largest and best-preserved cliff dwelling in the Park. The Spruce Tree House cliff dwelling was constructed by the ancestors of the Pueblo peoples around 800 years ago in a naturally formed alcove about 216 feet wide by 89 feet deep.

The nose of the alcove contains a thin arch delineated by a persistent curved crack (called Richardson crack), which extends approximately 270 ft north from above the south end of the alcove. The crack is up to several feet wide in some areas, and the arch cross-section is roughly 10 ft thick by 20 ft high. An investigation in the 1960s led to a stabilization program that included rockbolt installation and a thorough cleaning and grouting of the crack.

Rockfalls from the arch and adjacent rock surfaces highlighted the need for a new assessment of arch stability and additional support efforts.

ITASCA’s Role

The U.S. National Park Service retained ITASCA to investigate the arch and conduct a stability assessment through three-dimensional numerical modeling of multiple remediation options to provide a recommendation on the optimal solution and an estimate of construction costs. The site investigation included shallow coring, surveying joint locations, crack monitoring, and joint characterization near Spruce Tree House Alcove.

Featured ITASCA Software

3DEC

Results

ITASCA developed a 3DEC model based on the geometry from a LIDAR survey and estimations of the crack geometry developed from the site investigation. The initial mechanical properties used for the intact rock and the local arch were derived from lab test results. The model was calibrated, meaning sandstone strength properties were adjusted so that the arch was stable when ignoring the presence of the rockbolts installed in the 1960s. After the model was calibrated, several new rockbolt configurations were simulated to determine a practicable remediation solution. After the installation of rockbolts in the chosen configuration, the arch was predicted to be stable up to a Strength Reduction Factor (SRF) of 3.

The 3DEC model of the alcove nose explicitly included the Richardson crack and several shale seams. Velocity contours are shown at a supported SRF of 3.

References

Brose, A., Loy, A., Peterson, L., Mason, J.A., & Peterson, R. (2018). Supporting A National Treasure: Spruce Tree House Alcove Arch Stabilization. GeoStrata Magazine, 22(6), https://doi.org/10.1061/geosek.0000236.