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Timpanogos Cave National Monument Rockfall Mitigation Measures Ut Fltp Tica 10(992)

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Timpanogos Cave National Monument Rockfall Mitigation Measures Ut Fltp Tica 10(992) TIMPANOGOS CAVE NATIONAL MONUMENT ROCKFALL MITIGATION MEASURES UT FLTP TICA 10(992) GEOTECHNICAL DESIGN REPORT Report # UT-PX-TICA-13-01 Geotechnical Services Branch July 2013 TABLE OF CONTENTS REPORT PAGE 1.0 INTRODUCTION ................................................................................................................... 1 1.1 Scope of Rockfall Hazard Evaluation ....................................................................... 1 1.2 Previous Reports and Investigations ......................................................................... 2 2.0 GEOLOGY AND SEISMICITY ........................................................................................... 3 2.1 Site Geology ................................................................................................................. 3 2.2 Regional and Local Seismic Setting ........................................................................... 3 2.3 Geologic Hazards ........................................................................................................ 4 2.4 NPS Geologic Hazard Policy ...................................................................................... 5 3.0 PROCEDURES AND RESULTS .......................................................................................... 7 3.1 Findings ........................................................................................................................ 7 4.0 ANALYSIS AND RECOMMENDATIONS ....................................................................... 12 4.1 Rockfall Analysis ....................................................................................................... 13 4.2 Rockfall Mitigation Recommendations ................................................................... 16 4.3 Rock Scaling Recommendations .............................................................................. 17 4.4 Rockfall Hazard Rating System Assessment .......................................................... 17 4.5 Construction Considerations ................................................................................... 19 5.0 DISCLAIMER/LIMITATIONS CLAUSE ......................................................................... 20 6.0 REFERENCES ...................................................................................................................... 21 APPENDICES Appendix A – Figures Appendix B – NPS Geologic Hazards Management Policy Appendix C – Rockfall Mitigation Methods Appendix D – Rockfall Analysis Data Appendix E – Rockfall Fence Details Appendix F - Special Contract Requirements Appendix G – Photos Page i 1.0 INTRODUCTION This report presents the findings of a visual reconnaissance and provides geotechnical recommendations to support the development of a rockfall hazard evaluation and associated mitigation measures within the Timpanogos Cave National Monument (TICA). TICA is located within the American Fork Canyon along Utah State Highway (SH) 92, approximately 3 miles east of Highland, Utah in Utah County. The main resource at the National Monument is the cave system providing access to Hanson Cave, Middle Cave, and Timpanogos Cave. To reach the cave, visitors must hike the strenuous, 1.5-mile-long cave trail, which rises steeply just over 1,000 feet. The Visitor Center and caves are open from late-May to late-September. A Project Vicinity Map and Project Location Map are presented in Appendix A. Regular rockfall events within the park coupled with seasonal visitation emphasizes the need for proactive, systematic evaluation of natural slope hazards. For example, rock scaling efforts are conducted by Park staff on an annual basis on the readily accessible slopes within the Park. In addition, the United State Geological Survey (USGS) has conducted several rockfall hazard assessments of specific locations within the Park, as well as locations within American Fork Canyon. To support ongoing efforts to improve the safety and performance of unstable natural slopes within the Park, this project focused on the following objectives: 1. Relate slope hazards and qualified risks with broader efforts to identify rockfall hazards within the Park; 2. Recommend a range of rockfall hazard mitigation strategies suited to Park resource management objectives and the effective use of maintenance resources, and apply these strategies toward site-specific hazard mitigation recommendations; and 3. Frame identified rockfall hazards and risks within the assest management architecture, emphasizing the need to treat specific rockfall hazard locations as assets requiring proactive, life-cycle type management. This study provides an assessment and prioritization of rockfall hazards for unstable natural slopes adjacent to the current location of the TICA Visitor Center and along the cave access trail. Recommended mitigation measures, and strategies for effectively deploying these measures, are provided along with order-of-magnitude cost estimates for general mitigation measures. 1.1 Scope of Rockfall Hazard Evaluation The rockfall hazard evaluation is focused on the current location of the TICA Visitor Center, located at the base of near-vertical rock cliffs comprised of highly fractured quartzite bedrock. The cliffs begin approximately 150 feet from the Visitor Center and extend to more than 600 feet in height. A talus slope is located below the cliffs and extends downslope to the Visitor Center. The talus slope is approximately 100-feet-high with an overall slope ratio of 1V:1H. Rockfall debris from the cliffs generally reach the vicinity of the Visitor Center and have occasionally impacted and penetrated the roof of the Page 1 Visitor Center. A 6-foot-high chain link fence is currently located between the toe of the talus slope and the Visitor Center. The fence has been damaged and is retaining material from numerous rockfall events. TICA is currently completing final design plans to move the Visitor Center away from these cliffs and increase parking capacity. The proposed location for the Visitor Center is at the east end of the existing parking lot. Relocating the Visitor Center will distance the structure from the runout zone of the active talus slope; thereby, providing greater protection from rockfall impacts. Relocation plans also include minor realignment of SH 92 in the vicinity of the Visitor Center to accommodate increased parking capacity. While the proposed site is better protected from rockfalls, the demolition and removal of the existing Visitor Center and associated structures increases the rockfall hazard to the proposed parking area. As a result, TICA has requested technical assistance with the evaluation of rockfall hazards and development of rockfall protection measures. A cursory hazard evaluation was also conducted of natural slopes along the length of the cave trail. Lower portions of the cave trail cross beneath near vertical cliffs comprised of intensely fractured Mutual Quartzite. Rocks falling from these cliffs commonly reach the vicinity of, or impact, the Visitor Center. Upper portions of the trail abut Deseret Limestone, which at this location consist of a sequence of fractured, interbedded limestones, dolomites, and quartzites. Steep gullies in the cliffs between the cave entrance and exit have been the site of numerous rockfalls. These rockfalls resulted in one fatality in 1933 and several near misses, which has prompted the construction of suspended, weighted fences to attenuate rockfall, as well as rockfall shelters at the cave entrance and exit in 1976. During the summer months, thousands of visitors per day access the trail and visit the cave system. Areas along the trail with known, significant rockfall hazards are marked with a red stripe. Trail users are advised to be cautious and not stop to rest in these areas. 1.2 Previous Reports and Investigations Geotechnical personnel from CFLHD were originally consulted for technical expertise on this project in August 2009. A trip report was prepared during the initial consultation which provided preliminary geotechnical recommendations on a rockfall catchment system. The recommendations were based on a visual inspection and estimates of rock slope geometry and characteristics. Page 2 2.0 GEOLOGY AND SEISMICITY American Fork Canyon is a deeply incised canyon that transects the Wasatch Mountain Range in central Utah. Specifically, the Canyon is within the deeply dissected Wasatch Front of northern Utah. The Wasatch Range is characterized by rugged mountain faces and narrow, east-west trending, steep sided canyons. Rapid regional uplift has caused many of the streams that drain the west side of the Wasatch Front to erode headward, carving back canyons, until they reach the mountain crests. The Wasatch Range is an uplifted block that extends approximately 120 miles. The range is 8- to 16-miles-wide and is bounded on the west by the scarp of the seismically active Wasatch Front. The Wasatch Front rises dramatically from the valley floor and separates the geologic province of the Basin and Range to the west from the Middle Rocky Mountain province to the east. The Wasatch range is geologically complex and is characterized by the normal faults along which the Basin and Range deformation occurred. There are many parallel ranges throughout the Timpanogos region due to the extensional tectonics pulling the crust apart in an east-west oriented pattern. Many westward flowing, high gradient, parallel streams dissect the Wasatch Front into isolated peaks separated by deep, narrow
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