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Engineering geology considerations for park planning, Antelope Island State Park, Davis County, Utah

October 11, 2000

Report: 00-1
In the mid-1980s, historically high levels of Great Salt Lake caused damage to park facilities on Antelope Island and destroyed the causeway linking the park to the mainland. Information on the engineering geology of Antelope Islandcan be used to improve park facilities and reduce the risk from geologic hazards and poor construction conditions. Certain characteristics of the geologic environment need to be considered in park planning. During wet cycles, Great Salt Lake may reach static levels of 4,217 feet (1,285.3 m), and wave- and wind-elevated levels locally may reach 6.5 feet (2 m) higher. A probabilistic assessment of the earthquake ground-shaking hazard along the Wasatch Front indicates that peak ground accelerations of approximately 0.20 to 0.30 g have a one-in-ten chance of being exceeded in 50 years on the island. A slope-failure hazard exists locally in colluvial and Lake Bonneville deposits, along the modern shore, and beneath cliffs. Flash-flood and debris-flow hazards exist on alluvial fans. Areas in the southern two-thirds of the island may have a relatively high potential for radon emission. Particular soil types on the island may be expansive, compressible, erodible, impermeable, or susceptible to liquefaction or hydrocompaction. The distribution of most geologic hazards can be defined, and many locations on the island have conditions suitable for construction. Lacustrine sand and gravel deposits are wide-spread and have engineering characteristics that are generally favorable for foundations. However, facilities and roads built close to the modern shoreline may be susceptible to lake flooding and erosion, slope failures, shallow ground water, and burial by active sand dunes. Well-graded (poorly sorted) alluvial-fan deposits are generally most suitable for wastewater disposal, although they may be subject to flooding or be underlain by low-permeability, fine-grained lacustrine deposits.