3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
discussed further in Section 3.5.3.2.4. Sandy deposits above the Bootlegger Cove Formation
are also potentially liquefiable where saturated from a perched aquifer (Updike and
Carpenter 1986).
The loose-to-medium-dense marine sands encountered across much of Knik Arm have the
highest liquefaction potential along the proposed bridge alignment. Several deep samples in
older deposits beneath the marine sands were also determined to be liquefiable; these soils,
however, are isolated within liquefaction-stable deposits. Little to no silty sand was
encountered at the surface along the Below-the-Bluff Roadway (KABATA 2006j). During
the 1964 earthquake, there were numerous examples of ground fissuring, lateral spreading,
and slumping in the tidal flats offshore of the Government Hill area, but little evidence of
liquefaction features such as sand boils or sand-filled fissures. The potential for liquefaction
also exists in the bluffs along Knik Arm where sands above the fine-grained Bootlegger Cove
Formation may be saturated from a perched aquifer (Updike and Carpenter 1986).
3.5.3.2.4 Slope stability
The bluffs along Knik Arm have been the sites of major catastrophic earthquake-induced
landslides in the past, and similar landslides are likely to occur in the future. The ability of
slopes to remain stable is continually altered by many geologic and man-made processes.
Both the west and east bluffs of Knik Arm are in a state of marginal stability, because erosion
from tides and currents are slowly cutting away the toes and slopes at an estimated rate of
about 0.5 foot per year (KABATA 2006n). While the west bluff exhibits evidence of active
slumping and mud flows, it is not known whether the sensitive clay unit of the Bootlegger
Cove Formation extends to this area; thus, the likelihood of large-scale translational
landsliding during an earthquake is unknown.
The steep bluffs bordering the east side of Knik Arm and Ship Creek have been mapped by
Dobrovolny and Schmoll (1974) as having mostly low-to-very low stability. The
1964 Elmendorf and Government Hill Elementary School slides are mapped as having
moderate stability because of the lower angles of the previously failed slopes. The Knik Arm
bluffs from about 1 mile north of Cairn Point to Government Hill, as well as the Ship Creek
bluffs, are mapped as having potentially large earthquake-triggered landslide risk because of
the presence of the Bootlegger Cove sensitive clay unit in the lower parts of the bluffs
(Dobrovolny and Schmoll 1974; Updike and Carpenter 1986).
3.5.4 Hazardous Materials and Contaminated Sites
An Initial Site Assessment (ISA) was prepared for the Study Area to determine the potential
for encountering petroleum, oil, and lubricants (POL)-contaminated sites or hazardous
materials if construction and operation of the proposed KAC project were to occur. The
objective of the ISA process is to evaluate, based on readily available information, whether
there are likely to be hazardous materials or petroleum products along the proposed project
alternatives or whether such materials or products would be likely to exist in the future
because of on-site or nearby activities or problems. Hazardous materials include ordnance
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