3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
transitions into a massive, very stiff-to-hard, silty clay beneath the east side of Knik Arm
(Figure 3.27). The transition from sand to clay near the east side may be of an
interfingering nature, or the sand unit may postdate the clay unit (Golder Associates 2003,
KABATA 2006j). The latter interpretation is plausible in light of the likely origin of the
submerged canyon (Cairn Point Trench) to the south, which lies at nearly the same
elevation as the base of the sand unit. If the canyon is an erosional remnant of the late
Pleistocene lower sea level stand, the sand unit may represent alluvium and marine sand
filling the canyon as the sea level rose, and the interfingering part belongs to the older
Bootlegger Cove Formation. It is also possible that the submerged bench at the head of the
canyon (the only location where interfingering layers have been encountered) represents
an early Holocene landslide deposit that slumped along a former east side bluff during a
lower sea level.
• Knik tills. The deepest unit encountered in borings beneath Knik Arm is predominantly a
hard, gravelly, sandy clay that may be a glacial till deposit associated with the Knik
Glaciation (KABATA 2006j).
Landslide deposits
Numerous landslide deposits occur along the Knik Arm and Ship Creek bluffs:
• Knik Arm Crossing – West Bluff. The west bluff area is characterized by numerous
shallow slump deposits as well as mud flows in the fine-grained units caused by spring
seepage at the interface between the lower sandy clay unit and the upper coarse-grained
deposits (KABATA 2006n).
• Knik Arm Crossing – East Bluff. The bluffs at the east end of the proposed Southern
Alignment and south to the POA exhibit multiple ravines, slough material, and localized
slump deposits (KABATA 2006j, 2006n). The proposed alignment east of the POA and
north of Government Hill would cross the toe of a 1964 earthquake-triggered deposit
known as the Elmendorf landslide, about 0.25 mile north of Government Hill, as well as
older landslide material consisting of wet, soft-to-loose, intermixed silts, clays, and sands
near the toe of the slope (KABATA 2006j, 2006n; Updike and Carpenter 1986).
• Anchorage approach alternatives. The Degan Alternative would cross primarily older
landslide material along the north bluff of Ship Creek. The Erickson Alternative would
cross a 1964 earthquake-triggered deposit known as the Government Hill Elementary
School slide consisting of jumbled blocks that rotated along Bootlegger Cove clays and
slurry flow material at the slide toe (Shannon & Wilson 1964, KABATA 2006n; Updike
and Carpenter 1986). Both alternatives cross a 1964 landslide deposit at the south bluff of
Ship Creek known as the First Avenue Slide (Shannon & Wilson 1994, 2005).
Artificial fill
A series of landfill deposits are located between Cairn Point and the POA. Elmendorf
operated a surface dump at the top of the bluff in this area in the 1940s and 1950s, and debris
from the landfill slumped down slope onto the beach. There are multiple locations along the
base of the bluff where debris is visible and being eroded by tidal action (KABATA 2006j).
12/18/07 3-115