Airport Liquefaction Susceptibility Analysis Report - ABAG ...

Fugro Consultants, Inc.
Project No. 04.79221200
and 1970s and extended the dike to the northwest using hydraulically-placed dredged sand and
gravel (the “sand and gravel dike”; URS, 2011). WLA (1999) identified at least four different
episodes of fill placement and three distinct fill units:
• Fill A: cohesive fill
• Fill B: loose to medium dense granular fills typically derived from dredged Merritt Sand
(identified as the primary liquefiable zone)
• Fill C: dense to very dense granular fills
The thickness of fill varies across the site inboard of the dike from roughly 8.5 to 13.5 feet in the
south to between 1 and 7 feet in the north (AGS, 2008). The fill material generally consists of
loose to dense sand, silty sand, medium stiff to stiff silt and clay, with variable amounts of
gravel. The thickness of the clay dike fill, composed of shelly Bay Mud, ranges between 10 and
20 feet. The thickness of the sand and gravel dike fill, composed of loose to medium dense
sand and gravel, ranges between 15 and 30 feet. The fill bodies typically thicken to the west
(WLA, 1999; URS, 2011).
The dike and artificial fill are underlain by YBM, which consists of soft, sandy clay to silty clay
with thin peat layers locally present. The thickness of YBM ranges from 0 to 25 feet (WLA,
1999). The YBM rests unconformably on late Pleistocene to early Holocene Merritt Sand and
OBM. The Merritt Sand consists of well sorted eolian sand that interfingers landward with
Pleistocene alluvial fan deposits that consist of silty and sandy clay with gravely lenses. The
Merritt Sand and Pleistocene alluvial fan deposits unconformably overlie OBM (Yerba Buena
mud), which consists of gray marine mud (low to high plasticity clay) with some sandy and
shelly horizons (WLA, 1999).
4.3 Buchanan Field Airport (CCR)
Buchanan Field Airport is located near the margin of Suisun Bay and the mouth of Walnut Creek
in northern Contra Costa County. The airport is built on a layer of artificial fill placed over alluvial
fan and estuarine sediments. The airport property is located on the distal portion of the
interfingering alluvial fans of Walnut Creek and Grayson Creek where they grade into the
estuarine environment of the San Francisco Bay margin. The major geologic units in the area
include Holocene and Pleistocene alluvial deposits, and Holocene estuarine deposits (Figure
5A; Lajoie and Helley, 1975; Helley and Lajoie, 1979, Witter et al., 2006). Maps of the historical
ecology of the area show that the airport property encompasses former estuarine, moist
grassland, and willow grove habitats (Figure 5B; SFEI, 2000). These habitats are characterized
by tidal or shallow groundwater conditions.
Aerial photography taken in 1939 (Figure 5D) shows the future airport property as farm land.
The already straightened channel of Walnut Creek and the winding channel of Grayson Creek
pass to the west and east, respectively, of the property. Topographic contours from show that
the land slopes gently to the north, underlain by the alluvial fan deposits of Walnut Creek, and to
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May 31, 2013