Airport Liquefaction Susceptibility Analysis Report - ABAG ...

Fugro Consultants, Inc.
Project No. 04.79221200
The liquefaction analyses performed generally calculated the highest settlement values along
the perimeter levees and dike along the western edge of the property. The levees and dike are
primarily composed of two distinct fill units: medium stiff Bay Mud fill, approximately 10 to 20
feet thick between stations 0+00 to 168+00; and loose to medium dense sand and gravel
approximately15 to 30 feet between stations 168+00 to 250+00 (URS, 2011; Figure 4A). The
foundation materials underlying the levees and dikes are composed of soft to medium stiff silty
clay (likely to be Young Bay Mud) along most of the alignment, with the exception of the
northern end of the alignment, where it is underlain by predominantly clayey sand and silty
sand. Of the levee and dike fill materials, the sand and gravel fill was calculated to be liquefiable
in all 5 scenarios while the Bay Mud fill yielded varying results.
In addition to the levee and dike fill, coarse grained layers within the Young Bay Mud and the
Merritt Sand were found to be liquefiable. Along the western portion of the property, the Merritt
Sand is located approximately 10 to 40 feet below ground surface. Along the eastern and
northern portions of the property, the borings suggest the presence of Merritt Sand, however the
deepest boring analyzed (36.5 feet below ground surface) does not fully penetrate through the
sand, leaving the thickness of the Merrit Sand in this portion of the property unknown. It is also
unclear what the nature, extent, and thickness of the Merritt Sand is below the main airport
terminals, where no subsurface data was evaluated.
6.1.3 Buchanan Field Airport
The liquefaction analyses performed for each of the five scenarios results in a calculated
settlement of between 1 and 4 inches (Table 3). The analyses used data available from 62
borings. Of the borings analyzed, only 10 were deeper than 30 feet and 31 were drilled to a total
depth of less than 10 feet (Table 2). The borings analyzed were concentrated within the
northern half of the property and outside of the property along the western property boundary.
No subsurface data was available for the southern half of the property.
Borehole data indicates that the surficial deposits at the airport are primarily composed of
artificial fill (subdivided into four distinct units; Fills A, B, C, and D), underlain by estuarine mud
in the northern portion of the property, and Holocene medial and distal alluvial fans deposits in
the southern portion of the property. Due to the sparse data available below 30 feet and
absence of subsurface borehole data for the southern half of the property, the nature and extent
of the contact between the estuarine mud and Holocene alluvial fan deposits is poorly
constrained.
Based on geologic and geomorphic mapping, Witter and others (2006) characterize Quaternary
deposits at Buchanan as having a moderate susceptibility to liquefaction, with a small cluster of
deposits at the north end having a very high susceptibility to liquefaction. These deposits likely
correspond to areas of mapped artificial fill over estuarine mud deposits. Given the relative
ages, densities, and textures of these deposits, Fill A (aggregate baserock composed of sandy
gravel with minor amounts of fill), Fill B (well graded sand with varying amounts of gravel), Fill C
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May 31, 2013