south of the building. The trench was sited to intersect
the projected trend of the apparent linear soil/rock
contact interpreted to lie beneath the western end of the
building (Figure G-1). The walls of the trench were
cleaned of the smeared soil coating, examined for
evidence of faulting, and logged.
(1) The trench exposed no soil-bedrock contact.
On the basis of the trench, it was concluded that the
entire building is underlain by alluvial fan channel
deposits consisting of cobbles and boulders of
unweathered basalt in a fine-grained matrix. These hard
channel deposits, which had a source east of the site,
may have caused "refusal" during the pre-development
geotechnical borings, leading to an interpretation that
bedrock had been encountered in the easterly portion of
the site. It was observed in the trench that the alluvial
deposits became finer-grained toward the west. Toward
the east, there was an increasing concentration of
cobbles and boulders, reflecting the deposition of
coarser material toward the upstream margin of the
alluvial fan. The coarser materials in the eastern portion
of the site were apparently interpreted as bedrock in the
original geotechnical investigation. Examination of the
trench did not reveal any evidence of faulting, and it
was concluded that the potential for ground rupture due
to faulting beneath the site was very low.
G-3
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