G-2.
Example 2 - Liquefaction Hazard Screening
extending at least to the penetration depth of the
borings. The nature of the soil materials encountered in
the borings is consistent with characteristics of the
This example illustrates the steps involved in screening
mapped geology.
a site for liquefaction hazard using the criteria described
in paragraph F-3. This example is based on a case
(3) Groundwater conditions. Groundwater was
history study for an existing building.
encountered at the time of drilling the borings at depths
varying between about 30 and 45 feet (9.1 and 13.7 m);
a.
Review of available information
it is not known to what extent, if any, tidal fluctuation
affected this range of variation. Based on the available
data, however, it is evident that groundwater elevations
(1) Site development. The building site is located
along the eastern boundary of the site are shallower than
in a metropolitan area of a moderately-to-highly active
those in the western portions of the site by as much as
tectonic region. The site development consists of two
about 6 to 8 feet (1.8 to 2.4 m).
high-rise office buildings connected by an elevated
bridge; both buildings were designed and constructed in
(4) Topographic conditions. According to U.S.
the early-1970s. Each building is rectangular-shaped
Geological Survey topographic maps and logs of soil
with a below-ground basement extending beneath the
borings drilled at the site during the design phase for the
footprint of the building. Available drawings and plans
buildings, the ground surface across the site varies
for the buildings indicate that the structures are
between elevations of about 25 and 34 feet (7.6 and 10.5
supported on systems of shallow foundations and
m) above mean sea level (MSL) and slopes very gently
footings situated beneath their respective basement-
downward to the south at a gradient of less than a
floor slabs. The foundation plans show the finished
degree. The only significant topographic change in the
basement-floor slabs to be approximately 30 feet (9 m)
site vicinity is at the waterfront along the bay situated
below the existing grade adjacent to the building. The
approximately 2500 feet (750 m) to the south and west.
drawings and plans indicate that the footings of the
perimeter walls are approximately 2.6 to 3.6 feet (0.3 to
(5) Historic earthquake effects. The buildings,
1.1 m) below the finished basement-floor slab. The
having been constructed in the early-1970s, have
column footings for both buildings are typically square,
experienced only relatively distant, moderate- to large-
with dimensions ranging between about 7 and 15 feet
magnitude earthquakes during their existence. The
(2.1 and 4.6 m); the bottoms of these footings are
ground motions from such earthquakes have been
generally about 5.5 to 7 feet (1.7 to 2.1 m) below the
merely felt in the site area, producing Modified Mercalli
finished basement-floor slab, although some extend as
Intensity (MMI) V effects, and therefore have not been
deep as 11.5 to 13 feet (3.5 to 4 m).
of consequence to the buildings or the site. During the
historical time period prior to construction of the
(2) Soil conditions. The site is situated in an area
buildings (i.e. since about 1800), the site experienced
mapped geologically as a Pleistocene-age formation,
ground shaking from several moderate to large
generally described as poorly consolidated, fine- to
earthquakes that reportedly produced MMI V-VII
medium-grained sand and/or sandstone that was
effects in the area. Two events in the nineteenth
deposited in nearshore marine, lagoonal, and non-
century, an estimated magnitude 6.5 in 1800 and an
marine environments. Eleven soil borings drilled at the
estimated magnitude 5.9 earthquake in 1862, reportedly
site during the buildings' original design phase
produced MMI VII effects in the site area; both of these
encountered predominantly fine sand, silty fine sand,
events are thought to have occurred on faulting in the
and fine sandy silt from the ground surface to the
offshore region west of the site area. There are no
maximum exploration depth of about 63 feet (19.2 m).
reports of ground failure distress for the site vicinity
The logs of the soil borings indicate that beneath an
associated with these or other historic earthquakes.
approximately 3-foot (1-m) thick surficial veneer of
sandy and clayey fill materials, the sands, silty sands,
and sandy silts encountered within depths of about 15
feet (4.6 m) below the ground surface are loose to
medium dense [7 < (N1)60 < 25]. Underlying these near-
surface sands and silts is a 12 to 15 foot (3.6 to 4.5 m)
thick stratum of dense to very dense poorly graded fine-
to medium-grained sand [35 < (N1)60 < 55]. This sand
stratum is in turn underlain by various thinner strata of
generally dense to very dense sands, silty sands and
occasional sandy silts [30 < (N1)60 < 60], interbedded
with very stiff to hard silty clays and clayey silts
G-4
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