$ Groundwater depth - The depth of the
density), the depth to the groundwater table, and the
groundwater table below the existing ground
amplitude and duration of ground shaking. As such,
surface is commonly reported on boring logs or
these factors can provide a basis for evaluating a site
water well logs; regional groundwater depth
for liquefaction hazard. For screening level
(elevation) contour maps may also be available
evaluations, criteria are given for assessing subsurface
and utilized if site-specific or nearby
soils and groundwater information available for a site.
measurements are not. Possible seasonal and
Screening criteria are not made a function of ground
historic fluctuations of groundwater levels should
shaking level because current understanding of
also be reviewed/considered.
liquefaction behavior does not preclude its occurrence
at any ground shaking level, although there are no
$ Building foundation - Available drawings and
reported/known cases of historical liquefaction for peak
other information on the proposed building
ground accelerations less than about 0.07g.
foundation system should be reviewed to ascertain
the type and depth of foundation (e.g., spread
(1) Sources of information. Sources of available
footings, piles).
information to be reviewed in conducting a screening
evaluation for liquefaction hazard include:
$ Site ground reconnaissance - Walkdown of the
site and buildings should be conducted to observe
$ Geologic maps - Large-scale (e.g., 1:24,000) or
and note the existing characteristics of the site
smaller-scale (e.g., 1:250,000) geologic maps are
(e.g., topography, especially slopes or free faces).
generally available for many areas from geologists
During the site reconnaissance, observations of
of regional U.S. Geological Survey offices, state
ground distress and/or building distress at the site
geological agencies, or local government
and nearby sites that may be related to
agencies. The geologic maps typically identify
geotechnical processes should also be recorded.
the age, depositional environment, and material
type for a particular mapped geologic unit.
(2) Screening criteria. It can be assumed that a
significant hazard due to liquefaction does not exist at a
$ Topographic maps - Similar availability as
site if, based on the review of available information,
geologic maps. These maps depict the general
one of the following screening criteria is met:
slope gradient and direction for the general site
vicinity and the presence of any significant nearby
(a) The geologic materials underlying the site are
free-face. Site grading plans may also be
either bedrock or have a very low liquefaction
available for review.
susceptibility according to the relative susceptibility
ratings that Youd and Perkins (1978) assigned based
$ Boring logs - Foundation engineering reports
upon general depositional environment and geologic
prepared for a facility typically contain logs of
age of the deposit. These susceptibility ratings are
geotechnical borings drilled at the site. The logs
shown in Table F-1.
typically contain information regarding the
stratigraphy (soil type), penetration resistance
(b) The soils below the groundwater table at the site
(density) and the depth at which groundwater was
are: stiff clays or clayey silts and have a clay content
encountered. The foundation engineering reports
(grain size< 0.005 mm or 0.0002 inches) greater than
may also contain laboratory test data such as grain
15 percent, liquid limit greater than 35 percent, or
size distributions, Atterberg limits, unit weights,
natural moisture content less than 90 percent of the
shear strength, etc.; these data are commonly
liquid limit (Seed and Idriss, 1982); or cohesionless
reported on the boring logs and reflected in the
soils (i.e. clean or silty sands, silts, or gravels) with a
soil descriptions given on the logs. In the absence
minimum normalized Standard Penetration Test (SPT)
of site-specific boring logs, logs for borings
drilled on an adjacent site may provide useful
screening information, as may logs of water wells
blows/foot); or cohesionless
drilled on site or nearby. If off-site information is
utilized, it is important to examine the
appropriateness of the off-site data by checking
the mapped geologic similarity of the sites (see
above).
F-13
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