slope performance, and level of ground shaking. The
soils that classify as clayey sand (SC) or clayey gravel
screening procedures involve a review of geologic and
(GC) with (N1)60 greater than 20. (The parameter
topographic maps, review of available data on the
(N1)60 is defined in paragraph F-4.) However, cohesive
subsurface conditions, and performing reconnaissance
soils that are highly sensitive based on measured soil
of the site and adjacent areas. Review of available
properties or local experience are not screened out. To
aerial photographs is desirable, especially if adequate
be classified as highly sensitive, a soil must possess
geologic and topographic maps are not available. In
each of the following property values: sensitivity
some areas, governmental agencies have prepared
greater than 4; liquid limit less than 40%; moisture
slope stability maps showing existing landslides and/or
content greater than 0.9 times the liquid limit; liquidity
relative slope stability. These should be reviewed if
index greater than 0.6; and (N1)60 less than 5 or
available. If appropriate, geologists and engineers in
government agencies knowledgeable of the
MPa (20 ksf). Areas of the U.S. with known highly
performances of slopes in the area should be contacted.
sensitive soils include some coastal areas of Alaska,
along the St. Lawrence River, some eastern and
(1) Screening criteria. It can be assumed that a
western coastal areas with estuarine soil deposits, and
significant hazard due to earthquake-induced
near saline lakes in the Great Basin and other arid
landsliding does not exist if all of the following criteria
areas. (Refer to Youd, 1998).
are satisfied:
(c) The groundwater table is at least 15 m (49 feet)
(a) The building site is not located within a pre-
below the ground surface, including considerations for
existing active or ancient landslide, and there are no
seasonal and historic groundwater level rises, and any
landslides on slopes of similar geometry and geology in
slopes or free-face conditions in the site vicinity do not
the site vicinity. The site is not located on, above, or
extend below the groundwater elevation at the site.
below a slope that displays cracking or other signs of
actual or incipient slope movement. There is not an
(3) Example. An example of screening for the
obvious hazard to the building from falling rocks or
hazard of liquefaction is given in Appendix G.
shallow soil flows on slopes located above the building.
c. Soil differential compaction. Information
(b) The site is not located adjacent to a shoreline.
sources to be reviewed in conducting a screening
evaluation for differential compaction are the same as
(c) The site is not located in a zone that has been
those identified above for the liquefaction potential
mapped as having a high landslide potential (static or
hazard. The site reconnaissance observations for the
seismic).
liquefaction potential hazard can be used for the
screening of the hazard of differential compaction.
(d) The building is located above a slope, is a
horizontal distance of at least three times the slope
(1) Screening criteria. It can be assumed that a
height from the toe of the slope, and is set back a
significant hazard due to differential compaction does
distance at least equal to the slope height from the top
not exist if the soil conditions meet both of the
of the slope. The geologic materials in the slope are
following criteria:
stiff cohesive (and nonsensitive) clays or clayey silts,
dense sands that do not have a significant liquefaction
(a) The geologic materials underlying foundations
potential, or bedrock. There are no obvious planes of
and below the groundwater table do not pose a
weakness in the slope, such as bedding planes dipping
significant hazard due to liquefaction.
out of the slope. If fill is present in the slope, there is
evidence that it has
(b) The geologic materials underlying foundations
and above the groundwater table are either: Pleistocene
in geologic age (older than 11,000 years); stiff clays or
clayey silts; or cohesionless sands, silts, and gravels
with a minimum (N1)60 of 20 blows/0.3 m (20
blows/foot).
d. Landsliding. The potential for landsliding or
downslope movement is dependent on slope geometry,
subsurface soil, rock and groundwater conditions, past
F-15
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