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The evaluation of liquefaction hazard is generally performed in several stages: (1) preliminary
geological/geotechnical site evaluation, (2) quantitative evaluation of liquefaction potential and
its potential consequences, and if necessary, (3) development of mitigation and foundation
remediation programs. The scope of the investigation required is dependent not only on the
nature and complexity of geologic site conditions, but also on the economics of a project and on
the level of risk acceptable for the proposed structure or development.
3.2 PRELIMINARY SITE INVESTIGATION
A preliminary site evaluation involves establishing the topography, stratigraphy, and location of
the ground water table at the project site. These geologic site evaluations must address the
following questions:
1. Are potentially liquefiable soils present?
2. Are they saturated and/or may become saturated at some future date?
3. Are they of sufficient thickness and/or lateral extent as to pose potential risk of damaging
ground deformations?
Sections 3.2.1 through 3.2.3 provide additional guidance regarding the questions to be addressed
in the preliminary evaluation. If this evaluation can clearly demonstrate the absence of a
liquefaction hazard at a project site, then it by itself may be sufficient. If some uncertainty
remains, however, then a more comprehensive geotechnical study should be undertaken. The
reference, “Screening Guide for Rapid Assessment of Liquefaction Hazard at Highway Bridge
Sites” (Youd 1998) provides useful information for preliminary hazard evaluations. In addition,
the reference, “Guidelines for Site-specific Seismic Hazard Reports for Essential and Hazardous
Facilities and Major and Special-occupancy Structures in Oregon” provides recommendations
for planning the site evaluation (OBGE/OBEELS 1997).
3.2.1 Potentially Liquefiable Soil Types
The quantitative liquefaction evaluation procedures in practice are based on the behavior of
predominantly sandy soils. These methods have been validated with field studies over the last
three decades, and a consensus has emerged regarding their application (Youd and Idriss 1997).
Understanding the liquefaction behavior of silty and gravelly soils has, however, substantially
lagged. Recommendations for these soils have been largely “rules of thumb” tempered by field
observations made after earthquakes. For example, cohesive soils with a fines content greater
than 30%, and whose fines either classify as “clays” based on the Unified Soil Classification
System (USCS), or have a plasticity index (PI) of greater than 30%, are not generally considered
potentially susceptible to soil liquefaction (Seed 1992; Youd and Idriss 1997).
The influence of fine-grained soil on the liquefaction resistance of predominantly sandy soils is a
topic that has received considerable attention over the past decade (Ishihara 1993, 1996;
Prakash and Dakoulas 1994). Laboratory testing of silts has been performed, but to a very
limited scale and with varying results (Chang 1990; Law and Ling 1992; Koester 1994; Singh
1994; Prakash et al. 1998; Guo and Prakash 1999). Recent examination of fine-grained soil
behavior during earthquakes and the results of laboratory tests reveal that uniformly graded loose
sandy soils that contain as much as 25% to 30% non-plastic to low plasticity fines may be highly
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