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3.4 LIQUEFACTION RESISTANCE: EMPIRICAL METHODS BASED
ON IN SITU PENETRATION RESISTANCE
The recommended method of characterizing a soil’s liquefaction resistance is based on in situ
tests because of the disturbance inherent in the sampling and laboratory testing of cohesionless
soils. Table 3.4 shows a comparison of the features of the SPT and CPT for assessment of
liquefaction resistance.
Table 3.4: Advantages and Disadvantages of the SPT and CPT for the Assessment of Liquefaction Resistance
(Youd and Idriss 1997)
FEATURE SPT CPT
Number of test measurements at liquefaction sites Abundant Abundant
Type of stress-strain behavior influencing test Partially drained, large strain Drained, large strain
Quality control and repeatability Poor to good Very good
Detection of variability of soil deposits Good Very good
Soil types in which test is recommended Non-gravel Non-gravel
Test provides sample of soil Yes No
Test measures index or engineering property Index Index
Although the SPT has historically been used for liquefaction assessments, the CPT is becoming
more common (Olsen 1997; Robertson and Wride 1997a,b) as an in situ test for site
investigation and geotechnical design, especially as the database of case histories grows.
Robertson and Campenella (1985) state that the most significant advantages of the CPT are its
simplicity, repeatability, and accuracy. The CPT also provides a continuous record, which is an
important feature for defining soil unit contacts accurately. The ability to measure pore water
pressures is another advantage. The advantages and disadvantages of both methods should be
kept in mind when making decisions for a given project. When the economics of a project
permit, the combination of SPTs and CPTs may offer a very reliable way to evaluate the
liquefaction susceptibility. Correction factors, such as those used for the SPT, also should be
applied to the CPT when appropriate.
Liquefaction susceptibility is usually expressed in terms of a factor of safety against its
occurrence. This factor is defined as the ratio between available soil resistance to liquefaction,
expressed in terms of the cyclic stresses required to induce liquefaction, and the cyclic stresses
generated by the design earthquake. These parameters are commonly normalized with respect to
the effective overburden stress at the depth in question. Evaluation of the resistance of soils to
cyclic pore pressure generation or triggering of soil liquefaction is generally accomplished using
the following six steps.
1. Evaluation of soil geology, including assessment of soil types, stratigraphy, site and
project geometry, water table and other hydrologic conditions.
2. Evaluation of static stresses at particular points of interest. At the depths of interest,
evaluate the pre-earthquake in situ effective vertical stress (σ v ′), and the pre-earthquake
“driving” shear stress acting on a horizontal plane (τ hv ). For level ground conditions, τ hv
is zero. For very loose, contractive soils, the presence of driving shear stresses (due to
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