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The post-liquefaction strength of silty soils has been investigated in the laboratory by
cyclically loading the soil until r u = 100%, then monotonically loading the soil undrained.
It is interesting to note that numerous investigators have found that liquefied silty sands
and silts are dilative when sheared following liquefaction (Stark et al. 1997; Boulanger et
al. 1998; Dickenson et al. 2000). This behavior indicates that once the soil liquefies, large
strains can be mobilized at sloping sites and at large strain the strength of the soil
increases. This scenario assumes that the loading is fully undrained. In light of the
limitations associated with sampling cohesionless soils and laboratory testing of the postliquefaction
behavior of soil, it is not recommended that the strength gain due to dilation
be incorporated into design.
4.2.2.2 The Strength of Liquefied Sand from In Situ Test Data
Recognizing the difficulties associated with laboratory testing of cohesionless soil,
alternative methods have been proposed for evaluating the residual shear strength of a
fully liquefied deposit. Two procedures that are commonly used are: (1) residual strength
ratio methods (Stark and Mesri 1992; Baziar and Dobry 1995); and (2) a procedure
which is independent of the in situ vertical effective stress (Seed and Harder 1990).
The relationship between SPT data, (N 1 ) 60 , and residual shear strength (S r ) with vertical
effective overburden pressure (σ′ vo ) for silty soil deposits developed by Baziar and Dobry
(1995) is shown in Figure 4.2. The curves were developed from the back-calculation of
residual shear strengths from case studies where liquefaction failures had occurred.
Nearly all of the case histories were selected based on previous work by Stark and Mesri
(1992). The evaluation procedure developed by Baziar and Dobry is based on the use of
the SPT to evaluate the potential for large deformations during earthquakes in saturated
loose sandy silt and silty sand deposits and slopes. The method is based on laboratory
tests and case histories corresponding to earthquakes of less than M W 8.0. Charts relating
the normalized standard penetration resistance and residual shear strength to vertical
effective overburden pressure have been developed for use as screening tools in
liquefaction hazard evaluations (Figure 4.2).
Figure 4.2 can be used to evaluate the large ground deformation potential during
earthquakes due to shearing of saturated, non-gravelly silt-sand deposits having at least
10% fines. The figure is applicable to slopes, embankments, and level or almost level
sites prone to lateral spreading. Figure 4.2 suggests that silty soils with a measured (N 1 ) 60
versus σ′ vo profile plotting to the right of the chart cannot experience flow failure due to
their dilative behavior, and that lateral spreading generally cannot exceed 0.3 to 1.0 m (1
to 3 ft) for earthquakes of less than M W 8.0. Figure 4.2(b) indicates that for silty deposits
that have experienced large deformations or flow failures, the S r /σ′ vo ranges from about
0.04 to 0.20. The average value of S r /σ′ vo is 0.12.
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