Assessing and managing the risk of earthquake-induced liquefaction to civil infrastructure - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 121

• Simple static limit equilibrium approach coupled with Newmark (1965)

sliding block analysis (e.g. Olson & Johnson, 2008).

5.3.3 Flow failure

Flow failure occurs when the static shear stresses on sloping ground exceed
the residual shear strength of the soil. Displacements due to fl ow failures
can be very large (up to tens of metres). Flow failures have the potential
to signifi cantly disrupt infrastructure over a large area. The potential for
fl ow failure to occur is reasonably straightforward to evaluate. Standard
slope stability analysis procedures can be used, substituting the residual
undrained shear strength of the liquefi ed layer for its static properties,
where a factor of safety below unity indicates a fl ow failure hazard.
A number of empirically derived relationships exist for the estimation of
residual shear strength (e.g. Olson & Stark, 2002; Yoshimine et al. , 1999;
Stark & Mesri, 1992). These empirical relationships are signifi cantly depen-
dent on the dataset used, and the assumptions made for the back analysis
of an observed fl ow failure, such as void redistribution, pre-liquefaction
properties and geometry (Idriss & Boulanger, 2008). Residual shear strength
parameters can also be obtained from laboratory tests although such tests
are dependent on obtaining true undisturbed samples. As noted by Kramer
(1996) 'evaluation of the residual shear strength of a liquefi ed sand is one
of the most diffi cult problems in contemporary geotechnical earthquake
engineering practice'. This observation still holds true and appropriate con-
sideration of the effect of uncertainties and possible ranges of parameters
is an important consideration for a risk-based approach.

5.3.4 Settlement

Settlement due to liquefaction can develop both during the seismic event
and after the earthquake due to dissipation of excess pore water pressures
as part of the reconsolidation of the liquefi ed soil. Several methods have
been proposed to calculate liquefaction-induced ground deformations,
including numerical and analytical methods, laboratory testing-based
methods, and fi eld testing-based methods. Most of the available methods,
with the exception of numerical modelling, are limited to the prediction of
the post-liquefaction vertical movements.
Tokimatsu & Seed (1987) correlated laboratory data (cyclic triaxial and
simple shear tests) and SPTs, through empirical relationships between rela-
tive density and SPT blow counts, to develop chart solutions for the estima-
tion of limiting shear and post-cyclic volumetric strains. Ishihara &
Yoshimine (1992) used laboratory data to propose correlations between
the reconsolidation volumetric strain and the factor of safety against