Assessing and managing the risk of earthquake-induced liquefaction to civil infrastructure - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 115

5.2 Hazard identifi cation

The framework presupposes that the presence of soil susceptible to lique-
faction has been identifi ed, based on historical, geological or geotechnical
criteria (e.g. Youd & Perkins, 1978, Seed et al. , 2003). The hazard identifi ca-
tion stage should not only evaluate that there is potential for liquefaction
to occur and the potential lateral and vertical extent over which it will occur,
but also how this will manifest itself at a site, and how it could potentially
damage infrastructure.
The following two sections describe the assessment of liquefaction poten-
tial and consequent permanent ground deformation in more detail. Figure
5.1 provides a summary of the potential liquefaction hazards and potential
responses of infrastructure.

5.2.1 Liquefaction potential

The evaluation of liquefaction potential is based on the comparison of
seismic demand on a soil with capacity of the same soil to resist liquefaction.
The seismic demand is expressed in terms of the cyclic stress ratio (CSR),
which is the ratio of the induced horizontal cyclic stresses normalised by
the vertical consolidation stress (Equation 5.1).

τ
σ

a σ

σ

r

av max v d

CSR = =⋅ 065 . ⋅ ⋅ [5.1]

g MSF

′ ′

v v

where: a max is the peak ground acceleration at the surface, g is acceleration
due to gravity, σ v is the ratio of total to effective vertical stress in the soil

layer, r d is a depth-based stress reduction coeffi cient and MSF is a magni-
tude scaling factor.
The 'liquefaction resistance' is expressed in terms of the cyclic resistance
ratio (CRR), which is evaluated based on laboratory testing or, most com-
monly, on in-situ testing.
A database of case histories in which evidence of liquefaction has or has
not been observed is used to relate the earthquake-induced CSR for each
site and the in-situ test results available. A CRR curve that bounds the
conditions at which liquefaction has historically been observed can then be
drawn (e.g. Fig. 5.3), which allows the factor of safety against liquefaction
potential to be evaluated for a given in-situ resistance, and a given CSR.
It should be clarifi ed that the defi nition of liquefaction occurrence in the
above-described procedure is based on fi eld observations, mainly at ground
surface level, of typical liquefaction effects (sand boils, lateral spreading,
settlements, etc.) which is obviously rather subjective. Among the available
in-situ tests, the Standard Penetration Test (SPT) was the fi rst test used for

σ v /