Assessing and managing the risk of earthquake-induced liquefaction to civil infrastructure - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 118

and soil resistance (i.e. material properties, ground profi le, etc.). These
methods also employ outcomes of a PSHA, in the form of a seismic hazard
curve and associated deaggregation results, to account for the joint prob-
ability distribution of ground motion parameters and moment magnitude
of earthquake scenarios. The joint probability distribution is then integrated
with reliability-based liquefaction evaluation procedures. The outcome of
such a fully probabilistic analysis is a direct estimate of the return period
of liquefaction, rather than a factor of safety or probability of liquefaction
conditional upon ground shaking for a specifi c return period.
Whether probabilistic or deterministic approaches are used, there is
always a need for engineers to rigorously manage the uncertainties in their
calculations. Within a deterministic framework, engineering judgement can
be used to assign qualitative ratings to the probability of liquefaction. For
example, the simultaneous occurrence of several low-probability scenarios
that would give rise to liquefaction, would have a very low probability of
occurrence (Idriss & Boulanger, 2008). On major infrastructure projects, it
is often considered appropriate to use probabilistic approaches in conjunc-
tion with deterministic 'sense checks'.
5.2.3 Laboratory testing
Laboratory testing of 'undisturbed samples', typically simple shear, triaxial
or torsional cyclic tests, can be used to obtain a direct estimate of the cyclic
strength of saturated sands. However, laboratory testing is not used as
widely as the conventional empirical liquefaction evaluation method due
to its high cost and diffi culties associated with sample disturbance during
both sampling and reconsolidation. It is widely accepted (e.g. Seed et al. ,
2003) that cyclic simple shear and cyclic torsional shear tests are the pre-
ferred laboratory methods to evaluate the liquefaction resistance, as they
can reproduce more accurately the fi eld loading conditions compared with
cyclic triaxial testing.
5.2.4 Emerging practice - numerical modelling
Whilst numerical modelling of liquefaction (mainly fi nite difference and
fi nite element modelling) has been developing for over 30 years, it still
represents 'emerging practice' as its use is still limited due to time and data
constraints. The particular relevance of these techniques in the context of
this chapter is their use in exploring risk management options and in reduc-
ing uncertainties associated with risks that have been identifi ed as unac-
ceptable by infrastructure owners and operators. Within a risk management