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Recent developments in procedures for estimation of liquefaction potential of soils
Développements récents des méthodes d’estimation du potentiel de liquéfaction des sols
Katzenbach R., Clauss F., Rochée S.
Technische Universität Darmstadt, Institute and Laboratory of Geotechnics, Germany
ABSTRACT: Liquefaction of soils is associated with a loss of shear strength due to an increase of pore pressure. It causes important
damages during earthquakes and is insofar a high risk factor for buildings and infrastructures. A proper estimation of liquefaction is
required for a safe and economic design regarding earthquake resistance. In the last few decades various semi-empirical formulae
based on collected data from historical earthquakes had been suggested. This paper point out recent developments in this area in
relation with European Standards Eurocode 8 (2010) as well as methods described in Summary Report from the 1996 NCEER and
1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils (Youd et al. 2001) during last decade.
RÉSUMÉ : La liquéfaction des sols est associée à une perte de résistance au cisaillement due à une augmentation de la pression
interstitielle et peut être la cause de nombreux dommages lors de tremblements de terre. Il s’agit par conséquent d’un risque important
pour les bâtiments et infrastructures. Une estimation correcte du risque de liquéfaction est nécessaire afin d’obtenir un
dimensionnement à la fois sécurisé et économique. Différentes formules semi-empiriques basées sur des données collectées lors de
précédents tremblements de terre ont été suggérées ces dernières années. Cet article met en avant les développements récents dans ce
domaine, en relation avec la norme européenne Eurocode 8 (2010) et avec les méthodes décrites dans le rapport des workshops sur
l’évaluation de la résistance des sols à la liquéfaction du NCEER en 1996 et du NCEER/NSF en 1998 (Youd et al. 2001).
KEYWORDS: liquefaction, seismic design, Eurocode 8, Cyclic Stress Ratio, Cyclic Resistance Ratio
1 INTRODUCTION
Liquefaction of soils is associated with a loss of shear strength
of soil due to an increase of pore pressure. Liquefaction may
lead to important deformations and is insofar a high risk factor
for buildings and infrastructures. Different types of loading may
trigger liquefaction such as earthquakes, pile driving, train
traffic or blasting. The present study takes care of seismic
design and considers only earthquakes.
For a safe and economic design regarding earthquake
resistance, a proper estimation of liquefaction is required. In the
last few decades various semi-empirical formulae based on
collected data from historical earthquakes had been suggested
for a performance-based seismic design in liquefied zone for
seismically active areas.
It is quite obvious to see that the techniques used for
estimating liquefaction keep changing as the collecting data
increase with time. Consequently, divergences may be observed
between standards currently used for design of structures and
research reports. Recent developments in this area in relation
with European Standards, particularly the EN 1998 “Design of
structures for earthquakes resistance” (Eurocode 8), as well as
methods described in Summary Report from the 1996 NCEER
and 1998 NCEER/NSF workshops on evaluation of liquefaction
resistance of soils (Youd et al. 2001) and their actualizations
during last decade are presented here and discussed.
Procedures used for estimating potential of liquefaction
during earthquakes depend on slope angle with a distinction
between level ground sites and steeply sloping grounds
(Robertson and Cabal 2010). The present study deals
exclusively with procedures estimating the liquefaction
resistance of the soil during earthquakes for level ground sites
therefore with slope angles less than 5 degrees. It refers in the
European standards to section §4.1.4 of EN 1998 part 5
(Eurocode 8).
2 FIRST OBSERVATIONS
2.1
Main steps for estimating liquefaction potential for
seismic design
According EN 1998 (Eurocode 8), Youd et al. (2001) and
Robertson and Cabal (2010), screening criteria such as soil
properties or groundwater table are firstly used to determine
areas where liquefaction is more likely. In those areas,
quantitative estimations based on semi-empirical relationships
are then performed in three steps:
•
evaluation of the maximal available cyclic loading on the
site (Cyclic Stress Ratio CSR);
•
evaluation of the resistance capacity of the soil under
cyclic loads (Cyclic Resistance Ratio CRR);
•
comparison of CSR and CRR with the evaluation of the
Factor of Safety FS.
The steps were taken from the Simplified Procedure of Seed
and Idriss (1971).
2.2
Factors affecting liquefaction and screening criteria
Likelihood of occurrence and type of liquefaction depend
according Day (2002) and Prakash and Puri (2012) on:
•
soil properties: particle size gradation, relative density,
particle shape;
•
loading characteristics: intensity and duration of seismic
shaking;
•
site conditions: groundwater table, lateral earth pressure
coefficient, preloading, aging and cementation …
Some of those factors may be used as screening criteria,
insofar the quantity and reproductability of collected data during
previous earthquakes are sufficient to conclude about the risk of
liquefaction.
Important factors are water depth and saturation of the soil.
Presence of water is one necessary condition for triggering of
liquefaction. Moreover, liquefaction risks decrease with an
