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Analytical Evaluation of liquefaction potential of a site is based
originally on the pioneering work by H Bolton Seed and Idriss
CSR
Analytical Evaluation of liquefaction potential
of a site is based originally on the pioneering work by H Bolton Seed and Idriss
(1971) The “simplified procedure” originally developed involves the calculation
of the Factor of Safety obtained by determining the Cyclic Resistance Ratio and
Cyclic Stress Ratio of the site soils. The method has been modified and improved
by several researchers. The current “simplified procedure” calculates the factor
of safety, FS, against liquefaction in terms of the cyclic stress ratio, CSR
(the demand), and the cyclic resistance ratio, CRR (the capacity), according to
the formula:
FS=(CRR7.5/CSR).MSF.Kσ
.Kα
where:
CRR7.5
is the cyclic resistance ratio for magnitude 7.5 earthquakes, MSF is the
Magnitude scaling factor, Kσ is the overburden correction factor, and Kα is the
correction factor for sloping ground. CSR is estimated using the Seed and Idriss
(1971) equation multiplied by 0.65:
CSR=0.65(a/g).σvo/σ'vo.rd
where:
a is the peak horizontal acceleration at
the ground surface generated by the Earthquake, g is acceleration due to
gravity, σvo and σ’vo are the total and effective overburden stresses,
respectively, and rd is the stress reduction coefficient. Other than the purely
empirical grain size comparisons, the three commonly used methods to evaluate
the liquefaction resistance, CRR, Gutierrez Ref 6] are:
1) Using the Standard Penetration Test (SPT),
2) Using the Cone Penetration Test (CPT), and 3)
Using Seismic Shear wave velocity
Associated uncertainties in the development of
probabilistic methods for liquefaction risk analysis based on the “simplified”
method are:
1) the uncertainty in demand particularly the
maximum acceleration amax and the earthquake magnitude Mw , required to estimate
the magnitude scaling Factor MSF and
6 ] M. Gutierrez, J. M. Duncan, C. Woods and M.
Eddy “ Development of a Simplified Reliabilty-Based Method for Liquefaction
Evaluation” Civil and Environmental Engineering Virginia Polytechnic Institute &
State University i
2) the uncertainty in the capacity CRR . For CRR,
the uncertainties are due to natural variability of the soil and geotechnical
properties, in-situ testing procedures, and most importantly the simplified
method. Gutierrez 4 ]
Recent researches into this field have resulted
in further refinements in the procedure particularly in both the “deterministic”
and “Probabilistic” determination of liquefaction potential.
In a recent groundbreaking publication by
Raymond Seed et al known as the Queen Mary Paper,”ref 3] refinements in the
procedure over that of the “simplified” Seed (Senior) procedure have been
proposed.
New models presented and described in this
specific research paper deal explicitly with the issues of:
(1) Fines content (FC),
(2) magnitude-correlated duration weighting
factors (DWFM), and
(3) Effective overburden stress (Kσ effects),
and they provide both
(1) An unbiased basis for evaluation of
liquefaction initiation hazard, and
(2) Significantly reduced overall model
uncertainty.
source : http://www.ce.washington.edu/~liquefaction/html/main.html
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