Environmental Engineering Reference
In-Depth Information
(e) If soils are close to the boundaries of these criteria, and the consequences of liquefac-
tion are great enough, err on the side of caution and/or carry out laboratory tests.
12.4.3
The “simplified procedure” for assessing liquefaction resistance of a soil
12.4.3.1
Background to the simplified method
The most widely accepted, simplest and most practical method of assessing whether there
is a potential for liquefaction for horizontal ground conditions was developed originally
by Professor H.B. Seed and his co-workers (Seed and Idriss, 1971; Seed, 1979b; Seed and
Idriss, 1982; Seed et al., 1985b; Seed and De Alba, 1986). It was reviewed by a panel of
experts and reported in USNRC (1985).
The method is semi-empirical and is based on the maximum acceleration induced by the
earthquake a
max
, the SPT “N” value corrected for the SPT hammer energy and for over-
burden pressure (N
1
)
60
, earthquake magnitude (M), and fines content of the soil (% pass-
ing 0.075 mm). It is based on recorded cases of liquefaction during earthquakes in USA,
Japan and China.
Since then, there has been a gradual development of refinements of the method, including
greater application of the cone penetration test, modification to earthquake magnitude
corrections and to the corrections for high overburden stress, static shear stresses and age
of the soil deposit.
In 1996 a NCEER workshop on the evaluation of Liquefaction Resistance of Soils was
attended by 21 experts, who reached a consensus report on the state of the practice at the
time. This is reported in NCEER (1997) and Youd et al. (2001).
While there have been some further developments since 1997, they have not significantly
altered the understanding of the method, so what is produced here is a summary of the pro-
cedure as set out in Youd et al. (2001). Readers are urged to read that paper and to seek
later papers to keep up with current practice.
It should be noted that the original method was applicable to level or gently sloping
ground, underlain by Holocene age alluvial or fluvial sediments to a shallow depth
(
15 m). NCEER (1997) and Youd et al. (2001) say their paper should be applied only to
these conditions but give corrections factors for larger overburden stresses and for situa-
tions where there are static shear stresses, such as in dams and their foundations.
In reality, the method is used extensively for dams and their foundations, but readers
need to be aware of the larger uncertainties which apply in these conditions than for level
ground and should seek expert advice for important decision making, particularly in mar-
ginal cases.
12.4.3.2
The simplified method - outline
The method requires the calculation or estimation of two variables for evaluation of liq-
uefaction resistance of soils:
(a) The Cyclic Stress Ratio, CSR, which is a measure of the cyclic load applied to the soil
by the earthquake;
(b) The Cycle Resistance Ratio, CRR, which is the capacity of the soil to resist liquefaction.
The CRR is estimated from Standard Penetration Tests (SPT). Cone Penetration Tests
(CPT) or, less frequently, the shear wave velocity.
If the CSR is greater than the CRR, liquefaction is likely to occur.
For most purposes a simple comparison of CSR with CRR with engineering judgement
will be sufficient. However there are methods available which allow for the uncertainty in
the boundary between liquefiable and non liquefiable soils. These include Liao, Veneziano
and Whitman (1988) and Youd and Noble (1997). The latter indicate that the base curve
