Civil Engineering Reference
In-Depth Information
3.
Soil type:
In terms of the soil types most susceptible to liquefaction, Ishihara
(1985) states: “The hazard associated with soil liquefaction during earthquakes has been
known to be encountered in deposits consisting of fine to medium sand and sands contain-
ing low-plasticity fines. Occasionally, however, cases are reported where liquefaction
apparently occurred in gravelly soils.”
Thus, the soil types susceptible to liquefaction are nonplastic (cohesionless) soils. An
approximate listing of cohesionless soils from least to most resistant to liquefaction is clean
sands, nonplastic silty sands, nonplastic silt, and gravels. There could be numerous excep-
tions to this sequence. For example, Ishihara (1985, 1993) describes the case of tailings
derived from the mining industry that were essentially composed of ground-up rocks and
were classified as rock flour. Ishihara (1985, 1993) states that the rock flour in a water-sat-
urated state did not possess significant cohesion and behaved as if it were a clean sand.
These tailings were shown to exhibit as low a resistance to liquefaction as clean sand.
Seed et al. (1983) stated that based on laboratory testing and field performance, the
great majority of silts and clays will not liquefy during earthquakes. An exact dividing line
between liquefiable and non-liquefiable cohesive soils does not exist. Some guidelines
are as follows (Seed and Idriss 1982; Youd and Gilstrap 1999; Bray et al. 2004; Bray and
Sancio 2006; Boulanger and Idriss 2006):
●
There must be significant earthquake shaking, and the cohesive soil should be non-
cemented and in a loose state.
●
The cohesive soil should have low plasticity characteristics, such as a plasticity index
less than about 7 (PI
7).
●
The water content should be high relative to the liquid limit, such as a liquidity index
near or greater than one.
Even if the cohesive soil does not liquefy, there can still be the possibility of a signifi-
cant undrained shear strength loss due to the seismic shaking. The loss of strength due to
cycles of seismic loading has been termed cyclic softening.
4.
Soil relative density D
r
:
Based on field studies, cohesionless soils in a loose rela-
tive density state are susceptible to liquefaction. Loose nonplastic soils will contract during
the seismic shaking which will cause the development of excess pore water pressures. As
indicated in Sec. 6.2, upon reaching initial liquefaction, there will be a sudden and dramatic
increase in shear displacement for loose sands.
For dense sands, the state of initial liquefaction does not produce large deformations
because of the dilation tendency of the sand upon reversal of the cyclic shear stress. Poulos
et al. (1985) state that if the in situ soil can be shown to be dilative, then it need not be eval-
uated because it will not be susceptible to liquefaction. In essence, dilative soils are not sus-
ceptible to liquefaction because their undrained shear strength is greater than their drained
shear strength.
5.
Particle size gradation:
Uniformly graded nonplastic soils tend to form more unsta-
ble particle arrangements and are more susceptible to liquefaction than well-graded soils.
Well-graded soils will also have small particles that fill in the void spaces between the large
particles. This tends to reduce the potential contraction of the soil, resulting in less excess pore
water pressures being generated during the earthquake. Kramer (1996) states that field evi-
dence indicates that most liquefaction failures have involved uniformly graded granular soils.
6.
Placement conditions or depositional environment:
Hydraulic fills (fill placed
under water) tend to be more susceptible to liquefaction because of the loose and segregated
soil structure created by the soil particles falling through water. Natural soil deposits
formed in lakes, rivers, or the ocean also tend to form a loose and segregated soil structure
and are more susceptible to liquefaction. Soils that are especially susceptible to liquefac-
tion are formed in lacustrine, alluvial, and marine depositional environments.
