Civil Engineering Reference
In-Depth Information
6.1 INTRODUCTION
This chapter deals with the liquefaction of soil. An introduction to liquefaction was pre-
sented in Sec. 3.4. The concept of liquefaction was first introduced by Casagrande in the
late 1930s (also see Casagrande 1975).
As mentioned in Sec. 3.4, the typical subsurface soil condition that is susceptible to liq-
uefaction is a loose sand, which has been newly deposited or placed, with a groundwater
table near ground surface. During an earthquake, the application of cyclic shear stresses
induced by the propagation of shear waves causes the loose sand to contract, resulting in an
increase in pore water pressure. Because the seismic shaking occurs so quickly, the cohe-
sionless soil is subjected to an undrained loading (total stress analysis). The increase in pore
water pressure causes an upward flow of water to the ground surface, where it emerges in
the form of mud spouts or sand boils. The development of high pore water pressures due to
the ground shaking and the upward flow of water may turn the sand into a liquefied condi-
tion, which has been termed
liquefaction.
For this state of liquefaction, the effective stress
is zero, and the individual soil particles are released from any confinement, as if the soil
particles were floating in water (Ishihara 1985).
Structures on top of the loose sand deposit that has liquefied during an earthquake will
sink or fall over, and buried tanks will float to the surface when the loose sand liquefies
(Seed 1970). Section 3.4 has shown examples of damage caused by liquefaction. Sand
boils, such as shown in Fig. 3.19, often develop when there has been liquefaction at a site.
After the soil has liquefied, the excess pore water pressure will start to dissipate. The
length of time that the soil will remain in a liquefied state depends on two main factors:
(1) the duration of the seismic shaking from the earthquake and (2) the drainage conditions
of the liquefied soil. The longer and the stronger the cyclic shear stress application from the
earthquake, the longer the state of liquefaction persists. Likewise, if the liquefied soil is
confined by an upper and a lower clay layer, then it will take longer for the excess pore
water pressures to dissipate by the flow of water from the liquefied soil. After the lique-
faction process is complete, the soil will be in a somewhat denser state.
This chapter is devoted solely to
level-ground liquefaction.
Liquefaction can result in
ground surface settlement (Sec. 7.2) or even a bearing capacity failure of the foundation
(Sec. 8.2). Liquefaction can also cause or contribute to lateral movement of slopes, which
is discussed in Secs. 9.4 and 9.5.
6.2
LABORATORY LIQUEFACTION STUDIES
The liquefaction of soils has been extensively studied in the laboratory. There is a consider-
able amount of published data concerning laboratory liquefaction testing. This section pre-
sents examples of laboratory liquefaction data from Ishihara (1985) and Seed and Lee (1965).
6.2.1
Laboratory Data from Ishihara
Figures 6.1 and 6.2 (from Ishihara 1985) present the results of laboratory tests performed on
hollow cylindrical specimens of saturated Fuji River sand tested in a torsional shear test
apparatus. Figure 6.1 shows the results of laboratory tests on a saturated sand having
a medium density (
D
r
47 percent), and Fig. 6.2 shows the results of laboratory tests
on a saturated sand in a dense state (
D
r
75 percent). Prior to the cyclic shear testing,
both soil specimens were subjected to an effective confining pressure
0
of 98 kN/m
2
(2000 lb/ft
2
). The saturated sand specimens were then subjected to undrained conditions
