LIQUEFACTION - Geotechnical Earthquake Engineering - page 156

the soil liquefies (i.e., when u e 0 becomes equal to 1.0). If the sand had been tested in a

loose or very loose state, the loss of shear strength upon liquefaction would be even more
sudden and dramatic. For loose sand, this initial liquefaction when u e 0

becomes equal to
1.0 coincides with the contraction of the soil structure, subsequent liquefaction, and large
deformations. As such, for loose sands, the terms initial liquefaction and liquefaction have
been used interchangeably.
For dense sands, the state of initial liquefaction ( u e 0 1.0) does not produce large

deformations because of the dilation tendency of the sand upon reversal of the cyclic stress.
However, there could be some deformation at the onset of initial liquefaction, which is
commonly referred to as cyclic mobility.

6.2.2 Laboratory Data from Seed and Lee

Figure 6.3 (from Seed and Lee 1965) shows a summary of laboratory data from cyclic tri-
axial tests performed on saturated specimens of Sacramento River sand. Cylindrical sand
specimens were first saturated and subjected in the triaxial apparatus to an isotropic effec-
tive confining pressure of 100 kPa (2000 lb/ft 2 ). The saturated sand specimens were then
subjected to undrained conditions during the application of the cyclic deviator stress in the
triaxial apparatus (see Sec. 5.5.2 for discussion of cyclic triaxial test).
Numerous sand specimens were prepared at different void ratios ( e i initial void ratio).
The sand specimens were subjected to different values of cyclic deviator stress dc , and the
number of cycles of deviator stress required to produce initial liquefaction and 20 percent
axial strain was recorded. The laboratory data shown in Fig. 6.3 indicate the following:

1. For sand having the same initial void ratio e i and same effective confining pressure, the
higher the cyclic deviator stress dc , the lower the number of cycles of deviator stress
required to cause initial liquefaction.
2. Similar to item 1, for a sand having the same initial void ratio e i and same effective con-
fining pressure, the cyclic deviator stress dc required to cause initial liquefaction will
decrease as the number of cycles of deviator stress is increased.
3. For sand having the same effective confining pressure, the denser the soil (i.e., the lower
the value of the initial void ratio), the greater the resistance to liquefaction. Thus a dense
soil will require a higher cyclic deviator stress dc or more cycles of the deviator stress
in order to cause initial liquefaction, as compared to the same soil in a loose state.
4. Similar to item 3, the looser the soil (i.e., the higher the value of the initial void ratio),
the lower the resistance to liquefaction. Thus a loose soil will require a lower cyclic
deviator stress dc or fewer cycles of the deviator stress in order to cause initial lique-
faction, as compared to the same soil in a dense state.

6.3 MAIN FACTORS THAT GOVERN
LIQUEFACTION IN THE FIELD

There are many factors that govern the liquefaction process for in situ soil. Based on the
results of laboratory tests (Sec. 6.2) as well as field observations and studies, the most
important factors that govern liquefaction are as follows:

1. Earthquake intensity and duration: In order to have liquefaction of soil, there
must be ground shaking. The character of the ground motion, such as acceleration and
duration of shaking, determines the shear strains that cause the contraction of the soil par-
ticles and the development of excess pore water pressures leading to liquefaction. The