Civil Engineering Reference
In-Depth Information
FIGURE 6.3
Laboratory test data from cyclic triaxial tests performed on Sacramento River sand. The plot-
ted data represent the cyclic deviator stress versus number of cycles of deviator stress required to cause ini-
tial liquefaction and 20 percent axial strain. (
Initially developed by Seed and Lee 1965, reproduced from
Kramer 1996.
)
most common cause of liquefaction is due to the seismic energy released during an earth-
quake. The potential for liquefaction increases as the earthquake intensity and duration
of shaking increase. Those earthquakes that have the highest magnitude will produce
both the largest ground acceleration and the longest duration of ground shaking (see
Table 2.2).
Although data are sparse, there would appear to be a shaking threshold that is needed to
produce liquefaction. These threshold values are a peak ground acceleration
a
max
of about
0.10
g
and local magnitude
M
L
of about 5 (National Research Council 1985, Ishihara 1985).
Thus, a liquefaction analysis would typically not be needed for those sites having a peak
ground acceleration
a
max
less than 0.10
g
or a local magnitude
M
L
less than 5.
Besides earthquakes, other conditions can cause liquefaction, such as subsurface blast-
ing, pile driving, and vibrations from train traffic.
2.
Groundwater table:
The condition most conducive to liquefaction is a near-sur-
face groundwater table. Unsaturated soil located above the groundwater table will not liq-
uefy. If it can be demonstrated that the soils are currently above the groundwater table and
are highly unlikely to become saturated for given foreseeable changes in the hydrologic
regime, then such soils generally do not need to be evaluated for liquefaction potential.
At sites where the groundwater table significantly fluctuates, the liquefaction potential
will also fluctuate. Generally, the historic high groundwater level should be used in the liq-
uefaction analysis unless other information indicates a higher or lower level is appropriate
(Division of Mines and Geology 1997).
Poulos et al. (1985) state that liquefaction can also occur in very large masses of sands
or silts that are dry and loose and loaded so rapidly that the escape of air from the voids is
restricted. Such movement of dry and loose sands is often referred to as
running soil
or
run-
ning ground.
Although such soil may flow as liquefied soil does, in this text, such soil
deformation will not be termed liquefaction. It is best to consider that liquefaction only
occurs for soils that are located below the groundwater table.
