Environmental Engineering Reference
In-Depth Information
soft to medium consistency clays from the horizontal excitation will be essentially pure
shear (Zeevaert, 1972).
Increased settlements
of existing structures may result. An example of partial liquefaction
is a building in Mexico City founded on soft silty clays that was not undergoing signifi-
cant settlements until the July 28, 1957 earthquake. During the quake the building settled
5 cm and continued to settle for years afterward at rates of 3 to 5 cm/year. The shear forces
from the earthquake-induced seismic waves reduced the shear strength of the clay and
significantly increased compressibility by the phenomenon of “strain softening”.
Rupture of foundation members
may result as the seismic shear forces cause buildings to
translate horizontally, imposing high earth pressures on walls and bending forces on piles
and piers, especially where soft clays are penetrated. Softening of the clay due to the cyclic
strains may be a factor in inducing rupture.
Occurrence of Liquefaction
Geographic Distribution
Incidence of liquefaction is not great in comparison with the large number of earthquakes
that occur annually. Studies of earthquake records have produced relatively few cases
where liquefaction was reported, even though the records extended back to 1802 (Seed,
1975; Christian and Swiger, 1975). Known cases of liquefaction were reported for 13 loca-
tions of which two were earth dams. Magnitudes were generally greater than 6.3.
Japan
: Mino Qwari (1891), Tohnankai (1944), Fukui (1948), Niigata (1964), and
Tokachioki (1968)
●
United States
: Santa Barbara (1925, the Sheffield Dam), El Centro (1940), San
Francisco (1957), San Fernando (1971), Van Norman Reservoir Dam, San
Francisco (1971)
●
Others
: Chile (1960), Alaska (1964), and Caracas (1967)
●
Since the 1975 reports, liquefaction has been reported to have occurred in Loma Preita
(1989), California, Kobe, Japan (1994), and Izmit, Turkey (1999). In recent years, the poten-
tial for liquefaction and analytical procedures have received considerable attention from
investigators. Many municipalities have prepared liquefaction zonal maps.
Geologic Factors and Susceptibility
Geologic factors
influencing the susceptibility to liquefaction include sedimentation
processes, age of deposition, geologic history, water table depth, gradation, burial depth,
ground slope, and the nearness of a free face.
The potential
susceptibility
for soils of various geologic origins (see
Chapter 7)
in terms
of age are summarized in
Table 11.10.
Susceptibility is seen to decrease as the age of the
deposit, which reflects prestressing by removal of overburden or densification by ancient
earthquakes, increases. The greatest susceptibility is encountered in coastal areas where
saturated fine-grained granular alluvium predominates, often with limited confinement,
and where recent alluvium appears more susceptible than older alluvia. Offshore lique-
faction must be considered since the seafloor can become unstable from earthquakes or
wave forces during large storms (see
Section 11.3.4)
.
Factors of Liquefaction Potential
General
Gradation
: As shown in
Figure 11.34,
fine sands and silty sands are most susceptible, espe-
cially when they are poorly graded. Permeability is relatively low and drainage slow.
