Earthquakes - Geotechnical Engineering Investigation - page 943

11.3.3 Subsidence and Liquefaction

General
Earthquake-induced vibrations can be the cause of several significant phenomena in soil
deposits, including:

Compaction of granular soils resulting in surface subsidence, which at times
occurs over very large areas

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Liquefaction of fine sands and silty sands, which results in a complete loss of
strength and causes structures to settle or even overturn and slopes to fail

●

Reduction in strength in soft, cohesive soils (strain softening), which results in the
settlement of structures that can continue for years and also results from a form
of liquefaction

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Subsidence from Compaction
Causes
Cyclic shear strains densify granular soils, resulting in subsidence. Horizontal motions
induced by shocks cause compaction as long as the cycles are relatively close together,
even if the cyclic shear strains are relatively small. Vertical accelerations in excess of 1 g are
required to cause significant densification of sands, which is far greater than most surface
accelerations during earthquakes. This has been demonstrated by laboratory tests
(Whitman and DePablo, 1969).

Susceptibility Factors
As noted in the discussion of liquefaction below, the susceptibility of soils to compaction
during ground shaking depends on soil gradation, relative density or void ratio, confin-
ing pressure, amplitude of cyclic shear stress or shear strain, and number of stress cycles
or duration.
Compaction subsidence and liquefaction are closely related; the major difference in
occurrence is the ability of the material to drain during cyclic loading. Compaction occurs
with good soil drainage.

Occurrence
New Madrid Events of 1811 and 1812 : Ground subsidence extended over enormous areas,
and was reported to be as high as 15 to 23 ft in the Mississippi valley.
Homer, Alaska (1964) : A deposit of alluvium 450 ft in original thickness subsided 4 ft
(Seed, 1970, 1975).
Niigata, Japan (1964) : Many structures underlain by sand settled more than 1 m (Seed,
1970, 1975).

The Liquefaction Phenomenon
Cyclic Liquefaction in Granular Soils
Defined : Cyclic liquefaction refers to the response of a soil, subjected to dynamic loads or
excitation by transient shear waves, which terminates in a complete loss of strength and
entry into a liquefied state. (Cyclic liquefaction differs from the liquefaction that occurs
during the upward flow of water under static conditions.)
Described : If a saturated sand is subjected to ground vibrations it tends to compact and
decrease in volume; if the sand cannot drain rapidly enough, the decrease in volume
results in an increase in pore pressure. When the pore pressure increases until it is equal