Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 244

loading. It results from the tendency of soils to decrease in volume when subjected
to shearing stresses. Liquefaction occurs when the soil beneath the surface actually
behaves like a fluid under strong shaking (Brandes, 2003, Yeats and Gath, 2004).
Liquefaction refers to a process resulting in a loss of shear strength of soil due to
the excess of pore water pressure and to the cyclical shearing. The soil particles
initially have large voids between them. Due to shaking, the particles are relatively
displaced against each other and tend to more tightly pack, decreasing the void
volume. The water, which occupies the voids, being incompressible, comes under
increasing pressure and migrates upward to the surface, where the pressure is
relieved (Fig. 7.23) (Dobry, 1989).
As far as the structural behavior is concerned, the liquefaction during the
earthquakes produces the buoyant rise of buried structures such as tanks, ground
settlement and inclination of buildings (see also Figure 3.6) and failure of retaining
walls.
The failure of ground by large displacements due to the liquefaction can be
vertical, horizontal or a combination. During the failure, the surface layer
commonly breaks into large blocks (Fig. 7.24), which move back and forth and up
and down in the form of ground waves, as they progressively migrate horizontally
(Youd, 2003). Ground oscillation occurs on the flat terrain in response to inertial
forces within the broken blocks.
Figure 7.23 Effects of liquefaction: Migration of water toward the surface and
inclination of buildings (modified after Dobry, 1989)