Pore Pressure Coefficients
Liquefaction is a
phenomenon in which loose saturated sand loses a large percentage of its shear
strength and develops characteristics similar to those of a liquid. It is
usually induced by cyclic loading of relatively high frequency, resulting in
undrained conditions in the sand. Cyclic loading may be caused, for example, by
vibrations from machinery and, more seriously, by earth tremors.
Loose sand tends to
compact under cyclic loading. The decrease in volume causes an increase in pore
water pressure which cannot dissipate under undrained conditions. Indeed, there
may be a cumulative increase in pore water pressure under successive cycles of
loading. If the pore water pressure becomes equal to the maximum total stress
component, normally the overburden pressure, the value of effective stress will
be zero, i.e. inter particle forces will be zero, and the sand will exist in a
liquid state with negligible shear strength. Even if the effective stress does
not fall to zero the reduction in shear strength may be sufficient to cause
develop at any depth in a sand deposit where a critical combination of in-situ
density and cyclic deformation occurs. The higher the void ratio of the sand
and the lower the confining pressure the more readily liquefaction will occur.
The larger the strains produced by the cyclic loading the lower the number of
cycles required for liquefaction.
PORE PRESSURE COEFFICIENTS
Pore pressure coefficients are used to
express the response of pore water pressure to changes in total stress under
undrained conditions and enable the initial value of excess pore water pressure
to be determined. Values of the coefficients may be determined in the
laboratory and can be used to predict pore water pressures in the field under
similar stress conditions.
Consider an element of soil, of volume V
and porosity n, in equilibrium under total principal stresses ? s1? s2,
? s3, as shown in Figure,
the pore pressure being uo. The element is subjected to equal increases in
total stress ? s3
in each direction, resulting in an immediate increase u3 in pore pressure.