32
Coastal Engineering: Theory and Practice
Hence
7tH
1
«max(-d) - T ^kd
Tttmax(—d)
'R
H sinh kd
(2.24)
(2.25)
2.14 Liquéfaction of Sands
2.14.1 General
When saturated sand is subjected to loading without drainage, the pressure
in the pore fluid may, under certain circumstances, approach or equal the
total stress to which the sand is subjected. The total stress a minus the
pore pressure, pp is termed the effective stress a', or
a' = cr — pp
(2.26)
Under such condition, the effective stress approaches 0. Shear strength
and deformation of soils are controlled by the effective stress; as it
approaches 0, the shear strength likewise approaches 0. At this point the
sand becomes fluidized or liquéfaction occurs and the sand is unable to
support loads.
Under cyclic loading conditions, soils will expérience an increase in
strain with each load application. Laboratory studies hâve shown that
the pore pressures in saturated, un-drained sands build up progressively
under the action of these cyclic stresses. This is true for even dense sands,
although in dense sands, the magnitude of an increase in pore pressure with
each stress reversai may be quite small. However, with a sufficient number
of stress reversais, liquéfaction may occur, provided the excess pore water
pressures do not dissipate between load applications.
Cyclic stresses produced by waves can also induce pore pressures in
marine sédiments, and some evidence of liquéfaction under such conditions
has been observed. At any instant, the pore pressures in sédiment will
dépend on the State of denseness, the permeability of the sédiment, and
the induced cyclic stress ratios resulting from the waves. Permeability is
important because waves hâve much longer periods than earthquakes, and
the pore pressures can change due to redistribution within the sédiment as
well as dissipation at free-draining boundaries. Because the problem is one
of partial drainage rather than no drainage, it is more complicated than
the undrained earthquake problem.
Coastal Engineering: Theory and Practice
Hence
7tH
1
«max(-d) - T ^kd
Tttmax(—d)
'R
H sinh kd
(2.24)
(2.25)
2.14 Liquéfaction of Sands
2.14.1 General
When saturated sand is subjected to loading without drainage, the pressure
in the pore fluid may, under certain circumstances, approach or equal the
total stress to which the sand is subjected. The total stress a minus the
pore pressure, pp is termed the effective stress a', or
a' = cr — pp
(2.26)
Under such condition, the effective stress approaches 0. Shear strength
and deformation of soils are controlled by the effective stress; as it
approaches 0, the shear strength likewise approaches 0. At this point the
sand becomes fluidized or liquéfaction occurs and the sand is unable to
support loads.
Under cyclic loading conditions, soils will expérience an increase in
strain with each load application. Laboratory studies hâve shown that
the pore pressures in saturated, un-drained sands build up progressively
under the action of these cyclic stresses. This is true for even dense sands,
although in dense sands, the magnitude of an increase in pore pressure with
each stress reversai may be quite small. However, with a sufficient number
of stress reversais, liquéfaction may occur, provided the excess pore water
pressures do not dissipate between load applications.
Cyclic stresses produced by waves can also induce pore pressures in
marine sédiments, and some evidence of liquéfaction under such conditions
has been observed. At any instant, the pore pressures in sédiment will
dépend on the State of denseness, the permeability of the sédiment, and
the induced cyclic stress ratios resulting from the waves. Permeability is
important because waves hâve much longer periods than earthquakes, and
the pore pressures can change due to redistribution within the sédiment as
well as dissipation at free-draining boundaries. Because the problem is one
of partial drainage rather than no drainage, it is more complicated than
the undrained earthquake problem.