2. Characteristics and Motion of Sédiments
33
2.14.2 Soil types susceptible to liquéfaction
Most liquéfaction studies hâve centered on sands as the liquéfiable material.
Again, this is probably the direct resuit of earthquake-induced liquéfaction
studies. Earthquake shocks are of high frequency and short duration, and
they can induce high stresses in the soils. Sands are the materials, which
seem most readily to liquefy under these conditions. Sands, of course, are
cohésion less materials, that is, they hâve no shear strength at zéro effective
stress, and it is this property, which allows liquéfaction to occur. Sédiments
which are predominantly silt with little or no clay are also cohésion less; this
type of material was involved in the problems mentioned earlier regarding
the 3 m diameter pipe. Generally, clays are not susceptible to liquéfaction,
although the Mississippi Delta material, which lost strength during storms
to the point that instruments sank, was clay; however, it was extremely
weak (un-drained shear strength of approximately 366kg/m2).
By and large, sands and coarse silts should be suspected of being
liquefaction-prone materials. The in situ state of denseness must also be
considered in the détermination of liquéfaction. Loose materials, especially
those in a metastable particle arrangement, will liquefy more readily than
dense ones, but if a storm is long enough (with a large enough number of
wave-induced stress cycles), even dense materials are capable of liquefying.
[It is emphasized that the analyses and test methods, which follow, rather
than empirical methods, should détermine liquéfaction potential, since most
empirical methods were developed for earthquake analyses].
2.15 Shields Curve
It was shown by Shields [1936] that critical shear stress value could be
expressed as a function of the Reynolds number.
, _Tc\
= /(fie)
(2.27)
\Pa
where,
Re' . Reynolds number = v*D/v
Tcr: critical shear stress
v: kinematic viscosity
u*: shear stress velocity.
From Eq. (2.27), the critical shear stress (rcr) can be found by an itérative process. In Fig. 2.4 Unes are drawn indicating where, the above équation
holds for one particular grain diameter. With the help of these fines it is
33
2.14.2 Soil types susceptible to liquéfaction
Most liquéfaction studies hâve centered on sands as the liquéfiable material.
Again, this is probably the direct resuit of earthquake-induced liquéfaction
studies. Earthquake shocks are of high frequency and short duration, and
they can induce high stresses in the soils. Sands are the materials, which
seem most readily to liquefy under these conditions. Sands, of course, are
cohésion less materials, that is, they hâve no shear strength at zéro effective
stress, and it is this property, which allows liquéfaction to occur. Sédiments
which are predominantly silt with little or no clay are also cohésion less; this
type of material was involved in the problems mentioned earlier regarding
the 3 m diameter pipe. Generally, clays are not susceptible to liquéfaction,
although the Mississippi Delta material, which lost strength during storms
to the point that instruments sank, was clay; however, it was extremely
weak (un-drained shear strength of approximately 366kg/m2).
By and large, sands and coarse silts should be suspected of being
liquefaction-prone materials. The in situ state of denseness must also be
considered in the détermination of liquéfaction. Loose materials, especially
those in a metastable particle arrangement, will liquefy more readily than
dense ones, but if a storm is long enough (with a large enough number of
wave-induced stress cycles), even dense materials are capable of liquefying.
[It is emphasized that the analyses and test methods, which follow, rather
than empirical methods, should détermine liquéfaction potential, since most
empirical methods were developed for earthquake analyses].
2.15 Shields Curve
It was shown by Shields [1936] that critical shear stress value could be
expressed as a function of the Reynolds number.
, _Tc\
= /(fie)
(2.27)
\Pa
where,
Re' . Reynolds number = v*D/v
Tcr: critical shear stress
v: kinematic viscosity
u*: shear stress velocity.
From Eq. (2.27), the critical shear stress (rcr) can be found by an itérative process. In Fig. 2.4 Unes are drawn indicating where, the above équation
holds for one particular grain diameter. With the help of these fines it is