Civil Engineering Reference
In-Depth Information
there is no change of pore pressure or the pore pressures change from one steady state
to another, both of which are determinable. During an event which is undrained there
is no change of water content but pore pressures change and are unknown; in saturated
soil there is no change of volume.
Whether a particular loading or unloading event is drained or undrained depends
on the rate of loading and the rate at which drainage can occur; this depends on,
among other things, the permeability. Table 6.2 gives typical values for the coefficient
of permeability for soils with different gradings and Table 6.3 gives durations of typical
engineering constructions. In both cases the variations are very large.
Drained loading: effective stress parameters
If the soil is coarse grained and the permeability is relatively large, or if the rate of load-
ing is relatively slow, drainage will occur during the period of construction. This case
is called drained. Pore pressures are in equilibrium and can be found from the position
of the water table or from a steady state seepage flownet, as described in Chapter 14.
Since pore pressures are known effective stresses can be determined. These analyses
are known as effective stress analyses and the parameters are known as effective stress
parameters. Typical effective stress parameters include the critical state friction angle
φ
c
and the bulk modulus K
.
Undrained loading: total stress parameters
If the soil is fine grained and the permeability is relatively small or if the rate of loading
is relatively fast there will be no drainage during the period of construction. This case is
called undrained. Pore pressures are unknown but, in saturated soil, there is no change
of volume. Since pore pressures are unknown only total stresses can be determined.
These analyses are known as total stress analyses and the parameters are known as
total stress parameters. Typical total stress parameters include the undrained strength
s
u
and the undrained Young's Modulus
E
u
. In saturated soil there is no volume change
so the undrained bulk modulus
K
u
must be infinite and, from Eq. (3.31), the undrained
Poisson's ratio
ν
u
must be 0.5.
Consolidation
If the loading is undrained pore pressures in the vicinity of the structure change but pore
pressures far from the structure do not. As a result there will be hydraulic gradients
and seepage of water until the pore pressures are everywhere in equilibrium. This will
cause changes of effective stress and ground movements. This process is consolidation
(see Chapter 15) and you will need to obtain parameters for analyses of movements
due to consolidation.
In most cases in practice the loading or unloading will be neither fully drained nor
fully undrained and there will be some drainage and some pore pressure changes during
construction. There are complex coupled analyses which account for partial drainage,
but for routine geotechnical design it is necessary to consider the soil to be either
fully drained, in which case the analyses will be effective stress, or fully undrained, in
