Civil Engineering Reference
In-Depth Information
which must adjust to maintain constant water pressure at any horizon in the ground.
Since the air is continuous air pressures are constant throughout the soil. Pore air
pressures and pore water pressures are not equal. The degree of saturation over which
both water and air are continuous varies typically between about
S
r
=
0.25 to about
S
r
=
0.85 and this is the situation most common in practice.
26.5 Pore pressure and suction in unsaturated soil
The very important principle of effective stress was discussed in Chapter 6. This states
that there is an effective stress which controls the behaviour of soil including strength
and stiffness. The effective stress
σ
in saturated soil is related to the total stress
σ
and
the pore pressure
u
by
σ
=
σ
−
u
(26.7)
In an unsaturated soil the simple effective stress principle and equation (Eq. (26.7))
does not work. There must be some stress which controls its behaviour and this ought
to be some combination of total stress
σ
, pore water pressure
u
w
, pore air pressure
u
a
and degree of saturation.
Current practice is to consider the net stress and the matrix suction defined as:
net stress
=
(
σ
−
u
a
)
(26.8)
matrix suction
=
(
u
a
−
u
w
)
(26.9)
If the pore air pressure is atmospheric (i.e.
u
a
0) the net stress is the same as the
total stress and a negative pore water pressure gives rise to a numerically equal positive
suction. There is not, at present, a simple and satisfactory theory which can be used
to determine soil behaviour from the net stress and the matrix suction or the degree of
saturation and this is the subject of much current research.
=
26.6 Desaturation and water retention
If a saturated soil is subjected to an increasing suction there will be a critical suction
at which the water cavitates or boils and water vapour forms bubbles. In normal
circumstances water cavitates at room temperature at a suction of about 100 kPa but
in fine grained soils the water in the very small pore spaces can sustain much larger
suctions without cavitation.
If the suction is increased still further there will be a critical suction at which air is
drawn into the pore spaces. The suction at which air can enter the soil depends on
the size of the pore spaces. The analysis is similar to that in Sec. 6.4 for suctions in
saturated soil. Figure 26.4 shows the surface of the soil. The pore water pressure is
u
w
, the pressure in the external air is
u
a
, the diameter of the pore spaces in the soil is
d
v
1)
d
s
where
d
s
is the mean grain diameter and
T
is the surface tension force
between water and the material of the soil. For equilibrium
=
(
v
−
d
v
4
u
w
)
π
T
π
d
v
=
(
u
a
−
(26.10)
