Civil Engineering Reference
In-Depth Information
Skempton (
1954
) showed that the ratio of the pore pressure change to the change in the total major
principal stress gives another pore pressure coefficient b:
∆
∆
u
∆
∆
σ
σ
∆
∆
σ
σ
3
3
= =
B
B
=
A
1
−
σ
1
1
1
The coefficient
B
can be used to determine the magnitude of pore pressures set up at any point in an
embankment if it is assumed that no drainage occurs during construction (a fairly reasonable thesis if the
construction rate is rapid). Now
u
z
γ
=
u
γ
i.e.
u
B
∆
σ
0
1
r
u
=
+
γ
z
γ
z
A reasonable assumption to make for the value of the major principal stress is that it equals the weight
of the material above the point considered. Hence
u
0
∆
σ
=
γ
z
and r
=
+
B
1
u
γ
z
For soils placed at or below optimum moisture content (see Chapter
14)
, u
0
is small and can even be
negative. Its effect is of little consequence and may be ignored
so that the analysis for stability at the end
of construction is often determin
e
d from the relationship
r
u
=
B
.
The pore pressure coefficient
B
is determined from a special stress path test known as a dissipation
test. Briefly, a sample of the soil is inserted in a triaxial cell and subjected to increases in the principal
stresses
Δ
σ
1
and
Δ
σ
3
of
m
agnitudes approximating to those expected in the field. The resulting pore
pressure is measured and
B
obtained.
Steady seepage
It is easy to determine r
u
from a study of the flow net (Fig.
13.14)
. The procedure is to trace the equipo-
tential through the point considered up to the top of the flow net, so that the height to which water would
rise in a standpipe inserted at the point is h
w
. Since u
=
γ
w
h
w
:
z
h
w
Upper flow line
Fig. 13.14
Determination of excess head at a point on a flow net.
