Civil Engineering Reference
In-Depth Information
Fig. 11.17
Typical effective stress paths obtained from consolidated undrained triaxial tests on a normally
consolidated clay.
The K
o
line
For a soil undergoing one-dimensional consolidation the ratio
′
′
σ
3
/
is again constant and its value is given
the symbol K
0
. Plotting the maximum shear stress points of these stress circles enables the stress path for
one-dimensional consolidation, the K
0
line, to be determined; this line is inclined at angle
β
to the normal
stress axis.
K
0
is the coefficient of earth pressure at rest. For consolidation work K
0
may be defined for a soil with
a history of one-dimensional strain as the ratio:
Lateral effective stress
Vertical effective stress
K
0
=
11.8.2 Stress paths in the oedometer
Figure
11.18
shows the stress conditions that arise during and after the application of a pressure incre-
ment in the consolidation test. Initially the sample has been consolidated under a previous load and the
pore pressure is zero; the Mohr circle is represented by (p, q) the point X, circle I. As soon as the vertical
pressure increase,
Δ
σ
1
, is applied, the total stresses move from X to Y (circle 1). As the soil is saturated
Δ
u
=
Δ
σ
1
and the effective stress circle is still represented by point X. As consolidation commences the
pore water pressure,
Δ
u, begins to decrease and
∆
′
σ
1
begins to increase. The consolidation is one-
dimensional and therefore an increase in the major principal effective stress,
∆
′
σ
1
, will induce an increase
′
=
′
in the minor principal effective stress
∆
3 0
K
. Hence the effective stress circles move steadily towards
point Z (circles II, III and IV), where Z represents full consolidation.
The total stress circles can be determined from a study of the effective stress circles. For example the
difference between
Δ
σ
1
and
∆
′
σ
∆
σ
σ
1
for circle III represents the pore water pressure within the sample at that
time: hence
Δ
σ
3
at this stage in the consolidation is
∆
′
σ
3
for circle III plus the value of the pore water
pressure. It can be seen therefore that
Δ
u decreases with consolidation and the size of the Mohr circle
for total stress increases until the point Z is reached (circles 2, 3 and 4). Obviously circles 4 and IV are
coincident.
