Civil Engineering Reference
In-Depth Information
The time taken for consolidation to occur in the triaxial test generally gives a good indication of the
necessary rate of strain for the undrained shear part of the test, but it is not advisable to use this time to
determine c
v
unless there are no filter drains.
The consolidation characteristics of a partially saturated soil are best obtained from the triaxial test,
which can give the initial pore water pressures and the volume change under undrained conditions. Having
applied the cell pressure and noted these readings, the pore pressures within the sample are allowed to
dissipate while further pore pressure measurements are taken; the accuracy of the results obtained is much
greater than with the consolidation test as the difficulty of fitting the theoretical and test curves when air
is present is largely removed. The dissipation test is described by Bishop and Henkel (
1962)
.
Example 12.1:
Consolidation test
Results obtained from a consolidation test on a clay sample for a pressure increment of
100-200 kPa were:
Thickness of sample (mm)
Time (min)
12.200
0
12.141
1
4
12.108
1
12.075
2
4
12.046
4
11.985
9
11.922
16
11.865
25
11.827
36
11.809
49
11.800
64
(i) Determine the coefficient of consolidation of the soil.
(ii) How long would a layer of this clay, 10 m thick and drained on its top surface only,
take to reach 75% primary consolidation?
(iii) If the void ratios at the beginning and end of the increment were 0.94 and 0.82
respectively, determine the value of the coefficient of permeability.
Solution:
(i) The first step is to determine t
90
. The thickness of the sample is plotted against the
square root of time (Fig.
12.8)
and if necessary the curve is corrected for zero error
to establish the point E. The 1.15 line is next drawn from E and where it cuts the
test curve (point F) it gives √t
90
=
6.54. Hence t
90
=
42.7 min.
c t
H
v
T
=
2
Mean thickness of sample during increment (corrected initial thickness 12.168)
