Civil Engineering Reference
In-Depth Information
Fig. 12.6
Typical consolidation test results.
in the sample. In the majority of tests this effect is absent and points A and B are coincident. Initial
compression is not considered to be due to any loss of water from the soil and should be treated as a
zero error for which a correction is made.
●
BC (primary compression)
All the compression in this part of the curve is taken as being due to the expulsion of water from the
sample, although some secondary compression will also occur. When the pore pressure has been
reduced to a negligible amount it is assumed that 100% consolidation has been attained.
●
CD (secondary compression)
The amount by which this effect is evident is a function of the test conditions and can hardly be related
to an
in situ
value.
The square root of time 'itting' method
It will be appreciated that the curve described above is an actual consolidation curve and would not be
obtainable from one of the theoretical curves of Fig.
12.4,
which can only be used to plot the primary
compression range. To evaluate the coefficient of consolidation it is necessary to establish the point C,
representing 100% primary consolidation, but it is difficult from a study of the test curve to fix C with
accuracy and a procedure in which the test curve is 'fitted' to the theoretical curve becomes necessary.
A method was described by Taylor (
1948)
. If the theoretical curve U against
√
T is plotted for the case
of a uniform initial excess pore pressure distribution, the curve will be like that shown in Fig.
12.7a
. Up
to values of U equal to about 60%, the curve is a straight line of equation U
=
1.13
√
T, but if this straight
line is extended to cut the ordinate U
=
90% the abscissa of the curve is seen to be 1.15 times the abscissa
of the straight line. This fact is used to fit the test and theoretical curves.
With the test curve a corrected zero must first be established by projecting the straight line part of the
primary compression back to cut the vertical axis at E (Fig.
12.6)
. A second line, starting through E, is now
drawn such that all abscissas on it are 1.15 times the corresponding values on the laboratory curve, and
the point at which this second line cuts the laboratory curve is taken to be the point representing 90%
primary consolidation (Fig.
12.7b
).
To establish c
v
, T
90
is first found from the theoretical curve that fits the drainage conditions (the curve
m
=
1); t
90
is determined from the test curve:
c t
H
v
90
2
T
=
90
i.e.
T H
t
2
90
c
v
=
90
