Civil Engineering Reference
In-Depth Information
11.4 Application of consolidation test results
The range of pressure generally considered in a settlement analysis is the increase from p
1
(the existing
vertical effective overburden pressure) to p
2
(the vertical effective pressure that will operate once the
foundation load has been applied and consolidation has taken place), so that in the previous discussion
e
1
represents the void ratio corresponding to the effective overburden pressure and e
2
represents the final
void ratio after consolidation. In some text books and papers the initial void ratio, e
1
, is given the symbol e
0
.
Obtaining a test sample entails removing all of the stresses which are applied to it, this reduction
in effective stress causing the sample to either swell or develop negative pore water pressures within
itself. Owing to the restraining effect of the sampling tube most soil samples tend to have a negative
pore pressure.
In the consolidation test the sample is submerged in water to prevent evaporation losses, with the result
that the negative pore pressures will tend to draw in water and the sample consequently swells. To obviate
this effect the normal procedure is to start the test by applying the first load increment and then to add
the water, but if the sample still tends to swell an increased load increment must be added and the test
readings started again. The point e
1
is taken to be the position on the test e-p curve that corresponds
to the effective overburden pressure at the depth from which the sample was taken; in the case of a
uniform deposit various values of e
1
can be obtained for selected points throughout the layer by reading
off the test values of void ratio corresponding to the relevant effective overburden pressures. Generally
the test e-p curve lies a little below the actual
in situ
e-p curve, the amount of departure depending upon
the degree of disturbance in the test sample. Bearing in mind the inaccuracies involved in any analysis,
this departure from the consolidation curve will generally be of small significance unless the sample is
severely disturbed and most settlement analyses are based on the actual test results.
An alternative method, mainly applicable to overconsolidated clays, was proposed by Schmertmann
(
1953)
, who pointed out that e
1
must be equal to wG
s
, where w is the
in situ
water content at the point
considered, and that in a consolidation test on an ideal soil with no disturbance, the void ratio of the
sample should remain constant at e
1
throughout the pressure range from zero to the effective overburden
pressure value. Schmertmann found that the test e-p curve tends to cut the
in situ
virgin consolidation
curve at a void ratio value somewhere between 37 and 42% of e
1
and concluded that a reasonable figure
for this intersection is e
=
0.42e
1
.
In order to obtain the corrected curve, with disturbance effects removed, the test sample is either
loaded through a pressure range that eventually reduces the void ratio of the sample to 0.42e
1
or else
the test is extended far enough for extrapolated values to be obtained, at least one cycle of expansion
and recompression being carried out during the test. The approximate value of the preconsolidation
pressure is obtained and the test results are put in the form of a semi-log plot of void ratio to log p (Fig.
11.10b
). The value of e
1
is obtained from wG
s
, w being found from a separate test sample (usually cuttings
obtained during the preparation of the consolidation test sample). It is now possible to plot on the test
curve (point A) and a horizontal line (AB) is drawn to cut the ordinate of the existing overburden pressure
at point B; a line BC is next drawn parallel to the mean slope of the laboratory rebound curve to cut the
preconsolidation pressure ordinate at point C, and the value of void ratio equal to 0.42e
1
is obtained and
established on the test curve (point D). Finally points C and D are joined. The corrected curve therefore
consists of the three straight lines: AB (parallel to the pressure axis with a constant void ratio value e
1
),
BC (representing the recompression of the soil up to the preconsolidation pressure), and CD (representing
initial compression along the virgin consolidation line).
Apart from the elimination of disturbance effects the method is useful because it permits the use of a
formula similar to the compression index of a normally consolidated clay:
C
p
p
H
2
ρ
c
=
log
10
1
+
e
1
1
where C is the slope of the corrected curve (generally recompression). If the pressure range extends into
initial compression the calculation must be carried out in two parts using the two different C values.
