Solving Stress-Deformation Problems with Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 745

Figure 14.17 Adjustment of laboratory test data to compensate for compressibility of oedometer
apparatus.

initial void ratio. The horizontal line should then gradually
curve downward and join the recompression curve adjusted
for the compressibility of the apparatus.

The corrected swelling pressure can be determined using
the following procedure. Locate the point of maximum cur-
vature where the void ratio versus pressure curve bends
downward onto the recompression branch (Fig. 14.18). It
is also suggested that the point of maximum curvature be
located as the bisector of a horizontal line through the ini-
tial void ratio and the slope of the recompression curve,
as shown in Fig. 14.19. A horizontal line and a tangen-
tial line are drawn at the point of maximum curvature. The
corrected swelling pressure is designated as the intersection
of the bisector of the angle formed by the horizontal and
tangential lines and a line tangential to the recompression
curve. The recompression line can be determined from the
adjusted rebound curve and drawn tangent to the loading
curve.
The need to apply a sampling disturbance correction to the
constant-volume swelling pressure measured in the labora-
tory is revealed in several ways. First, it would be anticipated
that such a correction is necessary as a result of early soil
mechanics experiences in determining the preconsolidation
pressure for saturated clay soils. Second, attempts to use
swelling pressure meaurements that have not been corrected
for sampling disturbance have resulted in predictions of total
heave that are too low. Predictions of heave made using
corrected swelling pressures are often as much as twice

14.4.9 Correction for Sampling Disturbance
A correction for sampling disturbance can be applied to
the measured compression curve that has been adjusted for
compressibility of the oedometer. Sampling disturbance
increases the compressibility of the soil. The increase in
compressibility is most clearly observed when loading goes
from the in situ void ratio onto the recompression curves
and when the loading goes from the recompression curve
onto the virgin compression branch. Sampling disturbance
does not allow the laboratory specimen to return to its in
situ state of stress at its in situ void ratio.
Casagrande (1936) proposed an empirical construction
that can be applied to the laboratory-measured compression
curve for the determination of preconsolidation pressure.
The empirical construction was necessary in order to account
for the effect of sampling disturbance. Other construction
procedures have also been proposed for the determination
of preconsolidation pressure (Schmertmann, 1955). A
modification of Casagrande's construction is suggested for
determining the corrected swell pressure when the constant-
volume test procedure is used.