Solving Stress-Deformation Problems with Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 765

Figure 14.47 Void ratio changes corresponding to corrected and uncorrected swelling pressures.

soil problem also increases when the swelling pressure is
increased. The change in void ratio corresponding to the
corrected swelling pressure can be computed from Eq. 14.14
by substituting P s for P 0 (also see Fig. 14.47):

The above equation is shown graphically in Fig. 14.48
for various ratios of swelling pressure, P s /P s and P f /P s .In
other words, the initial and final stress states are referenced
to the uncorrected swelling pressure. The plot shows that
the relative difference between the two heave predictions
increases as the difference between the corrected and uncor-
rected swelling pressures increases (i.e., as P s /P s increases).
It is not uncommon for the corrected swelling pressure to
be two to three times greater than the uncorrected swelling
pressure (i.e., Fredlund, 1983). The relative difference in
heave also increases as the overburden pressure increases
(i.e., P f /P s increases). The difference in the prediction of
total heave can also be substantially different depending on
whether the corrected or uncorrected swelling pressure is
used in the analysis.

log P f
P s

e =

C s (14.29)

The change in void ratio corresponding to the uncorrected
swelling pressure P s is written as

log P f
P s

e = C s (14.30)

The difference in heave or in the change in void ratio can
be obtained by subtracting Eq. 14.30 from Eq. 14.29:

14.5.13 Example with Wetting from Top
to Specified Depth
Figure 14.49 shows the variables involved in studying the
effect of wetting of the soil from the ground surface to a
specified depth (e.g., by flooding the surface). For example,
an insufficient amount of water infiltration into the ground
may result in the wetting of only a portion of the active
depth. Let H r be the portion of the active depth that has
been wetted (i.e., rH ). The wetted zone is also subdivided
into j number of layers for the computation of total heave.

log P s
P s

e =

e − C s (14.31)

Referencing the difference between the two void ratio
changes to the change in void ratio when using the corrected
swelling pressure gives

log P s P s
log P f P s

e
e

e −

= (14.32)