Solving Stress-Deformation Problems with Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 740

The laboratory information desired by the geotechnical
engineer for predicting the amount of heave is an assessment
of (1) the in situ state of stress and (2) the swelling properties
with respect to changes in net normal stress. An understand-
ing of the compressibility properties under increased loading
conditions may also be relevant. A demanding testing facil-
ity and program would be required to completely assess
all possible variables affecting volume change behavior in
expansive soils. It is important, however, to develop rela-
tively simple, rapid, and economical procedures to obtain
the basic information for calculating potential heave.
Several laboratory testing procedures have been used in
engineering practice to obtain the necessary information for
the prediction of heave. The one-dimensional oedometer test
is commonly used to determine the present in situ state of
stress. Free-swell and the constant-volume oedometer test
methods can be used along with associated correction pro-
cedures. There are several oedometer test procedures that
have been proposed for the measurement of the swelling
pressure of a soil. The constant-volume and free-swell test
procedures are first described followed by a presentation of
several comparative studies of swelling pressures measured
using other test procedures.
The oedometer test translates the in situ stress state onto
the net total stress plane. The in situ stress state is referred
to as the “corrected” swelling pressure P s , which is equal to
the sum of the overburden pressure and the matric suction
equivalent. Figure 14.10 shows typical laboratory oedometer
test results extending to the virgin compression branch. The
entire laboratory loading curve is often in its entirety on the
recompression portion with applied loads not even reaching
the preconsolidation pressure of the soil. In other words, the
preconsolidation pressure of many desiccated soils, P c ,may
exceed the loading capacity of the oedometer apparatus.
The preconsolidation pressure refers to the maximum
stress to which the soil has come to equilibrium in the
past. Figure 14.10 also shows the relative position of the
corrected swelling pressure P s to the preconsolidation
pressure P c . The corrected swelling pressure P s is located
approximately at the intersection between the constant-void-
ratio line and a line representing the recompression branch.
In turn, the preconsolidation pressure P c is located at the
intersection between the recompression branch and the
virgin compression branch of the soil.
Methods for determining the preconsolidation pressure
have been described in many soil mechanics textbooks and
are not repeated herein. The overconsolidation ratio (OCR)
for an unsaturated, expansive soil can be defined as the ratio
of the preconsolidation pressure P c to the corrected swelling
pressure P s . The corrected swelling pressure P s is a repre-
sentation of the in situ stress state of the soil:

Figure 14.9 Representation of in situ stress state when soil has
undergone complex history of drying and wetting.

swelling and shrinking soils (i.e., expansive soils or high-
volume-change soils).

14.4.3 In situ Stress State
The prediction of heave requires information on the present
in situ stress state and possible future stress states. The dif-
ference between the present and future stress states is one
of the main variables controlling the amount of potential
volume change or heave. Consequently, it is important that
the present in situ stress state be accurately assessed. It is
necessary to correct laboratory test results for the effect of
sampling disturbance when using oedometer test results to
assess the present in situ stress state.
The one-dimensional oedometer test procedure appears to
be satisfactory for analyzing most swelling soil problems
encountered in geotechnical engineering practice. However,
the oedometer tests must be performed and interpreted in a
proper manner in order to assess the in situ stress state of
the soil.
It is difficult to obtain representative, undisturbed soil
samples when a soil mass is highly fissured and cracked.
Under these conditions it is prudent to make modifications
to a conventional heave analysis. There are important three-
dimensional effects that must be taken into consideration.
The in situ state of stress is somewhere along either the
wetting or drying portion of the void ratio-stress state vari-
able relationship when a soil is sampled for laboratory test-
ing (Fig. 14.9). The soil in the field has been subjected
to numerous cycles of wetting and drying. The soil has a
specific net normal stress and matric suction at the time
of sampling and the geotechnical engineer must attempt to
interpret the stress state of the soil on the basis of the labo-
ratory test results.

P c
P s

OCR = (14.13)