Consolidation and Swelling Processes in Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 809

CHAPTER 16

Consolidation and Swelling Processes in Unsaturated Soils

16.1 INTRODUCTION 16.2 STRESS AND SEEPAGE UNCOUPLED

AND COUPLED SYSTEMS

The application of total stress (or a load) to an unsaturated soil
can result in the generation of excess pore-air and pore-water
pressures. The excess pore-air and pore-water pressures will
dissipate with time and eventually the pore pressures return to
the equilibrium values that existed prior to loading. The dissi-
pation process of pore pressures is called “consolidation” and
the process results in volume change as excess pore pressures
are dissipated.
It is also possible for excess pore fluid pressures to be
generated as a result of changes in boundary conditions.
For example, the infiltration of rainwater at ground surface
can initiate water movement into the soil along with associ-
ated soil swelling or soil collapse. Water moves into the soil
because of a difference in the hydraulic head (or pore-water
pressure) at ground surface and the hydraulic head imme-
diately within the soil mass. The drying of a soil from the
ground surface also initiates moisture movement that can be
accompanied by volume decrease. The drying of the soil
can be visualized in terms of a moisture flux at ground
surface or a change in the pore-water pressure boundary
condition.
The “swelling” process is usually associated with a change
in moisture flux boundary conditions whereas the consolida-
tion process is usually associated with a change in the exter-
nally applied total stresses. In the case of a swelling process,
the pore pressures (i.e., pore-water pressure, pore-air pres-
sure or both) are below the equilibrium state and therefore
will increase toward an equilibrium state. The consolidation
process has a decrease in pore pressures with time while
the swelling process has an increase in pore pressures with
time. The consolidation and swelling processes are essen-
tially equivalent and opposite processes from a theoretical
standpoint. However, the swelling process is not usually ini-
tiated through a change in the total stresses (i.e., unloading of
the soil). Rather, it is usually environmental changes related
to precipitation that directly change the pore-water pressure
conditions at the ground surface and initiate a process of
pore-water pressure changes throughout the soil mass.

The dissipation of the pore-water and pore-air pressures
resulting from stress changes can be analyzed in an “uncou-
pled” or “coupled” manner. The uncoupled solution assumes
that the stress-deformation analysis and the pore fluid dissi-
pation processes are solved in an independent manner. The
coupled solution assumes that the stress-deformation and
pore fluid dissipation processes are solved simultaneously.
There is also another distinctive feature related to the
uncoupled and coupled solutions. When the uncoupled
approach is adopted for consolidation, the initial excess
pore fluid pressures (e.g., pore-water pressure and pore-air
pressures) are computed using the pore pressure parameters
presented in Chapter 15. The pore pressure parameters allow
the generated excess pore fluid pressure to be instantaneously
generated and used to initiate subsequent processes. When
using a fully coupled approach, the compressibility of the
pore fluids becomes part of the overall formulation and the
calculation of the generated excess pore pressures becomes
part of the solution.
The coupled approach is considered to be more rigor-
ous and correct for the simulation of the consolidation pro-
cess. However, the uncoupled approach is generally easier
to solve and sufficiently accurate for most engineering prob-
lems. The fully coupled consolidation process has revealed
the “Mandel-Cryer” effect, which suggests that it is a more
accurate solution. The Mandel-Cryer effect is related to the
Poisson's ratio of the soil and may only be significant for
problems where the induced pore pressures are the result of
external loadings (Vu and Fredlund, 2002).

16.2.1 Combining Stress, Water, and Air
for One-Dimensional Consolidation
The theory of consolidation for a saturated soil has been
viewed as one of the pinnacle analyses in classical soil
mechanics. The theory of consolidation has been exten-
sively used for design and control when dealing with satu-
rated soils. Case histories have verified that the mathematical