Theory to Practice of Unsaturated Soil Mechanics - Unsaturated Soil Mechanics in Engineering Practice - page 28

she cannot take the unsaturated soil portion of the problem
into consideration in the analysis. A more relevant question
is, “How accurate must the SWCC be known for the prob-
lem at hand?” To answer this question, the engineer must be
aware of the factors that affect the assessment of the SWCC.
The risks associated with the engineering design need to
be taken into consideration. If the risks associated with a
hazard are low, an estimation of the SWCC and subsequent
unsaturated soil properties are justifiable. If the risks are
high, it is prudent to more accurately assess the SWCC (i.e.,
possibly the desorption and adsorption curves) and subse-
quent unsaturated soil properties.
Saturated-unsaturated soil problem analyses are generally
viewed as the solution of a PDE within the context of
a boundary value problem. As such, there will be a pri-
mary variable that is solved for in the PDE solution. In the
case of a water flow seepage analysis, the primary vari-
able is “hydraulic head” h . The hydraulic head variable
generally has a reduced sensitivity to the unsaturated soil
properties. Therefore, it is said that hydraulic heads can be
computed with reasonable accuracy because heads are quite
insensitive to the input unsaturated soil property functions.
Hydraulic heads are used as input to a slope stability analysis
and, as such, it is possible to perform meaningful saturated-
unsaturated seepage analyses and use the results in a slope
stability study. On the other hand, if the amount of water
flow (i.e., water flux) in a seepage analysis is of interest to
the engineer, then the saturated-unsaturated soil properties
play a prominent role. Water infiltration or moisture flux
is of primary importance in the design of “cover” systems
and as such the soil properties need to be determined with
greater accuracy.

The engineer must be aware that there are drying and
wetting branches associated with the SWCC. The drying
and wetting curves can be considerably different and this
phenomenon is referred to as “hysteresis.” It is necessary
to determine a suitable drying and wetting curve for engi-
neering problems where the variable of interest from the
modeling exercise is significantly dependent upon the unsat-
urated soil property functions. It is important when per-
forming numerical modeling to ensure that the appropriate
bounding curve is used for solving the engineering prob-
lem. The desorption and adsorption curves can be assumed
to have a similar shape (i.e., congruent) apart from a lat-
eral shift of the inflection point on the drying and wetting
curves.

1.7.7 Challenge for the Future
There are many challenges that still lie ahead before it can
be said that engineers clearly understand how best to apply
unsaturated soil mechanics in engineering practice. How-
ever, one challenge that is important to address is the need
to establish protocols for various geotechnical engineering
problems associated with unsaturated soil mechanics.
Reliance on past experience and empirical practices will
be followed by engineers when there are no distinct stan-
dards of practice or protocol for addressing various unsat-
urated soil problems. The end result will be a lower level
of engineering practice with difficulties being encountered
that will likely be settled through the litigation process. It
is the responsibility of those involved in geotechnical engi-
neering practice to clearly define methodologies or proto-
cols associated with prudent engineering practice for various
unsaturated soil problems.