Theory to Practice of Unsaturated Soil Mechanics - Unsaturated Soil Mechanics in Engineering Practice - page 16

Figure 1.17 Potential remedial measures for a house founded upon piles placed through expan-
sive soils (Hamilton, 1977).

a favored engineered solution around the 1980s, particularly
for waste containment facilities and the remediation of
contaminated sites. Mining operations have two streams of
waste material that need to be properly handled to mitigate
damage to the environment. These two streams are waste
rock and mine tailings. Cover systems provide a potential
solution for both streams of waste materials.
The design of cover systems is easy to understand. How-
ever, there are numerous issues and challenges associated
with various aspects of design. There are issues related to
the required input information and the solution of the mois-
ture flow partial differential equation. There are a large
number of assumptions that need to be made at various
stages of the design process. These assumptions can signif-
icantly influence the final performance of the cover design.
The quantification of unsaturated soil properties for each
material involved (e.g., the permeability functions and the
water storage functions) has proven to be a challenge for the
geotechnical engineer (M.D. Fredlund Fredlund, 2000a, b;
D.G. Fredlund, 2007b).
Shackelford (2005) presented a summary of the primary
current and future environmental issues associated with
cover systems. The issues mentioned were (1) long-term
performance of waste containment systems, (2) alternative
barriers or covers (i.e., alternative to clay covers) and
barrier materials, (3) innovative barriers (covers) and barrier
materials, (4) forms of waste materials, (5) significance
of biological waste processes, (6) the role of numerical
modeling in cover design, and (7) professional identity.
The design of a cover system depends on the ability to
predict moisture fluxes in and out of the ground surface
as well as moisture fluxes through the bottom of the
cover system. The design analysis can be viewed as a
“flux-driven” problem because of the importance of the

surrounding weather conditions. Problems involving the
predictions of hydraulic head are known to be easier to
solve than problems involving the prediction of moisture
fluxes (D.G. Fredlund, 2007b). The boundary conditions at
ground surface need to be described in terms of a moisture
flux when performing the cover design. The ground
surface has moisture either coming down in the form of
precipitation or going up in the form of evaporation and
evapotranspiration. The quantification of the net moisture
flux boundary conditions at the ground surface has also
proven to be a challenging analysis.
A cover system can be viewed as a thin interface placed
between the overlying atmosphere and the underlying soil
strata, as shown in Fig. 1.18. The climate imposed on top
of the cover can vary widely from arid to humid condi-
tions. A particular cover system cannot function in a similar
manner over a wide range of climatic conditions. Rather, it
would be expected that a particular cover could only perform
satisfactorily under certain climatic conditions.

Atmosphere with temperature and
rainfall as the primary driving variables

Moisture flux boundary

Cover system

Waste material Waste material

Figure 1.18 Soil cover system used as interface between waste
material and atmosphere.