Ground Surface Moisture Flux Boundary Conditions - Unsaturated Soil Mechanics in Engineering Practice - page 274

Cover systems have increasingly become a viable solution
to mitigate environmental damage. Cover systems became
a common solution around the 1980s for the remediation of
contaminated sites and waste containment facilities. Mining
operations have two main streams of waste material that
need to be properly handled to reduce distress to the envi-
ronment. These two streams of materials can be referred to
as waste rock and mine tailings. Cover systems provide a
potential solution for both streams of waste materials.
The design of cover systems is conceptually easy to under-
stand but there are numerous issues and challenges associ-
ated with various aspects of the engineering analysis. There
are issues related to the required input information and the
solution of the mathematical equations (e.g., the moisture
flow partial differential equation). There are also a large
number of assumptions that must be made at various stages
of the design process. These assumptions can significantly
influence the outcome of the cover system design. The quan-
tification of unsaturated soil properties for each material
involved (e.g., the permeability functions and the water stor-
age functions) has also proven to be a challenge for geotech-
nical engineers (Fredlund, 2007b).
The design of cover systems depends upon the ability to
predict moisture fluxes in and out of the ground surface as
well as moisture movement through the unsaturated soils
comprising the cover system. The analysis can be viewed as
a “flux-driven” problem. Problems involving the predictions
of hydraulic head are generally easier to solve than problems
involving the prediction of moisture fluxes. The boundary
conditions at the ground surface must be described in terms
of a moisture flux when undertaking a soil cover design. The
quantification of the ground surface moisture flux boundary
conditions is coupled with the solution of moisture move-
ment in the underlying unsaturated soil.
A cover system can be viewed as a thin interface placed
between the atmosphere and the underlying soil strata, as
shown in Fig. 6.1. The climate imposed on top of the cover
can vary widely from arid to humid conditions. A particular
soil cover system cannot be expected to function in a similar
manner when subjected to a wide range of climatic condi-
tions. Rather, it would be anticipated that a particular type
of soil cover can only perform satisfactorily when subjected
to a limited range of climatic conditions. Consequently, soil
cover systems need to be designed for each site by taking
into consideration the available soil types for the soil cover
and the imposed weather conditions.
Cover systems can function quite differently under dif-
ferent climatic situations. For example, a cover system may
operate in a “store-and-release” mode in one situation but
might function as a saturation oxygen barrier in another case.
The location of the water table in the underlying materials
can also have an influence on the functionality of the soil
cover system. All elements of the cover system (i.e., atmo-
sphere, soil cover, and underlying soils) are highly variable
and material responses are generally highly nonlinear and
hysteretic (Shackelford, 2005; Fredlund, 2006).

Figure 6.1 Covers used as an interface between material to be
protected and climatic environment.

The challenges associated with the analytical design of
a soil cover system are formidable but not impossible to
accommodate. It might be difficult to obtain the same deci-
sion from two geotechnical engineers regarding whether a
cover system will function satisfactorily in a particular cli-
mate. The design of soil cover systems involves the use of
SWCCs. The SWCCs are hysteretic in character and as such
engineering protocols need to be established for the calcula-
tion of water movement in and out of the unsaturated soils
near the ground surface.

6.2 CLIMATIC CLASSIFICATION FOR A SITE

The climatic context should first be evaluated for any site for
which water balance or water infiltration is to be quantified.
The average climatic conditions as well as likely variations
in the local climate should be understood prior to embarking
on the numerical modeling of moisture movement.
Store-and-release soil covers placed at the ground surface
are known to have the potential to function satisfactorily
in arid or semiarid environments. However, there are situa-
tions where soil cover systems have been designed and used
where the climate is not arid or semiarid. The geotechnical
engineer should have an appreciation of the significance of
variations in the climate from one year to the next.
Climatic classifications are based primarily upon an
approximate water balance calculation referenced to the
ground surface. Calculations of climate classification pro-
vide the geotechnical engineer with guidance regarding the
suitability of a particular type of cover system. The cli-
matic classification provides information on whether a
particular type of cover system has the potential to operate
in a satisfactory manner. It is possible to proceed with the
cover design once it has been decided that a particular cover
system might function in a satisfactory manner.
The climate in a particular area is commonly evaluated
using the Thornthwaite climate classification system (Thorn-
thwaite, 1931, 1948). Thornthwaite published the first