Environmental Engineering Reference
In-Depth Information
Shackelford (2005) made several comments that are note-
worthy with regard to the difficulties associated with the
modeling of cover systems. He stated:
highly nonlinear mathematical functions. Needless to say,
the numerical modeling challenges are significant and appear
to exceed those of most other unsaturated soil mechanics
applications.
The ability of models to accurately predict field performance
of engineered systems has been and will continue to be an
issue facing geotechnical engineers. This issue is particularly
important when dealing with environmental problems because
models are often used as a tool to predict the future impacts
and resulting risk from engineering activities related to envi-
ronmental protection.
The uncertainty in the accuracy of the model predictions
can be considerable, particularly in cases where the time-
frame for the prediction is long, such as the disposal of
radioactive wastes with design lives ranging from hundreds
to thousands of years.
- uncertainty is associated not only with the comprehen-
siveness and accuracy of the physical, chemical, and biolog-
ical processes upon which the model is based, but also with
the accuracy of the input data and the lack of knowledge con-
cerning the changes in the material properties and processes
that may occur with time. As a result, predictions made with
existing models generally cannot be considered reliable until
and unless the predictions are verified by comparison with
field data.
- the problems in trying to predict the field performance of
engineered systems, particularly when the processes involved
are highly nonlinear and the properties of the systems are
time dependent. These examples also illustrate why evalua-
tion and verification of numerical models will continue to be
an important issue facing geotechnical engineers, particularly
within the realm of environmental issues.
6.4.1 Basic Functionality of Cover Systems
The first designed cover systems were predominantly made
of compacted clays. The intent was to construct a rela-
tively impervious cover over waste materials. However, the
primary problem encountered with clay covers was the for-
mation of cracks associated with drying and desiccation.
Cracks had the potential to make the clay covers quite per-
meable to water. As a result, a new generation of cover
systems called “alternatives covers” became more common.
The name alternative covers was used because they were
an alternative to clay covers. These cover systems were
also referred to as store-and-release covers and ET (evapo-
transpirative) covers.
The engineering design of a Store-and-Release cover sys-
tem involved the application of unsaturated soil mechanics
principles (Fig. 6.55). The covers could change in degree of
saturation with time and function in a manner that accommo-
dated environmental fluctuations. Reductions in the degree
of saturation reduced the hydraulic conductivity (or coef-
ficient of permeability) of the cover system as long as the
surface soil did not crack due to desiccation drying. A reduc-
tion in the degree of saturation increased the water storage
capacity of the cover soil. The intent was to have a soil
cover that buffered extreme climate forcing factors by stor-
ing water during wet periods and releasing it back to the
atmosphere during dry periods.
Alternative covers can be constructed using a variety of
soil types and often make use of sand and silt soils. The
covers are designed on the basis of water storage and water
release (i.e., a water balance design). There must be sufficient
capability for the water that infiltrates the cover on an annual
basis to essentially be removed during the same year. Store-
and-release soil covers are most suitable in arid and semiarid
climates. The cover design must also take into consideration
the distribution of precipitation throughout the year as well as
the distribution of thermal energy to drive evaporation. Stated
Numerical modeling associated with cover design has pre-
sented many challenges to geotechnical engineers. There
appears to have been significant problems related to con-
vergence when solving the partial differential moisture flow
equation. There are also questions related to the assump-
tions made at various stages of the analysis. The numerical
modeling analysis is handled as a transient seepage analysis
that may extend over a time period of several years (e.g.,
typically 10 years) using time steps on the order of a few
minutes. The weather conditions can vary from near-average
conditions to extreme conditions and the soil properties can
vary from being almost constant to taking on the form of
Top of the store-and-release cover
Saturated
Evapotranspiration
Water storage
Emptying
Summer
Winter
Summer
Winter
Summer
Seasons of the year
Dry
Figure 6.55 Concept of store and release used in the design of alternative cover systems.
 
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