Ground Surface Moisture Flux Boundary Conditions - Unsaturated Soil Mechanics in Engineering Practice - page 273

CHAPTER 6

Ground Surface Moisture Flux Boundary Conditions

6.1 INTRODUCTION Weather forecasting has been an area of intense research

over the past decades. Weather forecasting has as its primary
objective the ability to predict the temperature, wind, storms,
and precipitation of various areas of the world. Weather fore-
casting has proven to be a challenging science due largely
to the randomness of the processes involved and the com-
plexities of compressible mixture flow.
There are many near-ground-surface engineered structures
(e.g., soil covers) that require the input of likely weather
conditions as part of the design process. These engineered
structures rely on the statistical evaluation of past weather
conditions recorded at nearby weather stations. An evalua-
tion of the past 10 or more years of weather information is
necessary in order to establish a weather “finger print” for
a particular location under consideration.
The evaluation of moisture movement upward from the
ground surface can first be viewed in terms of the potential
amount of evaporation that could occur. Potential evapo-
ration, PE, from a body of water or a saturated ground
surface is easier to compute than the calculation of actual
evaporation, AE, from a soil surface. The state of stress in
the soil at ground surface can significantly affect the actual
rate of evaporation. It is important to understand the dif-
ference between PE calculations and AE calculations. It
is the determination of AE that is of primary interest to
the geotechnical engineer when attempting to determine the
net moisture flux at the ground surface. Net moisture flux
becomes a Neumann-type boundary condition that can be
applied during the numerical simulation of many unsaturated
soil mechanics problems.
The calculation of net moisture flux (i.e., actual in-
filtration) at the ground surface is required as part of the
assessment of the water balance at a site. The area under
consideration might be a mining site or some other resource
development operation. Soil cover systems have proven to
be invaluable in limiting and controlling the rate of water
infiltration into underlying soils during the past several
decades. The design of soil cover systems depends on the
calculation of the components of moisture movement that
give rise to net infiltration at ground surface.

The quantification of moisture flux boundary conditions is
an important topic which was not historically addressed as
part of classic soil mechanics. There are two main types of
boundary conditions that can be applied when solving mois-
ture movement problems. It is possible to specify a hydraulic
head or a flow rate along the boundaries of the geometry.
Hydraulic heads specified at the boundary of a problem are
called Dirichlet boundary conditions. Flow rates specified
across the boundary of a problem are referred to as Neumann
boundary conditions. Textbooks on saturated soil mechan-
ics explain how to apply a hydraulic head along the geo-
metric boundary or a zero-flux condition which designates
an impervious boundary. However, these are not sufficient
boundary conditions when dealing with near-ground-surface
problems involving unsaturated soils.
The ground surface forms the uppermost boundary for
most soil mechanics problems. This boundary is periodi-
cally subjected to precipitation in the form of rain or snow.
At other times moisture moves upward from the ground sur-
face through evaporation and transpiration. The soil immedi-
ately below the ground surface is generally unsaturated. The
ground surface forms a continuously changing moisture flux
boundary. There are other components such as “runoff” that
also need to be evaluated in conjunction with precipitation
in order to determine the “net moisture flux” at the ground
surface.
The determination of the net moisture flux at the ground
surface is complex and involves numerous assumptions as
a series of calculations are performed. There are several
other disciplines such as agriculture and surface hydrol-
ogy that have had an interest in calculating the net flux at
the ground surface. Extensive research has been undertaken
in other disciplines and the results are of great interest to
applications in soil mechanics. It is important for geotech-
nical engineers to reevaluate theories from other disciplines
to ensure that the assumptions used as part of mathemat-
ical formulations are realistic for geotechnical engineering
problems.