Theory to Practice of Unsaturated Soil Mechanics - Unsaturated Soil Mechanics in Engineering Practice - page 8

75

°

60 °

30

°

0 °

Extremely arid
Arid

Semiarid

90 ° 60 ° 30 ° 0 ° 30 ° 60 ° 90 °

120 ° 150 °

Figure 1.5 Extremely arid, arid, and semiarid areas of the world [from Meigs (1953) and
Dregne (1976)].

impetus to utilize weather data for the calculation of moisture
flux conditions at the ground surface.
There are a number of factors that affect the calculation
of thermal and moisture fluxes at ground surface. However,
temperature and precipitation data are the primary variables
required for the calculation of flux boundary conditions. The
runoff of water at ground surface is also a challenging vari-
able to calculate. The net difference between the ground
surface moisture fluxes is called “net infiltration” and is the
amount that will flow into the unsaturated soil at ground
surface. Often it is the infiltration past a particular depth
(e.g., bottom of the “cover layer”) that is of primary interest
and its magnitude is referred to as “deep infiltration.” Terms
such as “percolation” and “recharge” are also used when
describing the infiltration phenomenon. The procedures that
can be used to calculate potential evaporation (PE), actual
evaporation (AE), and other water balance variables will be
described later in this topic.

time consuming and expensive. Consequently, a variety
of “estimation techniques” for unsaturated soil property
functions have been forthcoming from research studies in
various countries (Fredlund, 2000a, Vanapalli et al., 1996a).
It has become part of acceptable geotechnical engineering
practice to utilize “estimation techniques” when applying
unsaturated soil mechanics at the preliminary engineering
design stage. Estimation techniques have particularly found
widespread usage in the area of saturated-unsaturated
seepage modeling where the permeability functions and the
water storage functions are estimated from a SWCC and
the saturated hydraulic conductivity (Thieu et al., 2000).

1.3.1 Estimation Procedures for Unsaturated Soil
Properties
The estimation of unsaturated soil property functions pro-
vides a new philosophical framework (or paradigm) that has
been effective in expediting the implementation of unsat-
urated soil mechanics. The challenge is to determine the
estimation procedures for unsaturated soil property char-
acterization that best describe the actual unsaturated soil
properties. The estimation techniques are particularly attrac-
tive because direct unsaturated soil testing in the laboratory
becomes too costly for many engineering projects.
Figure 1.6 shows that one of several general approaches
can be used for the determination of unsaturated soil prop-
erty functions. The direct measurement of unsaturated soil
property functions appears to be possible only for special
cases that are often of a research or extremely important

1.3 DETERMINATION OF UNSATURATED SOIL
PROPERTIES

Advances in the development of laboratory testing equip-
ment for unsaturated soils along with the ability to measure
soil suction have prepared the way for the implementation
of unsaturated soil mechanics. It is now possible to
measure most unsaturated soil properties; however, the
direct measurement of unsaturated soil properties can be