Theory to Practice of Unsaturated Soil Mechanics - Unsaturated Soil Mechanics in Engineering Practice - page 26

1.7 ENGINEERING PROTOCOLS FOR
UNSATURATED SOILS

Flexible cover

0

225

200

150

1

175

A natural and logical progression leading to the development
of suitable engineering design protocols should involve
(i) development of a fundamental science with suitable
constitutive laws to describe the material behavior of
concern, (ii) verification of suitable laboratory testing
procedures to measure appropriate soil properties, and (iii)
designation of design procedures (i.e., protocols) that should
be followed in engineering practice (Fredlund and Rahardjo,
1993a). While this is the preferred approach, it is not the
pattern for the development of protocols for unsaturated soil
problems. More commonly, solutions have been proposed
and then later the most appropriate engineering design
procedures emerge. To a large extent, this has been the
history associated with the design of “cover systems” that
became an engineered solution for many situations in the
1980s. The emergence of similar engineering protocols can
be observed for other application areas in unsaturated soil
mechanics.
A variety of estimation techniques have become an accept-
able part of engineering protocols when applying unsatu-
rated soil mechanics. The question can be asked, “Are all
estimation procedures for unsaturated soil property func-
tions, equally suitable for all engineering problems?” Or, the
question could be phrased, “Under what conditions should
various estimating and soil testing procedures be used?”
These are questions for which practicing engineers desire
answers in order to utilize sound engineering protocols with
confidence.
A lack of designated high-level engineering protocols
makes practicing engineers more vulnerable to litigation. It
is also difficult for engineers to insist on adopting high-level
engineering practices in all situations. Engineering protocols
can be used to provide guidance and recommendations
for sound engineering practices for the implementation
of unsaturated soil mechanics. There is need for general
guidelines related to the estimation of the SWCC, and to a
lesser extent to the USPF, since the SWCC is the basis for
estimating unsaturated soil property functions.

200

L

250

2

-300

350 kPa

3

0 3 6 9 12

Distance from center of cover or slab, m

Figure 1.27 Contours of matric suctions throughout expansive
soil mass after 3 days of moisture evaporation from ground surface
(after Vu and Fredlund, 2004).

− 10

Day 1

− 20

Day 3

− 30

Day 5

− 40

6 10

0 2 4 8 12

Distance from center of cover, m

Figure 1.28 Vertical displacements along ground surface for 1,
3, and 5 days of evaporation to atmosphere (after Vu and Fredlund,
2004).

Vertical displacement, mm
−

25

0 0

− 5 − 10 15 − 20 − − 30 − 35

1

2

3

Figure 1.29 Vertical displacements along vertical section below
edge of slab on ground after 1, 3, and 5 days of evaporation (after
Vu and Fredlund, 2004).

1.7.1 Definition of Engineering Protocol
When considering the practice of unsaturated soil mechan-
ics, the word “protocol” can be defined as “procedures and
practices acceptable for prudent unsaturated soils engineer-
ing practice.” The term protocol is commonly used in con-
nection with computing procedures (e.g., telecommunica-
tions protocols or Internet protocols) or standards for global
acceptance (e.g., the Kyoto Protocol). The definition has a
sense of flexibility and is meant to set a temporary standard
that could be changed with time. The term is meant to reflect
a generally accepted, high standard of engineering practice.
Engineering protocols for unsaturated soil applications
should provide general guidelines for the implementation of

slab. The soil is shown to shrink away from the concrete
slab as evaporation takes place from the ground surface.
Similar plots to those presented above can be produced
for the case of infiltration of water at ground surface. The
boundary condition at the ground surface can be specified
in terms of a moisture flux or a specified head (or pressure)
condition. The microclimatic conditions at a specific site
could also be analyzed to provide realistic boundary condi-
tions for the site. There are a wide variety of unsaturated
expansive soils and collapsible soil problems that can be
studied as a result of combining a transient seepage analysis
with a nonlinear stress-deformation analysis.