Theory to Practice of Unsaturated Soil Mechanics - Unsaturated Soil Mechanics in Engineering Practice - page 11

any special site investigations, soil testing, and engineering
analyses, (ii) the human resistance to change, and (iii) the
political, regulatory, and litigation factors.
Slowness in the implementation of unsaturated soil
mechanics appears to have been related to the cost of
laboratory soil testing for the quantification of unsaturated
soil properties. The original soil mechanics paradigm
involving the direct measurement of soil properties
becomes excessively costly when measuring unsaturated
soil property functions. However, estimation procedures
utilized for determining unsaturated soil property functions
have significantly increased the use of unsaturated soil
mechanics in engineering practice. Proposed estimation
procedures allow for the assessment of unsaturated soil
property functions in a cost-saving manner, as illustrated in
Fig. 1.6 (Fredlund, 2000).
Unsaturated soil mechanics has developed through a series
of stages and today engineers are faced with the challenge
of implementing the science as part of geotechnical engi-
neering practice. Implementation of a new science is always
a challenge, but it is a challenge that can open the door to
unique technological applications and financial rewards.

they cause. Expansive soils have been called the 'hidden
disaster': while they do not cause loss of life, economically
these soils have become one of the United States costli-
est natural hazards.” Expansive soil problems have occurred
in many other countries of the world. In Canada, Hamilton
(1977) stated: “Volume changing clay subsoils constitute the
most costly natural hazard to buildings on shallow foun-
dations in Canada and the United States. In the Prairie
Provinces alone, a million or more Canadians live in com-
munities built on subsoils of very high potential expansion.”
It would appear that most countries in the world have
had problems with expansive soils. Many countries have
reported the nature of their problems at research conferences.
Some countries reporting expansive soil problems are Aus-
tralia, Argentina, Myanmar, China, Cuba, Ethiopia, Ghana,
Great Britain, India, Iran, Israel, Kenya, Mexico, Morocco,
South Africa, Spain, Turkey, and Venezuela. In general, the
more arid the climate, the more severe is the shrinking and
swelling soil problem.
There is also need for reliable engineering design asso-
ciated with compacted soils, collapsing soils, and residual
soils. Soils that collapse usually have an initially low den-
sity. The soils may or may not be subjected to additional
load, but when they are given access to water, the soil struc-
ture collapse occurs. The water causes an increase in the
pore-water pressures with the result that the soil volume
decreases. The process is similar to that occurring in an
expansive soil but the direction of volume change is oppo-
site. Examples of soil collapse have been reported in the
United States as well as other countries (Houston et al.,
2001; Johnpeer, 1986). Leaky septic tanks and water lines
are a common initiating factor in soil structure collapse.
Inhabited areas with steep slopes consisting of residual
soils are sometimes the site of catastrophic landslides which
claim many lives. A widely publicized case is the landslide
at Po-Shan in Hong Kong which claimed 67 lives (Commis-
sion of Inquiry, 1972). Similar problems have been reported
in South American countries and other parts of the world.
The soils involved are often residual genesis and have
deep water tables. The surface soils have negative pore-
water pressures which play a significant role in the sta-
bility of the slope. However, heavy, continuous rainfalls
can result in increased pore-water pressures to a significant
depth, resulting in the destabilizing of the slope. The pore-
water pressures along the slip surface at the time of failure
may be negative or positive.
There appear to be two main reasons why a practical sci-
ence has been slow in being developed for unsaturated soils
(Fredlund, 1979). First, there was lack of an appropriate sci-
ence with a theoretical basis. There was a lack of appreciation
of the engineering problems and an inability to place the solu-
tion within a theoretical continuum mechanics context. The
stress conditions and mechanisms involved, as well as the
soil properties that must be measured, did not appear to be
fully understood. The boundary conditions were related to the

1.5 NEED FOR UNSATURATED SOIL
MECHANICS

Success in the practice of soil mechanics can be traced
largely to the ability of engineers to relate observed soil
behavior to stress state conditions. This ability has led
to the transferability of the soil mechanics science in a
relatively consistent engineering manner. This has been
the case for saturated soils, but unsaturated soil mechanics
has lagged behind in its implementation. Difficulty has
been experienced in extending classical soil mechanics to
embrace unsaturated soils.
The question can be asked: “Why hasn't a practical sci-
ence developed and flourished for unsaturated soils?” A
cursory examination might suggest that there is no need
for such a science. However, this is certainly not the case
when the problems associated with expansive soils are con-
sidered. Jones and Holtz (1973) reported that, in the United
States alone, “each year, shrinking and swelling soils inflict
at least $2.3 billion in damages to houses, buildings, roads,
and pipelines—more than twice the damage from floods,
hurricanes, tornadoes, and earthquakes!” They also reported
that 60% of the new houses built in the United States will
experience minor damage during their useful lives and 10%
will experience significant damage—some beyond repair.
In 1980, Krohn and Slosson estimated that $7 billion is
spent each year in the United States as a result of damage
to all types of structures built on expansive soils. Snethen
(1986) stated: “While few people have ever heard of expan-
sive soils and even fewer realize the magnitude of the dam-
age they cause, more than one fifth of American families
live on such soils and no state is immune from the problem