Environmental Engineering Reference
In-Depth Information
contribute to the shear strength of the soil. The two seemingly
opposite attitudes or perceptions give rise to an important
question, “How constant and stable are negative pore-water
pressures with respect to time?” Or, a more probing question
might be “Has the engineer's attitude toward negative pore-
water pressures been strongly influenced by expediency?”
1.5.7 Ground Movements Involving Expansive Soils
There is no problem involving soils with negative pore-water
pressures that has received more attention than the predic-
tion of heave associated with the wetting of an expansive
soil. Light structures such as a roadway or a small building
are often subjected to severe distress as a result of changes
in the surrounding environment subsequent to construction
(Figs. 1.16 and 1.17). Changes in the climatic environment
may occur as a result of the removal of trees and grass
and the excessive watering of a lawn around a new struc-
ture. The zone of soil that undergoes volume change on an
annual basis has been referred to as the “active zone.” The
higher the swelling properties of the soil, the greater will be
the amount of heave to the structure.
It has been common practice to obtain undisturbed
soil samples from the upper portion of the profile for
one-dimensional oedometer testing in the laboratory. The
laboratory results are used to provide quantitative estimates
of potential heave. Numerous laboratory testing techniques
and analytical procedures have been used in engineering
practice. The prediction of heave in expansive soils is dealt
with in considerable detail later in this topic.
Figure 1.15 Analysis of bearing capacity for lightly loaded struc-
ture placed on unsaturated soil with negative pore-water pressures.
the footings. In many cases, the water table is at a consider-
able depth and the soil below the spread footings has a neg-
ative pore-water pressure. Undisturbed samples, held intact
by negative pore-water pressures, are routinely tested in the
laboratory to obtain a measurement of the shear strength of
the soil. The assumption is often made that the pore-water
pressure conditions in the field will remain relatively con-
stant with time and therefore the unconfined compressive
strength will also remain essentially unchanged. Based on
this assumption and a relatively high design factor of safety,
the bearing capacity of the soil is computed.
The above design procedure has involved soils with neg-
ative pore-water pressures. The engineer almost seems to
have been oblivious to the fact that the design procedure
assumes that there will be a long-term retention of negative
pore-water pressure in the soil. The geotechnical engineer has
taken a very different attitude toward the long-term retention
of negative pore-water pressures when dealing with slope
stability problems. In the case of a slope stability problem,
the geotechnical engineer has generally assumed that long-
term negative pore-water pressures cannot be relied upon to
1.5.8 Design of Soil Cover Systems and Capillary
Breaks
Soil cover systems have increasingly become a potential
solution to mitigate environmental damage. Covers became
Figure 1.16 Common ground movements associated with a house with basement foundation
constructed in expansive soils (Hamilton, 1977).
 
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