Theory to Practice of Unsaturated Soil Mechanics - Unsaturated Soil Mechanics in Engineering Practice - page 14

Figure 1.14 Potential instability of near-vertical excavation dur-
ing construction of a foundation.

assumption is often made that as long as the pore-water
pressure remains negative, there is little or no movement
of fluids downward from the waste pond. However, it has
been observed that there is often a mounding of the water
table below the waste ponds even when the intermediate
soil may remain unsaturated. Engineers now realize that
significant volumes of water and contaminants can move
through the unsaturated soil portion of the profile even
though the soil has negative pore-water pressures.

Figure 1.12 Effect of precipitation on man-made slope subjected
to rainfall.

Surface sloughing commonly occurs on relatively flat
slopes following prolonged periods of precipitation. These
failures have received little attention from an analysis
standpoint. One of the main difficulties appears to have
been associated with the assessment of pore-water pressures
in the zone above the groundwater table.
The slow, gradual, downslope creep of soil is another
aspect which has not received much attention in the liter-
ature. It has been observed, however, that the movements
occur in response to seasonal, environmental changes. Wet-
ting and drying and freezing and thawing are known to be
important factors. It would appear that an understanding
of unsaturated soil behavior is imperative in formulating a
mathematical assessment of these problems.

1.5.5 Stability of Vertical or Near-Vertical Excavations
Vertical or near-vertical excavations are often used for the
installation of a foundation or a pipeline (Fig. 1.14). It is
well known that the backslope in a moist silt or clay soil
will stand at a near-vertical slope for some time before fail-
ing. Failure of the backslope is a function of the soil type,
the depth of the excavation, the depth of tension cracks,
the amount of precipitation, as well as other factors. In the
event that the contractor should leave the excavation open
longer than planned or should a high precipitation period be
encountered, the backslope may fail, causing damage and
possible loss of life.
The excavations being referred to are in soils above the
groundwater table where the pore-water pressures are neg-
ative. The excavation of soil for the trench also produces a
temporary decrease in pore-water pressures and an increase
in the shear strength of the soil. With time, there may be a
gradual increase in the pore-water pressures in the backslope
and a loss in shear strength. The increase in the pore-water
pressure is the primary factor contributing to the instability
of the excavation. Engineers often place the responsibility
for ensuring backslope stability onto the contractor. Pre-
dictions associated with this problem require an in-depth
understanding of unsaturated soil behavior and moisture flux
conditions.

1.5.4 Mounding Below Waste Retention Ponds
Waste materials from mining and industrial operations are
often stored as a liquid or slurry retained by low-level
dykes (Fig. 1.13). Soil profiles with a deep water table
have often been considered to be ideal locations for
these waste ponds. The soils above the water table have
negative pore-water pressures and may be unsaturated. The

Waste effluent

Dyke

Clay

Clayey silt
(unsaturated)

Mounding of the
water table
after waste pond

1.5.6 Bearing Capacity for Shallow Foundations
The foundations for light structures are generally shallow
spread footings (Fig. 1.15). The bearing capacity of the
underlying soils is often computed based on the unconfined
compressive strength of the soil. Shallow footings can easily
be constructed when the water table is below the elevation of

Water table prior to waste pond

Figure 1.13 Mounding of water table below a waste pond due
to seepage through unsaturated soil zone.