Environmental Engineering Reference
In-Depth Information
Many engineering analyses involve soils from above and
below the groundwater table. Consequently, a part of the soil
will be unsaturated with negative pore-water pressures while
another part of the soil profile may be below the water table.
The geotechnical engineer may choose to ignore the benefits
associated with the negative pore-water pressures above the
water table in some cases. However, it has become increas-
ingly apparent that it is possible to improve on most analyses
by taking negative pore-water pressures into consideration
in the analysis.
Soil mechanics problems associated with plastic and limit
equilibrium analyses can be studied while making a variety
of assumptions relative to the handling of the unsaturated
soil portion of the profile. Common plastic and limit equi-
librium analyses are related to (i) bearing capacity, (ii) lateral
earth pressures, and (iii) slope stability. The analyses of
these problems are discussed later in this chapter. It is pos-
sible to ignore the benefits related to the unsaturated soil
portion; however, there is generally value in asking the ques-
tion, “How does the inclusion of unsaturated soil properties
and negative pore-water pressures influence the analysis?”
If the engineer desires to study the “trigger” mechanism
for potential slope stability failures, it becomes imperative
to ascertain potential changes in matric suction in the slope.
The computed changes in matric suction become input infor-
mation to a simulated slope stability analysis.
A range of assumptions can be made when analyzing the
unsaturated soil portion of problems involving the assess-
ment of shear strength. For example, it might be possible to
conduct a bearing capacity analysis assuming that all matric
suctions go to zero. This analysis could be followed by sub-
sequent analyses where the estimated unsaturated soil prop-
erties are input along with more realistic assumptions related
to long-term pore-water pressures. If this design protocol is
followed, the geotechnical engineer becomes aware of the
significant effect that modest matric suctions have on the com-
puted results. It is common to experience a twofold or more
increase in bearing capacity when even modest matric suction
is present. A realization of the significant influence of matric
suction can also lead to the consideration of design details
that tend to retain soil suction with time.
Lateral earth pressures associated with the “active” mode
can be greatly reduced with modest matric suctions in the
soil. On the contrary, the “passive” earth pressure resistance
can be greatly increased with matric suction. A more com-
plex mode involves the analysis of soil-structure interaction
that arises as a soil dries and wets due to moisture move-
ment. The changes in earth pressures on a wall are directly
influenced by changes in matric suction over time.
The stability of slopes involving unsaturated soils is prob-
ably the engineering application area that is of greatest rel-
evance in geotechnical engineering practice. If the water
table is quite deep, the entire potential slip surface may pass
through unsaturated soil. However, it is rainfall infiltration
that is most likely to produce a catastrophic landslide with
120
SA1
27
°
Tangent line
100
80
60
27
°
40
20
b
angle
φ
=
10.7
°
0
0
20
40
60
80
100
120
140
160
180
200
Soil suction, kPa
Figure 12.34
Shear strength estimation based on saturated shear
strength parameters and estimated SWCC for sample no. SA1.
120
SA2
27
°
Tangent line
100
80
60
27
°
40
20
b
angle
=
12.5
°
φ
0
0 0 0 0 0 0 0 0 0 0 0
Soil suction, kPa
Figure 12.35
Shear strength estimation based on saturated shear
strength parameters and SWCC for sample no. SA2.
be drawn tangent to the shear strength envelope represent-
ing average in situ suction conditions. Typical straight lines
shown on each shear stength envelope correspond to
φ
b
val-
ues of 10.7
◦
and 12.5
◦
. It should be noted that the use of these
φ
b
values in an analysis is somewhat conservative since a
portion of the shear strength is being neglected.
12.2.8 Application of Shear Strength of Unsaturated
Soils in Engineering Practice
Measuring the shear strength parameters in the laboratory
is time consuming and costly. As a result, there may not
be many engineering projects where the client is prepared
to pay for the costs associated with measuring the unsatu-
rated shear strength properties. On the other hand, research
studies to date have shown that any one of several proposed
estimation procedures may provide reasonable estimations
of the unsaturated soil shear strength properties.
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