Environmental Engineering Reference
In-Depth Information
more commonly, the water table will rise due to excessive
watering of the vegetation surrounding the building.
There are situations where the groundwater table is far
below the ground surface and the hydrostatic profile is not
reasonable to apply for design purposes. Measurements of in
situ suction below the footings of existing structures in the
vicinity may provide some indication for a reasonable design
matric suction value. Average matric suctions are generally
found to be a function of the microclimate in the vicinity
of the structure (Richards, 1967). The long-term suctions
immediately below shallow footings are often 100 kPa or
greater. These suctions can significantly contribute to the
shear strength of the soil. Decisions regarding the design
matric suction value depend on local experience and the
microclimate in the region.
Figure 12.66 illustrates the effect of various matric suction
values on bearing capacity. The following soil parameters
were selected for the analysis: (1) an effective angle of inter-
nal friction φ of 20 , (2) an effective cohesion c of 5 kPa,
and (3) a friction angle with respect to matric suction, φ b ,
of 15 . The density of the soil is 1830 kg/m 3 . The design is
for strip footings with widths of 0.5 and 1.0 m. The footings
are assumed to be placed at a depth of 0.5 m. The bearing
capacity factors from Fig. 12.64 are as follows: N γ
12.4.4 Total Stress Approach
The total stress approach is similar to the procedure commonly
used in geotechnical engineering practice. The geotechnical
engineer may not normally view his or her design as one
involving the behavior of an unsaturated soil. Let us assume
that the site under consideration involves a clayey soil with
a groundwater table well below the proposed depth for the
footings. Typical engineering practice can be described as fol-
lows. A field investigation is conducted in which samples are
obtained at a variety of depths. The samples are brought to the
laboratory, extruded, and tested for unconfined compressive
strength. The data are interpreted and a design compressive
strength is selected.
The compressive strength is divided by 2 to give an
undrained shear strength for the soil of c u . The angle of
friction is taken as zero and the bearing capacity equation is
solved. It is easy for the engineer to lose sight of the fact that
the soil had matric suction (or negative pore-water pressure)
that was holding the specimen together. The shear strength
of the soil specimen tested in the laboratory was a function
of the negative pore-water pressure in the soil (i.e., matric
suction). There was also a change in pore-water pressure
resulting from unloading the soil during sampling. Changes
in pore-water pressure during sampling may be small relative
to the in situ negative pore-water pressure, but the suction
changes result in an increase in the strength of the soil.
The shear strength measured in the laboratory reflects the
matric suction in the soil. The extended Mohr-Coulomb
failure envelope can be used to visualize the relationship
between the unconfined compression test results and the shear
strength defined in terms of matric suction. Figure 12.67
illustrates a possible stress path follo wed during an uncon-
fined compression test (i.e., stress path AB ), where the matric
suction is assumed to remain constant during the test. The
unconfined compression test results can be translated onto
the strength envelope to represent the stress state at failure.
The undrained shear strength c u can be mathematically
written in terms of
=
5 . 0,
N c =
8 . 0.
The computations show that for a footing width of 0.5m
and no matric suction in the soil, the bearing capacity is
182 kPa. About 48% of the bearing capacity arises from the
effective cohesion of the soil. When the matric suction is
increased to 100 kPa (i.e., total cohesion equal to 32 kPa),
the bearing capacity increases to 655 kPa. Now, 85% of the
bearing capacity is due to the total cohesion component. The
results show a similar trend for the wider footing.
The main observation is that the matric suction dramati-
cally increases the bearing capacity of the soil. It is useful
to construct a plot similar to that shown in Fig. 12.66 when
attempting to select a suitable allowable bearing capacity in
unsaturated soils.
17 . 5, and N q =
the extended Mohr-Coulomb fail-
2000
= 20
φ
°
1600
b = 15
φ
°
c
= 5 kPa
γ = 18 kN/m 3
1200
D f = 0.5 m
800
400
0
0
50
100
150
200
250
300
Matric suction, u a - u w (kPa)
Figure 12.66 Bearing capacity of strip footing for various values of matric suction.
 
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