Environmental Engineering Reference
In-Depth Information
becomes nonlinear with respect to both net total stress and
matric suction.
A permeability function must be specified for the swelling
soil and a moisture flux is designated at ground surface to
simulate either rainfall conditions or evaporation conditions
from the soil around the flexible slab. A transient analysis was
first run for the case of evaporation from the soil surrounding
the impervious surface slab. Matric suction values along the
ground surface are shown for elapsed times of 1, 3, and 5
days in Fig. 1.25. Matric suctions for the same elapsed times
along a vertical section below the edge of the impervious slab
are shown in Fig. 1.26. The above two figures provide an
indication of the change in matric suction along vertical and
horizontal directions from the edge of the slab. Numerous
other graphs could be plotted to depict changes in matric
suction with time at any location in the soil mass. Figure 1.27
shows the matric suction contours throughout the soil mass
after 3 days of evaporation from the soil surface.
Changes in matric suction can be combined in a cou-
pled or uncoupled manner with a stress-deformation analy-
sis. Figure 1.28 shows the vertical displacements along the
ground surface for 1, 3, and 5 days of evaporation from the
ground surface. Figure 1.29 shows the vertical displacements
along a vertical section below the edge of the impervious
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.5
1.0
1.5
2.0
2.5
3.0
3.5
F
s
by Morgenstern-Price
Figure 1.23
Comparison of factors of safety,
F
s
, computed using
dynamic programming and limit equilibrium methods of analyses.
of considerable interest in geotechnical engineering. These
problems bring together an unsaturated, transient seepage
analysis and an unsaturated soil stress-deformation analysis.
Saturated soils may also be a part of the overall analysis.
The seepage and stress-deformation analysis can be brought
together in either a coupled or an uncoupled manner (Vu
et al., 2002). The nonlinearity associated with the unsatu-
rated soil, stress-deformation analysis presents challenges in
solving this problem. The initial stress state and pore-water
pressures must be established prior to the start of the model-
ing process because of the nonlinearity in the soil properties.
The seepage analysis boundary condition for an uncoupled
analysis of a slab-on-ground problem are shown in Fig. 1.24.
The example problem involves a slab on ground subjected
to the influences of a moisture flux boundary condition (Vu
and Fredlund 2004, 2004). The expansive soil layer is 3m
deep and has a swelling index of 0.15. The initial matric suc-
tion is assumed to be 400 kPa at the base of the expansive
soil layer. The swelling index
C
s
is converted to an equiva-
lent elasticity function for the unsaturated soil. The elasticity
function means that the incremental Young's modulus for
the material is a function of the stress state. The analysis
Day 5
300
Day 3
200
Day 1
100
Initial
0
0
2
4
6
8
10
12
Distance from center of cover, m
Figure 1.25
Matric suction values along ground surface after
elapsed times of 1, 3, and 5 days (after Vu and Fredlund, 2004).
Matric suction, kPa
0
100
200
300
400
0
Day 1
Specified suction
or boundary flux
Day 3
Initial
Flexible cover or concrete slab
Flux = 0
1
0
Day 5
Flux = 0
1
2
2
Constant suction = 400 kPa
3
3
0
3
6
9
12
Distance from center of cover or slab, m
Figure 1.26
Matric suction values along vertical section below
edge of foundation slab after 1, 3, and 5 days (after Vu and Fred-
lund, 2004).
Figure 1.24
Seepage analysis boundary conditions for slab on
ground placed on expansive soil (after Vu and Fredlund, 2004).
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