Environmental Engineering Reference
In-Depth Information
Vertical displacement (mm)
0
25
50
75
100
125
0.0
E
c
= 20 GPa
e
0
= 0.962
C
s
= 0.090
μ
= 0.40
E
c
= 10 GPa
E
c
= 5 GPa
0.5
1.0
Measured
Predicted
1.5
2.0
2.5
Figure 16.51
Measured and predicted vertical displacements with depth below water line for
various values of Young's modulus for concrete slab,
E
c
.
Table 16.8 Results of Parametric Study
around the perimeter of the slab than under the interior of
the slab. The edge region of the slab is more strongly influ-
enced by climatic changes and imposed watering conditions.
Edge lift occurs when the matric suction in the soil decreases
due to the infiltration of water into the soil around the edges
of the slab. An uncoupled analysis was performed to solve
the required seepage and stress-deformation conditions cor-
responding to edge drop and edge lift.
An example problem is analyzed to illustrate soil shrink-
age around the edge of a cover as a result of evaporation
from the surrounding soil surface (Fredlund et al., 2006).
The saturated-unsaturated seepage analysis is first performed
to provide the negative pore-water pressures with elapsed
time. The pore-water pressure results are then input to the
stress-deformation analysis.
The selected geometry consists of a single soil region 12m
wide by 3m deep, as shown in Fig. 16.52. Either a flexible
cover or a more rigid concrete cover can be analyzed. Let
us first consider a flexible cover that extends 6m from the
left symmetrical boundary. The initial suction profile was
Variation of
Parameter
Range
Predicted Heave (%)
Poisson's ratio,
μ
0.35-0.45
±
7
Swelling index,
C
s
0.085-0.95
±
5
Initial void ratio,
e
0
0.92-1.0
±
2
Coefficient of earth
pressure at rest,
K
0
0.4-1.0
±
2
Young's modulus of
concrete,
E
c
5-20 GPa
±
3
modulus would suffer more differential heave. The cracking
of the slab may result in larger maximum heave values.
The results of the parametric study are summarized in
Table 16.8. It can be seen that swelling index and Poisson's
ratio are the factors that have the greatest effect on the solu-
tion. The value of Young's modulus for concrete appears to
control the shape of the deformed slab.
Flexible impervious cover
Evaporation
Evaporation
Datum
16.9 COMPUTER SIMULATION OF EDGE LIFT
AND EDGE DROP OF SLABS-ON-GROUND
(a)
Residential foundations such as slabs-on-ground are often
constructed on unsaturated, expansive soils and have to resist
deformations associated with changes in matric suction in the
soil. Residential foundations are lightly loaded and therefore
susceptible to changes in matric suction.
The soil beneath an impervious cover can deform into either
an edge-drop mode or an edge-lift mode (Fig. 16.52). Edge
drop occurs when the matric suction in the soil surround-
ing the slab gradually increases due to evaporation and/or
evapotranspiration. There is generally more volume decrease
Infiltration
Infiltration
Flexible
impervious cover
Datum
(b)
Figure 16.52
Deformation pattern associated with (a) drying,
edge-drop, and (b) wetting, edge-lift, modes.
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