Environmental Engineering Reference
In-Depth Information
Log
p kP
a
p
0
p
c
Adjustment of the
natural m.c. curve
∆
p
(
e
0
)
i
(
e
0
)
i
Soil at natural m.c.
(
e
0
p
0
)
∆
e
s
∆
e
c
FIGURE 10.33
Double consolidation test
e
-
p
curves and
adjustments for an overconsolidated soil.
(From Clemence, S.P. and Finbarr, A.O.,
Proceedings of ASCE,
Preprint 80-116, 1980,
22 pp. With permission.)
Collapse
Soil soaked after 24 hr
bedding at 1 kPa
For an increase in foundation stress
∆
P
, and wetted soil, the settlement
ρ
is estimated
from the expression
ρ
(
∆
e
s
/1
e
o
)
∆
e
c
/(1
e
o
)
(10.1)
10.5.4
Treatment and Support of Structures
Evaluate the Degree of Hazard and Risk
Degree of hazard is basically a function of the probability of significant ground wetting
and of the magnitude of the potential collapse, if the critical pressure that will cause col-
lapse at the natural water content is not approached. Sources of ground wetting have been
given in
Section 10.5.2
in the discussion of collapse causes.
Low-hazard
conditions exist, where potential collapse magnitudes are small and
tolerable, or the probability for significant ground wetting is low.
●
Moderate-hazard
conditions exist, where the potential collapse magnitudes are
undesirable but the probability of substantial ground wetting is low.
●
High-hazard
conditions exist, where the potential collapse magnitudes are unde-
sirable and the probability of occurrence is high.
●
Degree of risk relates to the sensitivity of the structure to settlement and to the impor-
tance of the structure.
Reduce the Hazard
Prevent ground wetting
and support structures on shallow foundations designed for an
allowable bearing value sufficiently below the critical pressure to avoid collapse at natu-
ral water content. The critical pressure is best determined by
in situ
plate-load tests, and
the allowable soil pressure is based on FS
2 to 3, depending upon the settlement toler-
ances of the structure.
In some cases, such as large grain elevators, where the load and required size of a mat
foundation impose bearing pressures of the order of the critical pressure, the structures
have been permitted to settle as much as 1 ft (30 cm), provided that tilting is avoided. This
solution has been applied to foundations on relatively uniform deposits of loess with nat-
ural water contents of the order of 13% in eastern Colorado and western Kansas. In col-
lapsible soils derived from residual soils, however, such solutions may not be applicable
