Environmental Engineering Reference
In-Depth Information
p
o
Existing overburden pressure
σ
vp
Preconsolidation pressure
o
a
e
o
C
r
A
Virgin
“Disturbed” sample
“Undisturbed” sample
a
C
r
a
C
c
a
log 1 cycle
1.0
~
=(0.4
e
o
)
B
Log pressure
Figure 6.38.
Void ratio versus log effective stress plot for a typical overconsolidated soil with the
Schmertmann (1955) method shown to correct C
c
for sample disturbance.
pressure
vo
from which the sample was taken, and the in-situ void ratio e
o
, drawing OA
parallel to the unloading curve with A on the preconsolidation stress
vp
; and joining A to
point B, which is on the consolidation plot at 0.4e
o
.
The preconsolidation pressure measured in the laboratory should be checked against
the history of the soil deposit, and indirectly checked using the relationship between
the undrained strength of clays in the deposit, and the OCR (and hence
vp
) using
Equation 6.14.
6.2.1.2
Within the embankment
As for the foundation, the materials within the embankment have three components to
their compression - initial “elastic”, primary consolidation, and creep. Considering each
type of material in the embankment:
6.2.1.2.1 Earthfill
Soils in modern embankment dams are compacted to a degree of saturation which is typ-
ically 90-95%, so their early behaviour is that of partially saturated soils. Much of the
consolidation process consists of the gas in the voids being compressed, and forced into
solution, rather than flow of water from the system. It therefore occurs relatively quickly
and, as discussed in Section 6.2.2, most of the settlement in dams occurs during construc-
tion. It is incorrect to try to predict the behaviour of these zones by carrying out conven-
tional oedometer consolidation tests (which use saturated soils), and classical Terzaghi
consolidation theory, because these do not properly model the partially saturated
behaviour, and impose zero lateral strain (K
o
) conditions, which are often not correct,
