Civil Engineering Reference
In-Depth Information
Figure 8.6
Changes of state due to creep or vibration.
along the NCL to N
2
where the yield stress is
p
y2
and unloading to R
2
where the
stress is
p
02
. The state of a soil can however move directly from R
1
to R
2
by creep in
fine-grained soils and vibration in coarse-grained soils. Moreover the position of the
NCL can shift as a result of soil structure. These mechanisms will be described further
in Chapter 16.
Figure 8.6 shows the state of a sample of soil initially normally consolidated at
R
0
where the stress
p
0
p
m
moving directly to R
1
where the stress is same, by
creep or vibration. From the definition of overconsolidation ratio in Eq. (8.6) the
overconsolidation ratios at R
0
and R
1
are both the same and are equal to 1.0 since
the stresses have not changed. This means that the overconsolidation ratio defined in
Eq. (8.6) does not properly describe the current state of a soil.
The state of a soil can be better described by the yield stress ratio
=
p
y
p
0
Y
p
=
(8.8)
where
p
0
is the current stress and
p
y
is the yield stress which is the stress at the intersec-
tion of the swelling line through R
1
with the NCL. Notice that as the state moves from
R
1
to R
2
in Fig. 8.6, either by loading, yielding and unloading or by creep or vibration
the yield stress ratio increases because the yield stress increases from
p
y1
to
p
y2
.
8.4 State of soils on the wet side and on the dry side
of critical
Clays may be normally consolidated or, depending on how far the state is from the
normal consolidation line, lightly or heavily overconsolidated. There is a critical over-
consolidation ratio, shown as a broken line in Fig. 8.7(a), which separates lightly
and heavily overconsolidated soils. (We will see later that this line is below the crit-
ical state line (CSL) which corresponds to states at which soil fails during shearing.)
The precise value for the critical overconsolidation ratio depends principally on the
