Civil Engineering Reference
In-Depth Information
Figure 16.3
Change of yield stress ratio due to deposition and erosion.
and it is similar to Fig. 16.l (b). The yield stress ratio
Y
0
is defined in Eq. 8.12 as
Y
0
=
σ
y
σ
(16.3)
The state at A can move to B only by loading along the normal compression line which
is part of the state boundary surface. The recoverable plastic water content change
δ
σ
ya
to
σ
yb
and a change in yield
w
p
is associated with a change of the yield stress from
stress ratio.
There are other ways in which the state and yield stress ratio of a natural soil can
change and I will discuss some of these in the following sections. Some involve changes
of the water content and others involve changes of the state boundary surface.
Notice that the overconsolidation ratio
R
0
of the soil at A is smaller than that at
B because the maximum past stress
σ
yb
and the current stresses are the same. In this case the overconsolidation ratio
R
0
is the
same as the yield stress ratio
Y
0
.
σ
ya
is smaller than the maximum past stress
16.4 Effects of volume changes due to vibration
or creep
If coarse grained soils are vibrated at constant effective stress they will compress and
there will be irrecoverable plastic volume changes and changes in the yield stress ratio.
Figure 16.4(a) shows the state path A
B corresponding to compression by vibra-
tion at constant effective stress; the yield stress has increased from
→
σ
yb
with a
consequent increase in the yield stress ratio. Figure 16.4(b) is the corresponding state
path normalised with respect to the critical stress
σ
ya
to
σ
c
; this assumes that both
σ
z
and
σ
h
σ
c
as the volume
remain constant so the change of the state is due to the increase of
decreases.
If soil, or any other material, is left under a constant effective stress there will
be continued deformations which are due to creep. In soils, creep is greatest in fine
grained soils. The effects of creep in fine grained soil illustrated in Fig. 16.5(a) are
similar to those due to compression by vibration of coarse grained soil except vibration
