Civil Engineering Reference
In-Depth Information
16.5 Influence of layering in sedimented soils
Natural sedimentation of soil through water or wind is often episodic and in a
natural soil there will be layers each representing different episodes of deposition.
Each layer has coarser grains at the bottom, deposited first, and finer grains at the
top, deposited last. (This feature is common in sedimented soils and is called graded
bedding.)
Overall the soil is well graded and at a particular effective stress a normally consoli-
dated reconstituted sample will have a relatively low water content
w
r
in Fig. 16.6(a).
In the natural sedimented soil each part of each layer is poorly graded and at the same
effective stress a normally consolidated natural sample which contains several layers
will have a relatively high water content
w
s
at S in Fig. 16.6(b). The slopes of both
normal compression lines are assumed to be approximately the same.
For a state at A in Fig. 16.6(a) there are two yield stresses, one
σ
ys
corresponding to
σ
yr
corresponding to reconstituted soil and there
are two possible yield stress ratios. Figure 16.6(a) illustrates compression of sedimented
and reconstituted samples of the same soil starting from the same state at A. They have
different yield stresses and they travel down different normal compression lines.
In theory the critical state lines for both sedimented and reconstituted samples are
the same because, by definition, it is the ultimate state reached after very large defor-
mations when the sedimented sample has become completely reconstituted by shear
straining. (In practice it is difficult to achieve these critical states in laboratory tests but
they may be reached in natural landslides.) The critical stress
sedimented bedded soil and the other
σ
c
and the state param-
eters
S
v
and
S
σ
are the same for both sedimented and reconstituted samples of the
same soil.
16.6 Influence of bonding and weathering
During vibration or creep compression only the water content of soil changes but
during bonding and weathering both the water content and the state boundary surface
may change. A detailed discussion of the effects of bonding and weathering is beyond
the scope of this topic and all I can do here is outline the basic features: for more
Figure 16.6
Compression of sedimented and reconstituted soil.
