Civil Engineering Reference
In-Depth Information
detailed discussion there are papers by Lerouil and Vaughan (1990) and Coop and
Atkinson (1993).
The principal mechanism of bonding is by deposition of additional material, often
calcium carbonate, from the groundwater. This has the dual effect of reducing the
water content and shifting the state boundary surface. However, the critical state
corresponds to relatively large straining when the soil is essentially reconstituted and
to reach these states bondingmust fracture. This means that the critical states of bonded
and unbonded soil will be about the same and the main influence of bonding will be
on the position of the state boundary surface and on yield and peak strength.
Figure 16.7(a) shows normal compression lines for bonded soil and for the same
soil after it has been reconstituted. Soil is sedimented at a state near A. The path
A
B represents a reduction in water content due to deposition of bonding material at
constant stress. At the same time the normal compression line moves to the right as the
yield stress increases due to bonding. The path B
→
C represents compression
of initially bonded soil and part of this is outside the normal compression line for
reconstituted soil. The yield point
Y
b
lies on the state boundary surface for bonded
soil but. after yield, the state moves towards the line for reconstituted soil as the
cementing breaks with continuing strain. Notice the relatively large compression from
Y
b
to C as the cementing fractures.
There is, however, only one critical state line so values of the normalising parameter
σ
c
can be obtained unambiguously. (This is the principal reason for selecting
→
Y
b
→
σ
c
as the
normalizing parameter rather than the equivalent stress on the normal compression
line
σ
e
; see Sec. 9.6.)
Figure 16.7(b) illustrates the state path A
C corresponding to the
loading path in Fig. 16.7(a) with stresses normalized with respect to
→
B
→
Y
b
→
σ
c
. (The path is
for loading with constant stress ratio.) Part of the path lies outside the state boundary
surface for reconstituted soil and the yield point
Y
b
lies on the state boundary surface
for bonded soil. The states at A and C lie at the same point on the state boundary
surface for reconstituted soil and the state B is overconsolidated. The distance between
the state boundary surfaces for bonded and reconstituted soil depends principally on
the strength and amount of the cementing.
Figure 16.7
Behaviour of bonded soil.
