Peak states - The Mechanics of Soils and Foundations - page 145

on shearing, it compresses and reaches its critical state without a peak, as in Fig. 10.2.
As a consequence peak states are associated with dense or overconsolidated soils on
the dry side which dilate on shearing.
In Fig. 10.4 the peak state lines have been terminated at low stresses at points such
as B 1 and B 2 and peak states at low stresses are not given by Eq. (10.1). This means
that the cohesion intercept c pe is not the shear stress which the soil can sustain at zero
stress and it is merely a parameter required to define the Mohr-Coulomb equation.
Since, in this case, these peak states apply equally for clean sand and reconstituted
clays this cohesion intercept should not be associated with cementing or interparticle
attraction in clays.
In order to take account of the different voids ratios e 1 and e 2 in Fig. 10.4, we can
make use of the normalizing parameter

σ c described in Chapter 9. Figure 10.5 shows
the peak state lines from Fig. 10.4 normalized with respect to

σ c . Now all the peak
state lines for different voids ratios reduce to the single line BA and A is the critical
state point.
The equation of the line BA is

τ p
σ c =

c p + σ p
σ c

φ p

tan (10.2)

where

c pe
σ c

c p = (10.3)

From Eqs. (9.8) and (10.3) the peak cohesion intercept c pe in Eq. (10.1) is
log c pe
c p

e − e

= (10.4)

C c

and so c pe decreases with increasing voids ratio and, for a given normal effective
stress, the peak strength decreases with increasing water content. From the geome-
try of Fig. 10.5

c p = φ c − φ p tan tan (10.5)

Figure 10.5 Normalized peak and critical states for shear tests.