Civil Engineering Reference
In-Depth Information
(e) Peak strength
The peak strength of soil discussed in Chapter 10 can be represented by a linear
Mohr-Coulomb criterion given by Eq. (10.6) which contains the parameter
φ
p
. This
φ
p
is a material
parameter. Alternatively the peak strength can be represented by a simple power law
criterion given by Eq. (10.14). This contains a parameter B which is also a material
parameter.
σ
c
which contains the water content, so
includes the critical stress
(f) G
0
The shear modulus for very small strain
G
0
was described in Sec. 13.6. It is related
to the current stress and overconsolidation ratio by Eq. (13.8) and so it is a state
dependent parameter. Equation 13.8 contains parameters
A
,
n
and
m
which are related
to the plasticity index
I
p
(Viggiani and Atkinson, 1995) and so they are material
parameters.
From these simple analyses there are a number of important parameters which are
material parameters and which are related to simple descriptions and classification
parameters, such as the Atterberg limits. Some state dependent parameters such as
G
0
are related to the current state through material parameters.
These relationships between soil parameters and soil descriptions and classifications
are helpful for preliminary design before a ground investigation has been completed.
They are also useful for validation of results obtained from laboratory and in situ tests.
If parameter values measured in tests differ from those derived from descriptions and
classifications you should find out why. It may be that the test results were wrong or
it may be that the soil has some special properties or the effects of its structure are
important.
18.9 States of soils in the ground
Most soils were deposited through natural geological processes then compressed and
swelled during subsequent deposition and erosion. For example London clay was
deposited in a shallow sea and the surface level was once of the order of 200 m
above present ground level; it is overconsolidated. In contrast the soils below the
nearby Thames marshes have the same nature as the London clay because that is
from where they were eroded, but the ground level has never been higher than the
present: they are normally consolidated. The ground conditions in the Thames estuary
are like those shown in Fig. 4.4 and the states of the clay soils vary with depth in
predictable ways.
Changes of stress and water content in soils during deposition and erosion were
described in Chapter 16. Figure 16.2(d) shows qualitatively the variations of water
content with depth in normally consolidated and overconsolidated soils. When soil
is freshly deposited its water content is close to its liquid limit. When soil is at
its plastic limit its undrained strength
s
u
is about 170 kPa, it has been compressed
under an effective stress of about 800 kPa which would correspond to a depth of
about 80 m. Figure 18.9(a) shows the variation of water content with depth for nor-
mally consolidated and overconsolidated soils. Near the surface the water contents
