Civil Engineering Reference
In-Depth Information
the probable variation of stiffness in the ground can be very large. For design it is
necessary to choose a value of stiffness which corresponds to the mean strains in the
ground, as discussed by Atkinson (2000).
18.8 Parameters related to description and
classification
Some parameters such as the critical friction angle
φ
c
are material parameters and
they depend only on the nature of the grains. Other parameters, such as undrained
strength
s
u
, are state dependent parameters and they depend both on the nature of
the grains and on the current state. There may even be parameters which have the
same value for all soils. In the following sections I will give some correlations between
values for some soil parameters and the soil descriptions and classifications described
in Chapter 5. Some of these are empirical but some arise from the definitions of the
parameters themselves.
(a) Undrained strength and liquidity index
Undrained strength is a state dependent parameter and it depends on voids ratio or
water content, as shown in Fig. 9.5. The critical state undrained strength of soil at its
liquid limit is about 1.7 kPa and at its plastic limit it is about 170 kPa. The liquidity
index
I
l
given by Eq. (5.10) defines the current water content in terms of the Atterberg
limits described in Sec. 5.6. There is a linear relationship between liquidity index and
the logarithm of undrained strength, as shown in Fig. 18.7.
(b) Critical state friction angle
The critical state friction angle
φ
c
is a material parameter and it depends on the nature
of the grains. For fine grained soils it varies with plasticity index
I
p
, ranging from
less than 20
◦
for high plasticity soils to about 28
◦
for low plasticity soils. For coarse
φ
c
depends on the shape and roughness of the grains and it ranges from
about 30
◦
for soils with smooth rounded grains to more than 40
◦
for soils with rough
angular grains. Typical values for
grained soils
φ
c
are given by Muir Wood (1991).
Figure 18.7
Variation of undrained strength with liquidity index.
