Civil Engineering Reference
In-Depth Information
8.7 Summary
1. The basic mechanism of compression in soils is by rearrangement of the grains.
In coarse-grained soils this may be accompanied by fracturing of the soil grains
and in fine-grained soils by compression or swelling of clay particles.
2. The behaviour of soil during isotropic compression and swelling is given by
ln
p
v
=
N
−
λ
(8.2)
ln
p
v
=
v
κ
−
κ
(8.3)
and
N
are material parameters; their values depend only
on the nature of the grains. Equation (8.2) is for normally consolidated soil and
Eq. (8.3) is for overconsolidated soil.
3. Equations (8.2) and (8.3) demonstrate that the stiffness of soil is non-linear (i.e.
the bulk modulus is not a constant) when it is both normally consolidated and
overconsolidated.
4. Equation (8.2) represents the normal compression line. The state of a soil cannot
usually lie outside this line and moves below the line on unloading when the soil
becomes overconsolidated. The yield stress ratio
Y
p
is given by
The parameters
λ
,
κ
p
y
p
0
Y
p
=
(8.8)
where
p
y
is the current yield stress.
5. Normally the state of soil is changed only by loading and unloading and the state
moves on the current swelling and recompression line or on the normal compres-
sion line. The state of a clay may also change due to creep and the state of a sand
may change due to vibration.
6. There is a critical state line which separates the wet side from the dry side. Lightly
overconsolidated clays and loose sands are on the wet side of the critical line while
heavily overconsolidated clays and dense sands are on the dry side.
7. The behaviour of soil during one-dimensional compression and swelling is similar
to that for isotropic loading and is given by
σ
z
e
=
e
0
−
C
c
log
(8.10)
σ
z
e
=
e
κ
−
C
s
log
(8.11)
The parameters
e
0
,
C
c
and
C
s
are material parameters.
Worked examples
Example 8.1: Analysis of an isotropic compression test
Table 8.1 gives results obtained
from an isotropic test. The data are shown plotted in Fig. 8.13. Scaling from the dia-
gram,
0.05. Projecting the lines back to
p
=
1.0 kPa (i.e. ln
p
=
λ
=
0.20 and
κ
=
0),
