3480
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
There is no reason at all to use a log scale for pressure when
illustrating the compression behaviour of residual soils. The
graphs have therefore been re-plotted using a linear scale in
Figure 2(b). These graphs show a very different picture; there
is no indication at all of “pre-consolidation” pressures. Those
inferred from the log plot are not soil properties; they are purely
the product of the way the data are plotted
B9-3M OCR = 4.0
B9-4M OCR = 3.6
B7-5M OCR = 3.4
B8-7M OCR = 3.4
B8-8M OCR = 3.3
B7-9M OCR = 1.1
2.0
1.6
1.2
0.8
0.4
10 100 1000 10000
Void ratio
0 500 1000 1500 2000
10
20
30
Compression (%)
Pressure (kPa)
(a) log scale
(b) linear scale
Figure 2. Misinterpretation of the e-log(p) graph (after Wesley,
2000).
A second example of the misleading nature of log plots is given
in Figure 3, which shows the results of oedometer tests on a
residual clay found in the Auckland region of New Zealand.
The graph using a log scale suggests the existence of a pre-
consolidation pressure at about 600 kPa, while the linear plot
shows no trace of this; in fact the behaviour is almost linear.
While residual soils, by definition, cannot have pre-
consolidation pressures because they are not formed by a
consolidation process, they may still show a significant increase
in compressibility at certain stress levels. This arises because
some residual soils are highly structured and at a certain stress
level this structure begins to collapse causing the increased
compressibility This stress is best termed a vertical yield
pressure rather than a pre-consolidation pressure.
Some residual soils can show extremely variable
compression behaviour, such as that illustrated in Figure 4
which shows oedometer tests on three samples of clay derived
from the weathering of andesitic volcanic ash. When plotted
using a log scale, the behavour appears similar, and yield
pressure could be inferred from all three graphs. However,
when re-plotted using a linear scale the picture is very different.
0 500 1000 1500
Pressure (kPa)
(b) linear plot
(a) log plot
Vertical strain (%)
Vertical strain (%)
5
10
14
Figure 3. Behaviour of an Auckland residual soil (after Pender
et al, 2000)
Pressure (kPa)
10 50 100 500 1000 5000
0 100 200 300 400 500
2
Void ratio
Pressure (kPa)
Compression (%)
A
A
B
B
C
C
(a) log scale
(b) linear scale
4
6
8
10
12
14
Figure 4. Behaviour of volcanic ash soils
