3481
Technical Committee CFMS /
Comité technique CFMS
It is now seen that only Sample A shows a yield pressure, of
about 250 kPa. Sample B shows almost linear behaviour, while
Sample C shows steadily decreasing compressibility, or “strain
hardening” characteristics.
A general representation of soil compressibility, especially
over the pressure range of interest to geotechnical engineers, is
shown in Figure 5. This gives a far more realistic picture than
the conventional e-lot(p) plot. The almost universal use of the
log plot has created the belief that the compressibility of all
soils can be adequately represented by two straight lines on a
log graph, which is certainly not the case.
Pressure (linear scale)
Strain
Linear
Yielding (strain softening)
Strain hardening
Vertical yield
pressure
Yield from
structural breakdown
or preconsolidation
Strain hardening is typical
of dense soils with low
liquidity index
Strain softening is typical
of non-dense soils with high
liquidity index
Figure 5. A better representation of soil compressibility, valid
for all soils. (after Wesley, 2010).
It is a regrettable that the profession and those who teach soil
mechanics have not taken more notice of what Professor Nilmar
Janbu has been saying for many years. His message is
summarised in the following statement (Janbu, 1998):
“---
it remains a mystery why the international profession
still uses the awkward e-log p plots, and the incomplete
and useless coefficient C
c
which is not even determined
from the measured data, but from a constructed line
outside the measurements
---”.
Janbu made the above comments based on experience
with sedimentary soils. The mystery remains even
greater with residual soils. There is little doubt that if
teachers of soil mechanics always plotted results of
oedometer tests on undisturbed soils using both linear
and log scales they would very quickly realise how
misguided the continued use of the log scale is.
4 INFLUENCE OF HIGH PERMEABILITY
The high permeability of residual soils is caused by various
factors, including their relatively coarse nature, the presence of
unusual clay minerals, and particular forms of micro structure.
The high permeability has various practical implications and
students should be made aware of these in basic soil mechanics
courses. Only two will be described here; the first is the
determination of the coefficient of permeability from
oedometer tests, and the second is the short and long term
stability of cut slopes in clay.
Figure 6 shows typical root time graphs from conventional
oedometer tests on residual soils. According to one dimensional
consolidation theory these graphs should show an initial linear
section, from which the well known Taylor construction can be
used to determine the coefficient of consolidation. The graphs
in Figure 6 do not display this linear section, simply because
the pore pressure dissipates almost as soon as the load
increment is applied, and the shape of the graphs is a creep
phenomenon unrelated to the rate of pore pressure dissipation.
Volcanic ash soil
Waitemata clay
(w
Tropical red clay
eathered sandstone)
0 2 4 6
.
time
min
2
4
6
8
10
Compression (%)
Sample thickness = 2.0cm
Figure 6 Root time graphs from tests on residual soils.
It is not difficult to show that the highest value of the
coefficient of consolidation that can be reliably determined
from an oedometer test with a sample thickness of 2.0cm is
approximately 0.1m
2
/day (= 0.012cm
2
/sec.). Readings taken in
the first minute will only lie on a straight line if the c
v
value is
less than 0.1m
2
/day; many residual soils have higher values.
Because most geotechnical engineers and laboratory
technicians are unaware of this, the Taylor construction
continues to be regularly applied to graphs such as those in
Figure 6, and erroneously low values of c
v
are determined.
5 SLOPE STABILITY
The main trigger for slips or landslides in residual soil slopes is
intense and prolonged rainfall, a fact that reflects the relatively
high permeability of such soils. In the case of cut slopes,
therefore, it is very unlikely that behaviour during excavation
will be undrained. It is much more likely that a new long term
seepage pattern will develop as excavation proceeds. However,
this pattern will only be an average state, and there will be
frequent changes with time reflecting the weather changes. This
situation is illustrated in Figure 7, alongside the commonly
assumed behaviour of sedimentary soils. In residual slopes
changes in the water table and pore pressure occur in both a
regular seasonal pattern and in a random and unpredictable
manner as a result of sudden storm events. The challenge to the
geotechnical engineer is to estimate the worst case situation.
A further significant feature of slopes in residual soils is that
they are often much steeper than those in sedimentary soils.
This means that water tables may also be relatively steep, and if
analytical methods are used to assess stability, then care is
