The origin and geotechnical properties of volcanic soils and their role in developing f lank and sector collapses - Volcanic Rock Mechanics - page 150

the following stress-strain parameters: compression
index c c = 0.287, average coefficient of compressi-
bility a v = 0.087 MPa -1 and average confined Young
modulus e m = 27.8 MPa. Preliminary results on the
consolidation behaviour of the summit soil sam-
ples (sa, se and sk) show no volume increase
during the initial unloaded saturation phase. The
consolidations of the undisturbed samples indicate
total vertical strains ranging from about 4.5% for
a normal load of 100 kPa up to 12.5% for a maxi-
mum normal load of ∼450 kPa. in contrast, for
remoulded samples, the measured absolute vertical
strains range between 5 and 27% for the normal
load range of 49-371 kPa. consolidation curves
indicate that primary compressions were achieved
instantly after loading.

normal loads the angle of internal friction of soil
Rs1 decreases to ∼30° and this is very similar to
the extrapolated values from the hydrothermally
altered soil ( Fig. 3b ) . Figure 4 shows the normal-
ised shear strength and residual mobilised angles
of internal friction of remoulded samples tested
in ring shear conditions. in spite of the low and
small range of normal loads allowed by the exper-
imental settings, a relationship with increasing
normal load is clearly visible, as is the difference
in residual shear strength of the residual soils
tested.

3.2.4 Triaxial tests
Triaxial test results fall into three main groups:
permeability, volumetric behaviour, and stress-
strain behaviour during failure. For the first,
constant flow-rate permeability test, carried out
prior to consolidation of the probes, provide a
permeability range, k = 1.04 ⋅10 -4 -4.8 ⋅10 -4 cm/s.
These values are in agreement with those from
the oedometer tests. For the second set of results,
volumetric changes have been collected during the
confining phase of the tests. The results, shown
in Figure 5, indicate the lack of an explicit yield
strength or collapse. however, there is a clear con-
trast in behaviour above an effective confining
stress of around 700 kPa. Beyond this value the
e-log(p′) curves are parallel and define a normal
compression line. again, these results are in good
agreement with values from oedometer tests, and
the Rs1 shows values close to those of bonded
residual soils (Blight, 1997).
The analysis of the stress-strain behaviour
starts with the undrained tests results are shown
in Figure 6. Generally, the maximum shear stress
increases rapidly up to failure, which occurs at

3.2.3 Shear tests
The results obtained from the drained direct shear
tests can be used to produce Mohr-coulomb enve-
lopes in order to determine the strength param-
eters of the soil ( Fig. 3 ) . hydrothermally altered
soils, tested at low normal loads, show no differ-
ence between peak and residual shear strength.
These soils have low-cohesion values (∼100 kPa)
and angles of internal friction in the range 16-34°.
Weathered soil Rs1 shows distinctive peak and
residual behaviour as well as a clear decrease in
strength relative to increasing confining pressure.
at low normal loads sample Rs1 has an angle
of internal friction ∼40°, clearly higher than the
hydrothermally altered soils. however, for higher

Figure 3. Mohr envelopes from direct shear test results. Figure 4. Ring shear test results.