Environmental Engineering Reference
In-Depth Information
The plausibility of widespread residual soils is
high if they develop at the top of large ignimbrite
units, hence weak halloysitic residual soils such as
the one tested here, appear to be perfect candidates
for shear surfaces to develop through causing large
flank island collapses. in the case of the hydrother-
mally altered soils from advanced argillic altera-
tion, exposed at the summit region, it is harder to
determine their extension at depth and hence their
significance in sector and flank collaspes. Taking
the hydrothermal conditions described for stra-
tovolcanoes (e.g. Frank, 1995; Zimbelman et al.,
2005), alunite minerals in association with kaolinite
form in surface steam-heated environments, but
there is also evidence to support that these miner-
als form in deep magmatic hydrothermal environ-
ments too. Frank (op. cit.) suggests that superficial
zones of alteration go down possibly to a depth of
1000 m. Following more extensive hydrothermal
models for volcanic hydrothermal systems as well
as for epithermal and porphyry ore deposits (e.g.
sales and Meyer, 1948), it is also possible that areas
of near-neutral, intermediate-argillic, montmoril-
lionite-rich alteration are present surrounding and
grading into the advanced argillic, alunite-kaolinite
alteration of the central-system. These units appear
not to be exposed on Teide, although this may be
because recent lava fields have covered them.
in this study are therefore perfect candidates for
the potential failure surfaces of large volcanic
landslides.
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conclUsions
The origin of residual volcanic soil units and their
role in the development of large volcanic landslides
has been studied through units of weather and
hydrothermally altered soils exposed on Tenerife
island. These soils have been studied by carrying
out standard laboratory tests. The results highlight
the significance of the structure of the clay frac-
tion in controlling the shear strength. They also
highlight the presence of a bond in the weathered
residual soils. in fact, geotechnical tests revealed
that the mechanical behaviour of the residual soil
changes greatly if this bonding is broken. how-
ever, because the bonded structure generally fails
when the effective normal stress surpasses the yield
strength of the bonding, in the case of large vol-
canic landslides with thicknesses up to several hun-
dred meters, the high overburden easily exceeds this
value. in the light of the phenomena studied here,
weathered residual soils have a residual strength
value. This work brings to light the necessity to
test samples at high normal and confining stresses
in order to better constrain their real behaviour
within a volcanic edifice.
in any case, the low shear strength values of the
soil units analysed reduces the overall strength of
a volcanic edifice units such as the one analysed
