Environmental Engineering Reference
In-Depth Information
The origin and geotechnical properties of volcanic soils and their
role in developing flank and sector collapses
R. del Potro
Costa Rican Volcano Observatory (OVSICORI-UNA), Universidad Nacional, Costa Rica
Department of Earth Sciences, University of Bristol, United Kingdom
M. hürlimann
Technical University of Catalonia (UPC), Barcelona, Spain
aBsTRacT: Giant volcanic landslides are one of the most hazardous geological processes. still, the
mechanisms that trigger them remain unresolved. Recent studies suggest that the presence of weak vol-
canic materials is likely to play an important role. herein, we present a study of the weakening effect of
weathering and hydrothermal alteration of phonolitic lavas, pyroclasts and ignimbrites from Tenerife.
a comprehensive geotechnical characterisation of these materials reveals that, from weathering, the weak-
est units are porous, sandy-silty, non-plastic soils (sM) that are cohesionless, with high peak strengths and
significantly lower residual strengths. in the case of hydrothermal alteration, the weakest units are porous,
silty, clay-rich, medium plasticity soils (Mh) with low cohesion values and varying angles of internal
friction (17-45°). secondary mineralogy produced by alteration, mainly halloysites and the presence of
bonding in weathered soils and kaolinites or alunites in hydrothermally altered soils, appears to control
the behaviour of the soils.
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inTRoDUcTion
agents include morphological, geological, tectonic
and climatic factors, but for volcanic slopes addi-
tional elements such as magmatic intrusions apply.
Unfortunately, our poor knowledge on the strength
and mechanical behaviour of volcanic material has
limited modelling efforts to quantify the different
hypothesis. in this way, a number of complex, but
invalid, models have been published. For example,
Voight et al. (1983), who were the first to study large
volcanic landslides, assumed that an angle of friction
of 40° and negligible cohesion was representative
of the internal strength at Mount st. helens when
deep seated landslides developed in May 1980. sub-
sequent field characterisation and measurements of
volcanic materials confirmed that volcanic rocks had
strengths in excess of those assumed by Voight et al.
(op. cit.), and future workers were forced to include
unjustified empirical correlations in order to reduce
them (i.e. disturbance factor D > 0 in the hoek and
Brown criterion (hoek et al., 2002)). in the last dec-
ade a number of scientific studies have addressed this
issue and focussed on the geotechnical properties of
volcanic rock masses (hernández et al., 2006; Moon
et al., 2005; serrano et al., 2002a; Watters et al., 2000).
The findings of these studies provided a first order
approach to quantifying the mechanical properties
of volcanic rock masses. a normalised compilation is
provided by del Potro and hürlimann (2008).
large volcanic landslides are common phenom-
ena at most volcanoes worldwide. The deposits
and morphologies that characterise these events
have been identified in subaerial and submarine
environments worldwide (e.g. siebert, 1996). The
importance of studying large volcanic landslides is
highlighted by their potential to cause significant
damage to areas up to tens of kilometres from the
volcano. There is also the potential for damage on
a larger scale when landslides enter the sea and
generate a tsunami. Volcanic landslides are gener-
ally more voluminous than non-volcanic events.
The largest volcanic landslides, which can reach
volumes in excess of hundreds of cubic kilome-
tres, occur as volcanic islands flank collapses (e.g.
Masson et al., 2002; Moore et al., 1989), similar
but relatively smaller events, with volumes in the
range 0.1-20 km
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, have been observed at strato-
volcanoes worldwide (e.g. siebert, 1996; Vallance
et al., 1995). in many cases, collapses are recurrent
phenomena at one same volcanic edifice.
Despite a great recent effort to understand the
mobility and emplacement mechanisms of the large
debris avalanches little work has been devoted to
understanding the mechanism of initiation of such
huge landslides. For most landslides, destabilising
