Environmental Engineering Reference
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a few recent studies have used these values to test
the hypothesis of the different landslide driving/
triggering mechanisms (e.g. hürlimann et al.,
2000; Moon et al., 2005). Their preliminary results
show that a combination of increase in pore f luid
pressures and transient stress (regional seismic-
ity or large volcano tectonic events; i.e. caldera
formation) could cause flank or sector collapses.
however, it appears that volcanic rock masses
are too strong. herein, it has been suggested that
the presence of weak layers such as 'destructured'
pyroclastics (serrano et al., 2002b), residual soils
(hürlimann et al., 2001), hyaloclastites (schiffman
et al., 2006) or hot olivine cumulates (clague and
Denlinger, 1994) can be very significant in reduc-
ing the stability of a slope, and these have been
detected at many volcanoes which have experienced
instability. an alternative weakening mechanism
is hydrothermal alteration that can reduce lavas
to clay-rich soils (Watters and Delahaut, 1995)
and is thought to have been critical in the genera-
tion of many volcanic landslides (e.g. lópez and
Williams, 1993; opfergelt et al., 2006). in this study
we present a geotechnical study of the weakening
effect of weathering of phonolitic ignimbrites and
that of hydrothermal alteration of phonolitic lavas
and pyroclasts.
(sk1) and two from the finest sectors of sa and
sk (sa2 and sk2 respectively) were also collected.
Both samples of sa were collected from the extru-
sion location of a prominent fumarole.
The field survey on Tenerife revealed that, away
from the most recent crater of Teide volcano, wide-
spread residual soils (also called paleosols) are the
weakest materials. Red residual soil units are found
located directly above ignimbrites—pyroclastic
deposits produced by explosive eruptions of pho-
nolitic magmas. on Tenerife, these deposits have
a thickness of several decimetres and are found
as part of the central volcanic complex, exposed
by erosion on the scarps of large island flank col-
lapses. Weathering processes change the phonolitic
pyroclastic deposit into a residual soil. This soil
is then eventually covered by a lava flow or pyro-
clastic deposits which further modify the soil
through thermal processes. hence, such residual
soils are characterised by a double cementation
or bonding that is caused first by the lithifica-
tion processes during the deposition of the hot
pyroclastic material and secondly by the thermal
alteration ('baking') induced during the emplace-
ment of the hot unit overlaying the soil layer. Two
residual soil samples (Rs1 and Rs3) were collected
a remnant of the northern flank of the central
las cañadas edifice. samples Rs1 was cut from
the western lateral landslide scarp of the la oro-
tava near a site called 'Piedra de los Pastores'. The
outcrop is exposed along a forestry road and it is
2
Volcanic ResiDUal soils
Two common mechanisms for the genesis of vol-
canic residual soils are climatic weathering and
hydrothermal alteration. in order to attempt quan-
tifying the expected decrease in shear strength,
relative to the parent material, we have collected
several samples of volcanic residual soils in the
island of Tenerife. hydrothermally altered soils
are the weakest materials exposed on Teide vol-
cano—the most recently active volcano in the
central volcanic complex. Three hydrothermally
altered soil unit (sa, se and sk) have been col-
lected from inside the small summit crater (
Fig. 1
).
These soils are hydrothermally altered products
from phonolitic lava flows of the last summit erup-
tion (∼1 ka ago). Persistent hydrothermal activity
has changed lavas to clay-rich soils by sulphate
leaching. Units sa and se are light coloured soils
exposed along the summit fumarole field and are
both found as extensive units comprising a sig-
nificant volume of ∼5 cm sulphur crystals as well
as ∼20 cm clasts from the parent rock. Unit sk, on
the other hand, is a red, more homogeneous unit
found on the southern inner rim of the summit
crater. Undisturbed samples of sa and se (sa1,
se1a and se1b) were extracted from a depth of
between 10 and 40 cm using a 12 cm-wide metal
tube. in addition, remoulded samples from sk
Figure 1. simplified geological map of the central
part of Tenerife island. inset shows the location of the
island of Tenerife within the canary archipelago. The
thick black line denotes the position of the las cañadas
caldera wall and the dotted lines the sub-aerial extend
of the three most recent island flank collapses. The loca-
tion of the weathered residual soils is indicated by letter
a and the location of the hydrothermally altered soils is
indicated by letter B.
