Environmental Engineering Reference
In-Depth Information
3
GeoloGical anD GeoMechanical
conDiTions oF The PRe-FailURe
Volcanic eDiFice
are occasionally present with sizes ranging from 1
to 3 cm. secondary minerals are observed inside
them. Fracture zones and slikenside surfaces have
been identified.
Pressuremeter tests, borehole geophysics (sonic,
acoustic, televiewer camera, calliper) and labora-
tory tests have been carried out. laboratory and
geophysical results are not yet available at time of
writing this paper (2009, December).
The deformational properties of the hyaloclas-
tites were obtained from 16 pressuremeter tests car-
ried out at different depths in one of the boreholes
ranged from 10 MPa to 3212 MPa. Most frequently
intervals range from 50 to 80 MPa and from 125 to
135 MPa. a representative value of 129 MPa has
been considered for modelling purposes.
The simplified geological model for the pre-
failure edifice is shown in Figure 2. The materi-
als were grouped in those corresponding to the
emerged edifice (above sea level) and the submarine
edifice (below sea level). The following lithological
units have been distinguished as representative of
the simplified geological model of Tenerife island
flanks for geomechanical purposes:
Forming the flanks of the island:
1. lava flows (60%) and autoclastic breccias
(40%).
2. lava flows (70%) and autoclastic breccias
(30%).
3a. altered lava flows (90%) and pyroclastic
deposits (10%), below the water level.
3b. altered lava flows (80%), pyroclastic deposits
(10%) and dykes (10%), below the water level.
Forming the structural axis of the island:
4. lava flows (30%), autoclastic breccias (20%),
pyroclastic deposits (40%) and dykes (10%).
5. altered lava flows (40%), pyroclastic deposits
(30%) and dykes (30%), below the water level.
Forming the submarine edifice:
6a. hyaloclastites (70%) and pillow lavas (30%).
in order to analyze the mechanical behavior of the
flanks of the volcanic edifice, the geological, geo-
morphological and geomechanical representative
modeling of the pre-landslide edifice have been
prepared.
The different factors influencing volcanic land-
slide processes have been considered: morphology,
lithology, geological structure, hydrogeological con-
ditions, geomechanical properties, weathering and
alteration and state of stress. also the triggering
factors on stability have been studied: volcanic and
hydrothermal activity, dike intrusion and seismic
activity.
The geometrical model before the occurrence of
Güímar and la orotava rockslides has been assessed
considering paleomorphological data obtained from
the slopes and lateral scarps of the volcanic edifice
not affected by landsliding processes. The morpho-
logical features of the submarine slopes have been
obtained from bathymetric data analysis.
it has been assumed that the ground water con-
ditions for the pre-failure edifice could be similar
to those encountered before intensive ground water
exploitation of the island aquifers had taken place.
a water table at 600-700 m below surface was esti-
mated according with ground water records, with
exception of the coastal areas. in the central part
of the edifice due to the presence of a large number
of dykes the ground water levels should be higher.
Geological and geotechnical data were recorded
from field survey and from the extensive network
of small diameter tunnels, with a total length
over of 4000 km, excavated in the inland flanks
for ground water supply purposes. Geotechnical
properties of the volcanic materials of the emerged
edifice have been also obtained from engineering
geological surveys (seisdedos, 2008; González de
Vallejo
et al
., 2008).
With respect to the geological and geome-
chanical data of the submarine edifice, only mor-
phological and tectonic data are available from
oceanographic surveys. in the easter corner of
the island site investigations have been carried out
where submarine rocks are outcropping (
Fig. 1
).
Three boreholes have been drilled in hyaloclastites
reaching one of them 200 m depth.
hyaloclastites and basaltic lavas have been the
rock materials core drilled in the three boreholes,
although hyaloclastite has been the predominant
lithology. hyaloclastites are composed of clastic
particles of irregular shape and sizes ranging
from 0.5 to 3 cm, forming a green, grey or brown
coloured breccia. This material is poorly consoli-
dated and weakly cemented. Voids and vacuoles
Table 1.
hyaloclastites pressuremeter moduli (e
p
).
Depth
m
e
MPa
Depth
m
e
MPa
23.5
159
95
1833
35
48
96
235
38.5
169
102
262
47.5
60
103
10
60
58
107
903
62
81
107.5
123
74
145
118
335
89
824
118.5
3212
