Environmental Engineering Reference
In-Depth Information
concerning stratigraphy, there are two units to be
considered: the lower and higher: The lower unit
is characterized by a predominance of pyroclastic
and hydromagmatic deposits with subordinated
basaltic lava flows and a strong intrusion of dykes.
The higher unit has an accumulation of trachyba-
saltic and trachytic lava flows with subordinated
lapilli intercalations.
This edifice began its process of gravitational
destruction at the same time it was being built,
firstly with the landsliding of las Playas i, then el
Julan, then again las Playas ii and finally the great
collapse towards the n about 21,000 years ago:
the landsliding of el Golfo. avalanche deposits of
these landslides have been identified on the seabed.
cartographic purposes there are 3 units: the dorsal
emissions ss, emissions that fill el Golfo valley and
recent and subrecent emissions.
The emission points in this edifice are those
located along the structural axes.
Most of the emission points which filled the
valley of el Golfo are located at the headwall of
the landslide scarp. lava emissions were very large
and partly filled the depression, reaching depths
of 200-300 m (data obtained from water galleries).
They form the marine platform.
Recent and subrecent emisssions appear on the
ends of the structural axes. They form sharp and
rough scoriaceous lava flows (locally known as
“
malpaís”
) and well preserved volcanic cones.
2.3
Volcanism of the ridges or the structural axes
(158 Ka-2,5 Ka)
This third edifice develops from rifts that converge
in the center of the island with angles of 120°. For
3
GeoTechnics FoR The
consTRUcTion oF a TUnnel
The “Project for the improvement of the hi-50
road, section: Frontera-sabinosa (Verodal
connection)” involves the construction of a tunnel,
known as “The Verodal Tunnel”, 589.60 m long.
Both ends will be artificial tunnels.
only two rock drillings have been made, due to
rough orography, one at the beginning and another
at the end.
With this information along with geological
mapping, it is inferred that the tunnel will affect
two types of geotechnical units: basaltic pyroclasts
from el Golfo-las Playas edifice (PBG) and vol-
canism of the ridges (PBD) as well as basaltic lava
flows from el Golfo-las Playas edifice (cBG).
Two units can be distinguished within basaltic
lava flows: central massive area and upper and
lower scoriaceous areas.
3.1
Massive areas of the lava flows
The geotechnical characteristics of these types
of materials from the viewpoint of geomechanics
Table 1. Bieniawski rock mass classification for massive
areas of basaltic lava.
Bieniawski classifications (1989)
score
strength of intact rock
material (MPa)
90
7
RQD
40%
6
spacing of discontinuities
0.3-1
10
condition of
discontinuities
Rough undisturbed
19
Figure 1. landslides occurred in el hierro. a) Map of
the scars of the lanslide, indicating the resulting paleo-
shorelines after the collapses (modified from carracedo
et al., 1997; B) image of the seabed around el hierro
obtained from multibeam bathymetry data (modified
from Masson et al., 2002).
Groundwater
Dry
15
Rating adjustment for
discuntinuity orientation
10%
RMR
47
