Environmental Engineering Reference
In-Depth Information
Geotechnical characterization of el Verodal Tunnel
in el hierro, spain
M.c. lópez-Felipe
Estudios del Terreno S.L., Spain
J.T. Fernández-soldevilla
Obras Civiles de Tenerife S.L., Spain
aBsTRacT: el hierro island, 0,2 million years old, is the youngest of the canary islands. since it
was recognized Unesco Biosphere Reserve in 2000, the road infrastructure to be planned has to cause
a minimum impact on the landscape of the island. This applies to el Verodal Tunnel to be built in the
nW side of the island. Due not only to orography but also to administrative constraints, site studies
are scarce. Moreover, the implementation of the existing rock mass classifications for volcanic rocks
makes the design of the tunnel harder due to the great variability shown by these materials.
1
inTRoDUcTion
There are 3 units to be considered: lower, middle
and upper.
The lower section is characterized by a stack
of thin lava interspersed with pyroclastic levels
crossed by a strong network of dikes. The mid-
dle section, known as Tabular or “Plateau de san
andrés” consists of nearly horizontal lava flows.
The upper section is characterized by the emission
of pyroclastic rocks with subordinate lavas, known
as Group Ventejis Picos-Moles Volcanoes.
Due to the rapid growth of the island, slope
instabilities occurred. They gave place to the land-
slide of the west side of Tiñor edifice, known as
“The Tiñor landslide”. Though it is currently hid-
den by the subsequent emission of the el Golfo-las
Playas edifice and volcanism of the ridges, this
landslide has been inferred from surface geologi-
cal studies and information extracted from water
galleries.
The island of el hierro with its 268 km
2
and
1500 m altitude is the westernmost, smallest and
geologically youngest of the canary islands. Due
to its geographical position at western edge of the
known world, it was referenced as meridian 0 for
several centuries. it is the emerged part of an oce-
anic volcano, mainly basaltic composition, whose
submarine base stands on the atlantic abyssal
plain, about 3700-4000 m depth.
2
sTRaTiGRaPhY DescRiPTion
oF GeoloGical UniTs
Field criteria, as well as geochronological and pale-
omagnetic criteria have been used for the general
set of stratigraphy. Given these considerations, the
following volcanic edifices can be considered:
2.1
Tiñor ediice
2.2
El Golfo-Las Playas Edifice (545-176 Ka)
The magmatic activity in the area restarts WsW
Tiñor edifice after a rest period of 350 ka. The
resulting edifice is called “el Golfo-las Playas
edifice”, as it outcrops in this area.
This is the largest shield volcano in the evolu-
tion of the island with 20 km basal diameter and
2000 m high. however, the visible portion is about
600 m high. This is so because it was mostly plunged
into the ocean, (“landsliding of el Golfo”) and
because it is covered by recent eruptions.
lava flows are nearly horizontal in its central
area but reach strong dips towards the outer area.
2.2
el Golfo-las Playas edifice
2.3
Volcanism of the ridges or the structural
axes
Volcanism of the ridges ss
emissions that fill el Golfo valley
Recent-subrecent emissions
2.1
Tiñor Edifice (1,2-0,88 Ma) million years
it is the oldest emerged area of the island. it is
located in the ne area and inside el Golfo-las
Playas edifice headwall of the landslide scarp.
This edifice was mainly developed from ne rift.
