Deformational behaviour of pyroclastic rocks beneath the upper reservoir of the hydro-wind plant at El Hierro - Volcanic Rock Mechanics - page 55

energy as the water flows from one reservoir down
to the other, thus providing immediate and con-
tinuous electrical energy (5).
The existing diesel engine station at llanos
Blancos will supplement power generation in
exceptional circumstances, when wind and water
are insufficient to meet energy demands.
Besides being self-sufficient with clean energy,
the island will also have a guaranteed supply of
fresh water, one of the most prized resources, as
the system will work with water that has previously
been desalinated.
This project will turn the island of el hierro
into a worldwide reference in terms of alternatives
to fossil fuels.

The second theory explains the conical shape
of “la caldera” and the characteristics of the
deposits located in the eastern half of its perim-
eter, as being the result of a phreatomagmatic
eruption that came about when the dyke that
runs from north to south came into contact
with phreatic water that had accumulated in the
area.

3 siTe inVesTiGaTion

3.1 Site investigation works
Two investigation campaigns were carried out for
the Geological-Geotechnical study, the majority
of the site investigations being dedicated to study-
ing the locations for the upper and lower reser-
voirs. ( Table 1 ).

2 GeoloGical seTTinG

2.1 Geological setting
The age of the volcanic island of el hierro is esti-
mated at around 1.2 million years, making it the
youngest island in the canary archipelago. it was
formed by the successive overlapping of two sig-
nificant volcanic structures (Tiñor, 1.1-0.9 Myrs
old, and el Golfo, 0.5-0.1 Myrs old), and a lat-
ter dorsal ridge (carracedo, 2001), with three axes
that give the island its characteristic “Y” shape.
The upper reservoir is being built at “la cal-
dera”, a depression made up of volcanic materials
associated with the Tiñor structure, more spe-
cifically with its middle unit (the “tabular” unit,
formed by lava extrusions that dip 20-30° towards
the e-se) and upper layers (the “Ventejís” volcano
group, made up of pyroclastic rocks with a low
lava proportion).
Petrologically, the abovementioned extrusions
correspond to basaltic rocks. The geo-chemical
analysis of different samples of rock taken from
the perimeter of “la caldera” have enabled their
classification according to their content of sio 2,
as basic or ultra-basic rocks; with regard to their
content of Mgo, they are classified as basalts or
trachybasalts.

3.2 Boreholes and in-situ testing carried out
at the upper reservoir
eleven boreholes were drilled for the study of
the upper reservoir, using rotation for continu-
ous core recovery, whilst also carrying out pres-
suremeter tests using an elastemeter-2, model
4181 equipment (from oyo corporation, Japan)
( Table 2 ) and permeability tests. seven of the
boreholes were distributed across the bottom of
“la caldera” ( Fig. 2 ) to study the fill materials
and estimate the position of the bedrock; and
the four remaining boreholes were drilled in the
outer part of the eastern rim in order to analyse
the stability of the rim where previous geological
cartographies had revealed unfavourable geologi-
cal characteristics, such as the dipping of the lay-
ers towards the slope and the existence of at least
two layers of almagra interspersed amongst these
lava layers.

Table 1. Prospections carried out at the chosen loca-
tions for the reservoirs and buildings, and the route of
the pressure pipe.

Type of investigations *

2.2 Geological model of “La Caldera”
The study and investigation of “la caldera” for
the purposes of the construction of the upper res-
ervoir has given rise to at least two geological theo-
ries concerning its origins.
The first of these attributes its origins to the
collapse of a volcanic tube, with the materials that
formed the ceiling of this tube currently being
found in a fragmented condition as fill at the bot-
tom of the caldera.

location s. c. DPsh P.s.R T.e. s.e.V

Upper reservoir 11 11 9 9 2 6

lower reservoir 9 18 7 16 4 5

Pumping station 6 6 5 - - -

Turbine plant 4 2 - - - -

Pressure pipe - - - - - 3

* s, borehole; c, trial pit; DPsh, dynamic penetration;
P.s.R., seismic refraction survey; T.e., electrical tomogra-
phy profile; s.e.V., vertical electrical sounding.