Environmental Engineering Reference
In-Depth Information
Tazo, alojera areas at the northeast most part of
the island, the head areas of “el Golfo” and “las
Playas” depressions in el hierro and finally along
the deep ravines and coast cliffs of the “anaga”
and “Teno” massifs, the wall of the “cañadas”
caldera, head and edges of the “Güimar” and “la
orotava” valleys in Tenerife island. There, slope
gradient vary from 65-50º to 30-26º and almost
vertical in cliff areas. Most of these areas are the
result of previous landslides processes and conse-
quently, there is not expected an early occurrence
of giant mass wasting processes. Rocky debris and
small avalanches are expected to be the mainly sub-
sequent erosive mechanisms.
By contrast, accumulation of volcanic lava flows
and interbedded pyroclastic layer is the result of
cycles of continuous volcanic eruptions that may
evolve to the build-up of large steeped edifices with
poorly stabilized slopes and high risk of landslides.
The growth of such unstable volcanic edifices may
take place over previously collapsed areas filling
the consequent deeply eroded depressions with
the consequent possibility of later landslides. it is
assumed that the high gradient slopes originated
by accumulation of lavas and pyroclasts, constitute
zones of potential hazard of landslide and hence,
final discussion will be focused on them. examples
of these steep areas highlighted here are “el Julan”
(sW flank of el hierro island) which exhibit an
average gradient slope of 25º, the volcanic rift of
“cumbre Vieja” (s of la Palma island) with an
average gradient slope of up to 20º and “Teide
stratovolcano”, where the gradients of the flanks
Resistance parameters of rock masses such as
cohesion and friction angle may be deduced from
the hoek and Brown criterion (serrano and olalla,
1994) by means of the resistance-related properties
of intact rock and the geological conditions, namely
joints, fissures and alteration degree. Both cohesion
and friction angle could be quantified through the
Geological strength index or Gsi (e.g. hoek
et al
.,
1992) that may be easily estimated through the
method of Marinos & hoek (2000) and Marinos
et al
. (2005) or the RMR (the Rock Mass Rating
from Bieniawski, 1989), which depends on param-
eters such as the Gsi among others.
The Mohr-coulomb fits: cohesion and friction
angle of the rock masses for different rocks were
estimated by means of the hoek and Brown failure
criterion (hoek
et al
., 2002) based on previous geo-
mechanical data of intact rock (Rodriguez-losada
et al
., 2007; Rodriguez-losada
et al
., 2009).
2
The Role oF The loW ceMenTeD
PYRoclasTs
low cemented pyroclasts conform significant
deposits outcropping in all the islands in the form
of air fall salic deposits (pumice) or lipilli depos-
its, both being either slightly cemented or non-
cemented fragments, that could be considered as
typical examples of macroporous rocks. in this
sense, special and complex geotechnical behav-
iour is found for these materials. at low stress
levels pyroclasts behave like a rock with very high
deformation moduli; however, under high stress
levels the internal structure is destroyed and the
rock deformability dramatically increases, behav-
ing as a soil (Uriel & serrano, 1973; 1976). This
phenomenon is known as mechanical collapse and
the involved materials as mechanically collaps-
ible. Under isotropic compression, the pyroclastic
deposits react in one of the two following ways:
a) continuous increase of the specific weight and
stiffness with no failure processes and b) continu-
ous increase of the specific weight until it reaches
a threshold value at which, a sudden collapse takes
place.
Under laboratory experiences, it has been
noted that intact pyroclastic samples may experi-
ence at least two consecutive collapses: a first one
produced by breakage of particle contacts and a
Figure 1. location of “el Julan” and “san andres”
fault (el hierro), “cumbre Vieja” (la Palma) and
“Teide” (Tenerife).
