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contribution to geotechnical characterization of Basaltic pyroclasts
a.M. Malheiro
Civil Engineering Regional Laboratory, Azores, Portugal
J.F.V. sousa
Civil Engineering Regional Laboratory, Madeira, Portugal
F.M. Marques
Civil Engineering Regional Laboratory, Azores, Portugal
D.M. sousa
Civil Engineering Regional Laboratory, Madeira, Portugal
aBsTRacT: Both the azores and Madeira islands, located in north atlantic ocean, are of volcanic
nature. The present work focuses on the geotechnical characterisation of basaltic pyroclasts from the
azores and Madeira, in order to get some comparison among them. in order to characterize, evaluate the
geomechanical properties and get some more geotechnical data about basaltic pyroclasts, some samples
were collected in both archipelagos, to do some laboratory tests. in situ tests were also made with these
materials. Results include data on sPT tests, plate load tests, los angeles tests, in situ dry density and
specific weight tests, determination of particle size distribution, compaction and cBR tests and consoli-
dated drained (cD) direct shear tests. some correlations between several properties are presented, namely
between the strength and the deformability of volcanic materials. Finally, some considerations are made
about the potentially utilizations and problems related to engineering applications.
1
inTRoDUcTion
in this context, this paper aims to contribute for
a better knowledge of some geotechnical aspects
of basaltic pyroclasts, given its specificities, aiming
an eventual wider application, as well as to improve
the current uses.
Being the Madeira and the azores archipelagos
of volcanic origin, volcanic materials have always
been, not only the main construction material for
the local buildings, but also the only foundation
terrain of all the constructions in these islands.
The increasing development in civil engineer-
ing works has shown the need to know better the
geotechnical characteristics of volcanic materials,
as they have some particularities and differences
relatively to the non volcanic materials and besides,
studies on that subject are very scarce. one of the
existing volcanic materials that assume particular
importance due to its applicability in the scope
of civil engineering is basaltic pyroclasts. These
materials are often used in the azores, but its
exploration is conditioned in Madeira archipelago,
due to its reduced expression.
however, as it is of great interest to get a deeper
knowledge of its geotechnical characteristics, it
was considered important to make a survey of
some tests executed with this type of material in
both archipelagos, in order to be able to make some
eventual comparisons.
2
soMe GeoloGical caRacTeRisTics
Fragments of rock thrown out by volcanic explo-
sions are called
ejecta
, and accumulations of
such fragments are known as
pyroclastic rocks
(MacDonald 1972). This type of material has also
other designations, such
tephra, volcanic scoria,
“
bagacina
”, the common name given at the azores
islands or even “
areão
” as it is known at Madeira.
Pyroclastic deposits form directly from the fragmen-
tation of magma and rock by explosive volcanic activ-
ity. There are several ways to classify pyroclastic rocks.
The most important classification of tephra is the one
based on the size of the fragments (MacDonald 1972).
MacDonald (1972) defined Bombs (round to subangu-
lar) and Blocks (angular) as ejecta lager than 64 mm,
lapilli if the average diameter is between 2-64 mm and
ashes if the diameter is <2 mm.
