Environmental Engineering Reference
In-Depth Information
They also can be grouped into three genetic
types according to their mode of transportation
and deposition: Falls, Flows and surges.
Volcanic scoria are integrated in the first group.
explosive activity builds scoria, cinder or spatter
cones, or both, at the vent, with scoria-fall deposits
of limited aerial extent and volume being deposited
around and downwind of the vent. Those cones
well defined, are usually symmetric, with a height
that rarely exceeds a few hundreds meters. The
inclination of the slopes of these cones is normally
close to 33º (natural slope angle of loose scoria),
value that changes with the time, due to erosion
(Fraga 1988).
scoria-fall deposits are composed largely of
vesiculated basalt to near-basaltic magma. These
are the deposits characteristic of strombolian
explosive activity. such eruptions eject scoria and
relatively fluid lava spatter, and are often accompa-
nied by the simultaneous effusion of lava.
Deposits of scoria cones often consist of
rather poorly bedded, very coarse-grained with
metre-sized ballistic bombs and blocks.
scoria is typically dark gray to black in color,
mostly due to its high iron content. sometimes, oxi-
dation (of iron) by hot gases streaming preferentially
through the central part of the volcanic edifice,
leads to a deep reddish-brown color (scmincke
2006).
on what concerns mineralogical composition,
some comparisons were made between X ray
diffractometry analyses made with both Madeira
(lRec Madeira) and azorean samples (Fraga
1988). in both cases, mineralogy was similar:
the most abundant mineral was a plagioclase
(anorthite), in second place, pyroxens (augite) and
then olivines (Forsterite). in the samples of the red
basaltic pyroclasts, there was still the presence of
hematite (resultant from the iron oxidation).
they are formed by grains, volcanic scoria should
be treated as an aggregate (non crushed natural
aggregate) (Fraga 2009).
its properties depends on the size of the grain,
the shape, the porosity and petrological composi-
tion as well as the degree of packing among the
particles, compaction state of the deposit and
resistance of the referred particles (Vallejo et al.,
2006).
some geotechnical characteristics of the tested
materials are presented below. several Madeira
samples of basaltic pyroclasts were tested as well
as some samples from the azores.
The following laboratory tests were performed
with the Madeira samples: tests for grain size dis-
tribution, specific weight, compaction and cBR
tests (californian Bearing Ratio) and consolidated
drained (cD) direct shear tests.
consolidated drained direct shear tests were
made on 7 samples, being tested separately the
fractions with sizes inferior to 2.00 mm (sieve
asTM nº 10) and with the fractions of sizes inferior
to 4.75 mm (sieve asTM nº 4), in a shear box with
the following dimensions: 100 × 100 × 25 mm
3
.
For two of the samples, consolidated drained
direct shear tests were performed on the entire
samples, in shear box with the dimensions
300 × 600 × 200 m
3
, existing in the laboratory
of construction Materials of the Faculty of
engineering of the University of Porto.
some in situ tests were performed to determine
the dry density, and plate load tests to determine
the characteristics of natural deposits of basaltic
scoria.
in azorean samples the following laboratory
tests were performed: tests for grain size distribu-
tion, los angeles, cBR and specific weight tests.
The following in situ tests were also performed:
sPT, plate load tests and
in situ
dry density.
one should remark that the in situ tests in
Madeira were performed on natural deposits,
while the ones performed in the azores were
executed on landfill material, with exception of
the sPT tests which were performed on natural
terrain.
3
GeoTecnical chaRacTeRisTics
oF PYRoclasTs MaTeRials
Due to its volcanic nature, basaltic pyroclasts are
a frequently heterogeneous material, non plastic,
porous and with low density, with high levels of
water absorption, being more or less consolidated.
The loose pyroclastic deposits are, therefore, a
group of specific materials (on what regards its
geotechnical characteristics) as they don't have the
typical behaviour of a rock or of a soil; so they
should be analysed as an independent geotechni-
cal group. Being a rock, from a geological point
of view, its geotechnical behaviour approaches,
however, to that of a soil, with the particularity
of being almost indifferent to the water content.
especially due to this characteristic and because
3.1
Results of the tests performed
3.1.1
Madeira samples
on what concerns the pyroclastic materials of
Madeira island, the values obtained for the main
geotechnical properties are: specific weight = 24.7-
29.1 kn/m
3
, dry density = 12-14 n/m
3
, cohe-
sion = 0-0.1 MPa, friction angle = 29-50º.
one should emphasize that though these
materials are loose, in consolidated drained direct
shear tests a “cohesion” value was obtained which
is associated to the form and imbrication of the
