Environmental Engineering Reference
In-Depth Information
than 7 km and which includes the Jameos del agua
and the cueva de los verdes.
after studying all three cave formation proc-
esses, two types of cave can be classified according
to their position relative to lava flows:
- Type i: caves at the top or the base of a lava
flow.
- Type ii: caves inside a lava low.
Millimetric or centimetric cavities are treated
as part of the rock matrix when considering its
characteristics and therefore do not need a specific
study. The size of lava tubes will usually make it
necessary to study them individually and apart from
the rest of the lavatic massif. Decimetric and metric
cavities cannot be studied as part of the matrix
and their size does not allow an individualized
study; these cavities must then be included in a
geotechnical classification of volcanic massifs.
a way of assessing the importance of these
middle-sized cavities is through the cave index
(estaire, J.
et al
., 2008), defined as the quo-
tient between the sum of the volumes of the
massif 's connected cavities and the total volume
of the massif. This index allows the quantitative
assessment of cavity occurrence and, with it, cavity
importance when evaluating the geotechnical
properties of the massif as a whole.
of the matrix rock and, as opposite to most rock
massifs, very little on the fracture system.
2.2.1
Pyroclastic rock strength
Pyroclasts can be bound by two different proc-
esses: welding of the clasts at high temperatures
and cementation by interstitial fluids.
The mechanic behavior of this kind of rocks will
depend on five factors (serrano, a.
et al
. 2002):
- compaction.
- Degree of particle welding.
- imbrication of particles.
- Particle intrinsic strength.
- alteration.
The first three parameters are usually related:
the higher the imbrication and welding are, the
higher the compaction will be. This is not always
true, however, and sometimes low density deposits
with a very high degree of welding can be found,
or very compacted deposits with a low level of
particle joining.
The degree of compaction is the most influential
factor on the strength of a pyroclastic massif, and
close attention must be paid to this parameter when
studying massif properties. The best way would be
to measure the density of the material on site.
Welding degree and imbrication must be
assessed together. imbrication degree is related
to the number of contact points among particles,
while welding degree is related to the percentage
of total particle area that is in welded contact with
another particle. These parameters can only be
measured by direct observation on the field.
compression strength values in pyroclastic rocks
show a very wide range. obtaining laboratory data
for this type of rocks is difficult in many cases,
since they are composed of a conjoining of many
irregular fragments, making it difficult to sculpt
testing specimens. Data dispersion in this tests is
high, and many specimens are needed to obtain
reliable values (serrano, a.
et al
., 2008).
low density volcanic agglomerates have a
very specific mechanical behavior. at low tensile
strengths they behave like rocks with very high
deformation modulus, but at high tensile strengths
they crumble to dust and behave like soils, with
an important increase in their deformability. This
is known as a mechanical collapse (serrano, a.
1976).
Mechanical collapse can be explained by the
high porosity usually present in these materials
and the strong unions among grains due to their
high-temperature welding.
Unions among particles are highly rigid for low
loads but at higher loads these unions progressively
break until the material becomes a low density soil,
thus explaining its high secondary deformability.
2.1.3
Lava flow strength
compression strength values of volcanic rocks are
high, between strong rock and extremely strong
rock.
The high strength of this kind of rocks
means that the stability of a volcanic massif
will depend mainly on the shear strength of the
discontinuities.
Due to the behavior of liquid lava, flow
structures may appear in these rocks, mainly
because of mineral orientation. These structures
generate anisotropies that affect the geotechnical
properties of the rock.
The possibility of the existence of flow structures
must, then, be accounted for when measuring its
strength.
Drossy layers, from a geotechnical point of
view, behave like a granular soil, and their com-
pactibility will depend, among other factors, on
any consolidation due to overlaying lava flows and
to cementation by circulating fluids (Peiró 1997).
2.2
Pyroclastic rocks
Pyroclastic rocks show a complex behavior,
sometimes closer to that of a soil than to that of
a rock. Their geotechnical properties have little in
common with the rest of rock types. The behavior
of these massifs will depend mainly on the strength
