Environmental Engineering Reference
In-Depth Information
Rock mass classification schemes in volcanic rocks
M. Muñiz Menéndez
Master Mecánica de Suelos e Ingeniería de Cimentaciones (CEDEX), Madrid, Spain
J. González-Gallego
Laboratorio de Geotecnia (CEDEX), Madrid, Spain
aBsTRacT: Volcanic massifs, due to their origin, present certain characteristics that make their study
through a classic geomechanical classification not always appropriate. The presence of discontinuities,
the peculiar block shape and the presence of voids condition the behavior of lavatic massifs. Behavior of
pyroclastic massifs depends mainly on their matrix rock, making the use of existing geomechanical clas-
sifications inadequate. This work represents a first step in the development of a geomechanical classifica-
tion specific to volcanic massifs, accounting for all the properties that actually condition their behavior.
1
inTRoDUcTion
to those of non-volcanic materials (González de
Vallejo, l.i.
et al
., 2006). This calls for very atten-
tive and cautious study of these massifs, following
a methodology different to the usually employed.
The main distinguishing features of volcanic
massifs are their high structural and lithological
heterogeneities, with alternating materials of very
different properties, the presence of discontinuities
of varied origin, the existence of cavities of differ-
ent sizes and a great variability in layer thickness.
From a geotechnical perspective, volcanic rocks
are classified in two main groups: lavatic rocks,
formed by the cooling of lava flows, and pyroclastic
rocks.
From the second half of the 20th century
onwards, many geomechanical classifications have
appeared, with the original aim of easing decision-
making when considering support in subterranean
excavations.
nowadays, geomechanical classifications are
widespread in every undertaking related to a rock
massif, and are used in the design stages of a project
as well as in the construction stage. This makes it
necessary to know the limitations and difficulties
entailed by such classifications.
Most of these classifications are based on
quantifying several of the massif 's parameters and
variously computing them to obtain a quality index
for the massif, thus obtaining an approximation to
the geomechanical behavior of the massif through
direct observations and simple field tests.
numerical and empirical methods employing
values obtained from one of these classifications as
an entry parameter are more abundant every day,
forcing classifications to be very finely tuned, and
every one of their parameters carefully considered.
Volcanic rocks, due to their origin, show very
distinctive geomechanical characteristics. Volcanic
rock massifs are very heterogeneous in their
structural and lithological characteristics, making
the application of geomechanical classifications on
them especially difficult and, sometimes, impossible.
2.1
Lavatic rocks
The contact of a lava flow with the air can cause
its surface to break due to rapid cooling, creating
a drossy surface, while the inside of the flow
cools more slowly and presents a more orderly
structure.
Different volcanic episodes usually create a series
of lava flows separated by scoria or even soils. This
alternating of materials increases the heterogeneity
of the volcanic massif.
The main distinguishing features in these massifs
are their discontinuities, the presence of cavities
and their resistant properties.
2.1.1
Discontinuities
Volcanic massifs are affected by discontinuities
created by the different geological processes that
affect them.
These discontinuities can be classified according
to the geological process that originated them into
2
Volcanic Rocks
The geomechanical properties and geotechnical
behavior of these materials are completely different
