Environmental Engineering Reference
In-Depth Information
Relationships between porosity and physical mechanical properties
in weathered volcanic rocks
a. Pola, G.B. crosta, R. castellanza, F. agliardi, n. Fusi, V. Barberini, G. norini & a. Villa
Dipartimento di Scienze Geologiche e Geotecnologie, Università di Milano-Bicocca, Milano, Italy
aBsTRacT: Volcanic rocks are frequently found under weathered/altered conditions. Degradation
and transformation can occur both at the surface and at large depth causing a progressive change in the
physical mechanical properties. Degradation can cause an increase in porosity and this can control the
rock behavior. in this paper we discuss the relationships between porosity characteristics, micro-structure
and texture, and the mechanical behavior of lava at different degrees of weathering (lavas from the campi
Flegrei, italy). The performed laboratory tests include: uniaxial compression, indirect tension, and uniax-
ial compression with ultrasonic wave measurements. a description of the mechanical behavior is obtained
and a detailed description is performed through a series of pre and post failure non destructive analyses.
Porosity values have been related to stress and strain relationship, in addition pore size characteriza-
tion is presented in a companion abstract/manuscript. Results are interpreted in the key of degree of
weathering and its related characteristics. an empirical linking between the change in strength with the
degree of alteration is presented and discussed.
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inTRoDUcTion
abundant vesicles and pores, with different sizes,
that sometimes are filled with secondary miner-
als (al-harthi et al., 1999, Tillerson and nimick,
1984, hudyma et al., 2004). Previous studies on
similar rock lithologies suggest that the compres-
sive strength is controlled by total porosity, the
abundance of macropores and the pore structure
and size distribution as well as the type of forming
particles (luping, 1986; nimick, 1988; al-harthi
et al., 1999; Price et al., 1994; aversa and evange-
lista, 1998; avar et al., 2003; avar and hudyma,
2007; hudyma et al., 2004).
in this paper we present the initial results of
a study concerning the effect of alteration and
porosity characteristics on the physical mechanical
behaviour of a volcanic rock. This paper is directly
related to a companion paper presented at this
conference (Pola et al.) on the characterization of
porosity by means of different methodologies.
The analysis of the stability of volcanic edifices
or rock masses in volcanic rocks is often prob-
lematic because of the variability of the materi-
als (e.g. individual lava flows, pyroclastic deposits,
and interbedded units), their heterogeneity, the
presence of abundant voids and variable degree
of cementation. These characteristics make some-
times extremely difficult to reach a correct and
representative characterization of the physical
mechanical behavior.
Volcanic rocks are frequently composed of
both matrix material and pores, and they are often
found in altered/weathered conditions because of
the highly active volcanic environment and the
presence of hydrothermal conditions. Generally,
both the strength, the deformability and stiffness
of these rocks shows a dependence on the porosity.
Porosity can be formed by voids, between grains or
minerals, of different size and shape, with a par-
ticular frequency distribution of size and it can be
interconnected or disconnected. Various research-
ers investigated the physical mechanical behaviour
of rocks as a function of their porosity.
Dolostones, sandstones, limestones, dolerite and
granites are mainly characterised by microporos-
ity. on the other hand, volcanic rocks (e.g. basalts,
scorirae, lithophysae-rich tuffs, tuffs and pyroclastic
deposits in general, breccias) often present a brec-
ciated, porous or vesicular texture characterised by
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sTUDY aRea anD MaTeRials
The tested materials have all been collected from
the solfatara volcano in the campi Flegrei area
(campanian volcanic province, italy). large lava
samples, characterized by diverse degree of altera-
tion, have been collected and resampled in the lab-
oratory to perform physical mechanical tests.
a mineralogical and petrographical descrip-
tion of the materials have been completed through
