Environmental Engineering Reference
In-Depth Information
characterization and comparison of pore distribution
in weathered volcanic rocks by different techniques
a. Pola, G.B. crosta, F. agliardi, n. Fusi, V. Barberini & l. Galimberti
Dipartimento di Scienze Geologiche e Geotecnologie, Università di Milano-Bicocca, Milano, Italy
e. De Ponti
Struttura Complessa di Fisica Sanitaria, Azienda Ospedaliera San Gerardo, Monza, Italy
aBsTRacT: Volcanic rocks are widespread in different geological contexts and exhibit complex behav-
iors, from hard to extremely soft rocks, depending on mineralogy, porosity, and weathering. in particular,
their mechanical properties are influenced by the size and shape of pores. We report the results of porosity
characterization by different techniques, performed on lavas with different degrees of alteration, sampled
in the campi Flegrei area (italy) and characterized through petrographic analyses. Bulk-specific weight
measurements, water immersion and hg-porosimetry gave total and interconnected porosity values. anal-
ysis of thin sections provided 2D pore size and shape estimates and insight in pore relationships with rock
matrix and weathering. X-ray computer Tomography allowed complete 3D reconstruction of rock pores.
Different image processing methods for data extraction and analysis have been used to develop a standard
analysis procedure. Results in terms of measured porosity and pore size and shape distributions obtained
by different techniques are discussed.
1
inTRoDUcTion
provides 2D estimates of porosity with data con-
cerning pore size and distribution and their rela-
tionship with rock matrix and weathering.
X-ray tomography allows a complete 3D recon-
struction of porosity distribution both before and
after the performance of geomechanical tests. 3D
reconstruction of pores from X-ray tomography
has been performed for this study at different reso-
lution (from 5 to 650 µm). Pore geometry, inter-
connection and distribution can be analyzed and
introduced in numerical models or used to inter-
pret rock behavior observed in situ and during
laboratory tests.
Different methods of image analyses have
been used in this study and they could be easily
transported to other cases. The entire procedure
includes: image processing (noise reduction and
filtering), data extraction (grey-scale thresholding
and particle separation), and data analysis (meas-
urements and selection). Results are in terms of
measured porosity, geometrical distribution, dif-
ferences among the various adopted methods.
advantages and disadvantages are presented and
discussed.
Quantification of porosity and its geometry
(size, shape, distribution and frequency) is of pri-
mary importance to evaluate and interpret rock
behavior observed in situ and during laboratory
tests. in fact, physical-mechanical properties of a
Volcanic rocks are widespread in different geologi-
cal contexts and exhibit complex behaviors ranging
from hard to extremely soft rocks, depending on
mineralogy, cementation, porosity, and alteration/
weathering. nevertheless, despite their influence
on the geotechnical behavior of engineering struc-
tures and natural slopes, the relationships among
physical and mechanical characteristics of these
rocks are complex and still poorly understood.
Mechanical properties of weathered/altered vol-
canic rocks are greatly influenced by the size and
shape of pores. Recently, X-ray tomography and
two-dimensional images have been widely used to
characterize the structure and sizes of different
particles (e.g. clasts, and minerals). some works
describe automated methods to extract grain char-
acteristics (Butler et al., 2000), some others applied
image methodologies to quantify heterogeneity
in clastic rocks (Geiger et al., 2009). in this study
we describe results of porosity characterization
by different techniques: bulk weight and specific
weight measurements, water immersion, mercury
porosimetry, image analyses of thin sections, and
X-ray tomography.
Weighting, water immersion and mercury
porosimetry give information about total and
interconnected porosity. analysis of thin sections
