Rendiconti Online della Società Geologica Italiana

Volume 21 - Settembre 2012
COMITATO ORGANIZZATORE
Salvatore CRITELLI (Università della Calabria)
Paolo CAPPADONA (ONG e Autorità di Bacino Regione Calabria)
Gino Mirocle CRISCI (Università della Calabria)
Rosanna DE ROSA (Università della Calabria)
Carlo DOGLIONI (Presidente Società Geologica Italiana)
Rocco DOMINICI (Università della Calabria)
Elisabetta ERBA (vice-presidente Società Geologica Italiana)
Francesco GAGLIARDI (Università della Calabria)
Fausto GUZZETTI (Direttore CNR-IRPI)
Francesco MUTO (Università della Calabria)
Francesco PERRI (Università della Calabria)
Fabio Massimo PETTI (Società Geologica Italiana)
Rocco RONGO (Università della Calabria)
Franco RUSSO (Università della Calabria)
Maurizio SONNINO (Università della Calabria)
Franco VIOLO (Presidente ORG Calabria)
Alessandro ZUCCARI (Società Geologica Italiana)
In copertina: “Morpho-Bathymetry of the Mediterranean Sea”, CIESM, Ifremer Special Publication,
France. 1/3.000000 scale map.
RENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA
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Pubblicato online il 1 Settembre 2012.
Rendiconti Online della Società Geologica Italiana - vol. 21 (2012)
RENDICONTI Online della Società Geologica Italiana
pp. 1-793.
ISSN 2035-8008
RENDICONTI Online
della
Società Geologica Italiana
Volume 21 (parte prima)
86° Congresso Nazionale della Società Geologica Italiana
Arcavacata di Rende 18-20 Settembre 2012
NOTE BREVI E RIASSUNTI
A cura di: Salvatore Critelli, Francesco Muto, Francesco Perri,
Fabio Massimo Petti, Maurizio Sonnino, Alessandro Zuccari
ROMA
SOCIETÀ GEOLOGICA ITALIANA
2012
www.socgeol.it
Rend. Online Soc. Geol. It., Vol. 21 (2012), pp. 551-552.
© Società Geologica Italiana, Roma 2012
Geotechnical characterization and landslides in the weathered
granitoids of Calabria (southern Italy)
F. IETTO (*), M. PARISE (°), M. PONTE (*) & D. CALCATERRA (**)
Key words: Calabria, granitoids, weathering.
The main outcrops of granitoid rocks in the southern part of
the Italian peninsula are to be found in Calabria or, to be more
precise, in southern Calabria (Serre massif). The widespread
granitoid outcrops and the intensity of the mass wasting
processes (estimated in the order of 1000-1200 t/km2 of soil
eroded in the Serre mountains, IETTO & IETTO, 2004), directly
associated with the rapid uplift, have prompted a great deal of
scientific interest. This, in turn, has led to several geochemical,
petrographic, geomorphological, engineering-geological and
geotechnical studies of the weathered sequences. The literature
that, up to now, has studied the weathering processes of these
rocks in Calabria, even though prevailingly dealing with the
Serre mountains (MORESI, 1987; MONGELLI & MORESI, 1990;
CALCATERRA et alii, 1993; MONGELLI, 1993; MONGELLI et alii,
1996; LE PERA & SORRISO VALVO, 2000, CALCATERRA &
PARISE, 2005), has always implicitly or explicitly stated that
weathering occurred in an environment with a “Mediterranean
climate” and, consequently, during the Pleistocene, taking into
account the climatic variations of this period. This assumption is
the basis from which the various stages of maturity of these
processes have been looked for and studied area by area,
adopting classical methods of analysis or proposing some new
techniques. However, our previous studies allow us to state that
the beginning of the weathering processes in Calabrian granitoids
dates back at least to a pre-Tortonian age and that the present-day
outcrops of granitic alterites are the erosive residual of more
ancient mantles (IETTO & IETTO, 2004; IETTO et alii, 2007).
This paper is part of a wider research program aiming at the
geomechanical characterization of the crystalline massifs in
Calabria and at his correlation with landslides phenomena. Here
the first results are described focused on some sample-areas
located on the Ionian side of the central-southern Calabrian
granite massifs. The sample-areas are located near the village of
Fabrizia (in the Serre Massif) and near the village of Rossano
Calabro (in the Sila Massif).
In the Serre mountains, along the Molini torrent, right
tributary of the Allaro river, near the village of Fabrizia, two
granitoid lithotypes crop out having different litho-mechanical
characteristics, that, from bottom to top consist of:
 relatively fresh granitoids showing values of Schmidt
hammer rebounds in the range 20-50 (average: 42),
therefore being ascribable to class III (IAEG, 1981),
with 15<RQD>75%;.
 strongly disgregated and oxidised granitic saprolite,
whose geomechanical properties varies from those
typical of a class IV (Schmidt hammer rebounds: 11-22,
IAEG 1981) and, subordinately, of a class V (Schmidt
hammer rebounds: 0-16, IAEG 1981). Outcrops clearly
referable to class VI have not been so far identified.
The transition between saprolite and fresher rocks is not sharp
but gradual with depth. The maximum thickness of the
decomposed granitoids is in the order of 40-45 m. For the latter,
additional field and laboratory tests suggest:
_________________________
(*) Department of Earth Science – University of Calabria;
(**) Department of Hydraulic, Geotechnical and Environmental
Engineering – Federico II University of Naples;
(°) National Research Council - IRPI (Bari);
551
- internal friction angle: between 36° and 44°;
- NSPT: between 13 and 35, up to refusal (more than 50
blows) as depth increases;
- depth of water-table: about -13 m from ground level;
- P-wave velocities in the range of 900/1000 m/s in the
saprolite. Such a velocity range is coupled by
Lugeon permeability values, which usually show
values of Ke≥10-4 cm/s (to a depth -25 meters from
ground level) for as long as the grüs, permeable by
porosity, is considered. Ke, however, changes
rapidly, reaching values lower than 10-4 cm/s when
P-wave velocities increase to more than 1500 m/s, a
situation occurring where the uniformly porous rock
changes into terms provided with a mixed
permeability, deriving from porosity and jointing.
The slopes surrounding Fabrizia and the valley of the
Allaro river have a long history of landsliding and flooding
events. In fact in the great flood event of 1972-73, in this
area, there was the activation or reactivation of up to 7-8
landslides/Km2. Today, in the same area, the weathered rocks
evidence a wide variety of mass movements: debris flow-type
instabilities dominate in the saprolite (class V in IAEG,
1981), while translational slides are more significant in the
less weatherd rocks (class IV in IAEG, 1981). The sliding
surfaces are often located in the transition band between the
weathering mantle and the parent fresher rocks (in this area
corresponding, generally, to class III - IAEG, 1981). This
transitional zone, made up of an irregular mixture of saprolite
and rock volumes, holds a water-table that controls the
86° CONGRESSO SOCIETÀ GEOLOGICA ITALIANA
18-20 SETTEMBRE 2012, ARCAVACATA DI RENDE (CS
susceptibility to denudation processes. Therefore the depth of
mobilized material can also reach 25-30 m especially for
translational slides.
A second study area is located on the eastern side of the
Sila Massif, near the village of Rossano Calabro, in the Piana
dei Venti locality (730 m a.s.l.).
Fracturing at very high depth in rocky masses combined
to both current and past (pre-Tortonian) climatic predisposing
conditions have favoured the development of an intense
weathering (GUZZETTA, 1974; IETTO, 1975; CASCINI et alii,
1992; 1993), responsible for the transforming the granitoids
into highly-weathered low-cohesion rocks.
The penetration of meteoric waters mainly occurs through
faults and thrusts, from which the weathering processes have
been originated, spreading toward higher depths of the rocky
masses, hence causing typical edge rounding and being
responsible for the formation of boulders (TWIDALE, 1982).
The weathering profile of the outcropping granitoids has
been obtained according to the Hong Kong Geotechnical
Control Office approach (GCO, 1984), which, as well known,
is based upon six classes. Three measurement points (P 1, P2,
P3) on a 75 m-wide granitic outcrop have been considered.
The average rebound values obtained are, respectively, equal
to 40 (class III in GCO, 1984), 14 and 23 (class IV in GCO,
1984).
The mechanical parameters are highly variable; in fact the
weathered profile shows a complex pattern, whose more
representative terms range from “highly weathered rocks”
(class IV in GCO, 1984) to “completely weathered rocks”
(class V in GCO, 1984) and to “residual soil” (class VI in
GCO, 1984). Geotechnical laboratory tests (grain-size
analyses, direct shear tests) have been carried out on two
samples of residual soil. The soil has been classified as
“slightly silty sand and gravel”; direct shear tests have
provided the following results:
Sample 1: cohesion = 1.4 kPa – friction angle = 30°
Sample 2: cohesion = 4.5 kPa – friction angle = 33°.
Moreover, three granitoid blocks, belonging to boulders
isolated within the upper part of the investigated area, have
been subjected to uniaxial compressive strength tests. The
samples have shown an average UCS equal to 11 MPa.
According to DEERE & MILLER (1966), they are to be
ascribed to the “class E” (very low strength, less than 28
MPa).
Finally, some conclusive remarks are made, based on the
preliminary results so far obtained in the above study areas, as
well as on previous studies.
REFERENCES
CALCATERRA D., IETTO A. & DATTOLA L., (1993) – Aspetti
geomeccanici ed idrogeologici di ammassi granitoidi (Serre
calabresi) – Boll. Soc. Geol. It., 112, 395-422;
CALCATERRA D. & PARISE M. (2005) – Landslides types and
their relationships with weathering in a calabrian basin,
southern Italy – Bull. Eng. Geol. Environ, 64: 193-207;
CASCINI L., CRITELLI S., DI NOCERA S.& GULLÀ G. (1992) - A
methodological approach to landslide hazard assessment: a
case history. Procedings 6th International Symposium on
Landslides, New Zeland;
CASCINI L., CRITELLI S., DI NOCERA S., GULLÀ G. &, MATANO
(1993) - Grado di alterazione e franosità negli gneiss del
Massiccio Silano: l’area di San Pietro in Guarano (CS)”,
estratto da Geologia Applicata e Idrogeologia – volume
XXVII;
DEERE, D.U. & MILLER, R.P. (1966) - Engineering classification
and index properties for intact rock: Air Force Weapons
Laboratory Technical Report AFWL-TR-65-116, 277 p.
GEOTECHNICAL CONTROL OFFICE (1984) - Geotechnical manual
for Slopes, 2nd Edition. Geotechnical Control Office, Hong
Kong;
GUZZETTA G. (1974) - Ancient tropical weathering in Calabria.
Nature, 251, 302-303
I.A.E.G. (1981) – Rock and soil description and classification for
engineering geological mapping – Report by the I. A. E.G.
Commission on engineering geological mapping, Bull. Int.
Ass. Engng. Geol., 24, 235-274;
IETTO A. (1975) – Geologia e pianificazione territoriale e
urbana in Calabria. Mem. Soc. Geol. It., 14, 421-490;
IETTO A. & IETTO F. (2004) – Age and history of the
weathering of granitoids in southern Calabria (Italy) –
Geogr. Fis. Din. Quatern., vol. 27, 37-45;
IETTO F., DONATO F.F. & IETTO A (2007) – Recent reverse
faults and lanslides in granitoid weathered profiles, Serre
Mountains (southern Calabria, Italy). Geomorphology, 87,
196-206;
LE PERA E. & SORRISO VALVO M., (2000) – Weathering and
morphogenesis in a mediterranean climate, Calabria, Italy –
Geomorphology, 34, 251-270;
MORESI M, (1987) – L’alterazione dei graniti delle Serre
Orientali (Calabria) – Rend. Soc. Geol. Ital. di Miner. e
Petrol., vol. 42, 237-248;
MONGELLI G. & MORESI M, (1990) – Biotite-kaolinite
transformation in a granitic saprolite of the Serre (Calabria,
southern Italy) – Minr. Petrogr. Acta, vol. XXXIII, 273-281;
MONGELLI G., (1993) – REE and other trace elements in a
granitic weathering profile trom “Serre”, southern Italy –
Chemical Geology, 103, 251-270;
MONGELLI G., DINELLI E., TATEO F., ACQUAFREDDA P. &
ROTTURA A., (1996) – Weathered biotites from granitoidi:
the fractionation of REE, Th and transition elements and the
role of accessory and secondary phases – Miner. Petrogr.
Acta, vol. XXXIX, 77-93;
TWIDALE C. R. (1982) - Granite landforms, Elsevier,
Amsterdam.
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