Rend. Online Soc. Geol. It., Vol. 21 (2012), pp. 510-512, 3 figs.
© Società Geologica Italiana, Roma 2012
Geological investigation in the SE neighbourhoods of Rome aimed to an
evolutionary model of cavity networks for hazard zonation
G. BIANCHI FASANI (*), E. DI LUZIO (**), F. BOZZANO (***)
Key words: Geological modelling, cavity networks, void
migration process , Rome
migration outlined by BIANCHI FASANI et al (2011) was
discussed for the area under study in the light of the main results
from the geological modelling.
INTRODUCTION
The south-eastern areas of Rome (Fig. 1) developed over a
Middle Pleistocene volcanic multilayer featured by pyroclastic
units erupted from the Albani Hills (VENTRIGLIA 1971, 2002;
FUNICIELLO et alii, 2008; MAZZA et alii 2008; GIORDANO et alii,
2006).
The same area is featured by several underground networks of
man-made cavities excavated in welded volcanic ashes known as
“pozzolane” since the Roman Age. The “pozzolane” units were
intensively used to produce a concrete when mixed to carbonate
lime and water (opus caementicium). The concrete exhibits a
consistent cohesiveness due to the high content of silica and
alumina in the volcanic deposits.
The irregular and locally pervasive systems of underground
quarries have been excavated till to years 1950-1960, that is
during the last period of urban expansion. Such networks pose
today a serious safety risk due to their interaction with anthropic
activities and modern infrastructures.
The investigated area includes the territories of the 6, 7 and 9
municipalities (Fig. 1) for an overall extension of about 35 km2.
Cavities dataset were reported from literature (FUNICIELLO et
alii, 1995; VENTRIGLIA, 2002), while the geological modelling
was completed using stratigraphic logs from more than 1000
boreholes (VENTRIGLIA 1971, 2002 and unpublished data
provided by the municipalities involved in the study).
Updated isobaths and thickness maps were drawn for the
bounding surfaces of the main pyroclastic units (“Tufo Lionato”,
“Pozzolane Rosse”, “Pozzolane Nere”). Then, a geostatistical
analysis was performed in order to investigate cavities spatial
distribution and the correlation with the local stratigraphic
setting. Finally, the evolutionary model of void upwards
_____________
(*) Centro di Ricerca CERI, Previsione, Prevenzione e Controllo dei Rischi
Geologici, ‘‘Sapienza’’ University of Rome, P.zza U. Pilozzi 9, 00038,
Valmontone, RM, Italy; [email protected];
(**). CNR-ITABC, Istituto perle Tecnologie Applicate ai Beni Culturali,
Area della Ricerca Roma 1 - Montelibretti, Via Salaria Km. 29,300 - C.P.
10, 00016, Monterotondo St., RM.
(***) Dipartimento di Scienze della Terra, ‘‘Sapienza’’ University of
Rome, P.le A. Moro 5, 00185 Rome, RM, Italy
Fig. 1 – Google Earth map of the study area in south-eastern Rome. Yellow lines
include territories of VI,VII,IX municipalities. White lines are cross section
traces.
GEOLOGICAL SETTING AND MAIN
CONSIDERATIONS
The Middle Pleistocene volcanic units featuring the local
geological setting were the result of several eruptive phases
belonging to the Tuscolano-Artemisio phase of the Colli Albani
district (GIORDANO et alii, 2006; FUNICIELLO et alii, 2008 and
references therein). Pyroclastic units have a general subhorizontal attitude (Fig. 2). The “Pozzolanelle” and “Tufo
Lionato” uppermost units (Villa Senni Unit in FUNICIELLO et alii,
2008) display sharp thickness variations linked to paleo-drainage
networks. The underlying “Pozzolane Rosse” and “Pozzolane
Nere” units instead show main thickness variations across the
“Acqua Bulicante” Fault (Fig. 2). The “Pozzolane Nere” unit in
particular is greatly thinned or even absent in the footwall zone
due to post-faulting erosive process on horst areas.
All the aforementioned units lays above a basal, pyroclastic
sequence of layered tuffs (“Tufi Antichi” unit). Such sequence on
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86° CONGRESSO SOCIETÀ GEOLOGICA ITALIANA
18-20 SETTEMBRE 2012, ARCAVACATA DI RENDE (CS)
Fig. 2 – Cross section 3 (SW-NE direction, 10×vertical exaggeration). Surface projections of cavities are indicated.
its turn rests above the Plio-Pleistocene sedimentary substratum
of the Roman area.
Within such a geological scenario, two main levels of cavities
are found at different depths in the hanging-wall zone of the
Acqua Bulicante Fault, actually into the “pozzolane” layers; in
the foot-wall zone - i.e. in the westernmost areas of the study area
- cavities are found within the “Pozzolane Rosse” and “Tufi
Antichi” units. In both situations, The “Tufo Lionato” and
“Pozzolanelle” units act as cap rocks; therefore, the analysis of
their thickness variations is crucial for an hazard zonation.
MODEL OF VOID MIGRATION
An engineering-geology model of cavity networks was
already described by BIANCHI FASANI et alii (2011) in a
restricted zone within the study area. On the base of direct
explorations, the authors propose an evolutionary model of void
migration for cavities hosted within the “Pozzolane Nere” Unit.
The model can be therefore better applied in the hanging-wall
zone of the “Acqua Bulicante Fault”. According to it, collapses
processes are strictly linked to the local volcanic setting
consisting, of “pozzolane” layers alternated with palaeosoils (Fig.
3). The overall view of the phenomenon provides a first phase
characterized by the degradation of the palaeosoil overlying the
“Pozzolane Nere” unit determining the collapse of the entire
level (paleosoil 1 in Fig. 3a). Where these first collapses occur,
then the roof of the tunnel migrates upwards, till the Tufo
Lionato lithoid bank is exposed working as a sort of beam (Fig.
3b). The subsequent evolution is mainly characterised by the
formation of tension cracks in the central part of the intrados of
the cavity and the consequent collapse of shaped blocks from the
Tufo Lionato “plate” (Fig. 3c). This phenomenon implies that the
roofs composed of the “Pozzolanelle” bank are now exposed.
The last phase includes the multiple step collapses from the
Pozzolanelle bank until the ground level is approached and a
sinkhole is suddenly formed when the critical threshold of the
cover thickness is reached (Fig. 3d).
Where available information on cavities locations and depths
was included in cross section 1-6 to find those geological
conditions that - according to the evolutionary model -could
represent critical setting for the process of void upward
migration.
CONCLUSION
This study was undertaken in the south-eastern part of Rome
to investigate the relationship between the geological setting at
subsoil and cavities distribution. Moreover, critical conditions of
void migration were detected following the main lines of an
evolutionary model outlined in a previous work. Main conclusion
of this work are:
a) the spatial distribution of cavities seems to be influenced by
the geological setting of the hosting “pozzolane layers” and the
overlying cap rocks, both in terms of elevation of bounding
surfaces and units thickness;
b) geological cross sections bearing cavities locations and depths
allowed to spot many cases where critical conditions for the
process of upward void migration are present according to the
model.
REFERENCES
BIANCHI FASANI G., BOZZANO F. & CERCATO M. (2011) - The
underground cavity network of south-eastern Rome city
(Italy): an evolutionary geological model oriented to hazard
assessment. Bull Eng Geol Environ., 70, 533-542.
FUNICIELLO R., PRATURLON, A. & GIORDANO, G. (1995) - La
geologia di Roma, il centro storico. Memorie descrittive della
Carta Geologica d'Italia, 50, 550pp.
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18-20 SETTEMBRE 2012, ARCAVACATA DI RENDE (CS)
Fig. 3 - Evolutionary model for void migration towards surface in the study area from BIANCHI FASANI et alii (2011): a) original cavity within the
“Pozzolane Nere” Unit; cracks in the overlying paleosoil 1; b) enlarged cavity with cap migrated into the “Tufo Lionato” Unit; c) collapse of the “Tufo Lionato”
plate and cap migration into the “Pozzolanelle” Unit: d) sinkhole formation. Legend: 1) Anthropic deposits and “Pozzolanelle” Unit; 2) “Tufo Lionato” Unit; 3)
paleosoil 1; 4) “Pozzolane Nere” Unit; 5) paleosoil 2; 6) “Pozzolane Rosse” Unit
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