Elmo D. 2001
5
The Geomechanics Classification System (Bieniawski, 1989), Q index (Barton et al., 1974) and the
Geological Strength Index (Hoek, 1995), the latter implemented with the Hoek and Brown criterion (1997),
were used to determine guidelines for the rock mass behaviour and support systems. The DIPS® and
UNWEDGE® software packages enabled the study of the kinematic possibility of wedge failure to be
carried out, whilst a new methodology based on a probabilistic approach and the utilisation of the @RISK®
computer program was used to determine the engineering properties of the rock mass for a wider range of
conditions. Finally, elements concerning the analysis of the proposed excavation method were also
considered.
Acknowledgements
I would like to thank Dr. W. Murphy for his initial assistance in the preparation of this project. I thank very
much Mr. A.P. Poulsom who has kindly step in as my project supervisor in the second semester of my final
year and I also would like to express my appreciation for his lectures about the engineering of underground
structures. I also thank Mr. D. Giles and Mr. D. Fall for the general support they have provided not only with
regards to this project. Finally I would like to express my appreciation to all the staff of the Department of
Geology of the University of Portsmouth.
Disclaimer
This project was assembled with the help of the Department of Geology of the University of Portsmouth but
does not necessarily or entirely reflects the opinions of the components of his staff.
Elmo D. 2001
6
1 INTRODUCTION
1.1 Project description
This project is intended as an engineering appraisal for a road tunnel, 1.8 km long and 8 meters wide,
connecting two villages near the city of Bologna, in an area that is part of the regional park known as Parco
Regionale dei Gessi Bolognesi e Calanchi dell’Abbadessa, with the intention to provide an alternative route
between the Idice river valley and Bologna.
Taking into account the restrictive rules issued by the regional park authorities as far as it concerns
engineering works within the park boundaries, it is proposed to drive the road tunnel entirely within the
Miocene age marly deposits of the Epiliguri Rocks Formation, since, in order to prevent environmental
damages to the remarkable system of gypsum caves of the region, tunnelling and excavation processes
should not be undertaken within the deposits of the Gessoso Solfifera Formation.
In a general context, the first activity usually performed when designing and constructing a tunnel is a
characterization of the rock mass into which the tunnel is to be driven, with particular attention to the
following properties:
™ Topography of the area, the climate and the accessibility of the area.
™ Location of the tunnel with respect to the ground surface and rock formation boundaries.
™ Structural stability of the rock body, which is a function of seismic activities, faults and stresses
concentrations.
™ Hydrologic regime and its perturbation, which is a function of the permeability of the ground and the
ground water flow rates.
™ Potential for subsidence and other surface effects.
™ Rock types in the rock mass, their genesis and their homogeneity.
™ Degree of weathering of the rocks.
™ Geologic discontinuities and other defects.
™ Deformability characteristics under short- and long-term loading.
™ Strength characteristics in reference to a rational failure criterion.
™ In-situ stress and hydraulic and/or dynamic loads.
™ Geometric and mechanical properties of systematic and extensive discontinuities.
Elmo D. 2001
7
More specifically, the aims of this project are:
™ Provide an engineering characterisation of the rock mass in order to derive both quantitative and
qualitative data.
™ Provide guidelines for the engineering of the tunnel.
™ Present the relevant geological and hydrogeological aspects and the influences that they may have on
the engineering of the tunnel itself.
1.2 Geographic and geological setting
Bologna lies in the southern part of the Emilia-Romagna region, between the northern Apennines to the
south and the Po River Plain to the north. It is about 90 km from the Adriatic Sea to the east and about 100
km from the Tyrrhenian Sea to the southwest. The surrounding landscape is characterised by a considerable
lithological heterogeneity and by a variety of morphological structures, ranging from flat to hilly and
mountainous terrains. The climate of Bologna and its surroundings areas can be classified as sub-tropical
humid (Pinna, 1977) and normally pluviometric maxima occur in spring and autumn (AER, 1996-1997).
More specifically the villages of San Lazzaro di Savena and Castel dei Britti are located in the outskirts of
Bologna, on the initial hilly slopes, which overlook like a large balcony the city and the plain (Figure 1.1).
Figure 1.1: Project location and 3D representation
Elmo D. 2001
8
Stretching between the Zena and Idice rivers, this is a territory characterised by areas of great interest, both
naturalistical and historical, representing a unique heritage in the regional panorama.
The high solubility degree of the rocks of the Gessoso Solfifera Formation (Gypsum Formation) it is
responsible for the evolution of typically kartsic morphologies such as dolines and blind valleys. Karstic
phenomena are well developed below the ground surface, where more than 200 caves are interwoven
together, forming one of the widest systems of gypsum caves in Europe. The Grotta della Spipola, the
widest one, and the Grotta del Farneto, a remarkable archaeological site, represent the better known caves of
the area.
Approximately the Gessoso Solfifera Formation has on overall extent of 230 hectares, from the Savena
valley to the higher grounds of Miserazzano, including the disused quarries of Monte Croara, adjacent to the
blind valley of the Acquafredda. Two dolines, known as Buca dell' Inferno and Buca di Gaibola, are located
between the rivers Zena and Idice, the latter being characterised by sink holes leading to minor caves.
The Argille Scagliose Formation, in contrast, is characterised by arid, almost desertical, landscapes, forming
knife-edge, erosional features known as calanchi, which occur on 40° to 60° hill slopes. These exposed
slopes are subjected to extensive erosion, which creates intense dissecation on their scarp surfaces (Figure
1.2).
Figure 1.2: Calanchi - Locality near Castel dei Britti (Grid Ref. 4921350-1694350)
Elmo D. 2001
9
The Gessoso Solfifera Formation and the Argille Scagliose Formation are certainly the most extensive
lithological units of the region, but a full description of the landscape would not be complete without
mentioning the Miocene age marly rocks and the younger sediments deposited during the Pliocene and
Pleistocene. Although being almost completely covered by the vegetation, they represent marine sediments
deposited in a near-shore, shallow to deep-water marine environment. Located south of Bologna, this Plio-
Pleistocene intra-Apennine basin was morphologically represented by a gulf, associated with a sector of the
Apennine mountain chain greatly subsidence toward the northeast (After Cremonini and Ricci, 1982).
Fluvial and marine terraces occur at progressive elevations on the valley floors, revealing a strong uplift
since the Lower-Middle Pleistocene (Colotti and Pieruccini, 1999). The terraces, formed mainly by reddish
brown sandy soils, comprise sediments which were deposited approximately 700000 years ago, in relation
with both the changes in the sea level associated with the late glacial period and the processes related with
the formation of the Apennine mountain chain.
Elmo D. 2001
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2 ELEMENTS OF GEOLOGY OF THE BOLOGNA FOOTHILLS
The Bologna foothills, although of modest altitude and ordinary aspect, are characterised by a complex
structure, consisting of a variety of different sedimentary lithologies that are the result of a long and complex
geological history. A crono-stratigraphic sequence with an indication of the nature of the different deposits
and conditions of deposition is illustrated in Figure 2.1.
Figure 2.1: Cronostratigraphic sequence of the deposits in the areas surrounding Bologna
Elmo D. 2001
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2.1 Description of the lithological units
2.1.1 Argille Scagliose Formation
The Argille Scagliose Formation represents a rock body mainly formed by a clayey matrix characterised by
small and bright chips in which it is possible to observe fragments of rocks of different age and composition,
such as marls, calcareous rocks, sandy rocks and ophiolitic clasts.
The Argille Scagliose Formation forms large and tectonically deformed outcrops dominating all the southern
sector of the mapping area and those outcrops themselves can be also considered as parts of particular
erosional features known as calanchi (See also Paragraph 3.2). The deposits have a chaotic texture, as they
are formed by fine grained, dark bluish grey clays in which are enclosed fragments of other rocks, whose
dimensions range from few centimetres up to 1m, being at the same time highly fractured (Figure 2.2).
(a) (b)
Figure 2.2: Argille Scagliose Formation (Grid Ref.: 4921350-1694350)
a) Highly fractured marly clast whose dimensions are approximately 30cm x 30cm
b) Characteristic desiccated and fractured surface
The poor vegetation cover characterising the slopes within this formation could be related to the tendency of
these clayey materials to dry during the hot summer periods, resulting in the formation of a characteristic
fractured and desiccated surface. Successive winter rainfalls may enhance even further the instability of
these slopes as those clay materials increase their volume as a direct consequence of water absorption.
The diagenesis of these deposits has been associated
1
to high temperature and pressure, together with the
presence of warm fluids in which were diluted several different minerals, which eventually were later
1
"Parco regionale dei Gessi e Calanchi dell'Abbadessa" - Regione Emilia Romagna, 2000 - Page 34
Elmo D. 2001
12
deposited as crystals. Crystals of Calcite (CaCO
3
), which can be easily observed in form of pinkish white,
long and narrow striated veins, represent the most common mineral, whilst Pyrite (FeS
2
) is found as small,
nodular, goldenish brow crystals. Probably less frequent it is the Barite (BaSO
4
), which forms small, greyish
black crystals with a particular texture.
Since the beginning of the 20
th
century, the different hypothesis about the origin of the Argille Scagliose
Formation were related to studies which denied the possibility of movement of any rock body, even if those
same studies were then unable to explain the chaotic texture of these deposits. Others studies associated the
structure of the Argille Scagliose Formation to intrusions of magmatic rocks represented by the ophiolitic
clasts which can be found within the clayey matrix. Giuseppe Bianconi
2
gave the name to this formation in
the late 1840, whilst Luigi Bombicci was the first geologist who associated the origin of these deposits to
strong horizontal movements related to the formation of the Apennine mountain chain.
Different terms have been used to describe these deposits: allochthonous complex, undifferentiated
allochthonous complex, heterogeneous chaotic complex and tectonically deformed sedimentary melanges
(M. Cazzoli, 2000).
2.1.2 Epiliguri rocks Formation
The deposits of the Epiliguri Rocks formation conformably overlay the Argille Scagliose Formation and
along the left flank of the Zena River valley it is possible to observe, in proximity of a zone not entirely
cover by the vegetation, the gradational boundary between the Argille Scagliose and these light brownish
grey rocks, which were deposited approximately 23 million years ago in the Miocene period. For their
position, these deposits are known as Epiliguri, which literally means standing on the top of the Liguridi or
Argille Scagliose.
These deposits range from marl to calcareous sandstones with the upper strata formed by brown marly clays,
with a total thickness of 350 meters. Although this grade of variation, they have been mapped as belonging
to a single formation as the widespread vegetation cover has make it impossible to determine the exact
boundary between each different facies.
2
An excerpt from the original notebook of Luigi Bianconi describing these deposits is reported in "Parco
regionale dei Gessi e Calanchi dell'Abbadessa" - Regione Emilia Romagna, 2000 - Page 27
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