Geomechanical characterization of carbonate rock masses in

Geomechanical characterization of carbonate rock masses
in underground karst systems:
a case study from Castellana-Grotte (Italy)
Mario Parise1, Maria Addolorata Trisciuzzi2
1
National Research Council, IRPI, Via Amendola 122-I, 70126 Bari, Italy
e-mail: [email protected]
2
Technical University, Engineering Faculty of Taranto, Italy
ABSTRACT
Analysis of the stability conditions of rock masses in underground karst systems involves a study of the
breakdown processes that acted during the formation of the caves, integrated with a survey of the present
conditions of the walls and roof. Especially in show caves, and in caves frequently visited by speleologists
and researchers, evaluating the susceptibility related to rock falls is of paramount importance. In this
contribution we presents the results of a research carried out at the Castellana-Grotte show caves (Apulia,
southern Italy), where we performed a geomechanical characterization of the carbonate rock mass, aimed at
obtaining the necessary data for up-to-come stability analysis. The role played by gravity-related processes in
shaping the karst systems, and contributing to the evolution of the caves as they appear today, is well evident
throughout the whole underground system: the entrance is a wide opening at the ground surface, due to roof
collapse as the extreme consequence of the upward propagation of instability mechanisms. Similar features,
which however do not reach the surface, are also visible in many other rooms of the system. Our study
started by mapping the fallen blocks, and measuring the main morphometric parameters, at the same time
performing observations on the weathered surfaces, and ascertaining the presence of secondary deposits on
the blocks. Following this phase, that resulted in a detailed cartography of the fallen blocks, the rock mass
was surveyed and described by means of structural surveys dedicated to investigate the most relevant
discontinuity systems and their main features, according to the standards proposed by the International
Society of Rock Mechanics. The geomechanical characterization of the carbonate rock mass is crucial for
allowing further analysis devoted to the assessment of the stability conditions within the karst system.
KEY WORDS: rock mass, breakdown, discontinuity, karst, stability
Breakdown processes in caves
Natural collapse in caves generally occurs
through progressive failures of roof rock units,
whilst wall failures are less common. The
process of roof stoping (or cavity migration)
consists of the progressive failure of individual
beds or slabs of rock, developing upward, and
eventually reaching the ground surface. It
seems to be more rapid in thinly-bedded
limestones rather than in massive or thicklystratified carbonate rock masses. The roof and
floor of an opening in horizontally bedded
rock can actually suffer from buckling failure
when the rock plates are relatively thin and
when the in situ horizontal stress is high.
When the in situ horizontal stress is low, the
roof slabs in a similar opening can fail as a
result of the tensile stresses induced by
bending of the slabs under their own weight
(Hoek, Brown, 1980).
Stability problems in blocky jointed rock
are generally associated with gravity falls of
blocks from the roof and sidewalls (White,
228
Mario Parise, Maria A. Trisciuzzi
White, 1969). Weathering may be locally
important, decreasing the physical properties
of the rock, and favouring enlargement of the
fissures and joints, until causing detachment.
The main factors controlling the profiles of
cave passages are the structural and
lithological features of the host limestone, and
the past and present hydrology in the cave.
Starting from the cave passage profiles,
therefore, with particular regard to width and
height, and, looking at the overall shape of the
cross-section, it is possible to have a
preliminary guess about the processes that
have been active in the past to produce such a
shape.
Besides creating problems in the
underground setting, the presence of large
voids or cave in karst environments is a
significant geohazard even at the surface for
engineers, due to the notorious unpredictability
of their location and extent (Culshaw,
Waltham, 1987; Parise, in press). A classic
example of the unpredictable nature of karst
has been described for the Remouchamps
Viaduct in Belgium: on the initial ground
investigations, 31 boreholes found no cave, but
the subsequent excavation of the pier footings
found two unknown caves; this brought to a
second phase of investigation, with 308 new
boreholes that found no more caves (Waltham
et al., 1986).
The difficulties in estimating this geohazard
have been treated by Waltham (2002) in his
engineering classification of karst, that is
largely based on the three features most
relevant to engineers concerned with the
integrity of structural foundations in karst
terrains: sinkholes, rockhead and caves. Five
classes are defined on the basis of the typical
assemblages of morphological features, from
undeveloped karst to normal, mature, complex,
and extreme karst. Even though the classes can
be recognised in a climatic context, the high
variability of karst (also within a region) make
the classification not absolute. This means that
an area can be attributed to a certain class, but
some small sites may fall into a higher or
lower class.
In areas above show caves, it is therefore
very important to ascertain the overall
properties of the host rock mass, and the
stability at the surface, in order to allow safe
visits to tourists, and evaluate the possibility of
occurrence of subsidence and sinkhole events
at the surface. As part of a project devoted to
these aims, the paper describes in the
following the main features of the karst system
at
Castellana-Grotte,
Italy,
and
the
geomechanical characterization
of the
carbonate rock mass therein present.
The karst system at Castellana-Grotte
The Apulia region of southern Italy is
formed by Jurassic-Cretaceous limestones and
dolostones covered by Tertiary and Quaternary
clastic carbonates. It was interested since the
Lower Pleistocene by a general uplifting, until
it reached the present configuration (Doglioni
et al., 1994). The region is fragmented by high
dip, mostly NW-SE striking, faults into
uplifted and lowered blocks (Ricchetti et al.,
1988; Bosellini, Parente, 1994). Due to the
widespread presence of carbonate rocks,
surface and underground landforms were
extensively involved in karst processes that
produced an extensive network of underground
cavities and conduits. The landscape is
generally flat and characterized essentially by
landforms of karst origin, whose best
morphological expressions are identifiable on
the Murge Plateau of inland Apulia (Sauro,
1991). The lower part of south-eastern Murge
is on a coastal platform of Pleistocene
calcarenites resting over the Cretaceous
limestone bedrock: near the coastline, wide
cavities are prone to development by
dissolution at the interface between salt and
fresh water at either current or past sea levels.
A variety of karst features characterizes this
territory, both at the surface (dolines, poljes,
dry valleys, karst microforms, etc.) and at the
subsurface (Parise, 1999, 2006). The network
Geomechanical characterization of carbonate rock masses in underground karst system
of caves in the south-eastern Murge, in
particular, is among the most developed in
Apulia, and include the longest and most
famous karst system of the region, the
Castellana Caves (Fig. 1). First explored by
Professor Anelli in January 1938, the cave was
soon exploited as show cave (Anelli, 1938,
1957), whilst in the decades later the
explorations continued to add new passages to
the overall development, until reaching the
229
presently known length of 3,348 meters, with a
maximum depth of -122 meters (Parise et al.,
2002). As for most of the caves in this
territory, the Castellana Caves have a
prevailingly sub-horizontal pattern, with large
caverns, whose height ranges from a few
meters to some tens of meters, and intervening
corridors; development of the latter is
frequently controlled by the main discontinuity
systems in the rock mass.
Fig. 1. Longitudinal cross-section at the Castellana Caves.
The karst system at Castellana opens in the
Altamura Limestone formation, a stratified
limestone of Upper Cretaceous age (Parise,
Reina, 2002): it can be classified as an hard
rock with crystalline texture and isotropic
structure at the laboratory specimen scale,
whilst, at the rock mass scale, it can be
considered as an anisotropic rock due to
moderately spaced bedding planes (Lollino et
al., 2004). The rock mass is intensely
fractured, and locally show arching and
deformations in the limestone strata, induced
by the weight of the rock above (Photo 1).
Photo 1. Limestone strata deformed, due to the weight of
the rock above (photo courtesy of G. Ragone).
Breakdown processes have played a very
important role in the cave evolution in this
area, and at several sites have become the main
cause of widening and upward enlargement of
the original caves. This is well evident at the
Castellana Caves, starting from the cavern
(called Grave; see figure 1) at the entrance of
the system: it presents in fact a wide opening
due to the collapse of the roof, which was the
extreme consequence of the upward
propagation of the instability mechanisms. At
many other rooms in the Castellana Caves it is
possible to observe similar features, even
though not reaching the ground surface,
together with other mechanisms of breakdown,
from block to slab and chip breakdowns, to
major ceiling collapse: these processes often
result in thick fall deposits and in recurring
bell-shaped cross-sections of the caverns.
The karst systems at Castellana-Grotte are
multi-phase, having initially formed when the
limestone rock mass was saturated beneath a
water table, and later evolved when the water
table lowered. As a result, the original network
of tubular phreatic caves was modified by
subsequent phases of vadose caves, mostly
characterized by canyon-like features. In turn,
these passages later changed through
breakdown processes, and were partly or
completely filled by allogenic sediments and
secondary calcite deposits. In some rooms,
230
Mario Parise, Maria A. Trisciuzzi
where thickness of the strata is lower, the
progressive failures of the unstable roof easily
created an increasing pile of rock debris, and
the upward migration of the void. At these
locations, the original dissolutional cave may
therefore be at depth much greater, below the
several meters-thick debris. All of this makes
the cave systems extremely complicated as
regards the evaluation of stability phenomena.
In particular where the walls are fully
decorated with stalactites and flowstones, the
recognition of discontinuity systems in the
rock mass becomes very difficult.
Methods and results
To examine the breakdown processes that
worked in the formation of the present caverns
at
Castellana
Caves,
morphological
observations were carried out in the karst
system: distribution of detached blocks was
mapped, and the main morphometric
parameters (length, thickness, volume)
measured. Efforts have also been made to
identify the most likely source area of each
block. The morphometric parameters have
been then compared to the size of the rooms
and caverns where the blocks are present. This
phase of work was completed by structural
surveys dedicated to investigate the main
discontinuity systems in the carbonate rock
mass, and their main features, according to
internationally established standards. In situ
surveys were in particular focused on the
effects of weathering deriving from water
infiltrating from the surface, and on the other
properties that have been identified as
significant for any engineering classification of
limestones (Fookes, Hawkins, 1988; Anon,
1995; Waltham, Fookes, 2003).
Phase 1: Inventory of fall deposits
The Castellana Caves were opened to the
public soon after the discovery, and became
rapidly one of the most popular show caves in
Italy and Europe. Using the natural
underground karst systems as a public space,
even though along selected paths, strongly
changes the natural environment. At Castellana
this occurred by realizing the tourist passages
with intense use of concrete, and removing the
rocky debris present in the caves.
Nevertheless, the greater blocks and pieces of
rock debris were not moved.
Photo 2. Rock debris in the initial part of the karst
system, between Grave and Caverna dei Monumenti.
Rule meter for scale (case diameter 10 cm).
Photo 3. Fallen block at the end of the long Corridoio
del Deserto: note that the block, including the
speleothems above it at the time failure occurred, leaned
against the opposite wall; after the detachment,
stalagmites developed above the fallen block. At this site
the passage is very narrow, and the tourist pathway is
exactly below the block.
Starting from these considerations, we
performed a detailed inventory of the deposits
Geomechanical characterization of carbonate rock masses in underground karst system
produced by falls and breakdown in all the
karst system, that still occupy their original
position. The deposits were mapped and
described, taking into account their size and
shape, and performing an attempt in
correlating them with the likely detachment
zones (Photo 2). Further, the presence of
secondary deposits (concretions, speleothems,
flowstones) over the rockfall deposits was
observed, and their height measured (Photo 3).
Fig. 2. Inventory map of fallen deposits at the Caverna
dei Monumenti, the second largest room in the Castellana
Caves (see fig. 1 for location). The map covers a length
of about 100 meters along the NW-SE direction.
Every group of blocks has been marked
with a capital letter (indicating the room where
it is located) followed by a number to
discriminate it from the nearby groups. Within
each group, the single block was identified by
adding a small letter. Different forms were
prepared and filled during the field survey for
each complex, and for every significant block
as well; an example of form for one of the
main rooms in the Castellana Caves (Caverna
dei Monumenti) is shown as Table 1. At those
sites where it was not possible to identify the
single blocks, due to presence of a chaotic
mass of debris, only the form describing the
overall complex was compiled. Even though
time-consuming, this approach allowed us to
collect a great amount of data on the fall
deposits within the karst system, and to
produce for every room a detailed map, a
simplified sample of which is presented here
as Fig. 2. In this map, it is possible to note the
differences in the areas covered by the groups,
and, at the same time, in the density of the
231
blocks among the different groups; this latter is
generally due to concentration of debris in a
particular zone, depending upon the more
frequent detachment of rocks from source
areas characterized by intense jointing and/or
passage of water.
Phase 2: Geomechanical characterization of
carbonate rock masses
The second step in the study of the karst
system at Castellana was to develop a
geomechanical characterization
of the
carbonate rock mass where the cave opens.
This part of the work was performed following
the standards defined by the International
Society of Rock Mechanics for the description
of rock masses (ISRM, 1978). Therefore, all
the relevant parameters needed to make a
detailed description of the discontinuity
systems in the rock mass were observed,
measured,
mapped,
described
and/or
estimated:
strata
bedding,
spacing,
pervasiveness, roughness, wall resistance,
aperture, infilling material, presence of water,
number of discontinuity systems, size and
shape of the blocks. The above recalled ISRM
standards, however, were not specifically
designed for carbonate rock masses; due to this
reason, an effort was made to include in the
description further observations specific for
soluble rocks affected by karst processes. For
example, the presence of karst conduits along a
particular discontinuity system was indicated,
where present, as well as the preferential flow
of water in correspondence of specific
fractures or joints. Size of the karst features
observed was also measured.
Furthermore, some observations were
performed on the weathering condition of the
rock mass. The limestone is in fact frequently
characterized at its outer portion by very soft
and porous material, some mm thick, that
locally create a continuous coating over the
less weathered rock.
The thicker weathered zones are generally
found at those sites where the walls are at
contact with clastic sediments or they are
wetted by trickling or condense water and
where weathered material is protected against
mechanical erosion (Zupan Hajna, 2003). The
232
Mario Parise, Maria A. Trisciuzzi
contact with fine-grained sediments is
particularly important, since it contributes to
provide the moisture required for dissolution.
Table 1. Form for collection of data about fallen blocks in the Castellana cave system. The form refers to Caverna dei
Monumenti, the same cavern shown in Fig. 2.
FORM FOR CAVERN/ CORRIDOR/ LATERAL BRANCH
1
DATE OF SURVEY
CAVERN
CORRIDOR
LATERAL BRANCH
2
NAME
3
4
LETTER ASSOCIATED
13 March 2006
MONUMENTI
C
GENERAL DESCRIPTION: morphology
The Caverna dei Monumenti is a huge cavern (the second largest in the system, after the Grave) characterized by chaotic
rock debris of great size. Above the fallen materials, some meters-high stalagmites complexes have grown, cementing most
of the rock debris, and creating the main forms of the cavern. The rock walls are stratified, with sub-horizontal bedding.
5
6
BEDDING
STRIKE
DIP
SLOPE
N120
NE
4˚
N100
N
8˚
LOCATION OF MEASURE
POINT
NE wall
OBSERVATION OF THE MAIN ELEMENTS
6.1
VAULT: fractures, detachment areas, stalactites, etc.
The vault presents wide zones from which the rock were detached: they are recognizable for the overall half-moon shape,
strongly conditioned on one side by the most important discontinuity systems at Castellana Caves, that is the NW-SE
family. In addition to the central area in the vault (the zone where height of the cavern reaches its maximum), some minor
detachment areas can be identified. Several discontinuities in the vault are marked by lines of stalactites.
WALLS: bedding strata, presence or absence of peculiar elements, comparison with the other walls in the same
6.2
room/cavern/corridor/etc
The rock walls are stratified, with sub-horizontal bedding, and characterized on the north-eastern side by several conduits
located along the bedding planes.
6.3
6.3.1
PAVEMENT: deposits
OVERALL DESCRIPTION: location of groups with respect to the context
The largest groups are C2 and C3, both in terms of amount of rocky debris and size of single blocks. C2 derives from falls
from the NNW wall of the cavern, where the two narrow passages connecting it with the Grave are located. C3 deposits, on
the other hand, derive from falls from the vault, likely in more successive episodes. Several sub-groups can be identified
within C3. Minor, but still significant, groups mark the rest of the cavern.
Single block
6.3.2
CLASSIFICATION
Group
Corrosive moisture has been in fact invoked as
the main reason for limestone weathering in
several cases, including the drenching of clay
pebble surfaces (Davis, Mosch, 1988).
CODE
C1a; C2a; C3a; C3b; C3c; C3d;
C3e; C4a;C5a; C6a; C6b.
C1; C2; C3; C4; C5; C6.
At selected sites, characterized by
particular jointing in the rock mass, or
considered of greater importance due to
vicinity to the tourist passages, detailed geostructural surveys were performed by
Geomechanical characterization of carbonate rock masses in underground karst system
measuring several hundreds of discontinuities,
analysing them statistically and representing
by means of rose diagrams (Fig. 3) to highlight
the more frequent discontinuity systems and
their relation with the direction of the cavern
sidewalls.
233
mass. The main data from the surveys are
listed in Table 2. Besides bedding of strata,
four discontinuity systems have been identified
at each measurement station, the prevailing
system always being in the range N 130-150.
The sector mostly affected by instability,
corresponding to the second and third stretches
(B + C), has been analyzed at greater detail,
through determination of the joint roughness
coefficient (JRC) and the joint compression
strength (JCS). At this aim, in situ tests were
carried out on the exposed joint surfaces to
determine the roughness and the compressive
strength of the joint walls. Over 120 joint
profiles have been tested by means of a
profilometer, and the mean JRC value
estimated for each discontinuity system after
statistical treatment of the data. Mean JRC
ranges from 6-8 to 18-20 (Table 3), according
to ISRM standards (ISRM, 1978). The mean
value of the joint wall compressive strength,
JCS, as deduced by means of Schmidt hammer
tests, again performed according to ISRM
suggestions, has a mean value of 54 MPa. The
residual friction angle of the joints, φ’r, has
been deduced by means of tilt tests performed
on natural joint planes, and the corresponding
mean value is about 32°. Eventually, in order
to assess the peak friction angle, φ’p, the
empirical criterion of Barton has been used,
and the resulting value ranges between 43° and
57° (Table 3).
Fig. 6. Rose diagram (equal area projection) at
measurement station B in the Corridoio del Deserto: note
the marked prevalence of discontinuities belonging to the
NW-SE family, that is the main direction of development
of the Castellana karst system.
The main site object of this part of the
research was a 100 m-long stretch in the final
part of Corridoio del deserto, where three
separate survey lines have been established for
the geomechanical characterization of the rock
Table 2. Main data surveyed at the measurement stations in the Corridoio del Deserto.
Measurement
station
Length
(m)
Number of
discontinuities
Mean spacing
(m)
A
B
C
17
11.8
20
130
127
102
0.13
0.09
0.20
Main system
(spacing, in m, between
brackets)
N 140-150 (0.55)
N 140-150 (0.62)
N 130-140 (1.80)
Table 3. Joint roughness coefficient (JRC) and joint compressive strength (JCS) values at the Castellana Caves.
Discontinuity
system
bedding
N 015
N 100-130
N 125-140
N 150
Number of JRC
profiles
14
22
28
32
28
Mean JRC
JCS rebounds
6-8
8-10
16-18
6-8
18-20
78
52
46
150
62
φ’p
(°)
43
45
55
43
57
234
Mario Parise, Maria A. Trisciuzzi
Discussion and future perspectives
The geomechanical characterization of rock
masses is mandatory to any evaluation of rock
stability. In karst caves, it is important to
integrate the widely used approach and
standards with further observations concerning
the peculiarity of underground karst landforms
and processes. Combining the data from the
geomechanical characterization of rock masses
with direct observations on the breakdown
mechanisms acting in the cave may provide
crucial insights toward the comprehension of
the more likely instability process, and help in
identifying the sites more susceptible to rock
falls. There is no doubt that in show caves, and
in
caves
particularly
frequented
by
speleologists and researchers, this is an
important achievement.
The main mechanisms of instability that
have been observed at the Castellana Caves
are:
a) a progressive upward evolution of the
roof cave through detachment of
successive strata, until possibly reaching
the ground surface;
b) detachment of single block(s) from the
roof or the walls;
c) detachment of single slabs from the roof;
d) fall of overhanging rock shelf from the
walls.
Mechanism a) has been the main one acting
in the system, but acted in past geological
times, soon after the level at which the karst
system develops was left by the underground
river that made its way at greater depths. The
unsupported walls and roofs of the original
environments were at that time subjected to
collapses and progressive upward stoping.
Today, this mechanism is dormant, and no
massive fall has been observed recently.
Nevertheless, the presence of several
discontinuity systems identified in all the
measurement stations, together with some
properties of the rock walls (aperture of
fractures, weathering, water infiltration, etc)
points out to the necessity of monitoring the
sites where jointing is particularly widespread
and pervasive.
All the other mechanisms above are of
minor entity when compared to the first one.
They have to be considered still active, due to
the before described characters of the rock
mass. Mechanism b) may occur through falling
of rock wedges, depending upon orientation of
the discontinuities and geometry of the roof.
Wall failures (mechanism d), leading to
widening of the cave passages, occur where
portions of the wall are left unsupported as
shelves, and then fall. In some passages, a
typical step profile, showing successive steps
moving away from the middle line of the
pathway, may be in fact observed (Fig. 7).
Fig. 7. Cross-section at Caverna della Civetta. Note at
the north-eastern wall the presence of unsupported rock
shelves (marked by arrows), that might be subject to
future detachments from the rock mass.
In conclusion, it has to be stressed that the
geomechanical characterization of carbonate
rock mass in karst caves is crucial for allowing
further analysis devoted to the assessment of
the stability conditions within the karst system.
At this regard, a preliminary analysis has been
performed in one of the cavern of the
Castellana system by means of a discrete
element code (Lollino et al., 2004): the
numerical results have highlighted how the
gradual degradation with time of the limestone
Geomechanical characterization of carbonate rock masses in underground karst system
tensile strength (Diederichs & Kaiser, 1999)
seems to be the main factor controlling both
formation and propagation of vertical joints
within the rock strata overlying the cave roof.
This degradation essentially depends upon the
235
chemical and moisture weathering processes of
the rock mass. Further analysis are necessary
to ascertain whether these outcomes
characterize the whole karst system or are
limited to specific rooms and sites.
Acknowledgements
We acknowledge the Grotte di Castellana
s.r.l. for having permitted the access to the
Castellana Caves for this study. Giovanni
Ragone kindly provided the photograph of
Fig. 2.
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