Geomechanical characterization of carbonate rock masses in underground systems: a case study from Castellana-Grotte ()

Mario Parise 1, Maria Addolorata Trisciuzzi 2 1 National Research Council, IRPI, Via Amendola 122-I, 70126 , Italy e-mail: [email protected] 2 Technical University, Engineering Faculty of , 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 , 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 (, 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 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 rock can actually suffer from buckling failure through progressive failures of roof rock units, when the rock plates are relatively thin and whilst wall failures are less common. The when the in situ horizontal stress is high. process of roof stoping (or cavity migration) When the in situ horizontal stress is low, the consists of the progressive failure of individual roof slabs in a similar opening can fail as a beds or slabs of rock, developing upward, and result of the tensile stresses induced by eventually reaching the ground surface. It bending of the slabs under their own weight seems to be more rapid in thinly-bedded (Hoek, Brown, 1980). limestones rather than in massive or thickly- Stability problems in blocky jointed rock stratified carbonate rock masses. The roof and are generally associated with gravity falls of floor of an opening in horizontally bedded blocks from the roof and sidewalls (White, 228 Mario Parise, Maria A. Trisciuzzi

White, 1969). Weathering may be locally The difficulties in estimating this geohazard important, decreasing the physical properties have been treated by Waltham (2002) in his of the rock, and favouring enlargement of the engineering classification of karst, that is fissures and joints, until causing detachment. largely based on the three features most The main factors controlling the profiles of relevant to engineers concerned with the passages are the structural and integrity of structural foundations in karst lithological features of the host limestone, and terrains: , rockhead and caves. Five the past and present hydrology in the cave. classes are defined on the basis of the typical Starting from the cave passage profiles, assemblages of morphological features, from therefore, with particular regard to width and undeveloped karst to normal, mature, complex, height, and, looking at the overall shape of the and extreme karst. Even though the classes can cross-section, it is possible to have a be recognised in a climatic context, the high preliminary guess about the processes that variability of karst (also within a region) make have been active in the past to produce such a the classification not absolute. This means that shape. an area can be attributed to a certain class, but Besides creating problems in the some small sites may fall into a higher or underground setting, the presence of large lower class. voids or cave in karst environments is a In areas above show caves, it is therefore significant geohazard even at the surface for very important to ascertain the overall engineers, due to the notorious unpredictability properties of the host rock mass, and the of their location and extent (Culshaw, stability at the surface, in order to allow safe Waltham, 1987; Parise, in press). A classic visits to tourists, and evaluate the possibility of example of the unpredictable nature of karst occurrence of subsidence and events has been described for the Remouchamps at the surface. As part of a project devoted to Viaduct in Belgium: on the initial ground these aims, the paper describes in the investigations, 31 found no cave, but following the main features of the karst system the subsequent excavation of the pier footings at Castellana-Grotte, Italy, and the found two unknown caves; this brought to a geomechanical characterization of the second phase of investigation, with 308 new carbonate rock mass therein present. boreholes that found no more caves (Waltham et al ., 1986).

The karst system at Castellana-Grotte

The Apulia region of southern Italy is generally flat and characterized essentially by formed by Jurassic-Cretaceous limestones and landforms of karst origin, whose best dolostones covered by Tertiary and Quaternary morphological expressions are identifiable on clastic carbonates. It was interested since the the Murge Plateau of inland Apulia (Sauro, Lower Pleistocene by a general uplifting, until 1991). The lower part of south-eastern Murge it reached the present configuration (Doglioni is on a coastal platform of Pleistocene et al ., 1994). The region is fragmented by high calcarenites resting over the Cretaceous dip, mostly NW-SE striking, faults into limestone bedrock: near the coastline, wide uplifted and lowered blocks (Ricchetti et al ., cavities are prone to development by 1988; Bosellini, Parente, 1994). Due to the dissolution at the interface between salt and widespread presence of carbonate rocks, fresh water at either current or past sea levels. surface and underground landforms were A variety of karst features characterizes this extensively involved in karst processes that territory, both at the surface (dolines, poljes, produced an extensive network of underground dry valleys, karst microforms, etc.) and at the cavities and conduits. The landscape is subsurface (Parise, 1999, 2006). The network Geomechanical characterization of carbonate rock masses in underground karst system 229 of caves in the south-eastern Murge, in presently known length of 3,348 meters, with a particular, is among the most developed in maximum depth of -122 meters (Parise et al ., Apulia, and include the longest and most 2002). As for most of the caves in this famous karst system of the region, the territory, the Castellana Caves have a Castellana Caves (Fig. 1). First explored by prevailingly sub-horizontal pattern, with large Professor Anelli in January 1938, the cave was caverns, whose height ranges from a few soon exploited as (Anelli, 1938, meters to some tens of meters, and intervening 1957), whilst in the decades later the corridors; development of the latter is explorations continued to add new passages to frequently controlled by the main discontinuity the overall development, until reaching the systems in the rock mass.

Fig. 1. Longitudinal cross-section at the Castellana Caves.

The karst system at Castellana opens in the Breakdown processes have played a very Altamura Limestone formation, a stratified important role in the cave evolution in this limestone of Upper Cretaceous age (Parise, area, and at several sites have become the main Reina, 2002): it can be classified as an hard cause of widening and upward enlargement of rock with crystalline texture and isotropic the original caves. This is well evident at the structure at the laboratory specimen scale, Castellana Caves, starting from the cavern whilst, at the rock mass scale, it can be (called Grave; see figure 1) at the entrance of considered as an anisotropic rock due to the system: it presents in fact a wide opening moderately spaced bedding planes (Lollino et due to the collapse of the roof, which was the al ., 2004). The rock mass is intensely extreme consequence of the upward fractured, and locally show arching and propagation of the instability mechanisms. At deformations in the limestone strata, induced many other rooms in the Castellana Caves it is by the weight of the rock above (Photo 1). 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 Photo 1. Limestone strata deformed, due to the weight of breakdown processes, and were partly or the rock above (photo courtesy of G. Ragone). 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 the cave systems extremely complicated as progressive failures of the unstable roof easily regards the evaluation of stability phenomena. created an increasing pile of rock debris, and In particular where the walls are fully the upward migration of the void. At these decorated with and flowstones, the locations, the original dissolutional cave may recognition of discontinuity systems in the therefore be at depth much greater, below the rock mass becomes very difficult. several meters-thick debris. All of this makes

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 Photo 2. Rock debris in the initial part of the karst system, between Grave and Caverna dei Monumenti . mass, and their main features, according to Rule meter for scale (case diameter 10 cm). 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, Photo 3. Fallen block at the end of the long Corridoio even though along selected paths, strongly del Deserto : note that the block, including the changes the natural environment. At Castellana speleothems above it at the time failure occurred, leaned against the opposite wall; after the detachment, this occurred by realizing the tourist passages stalagmites developed above the fallen block. At this site with intense use of concrete, and removing the the passage is very narrow, and the tourist pathway is rocky debris present in the caves. exactly below the block. Nevertheless, the greater blocks and pieces of rock debris were not moved. Starting from these considerations, we performed a detailed inventory of the deposits Geomechanical characterization of carbonate rock masses in underground karst system 231 produced by falls and breakdown in all the blocks among the different groups; this latter is karst system, that still occupy their original generally due to concentration of debris in a position. The deposits were mapped and particular zone, depending upon the more described, taking into account their size and frequent detachment of rocks from source shape, and performing an attempt in areas characterized by intense jointing and/or correlating them with the likely detachment passage of water. zones (Photo 2). Further, the presence of secondary deposits (concretions, speleothems, Phase 2: Geomechanical characterization of flowstones) over the rockfall deposits was carbonate rock masses observed, and their height measured (Photo 3). 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, Fig. 2. Inventory map of fallen deposits at the Caverna aperture, infilling material, presence of water, dei Monumenti , the second largest room in the Castellana Caves (see fig. 1 for location). The map covers a length number of discontinuity systems, size and of about 100 meters along the NW-SE direction. shape of the blocks. The above recalled ISRM standards, however, were not specifically Every group of blocks has been marked designed for carbonate rock masses; due to this with a capital letter (indicating the room where reason, an effort was made to include in the it is located) followed by a number to description further observations specific for discriminate it from the nearby groups. Within soluble rocks affected by karst processes. For each group, the single block was identified by example, the presence of karst conduits along a adding a small letter. Different forms were particular discontinuity system was indicated, prepared and filled during the field survey for where present, as well as the preferential flow each complex, and for every significant block of water in correspondence of specific as well; an example of form for one of the fractures or joints. Size of the karst features main rooms in the Castellana Caves (Caverna observed was also measured. dei Monumenti) is shown as Table 1. At those Furthermore, some observations were sites where it was not possible to identify the performed on the weathering condition of the single blocks, due to presence of a chaotic rock mass. The limestone is in fact frequently mass of debris, only the form describing the characterized at its outer portion by very soft overall complex was compiled. Even though and porous material, some mm thick, that time-consuming, this approach allowed us to locally create a continuous coating over the collect a great amount of data on the fall less weathered rock. deposits within the karst system, and to The thicker weathered zones are generally produce for every room a detailed map, a found at those sites where the walls are at simplified sample of which is presented here contact with clastic sediments or they are as Fig. 2. In this map, it is possible to note the wetted by trickling or condense water and differences in the areas covered by the groups, where weathered material is protected against and, at the same time, in the density of the mechanical erosion (Zupan Hajna, 2003). The 232 Mario Parise, Maria A. Trisciuzzi contact with fine-grained sediments is provide the moisture required for dissolution. particularly important, since it contributes to

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 13 March 2006 CAVERN MONUMENTI 2 NAME CORRIDOR LATERAL BRANCH 3 LETTER ASSOCIATED C 4 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. STRIKE N120 N100 LOCATION OF MEASURE BEDDING 5 DIP NE N POINT NE wall SLOPE 4˚ 8˚ 6 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 PAVEMENT: deposits 6.3.1 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. CODE C1a; C2a; C3a; C3b; C3c; C3d; Single block C3e; C4a;C5a; C6a; C6b. 6.3.2 CLASSIFICATION Group C1; C2; C3; C4; C5; C6.

Corrosive moisture has been in fact invoked as At selected sites, characterized by the main reason for limestone weathering in particular jointing in the rock mass, or several cases, including the drenching of clay considered of greater importance due to pebble surfaces (Davis, Mosch, 1988). vicinity to the tourist passages, detailed geo- structural surveys were performed by Geomechanical characterization of carbonate rock masses in underground karst system 233 measuring several hundreds of discontinuities, mass. The main data from the surveys are analysing them statistically and representing listed in Table 2. Besides bedding of strata, by means of rose diagrams (Fig. 3) to highlight four discontinuity systems have been identified the more frequent discontinuity systems and at each measurement station, the prevailing their relation with the direction of the cavern system always being in the range N 130-150. sidewalls. 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 Fig. 6. Rose diagram (equal area projection) at measurement station B in the Corridoio del Deserto : note suggestions, has a mean value of 54 MPa. The the marked prevalence of discontinuities belonging to the residual friction angle of the joints, φ’r, has NW-SE family, that is the main direction of development been deduced by means of tilt tests performed of the Castellana karst system. on natural joint planes, and the corresponding mean value is about 32°. Eventually, in order The main site object of this part of the to assess the peak friction angle, φ’p, the research was a 100 m-long stretch in the final empirical criterion of Barton has been used, part of Corridoio del deserto , where three and the resulting value ranges between 43° and separate survey lines have been established for 57° (Table 3). the geomechanical characterization of the rock

Table 2. Main data surveyed at the measurement stations in the Corridoio del Deserto .

Main system Measurement Length Number of Mean spacing (spacing, in m, between station (m) discontinuities (m) brackets) A 17 130 0.13 N 140-150 (0.55) B 11.8 127 0.09 N 140-150 (0.62) C 20 102 0.20 N 130-140 (1.80)

Table 3. Joint roughness coefficient (JRC) and joint compressive strength (JCS) values at the Castellana Caves.

Discontinuity Number of JRC φ’ Mean JRC JCS rebounds p system profiles (°) bedding 14 6-8 78 43 N 015 22 8-10 52 45 N 100-130 28 16-18 46 55 N 125-140 32 6-8 150 43 N 150 28 18-20 62 57

234 Mario Parise, Maria A. Trisciuzzi

Discussion and future perspectives

The geomechanical characterization of rock All the other mechanisms above are of masses is mandatory to any evaluation of rock minor entity when compared to the first one. stability. In karst caves, it is important to They have to be considered still active, due to integrate the widely used approach and the before described characters of the rock standards with further observations concerning mass. Mechanism b) may occur through falling the peculiarity of underground karst landforms of rock wedges, depending upon orientation of and processes. Combining the data from the the discontinuities and geometry of the roof. geomechanical characterization of rock masses Wall failures (mechanism d), leading to with direct observations on the breakdown widening of the cave passages, occur where mechanisms acting in the cave may provide portions of the wall are left unsupported as crucial insights toward the comprehension of shelves, and then fall. In some passages, a the more likely instability process, and help in typical step profile, showing successive steps identifying the sites more susceptible to rock moving away from the middle line of the falls. There is no doubt that in show caves, and pathway, may be in fact observed (Fig. 7). 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 Fig. 7. Cross-section at Caverna della Civetta . Note at river that made its way at greater depths. The the north-eastern wall the presence of unsupported rock shelves (marked by arrows), that might be subject to unsupported walls and roofs of the original future detachments from the rock mass. environments were at that time subjected to collapses and progressive upward stoping. In conclusion, it has to be stressed that the Today, this mechanism is dormant, and no geomechanical characterization of carbonate massive fall has been observed recently. rock mass in karst caves is crucial for allowing Nevertheless, the presence of several further analysis devoted to the assessment of discontinuity systems identified in all the the stability conditions within the karst system. measurement stations, together with some At this regard, a preliminary analysis has been properties of the rock walls (aperture of performed in one of the cavern of the fractures, weathering, water infiltration, etc) Castellana system by means of a discrete points out to the necessity of monitoring the element code (Lollino et al., 2004): the sites where jointing is particularly widespread numerical results have highlighted how the and pervasive. gradual degradation with time of the limestone Geomechanical characterization of carbonate rock masses in underground karst system 235 tensile strength (Diederichs & Kaiser, 1999) chemical and moisture weathering processes of seems to be the main factor controlling both the rock mass. Further analysis are necessary formation and propagation of vertical joints to ascertain whether these outcomes within the rock strata overlying the cave roof. characterize the whole karst system or are This degradation essentially depends upon the limited to specific rooms and sites.

Acknowledgements

We acknowledge the Grotte di Castellana Ragone kindly provided the photograph of s.r.l. for having permitted the access to the Fig. 2. Castellana Caves for this study. Giovanni

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