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Geomechanical characterization of carbonate rock masses in underground karst systems: a case study from Castellana-Grotte (Italy) Mario Parise 1, Maria Addolorata Trisciuzzi 2 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 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 cave passages are the structural and integrity of structural foundations in karst lithological features of the host limestone, and terrains: sinkholes, 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 sinkhole 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 boreholes 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 show cave (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.

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