International Journal of Geoheritage. 2015, 3(2): 24-32 DOI: 10.17149/ijg.j.issn.2210.3382.2015.02.004 © 2015 Darswin Publishing House Stability determination of ceilings of some caves Ivaylo Ivanov Civil Engineering and Geodesy, Department of Geotechnics, University of Architecture, Sofia, Bulgaria Abstract: The karst areas in Bulgaria hide a number of potential hazards in case of insufficient engineering-geological investigations prior to construction. Some of those hazardous phenomena have been identified in almost all countries with karst locations. One of the biggest hazards is related to the sudden subsidence and collapse of ceil- ings in the karst caves which may bring about unforeseeable consequences and demolitions in the built areas. One of the well-known and interesting similar phenomena in Bulgaria is the Pro- hodna cave, over which the Karlukovo-Lukovit road passes by. The vibrations from the traffic as well as the additional impact of the blasts at the nearby rubble quarry could bring about problems leading to destruction of the cave’s arch. This paper describes the methods used by the author for determination of a coeffi- cient of stability of karst caves’ ceilings based on the theory of cracks and the critical width of caves. The methodology has been applied for assessment of stability of ceil- ings of two caves in the Karlukovo karst area – Prohodna and Svirchovitsa. Keywords: Bulgaria, Karlukovo karst area, cave ceilings, Griffit's cracks theory, stability, calculation 1 Introduction The karst areas in Bulgaria hide a number of potential hazards in case of insufficient engineering-geological investigations prior to construction. Some of those hazardous phenomena have been identified in almost all countries with karst locations. One of the biggest hazards is related to the sudden subsidence and collapse of ceilings in the karst caves which may bring about unforeseeable consequences and demolitions in the built areas (Sowers, G., 1996). One of the well-known and interesting similar phenomena in Bulgaria is the Pro- hodna cave, over which the Karlukovo-Lukovit road passes by. The vibrations from the traffic as well as the additional impact of the blasts at the nearby rubble quarry could bring about problems leading to destruction of the cave’s arch. This paper describes the methods used by the author for determination of a coeffi- cient of stability of karst caves’ ceilings based on the theory of cracks and the critical width of caves. The methodology has been applied for assessment of stability of ceil- ings of two caves in the Karlukovo karst area – Prohodna and Svirchovitsa. Author: Ivaylo Ivanov, Civil Engineering and Geodesy, Department of Geotechnics University of Architecture E-mail: [email protected] Ivaylo Ivanov: Stability determination of ceilings of some caves 25 2 Short geological characteristics of studied karst area The Kameno Pole-Karlukovo karst area (Popov, 1970) is situated in the Pre-Balkan region, comprising the outfalls of the Upper Cretaceous period in the river valley of the Iskar, between the towns of Roman and Cherven Bryag (Figure 1). Figure 1 Situation of Kameno pole – Karlukovo karst area The development of the karst in the region is related to the thick Campanian layers – Maestrichtian limestones, forming several suits (Dzhuranov S., M. Ivanov, & N. Yolkichev, 1993), distinguished by carbonaceous, clayey content, age and availability of specific fossils. The limestones in the area reach a thickness of over 200 m, the most strongly karsted is Kailaka formation. Downward to Mezdrs and Kunins formations, in parallel with the reduction of the carbonaceous content and the increase of the clayey component in the rocks, karsting sharply decreases and almost abates. The Ap- tian depositions - marls and sandstones of the Roman formation – serve as a common underlying formation for all of them. At the eastern side of the area the limestones are covered by Paleogene and Neogene deposits presented by clays and sands (Figure 2). In terms of tectonic, the area is situated in the Pre-Balkan tectonic region, at the border between the northern and southern Pre-Balkan and between its western and central part. These borders are fixed along large fault structures many of which cross each other in the vicinity (Ivanov, 1999). The Cretaceous depositions form nearly parallel and comparatively shallow syncline and not high anticline folds with ap- proximate direction west-east (Minchev, Pironkov, 1959/60). The syncline lowerings form the individual sub-areas of Kameno pole-Karlukovo karst area, namely Kar- lukovo sub-area, Kameno pole sub-area, and Gabarevo sub-area. 3 Short information about the studied caves The caves Bankovitsa and Svirchovitsa are situated in the Karlukovo karst sub-area, at the right bank of the Iskar River. They are formed in the limestones of Kaylashka suit (Figure 3). Prohodna cave is a karst bridge, 80 m long. It is part of a cave corridor of 26 INTERNATIONAL JOURNAL OF GEOHERITAGE Gabare subarea Kameno pole subarea Karlukovo subarea Figure 2 Geological map of Kameno pole - Karlukovo karst area 1 - 9. Lithological and stratigraphic units. 1. Kailaka formation (kK2m); 2. Mezdra formation (mzK2cp-m); 3. Dermantsi, Novachene and и Rumyantsevo formations (III,VIK2cp-m); 4. Kalentsi for- mation (kaK2st); 5. Roman formation (rmK1ap-a); 6. Malo Pestene formation (mpK1a); 7. Paleogene deposits; 8. Neogene deposits; 9. Quaternary deposits. Figure 3 Situation of the studied caves Ivaylo Ivanov: Stability determination of ceilings of some caves 27 impressive size, with approximate direction “east-west”. The total length of the bridge is about 80-100 m, the height at the east entrance is over 20 m, while the western en- trance is the highest one in Bulgaria – over 42 m (Vasilev H., & D. Sinyovsky, 2003). The cave forming possibly begins at the end of Pliocene and continues till the begin- ning of Pleistocene, which is connected with the change of the level of the Iskar River (Popov, 1985). Its age has been determined on the basis of geomorphological indica- tions and availability of old fluvial terraces. 4 Studies on the mechanism of destruction of the karst caves’ ceilings The issue of the stability and destruction of the karst caves’ ceilings has been poorly studied because of the comparatively rare manifestation of this karst deformation. The main mechanisms are nearly the same that can be seen in the destruction of the ceil- ings of the mine galleries which are comparatively better studied. The destruction of the karst caves’ and mine galleries’ ceilings may happen basically in two ways (Bak- lashov, 1988; Sowers, 1996). The first one is through exceeding the tensile strength in case of bending of the rocky layers of the ceiling which brings about appearance of a central crack and lay- er-by-layer detachment of rocky blocks, and also in case of exceeding the shear strength of the layers. The second way is through falling of some fragments from the ceiling and forming arches but it is typical mainly for strongly cracked rock mass (Figure 4). Figure 4 Principle mechanisms of destruction of the karst caverns ceilings in various conditions (Sowers, 1996). The author has studied mainly the first way, which is the most widespread way of ceiling destruction, and for that purpose he has used the theory of cracks (Griffith, 1921). According to this theory, the tensile stresses appearing in the middle of the ceiling lead to appearance and development of micro- and macro-cracks (Figure 5). The most hazardous of them are those developed at the middle of the arch and are approximately normally oriented towards it. According to Griffith’s criterion when 1/2 reaching the critical stress 3 2/ET c , the crack with a length of 2с begins to grow downward. Reaching the lower end of the ceiling, its development continues upward until reaching a horizontal abated surface (inter-layer crack, clayey band or the rock surface). According to the condition of Gordon-Cook, if the tensile strength 28 INTERNATIONAL JOURNAL OF GEOHERITAGE of the respective band does not exceed 1/5 of the tensile strength of the rock, then, in it, before the vertical crack, emerges a horizontal one, and the merger of them stops the vertical de- velopment of the crack. The merger of the two cracks proceeds with a de- tachment of a layer from the roof part of the cave under the impact of the dead weight. As a result, the horizon- tal crack grows and two overhanging ends of the ceiling are coming out. When the growing horizontal crack reaches the critical length lк, its un- controlled and unstable development begins (Jaeger, I., & W. Cook, 1969) and respectively, destruction of the ceiling or detachment of part of it. The magnitude of lк is determined according to the formula (Kositsin, Figure 5 Central crack development and collapse of 1980): the ceiling of karst cavern, according to the cracks theory (Griffith, 1921) 1/2 Kicr 1/4 lhк 2 , (1) 1/2 where Kiк (MPa.m ) – critical value of the coefficient of intensity of stresses, γ – bulk density of the rock (N/m3); h – thickness of the arch part or layer (m). In order to determine the critical length at which an uncontrolled destruction of the cave’s ceiling may occur, it is necessary an experimental determination of the critical value of the coefficient of intensity of stresses - Kicr to be carried out. The methodology implemented for the test of samples presumes determination of the ultimate load under which rock samples with artificially created beginning of crack are destructed. The sample is loaded in a press and bending stresses leading to destruction are created in its central part (Figure 6, 7). The value of the critical coeffi- cient of intensity of stresses КIcr is determined by the formula: 33 dcc3/2 22 KphIcr 4, 2 1 1 , (2) 2 dh where р – load (N/m); c, d, h – dimensions of sample (m).