ACTA GEOLOGICA UNIVERSITATIS COMENIANAE Nr. 51, 17-22 Bratislava 1996

Faculty of Natural Sciences /
Comenius University /
Prirodovedecka fakulta
S o u r c e : Title of Journal: Acta Environmentalica Universitatis Comenianae Published by: Comenius University - Faculty of Natural Sciences Bratislava Year: 1996, Number: 51
ACTA GEOLOGICA UNIVERSITATIS COMENIANAE Nr. 51, 17-22 Bratislava 1996
Engineering geological study of some medieval castles in Slovakia
Jan Vlcko - Rudolf Holzer - Anna Hyankova
Department of Engineering Geology, Faculty of Natural Sciences, Comenius University, Mlynska dolina, 842 15 Bratislava, Slovakia
Key words: engineering geological research, the protection and conservation, the historic sites
Abstract: Research work and practical studies aimed toward the protection and conservation of historic sites have shown important role of geosciences, namely engineering geology. The paper presents some results gained during an engineering geological research programme dealing with this topic.
I n t r o d u c t i o n A great number of medieval castle ruins (dating back mostly to the 13th century) located at steep ranging cliffs belongs to the typical landscape of Slovakia. Since the beginning of the 18th century the castles were loosing their primary defence function, many of them were destroyed by fire or during military actions and later abandoned. At present the majority of the castles have the character of ruins and they are in a rather poor state. Some of them endanger the safety of settlements (living places) and the traffic below, others are located in dense forests, in places difficult of access. The latter being part of nature reserves represent a hazard mostly to individuals (tourists).
Recently, a research programme has been initiated and sponsored by the Ministry of Environment, in which about 50 castles or castle ruins (from among 110) are going to be investigated in order to prevent their gradual disintegration due to both natural and man-made influence.
The paper presents some results of the engineering geological survey aimed at the restoration of some historic sites in Slovakia.
Works and techniques employed The preliminary stage of investigation carried out on a large regional basis, within a short period of time, with low financial costs requires a cost-effective approach. The experience gained from previous studies and practical works aimed toward the protection and conservation of historic sites proved that each historic site represents a completely different object of interest from geological, geotechnical and historical point of view. That was the real reason why no standardized methods and techniques were applied and the survey was strictly focussed at these priorities: l to undertake an engineering geological study of all rock slopes and to determine in the rock mass and in the castle walls those sections which represent a potential source of instability and to design the appropriate remedial works in designated sections, l to prepare graphic outputs that can be easily understood by specialists from branches dealing with historic building conservation, landscape architecture, etc.
The engineering geological survey was conducted largely by visual techniques without drilling and laboratory tests. The main accent was laid on engineering geological mapping and consequently engineering geological maps at a scale of 1: 500 were prepared from each site. Further activities were focussed on measuring the joint parameters, on location of past and active slope failures, rock falls, etc., on observation of weathering processes in rock mass and rock material and on karst processes. Special attention was paid to the character of anthropogeneous sediments and their thickness, which is quite different from the thickness found there at the time of construction. These sediments act now as a source of active pressure on the castle ruins and when accompanied by other natural and man-made factors they may cause damage.
Use was made of terrestrial photogrammetry for surveying and calculating joint orientation patterns in places difficult of access as well as in places which have been identified as potential sources of instability. Finally typical profiles from stereopairs were drawn.
Engineering geological factors influencing castle rock stability The majority of castle ruins is located on steep ranging slopes in areas of flysh and neovolcanic uplands and core middle mountains at a height of 350 to 700 m a.s.l. (Fig. 1). Due to intensive neotectonic movement of the West Carpathians in the periods of late Neogene and in the Quaternary a complicated geologic-tectonic structure developed. This development resulted in the formation of a highly dissected relief and a diverse climatic zonality of the territory.
Fig.. 1 Location of investigated sites : 1- core mountains, 2- Carpathian flysh and Klippen Belt zone , 3- Neogene volcanites, 4- Neogene tectonic depressions, 5-medieval castles
The effect of the climatic conditions at the above mentioned heights is quite evident, manifesting sudden daily and seasonal temperature fluctuations ranging from -20 to +20 oC, the daily temperature fluctuations range from 10 to 15 oC. The mean annual amount of precipitation varies between 600 to 800 mm. In this respect, the prolonged snow cover (100 to 140 days a year) is of great importance. The vertical climatic zonality and the variety of lithology control not only the exogeneous processes like weathering, erosion, slope movements, etc. but also superficial deposits formation and groundwater conditions.
The presence of overthrusted tectonic units is a typical feature of the West Carpathians and this has a substantial influence on the character and the state of the castle rocks. A very frequent case is when soft rocks mostly claystones and marlstones are overlain by rigid limestone-dolomitic complexes. Overthrust lines act, when affected by shear forces, as an important plane of weakness and/or potential shear planes. Such geological structure forms favourable conditions for deep-seated creep deformations inhibiting ridge loosening, block failures (block rifts and block fields) followed by rock toppling and rock falls. This geological structure was observed on several sites of interest. As an example we present the Uhrovsky hrad Castle located 130 km NE of the capital in the Strazovske vrchy Mts. The castle rock is formed by the Triassic dolomites, dolomitic breccias and limestones (Choc nappe). This formation is underlain by Neocomian marls, marly shales and marly limestones (Krizna nappe). Differential geotectonical development reflected in a mosaic of small structural-morphological elements as a result of an intensive Mesoalpine and mainly Late Alpine tectonic compression.
Due to uplift and horizontal stresses release, the existing fault system was rejuvenated and with the karst and weathering processes open tension cracks appeared (Fig. 2).
Fig. 2 Uhrovsky hrad castle - Upper Palace. Typical profile of the castle rock with the castle walls
The neotectonic fault system contributed dominantly to the general morhostructural division of the castle rock and resulted in an expressive cliff morphology. In places where the fault system and jointing has been intensively developed (two main fault and discontinuity systems were observed 270-305o/75-89o and 165-208o/68-80o - dip direction/dip) accompanied with favourable geological structure the deep seated creep movement occured and displacement of dolomite cliffs along subvertical tectonic or tension planes took place. This resulted in the destruction of the oldest part of the castle - the Romanesque chapel located in SW part of the castle rock. Across its outer arched wall a vertical crack 1 to 3 cm wide occurred and horizontal wall displacement took place. A part of castle walls attached to the chapel has already collapsed and the process of wall destruction is in a progress (Fig. 3). In the S part of the arc wall there can be seen the continuation of the vertical crack, which limits the rest of the outer arc wall. There is a serious danger that in the near future the top vault and the outer chapel walls might fall down. We may assume that the the presence of plastic rocks below the tectonically compressed dolomites caused deep seated deformations and associated with karst phenomena are the direct cause of the chapel and castle walls damage. Despite completely different engineering geological conditions the situation found at the Lietava Castle in Sulovske vrchy Mts. is similar (Fig. 4).
Fig. 3 Uhrovsky hrad castle (photo from 1993). Failures in the walls: 1 - minor, 2 - major, 3 - fallen down walls since 1972
The castle is located on a massive Paleogene carbonate conglomerates, which form a rather bizzare landscape with a number of cliffs. The basal conglomerates found in the bedrock are underlain by Cretaceous claystones and marlstones. This again led to deep-seated creep movement and subsequently to differentiated displacement of conglomerate cliffs along subvertical tectonic planes running deep into rock mass took place. The castle ruin shows a high degree of destruction and the displacement of several cliffs is quite evident from open cracks separating individual castle objects (Fig. 5). On the basis of terrestrial photogrammetry the orientation of faults and joints exhibited on places difficult of access was calculated and profiles (showing places of potential instability) were drawn by computer (Fig. 6). In order to determine the actual activity of creep movements we decided to install crack-gauge equipment (TM-71) in tension cracks which enable monitoring and facilitate the subsequent decision concerning remedial works.
Fig. 4 Lietava Castle
Fig. 5 Open cracks in the walls of Lietava Castle as the result of a creep movement
A very complicated geological structure developed in the Klippen-Belt zone of the West-Carpathians. It is characterized by intensive folding and extreme tangential compression and it reminds of a "tectonic megabreccia" (Matula 1965). Due to disharmonic folding rigid Jurassic limestones-sandstone strata are sunken into plastic Cretaceous marlstones in the form of blocks and lenses which in the present relief rise actively and form landscape dominants, on the top of which castles were built in the past.
The Lednica castle in Biele Karpaty Mts. represents a typical example of the fact that the physical state of the object is substantially influenced by the complex geotectonic structure of the bedrock. The geology of the Lednica castle Klippe (Fig. 7) represented on a sketch drawn on the basis of photogrammetric survey showing that important tectonic lines found in the bedrock continue in the castle walls in the form of open cracks and visible disturbances. It may be assumed that the process of crack formation is the result of a creep movement which is necessary to monitor so that appropriate remedial works may be designed.
Fig. 6 Typical profiles drawn from the photogrammetric survey from Lietava Castle
Failures of quite different origin brought about the destruction of the Cachtice castle in Cachtice Karpaty Mts.. The castle rock is formed by Triassic dolomites (Hauptdolomite) in which the main discontinuities are the bedding planes. A constant dynamic process of loosening and damages associated with the collapse can be observed on the castle ruins. The main reason for this process is an active earth pressure of anthropogeneous fills of great thickness occurring at critical places. Calculation proved that the coefficient of earth pressure at rest reaches 80% of limiting state of equilibrium.
The western part of the castle ruin manifests signs of destruction illustrated in Fig. 8. For restoration purposes we have delineated in the western castle walls zoning units in terms of their physical state, where the highly endangered parts (1) are affected by active earth pressure of anthropogeneous fills and by the presence of bedding planes (7) which have resulted in the formation of open cracks found in the castle walls (6). The stability of the castle walls (1) has been temporarily secured by means of supporting wooden beams. The zoning of the physical state of the castle objects has been carried out on all investigated castles in a similar way as in the case of Cachtice castle - either with the use of photogrammetric photography or in a ground plan.
Fig. 7 Lednica Castle. A typical sketch drawn from photogrammetric survey. Construction material: 1-sandstone, travertine;2-limestones, claystones;3- sandstones; 4- limestones, marlstones, travertines, sandstones; 5- tension cracks; 6- faults; 7- block slide; 8- boundaries between different construction materials; 9 - Jurassic limestones; 10- Cretaceous marls and marlstones; 11-slope sediments
Fig. 8 Zoning sketch of western walls at Cachtice Castle. Failures in the walls: 1- major; 2- moderate; 3- minor; 4- walls reconstructed in 1992; 5- fallen down walls; 6- open cracks; 7- faults; 8- Triassic dolomites; 9- supporting wooden beams
C o n c l u s i o n In this paper we have focussed our attention on the description of engineering geological and geotechnical factors causing the destruction of a number of castles and the presentation of the results of their engineering geological inventory. The results of engineering geological investigation proved that:
l the most intensive damage on castle walls occurred at the intersection of the main gravitational tectonic lines and at places where moving blocks occur, l the weathering of the stone-work (walls) and the underlying rocks as well as widening of open cracks by karst processes are sources of potential instability, l past earthquakes and other seismic effects (blasting) could promote rheologic creep movements, l the slope stability analysis gained by photogrammetric survey and joint set evaluation proved that attention has to be paid to the subgrade of the castle to a depth of 4 to 6 m where rocks are strongly weathered and jointed and the rock face shows several features of destruction, l the application of remedial works to those parts of the castle which were built on moving blocks seems to be very complicated and requires precise monitoring of slope movements and continual maintenance of the castle walls.
Naturally, there exist several other natural factors (vegetation cover) influencing the destruction processes but the greatest number of damage is caused by man (poor maintenance, tourism, environmental changes).
R e f e r e n c e s Vlcko, J. (1994): Engineering Geological Inventory of Selected Historical Monuments - Castle Cachtice (in Slovak). Research report, Dept.Eng.Geol., Bratislava
Hyankova, A. - Chabronova, J. - Vlcko, J. (1994): Engineering Geological Inventory of Selected Historical Monuments - Castle Lietava (in Slovak). Research report, Dept. Eng. Geol., Bratislava
Hyankova, A. - Vlcko, J. (1994): Engineering Geological Inventory of Selected Historical Monuments - Castle Lednica (in Slovak). Research report, Dept.Eng.Geol., Bratislava
Holzer, R. - Letko, V. (1995): Engineering Geological Inventory of Selected Historical Monuments - Uhrovsky hrad Castle (in Slovak). Research report, Dept. Eng. Geol., Bratislava.
*The research work was based on the results of the project of Faculty of Natural Sciences No. G-01.
*Delivered on the 11th January 1996
S o u r c e : Title of Journal: Acta Environmentalica Universitatis Comenianae Published by: Comenius University - Faculty of Natural Sciences Bratislava Year: 1996, Number: 51
Faculty of Natural Sciences / Comenius University / Prirodovedecka fakulta