Assessments on the stability of natural slopes prone to toe erosion, and man-made historical semi-underground openings carved in soft tuffs at Zelve Open-Air Museum (Cappadocia, Turkey)
Introduction
Turkey, a bridge between the Asian and European countries, is richly endowed with a cultural mosaic of various civilizations that have inhabited this land since Palaeolithic age. The Cappadocia Region of Central Anatolia (Figure 1), having a very distinct culture, is one of the attractive touristic sites in Turkey due to its spectacular and unique landforms and historical heritage. In this region, the structures carved into thick to massive tuffs survived and kept their original integrity for a number of centuries. Severe daily and seasonal changes in temperature, thermal isolation properties, self-supporting and easy carving of the tuffs at the Cappadocia Region have been the main reasons for extensive subsurface and multi-purpose use in the past and present. In the Iconoclastic Period (725–845 A.D.), the Cappadocia Region became the most important religious centre in Anatolia. The Göreme Valley was chosen as a centre for monastic education and chapels, basilicas, churches with one or several domes, and dwellings up to six storeys high were carved in the soft Cappadocia tuffs (Figure 2a). Underground cities and cliff settlements (semi-underground openings, which are openings carved next to cliffs to be used for residential and other purposes), most of which are more than 1500 years old, exist in the region (Figure 2b–c). Particularly, the area between Ürgüp, Göreme and Avanos towns (Figure 1) has recently become a museum of rock-cut structures, which are now converted into restaurants, hotels (Figure 2d–e) and shops. As well, modern underground openings in the region are being used for lodging, storage of fruits and potato (Figure 2f) and housing. The region was included in the World Heritage List in 1985 by UNESCO.
The Zelve Valley located between Göreme and Avanos (Figure 1) is one of the oldest locations for historical semi-underground cliff settlements in the Cappadocia Region and has a great historical, touristic and cultural importance. As well as being an important Christian settlement and religious centre between the 9th and 13th centuries, the rock settlements of Zelve were also home to the first seminars for priests (Gülyaz and Yenipınar, 1996). Although it has been closed to settlement due to some instabilities, which occurred in the 1950s and resulted in three casualties, it is still open to visits of local and foreign tourists as “Zelve Open-Air Museum”. It consists of three valleys (Figure 3a) and particularly in the first valley, there are a lot of rock-hewn openings in different sizes (Figure 3b) and three rock-carved ancient churches, namely Balıklı, Üzümlü and Geyikli, dating to pre-iconoclastic period before the 8th century. These exhibit some geo-engineering problems. The natural slopes in this valley are susceptible to toe erosion and rock falls. The semi-underground man-made historical structures of Zelve, which have been excavated in valley slopes and standing unsupported more than 1000 years, have been particularly deteriorating rapidly. The Geyikli Church, in the 1st valley, which was damaged due to a partial collapse in the past, totally collapsed in 2009 (Figure 3c), and therefore, its close vicinity was closed to visitors. Instability problems are increasing day by day due to enlargement of semi-underground openings as a result of spalling of the host rock, failure of some pillars and time-dependent decrease in rock strength, in addition to occasional slides along discontinuities and rock falls on the slopes in the 1st Valley in the Zelve Open-Air Museum.
The preservation as well as restoration of the historical sites requires basic understanding of the surrounding rock under various environmental conditions, and the stability of rock structures. Compared to underground structures, the surrounding rock of semi-underground structures (cliff settlements) is much more prone to physical and chemical weathering and degradation of rock properties. By considering the above mentioned problems and the importance of the Zelve Open-Air Museum in terms of its historical and cultural importance, in this study, the 1st Valley, which is the most critical part of the open-air museum, was selected for assessment of the geo-engineering characteristics of the tuff surrounding the semi-underground man-made openings that form the natural slopes, and for investigation of the effect of rate of toe erosion on the stability of steep valley slopes and structural stability of some selected historic settlements excavated as underground openings in different dimensions next to the cliffs of the valley. The research programme included surveying along the outcrops to characterize the rock mass conditions, particularly discontinuity characteristics, sampling for laboratory tests, periodic measurements to assess the rate of erosion at the toe of the cliffs and amount of spalling, measurement of the size of some selected openings carved in the soft tuff, evaluation of climatic conditions, studying on the mechanical response of the intact rock by testing rock specimens and estimation of rock mass strength, and investigation of the effect of toe erosion on the stability of natural slopes and structural stability of the underground openings selected from the valley using 2- and 3-D numerical codes.
Section snippets
Geology of the site and petrographical properties of the tuff
Central Anatolia constitutes a high plateau at an altitude ranging between 1300 and 1400 m asl which hosts a continental volcanic province with eruptions of Miocene to Upper Holocene age that was termed Anatolian Volcanic Province by Toprak et al. (1994). The province is surrounded by three old stratovolcanoes called Erciyes (3917 m), Melendiz (2935 m) and Hasandağ (3254 m) (Figure 1). The Cappadocia Region geologically covers approximately an area of 40,000 km2.
Cretaceous granitic and gabbroic rocks
Geomechanical properties of the Zelve tuff
In order to determine the physical and mechanical properties of the Zelve tuff, block samples were collected from the 1st valley. Since the study site is a natural museum and under protection, sampling directly from the walls of the openings is not allowed. Therefore, prime attention was paid to collect the blocks of rock fallen from the slopes of the valley and sidewalls of some openings. For this study's purpose, a total of 23 blocks in various sizes were collected from 8 locations in the
Rock mass characteristics and classification
Rock mass characteristics of the Zelve tuff were determined on the valley slopes. For this study's purpose, line surveys were carried out at accessible locations to systematically determine the discontinuity characteristics according to the methods suggested by ISRM (2007) and a total of 134 discontinuities were mapped at different locations in the first valley. The results of the scan-line surveys showed that the discontinuities in the Zelve tuff are primarily joints with some infilling
Geo-engineering problems observed in the Zelve Open-Air Museum
The observations in the first valley of the Zelve Open-Air Museum by Kasmer et al. (2008) indicated different modes of failure. These failures are considered to be associated with deterioration of tuffs, presence of discontinuities with high persistence and loss of overall strength of the tuff due to wetting–drying and freezing–thawing cycles. In addition, wind and rainfall are considered as strong erosion agents in the region. Besides, observations revealed that a seasonal stream flowing at
Assessments of erosion rate, amount of spalling and climatic conditions
Erosion resulting from different agents such as wind and water has a crucial importance not only on the formation of “fairy chimneys” but also on the stability of natural slopes of valleys in the Cappadocia Region. Particularly, the erosion is more important at the toe regions of steep valley slopes resulting in loss of support and slope failures when it reaches to a certain amount. By considering the importance of the erosion effect, in this study, rate of erosion was also measured and
Numerical modelling to assess stability of natural slopes prone to toe erosion and selected typical openings
Slopes in sedimentary and volcano-sedimentary rocks, such as tuffs, are prone to failure due to unfavourable discontinuity orientations, low rock mass strength, rapid weathering of soft units and erosion at the toe of the soft rocks. As indicated in the previous sections, instabilities both along natural slopes and in semi-underground openings in the Zelve Valley are mainly associated with (a) active role of degradation along discontinuities, (b) significant reduction of rock strength probably
Conclusions
In this study, an archaeological semi-underground cliff settlement, which was excavated in a soft tuff about 1500 years ago in the Cappadocia Region of Turkey and is recently being used as an open-air museum, was geotechnically evaluated. The evaluations were mainly focused on geo-engineering characteristics of the surrounding tuff and the stability of the natural slopes subjected to toe erosion and some selected typical semi-underground openings based on 2D and 3D numerical modelling. The main
Acknowledgements
The authors would like to express their sincere thanks to Professor Hasan Gercek of Bülent Ecevit University of Turkey for his valuable comments during the study and the revision of the article, Professor Ömer Aydan from Tokai University of Japan for his kind support for the measurement of erosion rate and his comments and suggestions, deceased Halis Yenipınar, past Director of Nevsehir Museum, and all the employees of the Zelve Open-Air Museum for their enthusiastic help in the field.
References (31)
- et al.
Geotechnical and geoenvironmental characteristics of man-made underground structures in Cappadocia, Turkey
Engineering Geology
(2003) Field-based experimental determination of the weathering rates of the Cappadocian tuffs
Engineering Geology
(2009)- et al.
Water-induced variations in mechanical properties of clay-bearing rocks
International Journal of Rock Mechanics and Mining Sciences
(2009) - et al.
Continuous and discontinuous stability analysis of the bell-shaped caverns at Bet Guvrin, Israel
International Journal of Rock Mechanics and Mining Sciences
(2002) - et al.
Empirical estimation of rock mass modulus
International Journal of Rock Mechanics & Mining Sciences
(2006) - et al.
Neogene ignimbrites of Nevşehir Plateau (central Turkey): stratigraphy, distribution and source constraints
Journal of Volcanology and Geothermal Resources
(1994) - et al.
The pre-ignimbrite (phreato) plinian and phreatomagmatic phases of the Akdag–Zelve ignimbrite eruption in Central Anatolia, Turkey
Journal of Volcanology and Geothermal Research
(1997) - et al.
Ignimbrites of Cappadocia (Central Anatolia—Turkey): petrology and geochemistry
Journal of Volcanology and Geothermal Research
(1998) - et al.
Engineering geological properties and durability assessment of the Cappadocian tuff
Engineering Geology
(1997) Annual Book of ASTM Standards—Soil and Rock, Building Stones, Section 4, Construction.V.04.08
(1994)
An experimental study on the investigation of long-term stability and environmental conditions of Derinkuyu Underground City
Studies on Derinkuyu Antique Underground City and its implications in geo-engineering
Deterioration of historical structures in Cappadocia (Turkey) and in Thebes (Egypt) in soft rocks and possible remedial measures
Engineering classification of rock masses for the design of tunnel support
Rock Mechanics
Cited by (18)
Variability of technical properties and durability in volcanic tuffs from the same quarry region – examples from Northern Hungary
2019, Engineering GeologyCitation Excerpt :Consistent results can be retrieved in the literature about other tuff types of diverse provenance, with similar porosity though, likewise being prone to freeze-thaw weathering when the majority of pores are below 10 μm. Significant deterioration of technical properties or complete disintegration might be recorded even after 20 freeze-thaw cycles (Auras and Steindlberger, 2005; Kaşmer et al., 2013; Di Benedetto et al., 2015). Similar considerations are applied also to tuffs from the same quarry region: previous studies documented that higher effective porosity does not necessarily mean lower durability, and the critical pore-size range plays a crucial role in controlling the resistance to freeze-thaw stresses (van Hees et al., 2003; Nijland et al., 2004).
An investigation into the development of toppling at the edge of fractured rock plateaux using a numerical modelling approach
2017, GeomorphologyCitation Excerpt :Styles et al. (2011) used Phase2 to back-analyze the Joss Bay Chalk cliff failure where the progressive development of a wave-cut notch at the base of a coastal cliff was modelled using simulated model excavation stages. Sturzenegger and Stead (2012) also used Phase2 to model the Palliser Rockslide, Canada, as a stepped failure surface, while Kaşmer et al. (2013) assessed the stability of natural slopes prone to toe erosion in Cappadocia, Turkey. Based on field evidence, two main simulations of the San Leo slope were conducted, in order to consider two possible processes leading to failure: (1) the softening of a relatively thin clay shale layer (5 m thick) and (2) the undermining of the rocky slab.
The relationship between weathering and welding degree of pyroclastic rocks in the Kilistra ancient city, Konya (Central Anatolia, Turkey)
2016, Journal of African Earth SciencesCitation Excerpt :Stones used in the construction of historic buildings are exposed to various direct or indirect atmospheric effects depending on climatic and seasonal conditions, resulting in partially or fully weathering (Fener and İnce, 2015). Weathering studies on historical structures constructed by or carved in pyroclastic rocks around the world have shown that pyroclastics are one of the most sensitive rock types against the atmospheric effects, and thus special care should be taken for the preservation of such sites (Fitzner, 1994; Binal et al., 1997, 1998; Topal and Doyuran, 1997, 1998; Topal and Sözmen, 2003; Steindlberger, 2004; Török et al., 2007; Ergüler, 2009; İnce, 2013; Kaşmer et al., 2013; Korkanç, 2013; Çelik et al., 2014; Di Benedetto et al., 2015; İnce et al., 2015; Kaljahi and Birami, 2015; Korkanç et al., 2015; Yavuz et al., 2015). The products of explosive volcanism, pyroclastic rocks are composed mainly of lithic clasts, crystals and juvenile fragments (i.e. pumice and glass shards).
Landslides and Cultural Heritage—A Review
2023, Heritage