can be estimated that the strengths essentially lie behind an assumed weathering zone of approx. 15 mm depth in the typical spans of unweathered Hilbersdorf porphyry tuff. According to the available results, compressive strengths from approx. 24 N/ mm² estimated and basis for further calculations. The value correspond to the literature data for Hilbersdorf porphyry tuff (e. g. Siedel 2006, KREISSL 2010, Wedekind et al. 2013).
However, in one case of the measurements, the depth of softening could not be finally detected. With a drilling depth of 15 mm, there is an estimated compressive strength of only approx. 15 N/ mm² at this point (column 1 middle hole #07_oed_1).
In order to provide proof of the load-bearing capacity of the columns, this minimum value of the compressive strength should be used as a maximum, additionally reduced by appropriate factors. The statically effective cross-section must be reduced at least by the amount of material loss plus 15 mm weathering depth.
Finally, two uncertainties have to be taken into account regarding the statements made above:
1. The two examination windows give solely partional insight to weathering condition of the column surfaces. Both, LANGE (2011) aswell as DU PUITS (2016) examined the columns before their sheathing and report massive surface losses of up to 5 cm depth.
2. The regression line was determined on the basis of dry-dried material. The literature does not provide information on the actual relationship between compressive strength and drilling resistance under the corresponding moisture conditions. KREISSL (2010) determined significant reduction in compressive strength on fully wet Hilbersdorf porphyry tuff material. In the case of the Hilbersdorf porphyry tuff dealt with here, the information about compressive strength as a funstion of relative humidity as possible in the town hall would have to be elaborated through additional investigations.
The case study of the town hall columns in Oederan compehensibly shows a valuation of the compressive strength of inorganic building material. The in situ determination is based on minimal invasive drilling resistance measurements on site correlated to laboratory parameters.
219References
Delgado-Rodriques, J. & Costa, D 2000: A new method for data correction in drilling resistance. Tests for the effect of drill bit wear. – Int. J. Restoration of Buildings and Monuments, 10: –18, Zürich.
DIN EN 1926 Prüfverfahren für Naturstein – Bestimmung der einachsigen Druckfestigkeit; 2006.
Du Puits V. 2016 Vorgezogener Ausschnitt aus dem Memorandum 1 – Chemnitz 03/2016.
Kreißl, S. (2010) Eigenschaften und Schadensphänomene des Hilbersdorfer Tuffs sowie Möglichkeiten der Steinergänzung mittels Mörtel. Diplomarbeit, 156 S.; http://opus.ba-glauchau.de/opus/volltexte/2010/1357/pdf/Diplomarbeit.pdf
Lange, M. 2011 Befunduntersuchung Rathaus Erdgeschoss – Schloss Kaufungen 06/2011.
Siedel, H. (2006) Sächsische „Porphyrtuffe” aus dem Rotliegend als Baugesteine: Vorkommen und Abbau, Anwendung, Eigenschaften und Verwitterung. Institut für Steinkonservierung e. V. Bericht Nr. 22, Mainz.
Wedekind, W., López-Doncel, R., Dohrmann R., Kocher M.& Siegesmund S., 2013. Weathering of volcanic tuffrocks used as natural building stone caused by moisture expansion. Environmental Earth Science. 69:1203-1224. DOI 10.1007/s12665-012-2158-1.
Wendler, E., Sattler, L. (1996): Bohrwiderstandmessung als zerstörungsarmes Prüfverfahren. – In: Wittmann, F. H. Gerdes, A. (Hrg.): Proc. 4. Intern. Koll. Werkstoffwissenschaften und Bauinstandsetzen (MSR IV’96): 145–159, Esslingen.
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HISTORICAL MAN-MADE CAVES IN JAPAN: VULNERABILITY OF ROCKS AND CULTURAL ASSETS IN THE UNDERGROUND ENVIRONMENT
Luigi Germinario, Chiaki T. Oguchi
IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.
– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –
VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.
Department of Civil and Environmental Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama-shi, 338-8570 Saitama-ken, Japan
Abstract
Many social and religious traditions of Japan are deeply rooted in the underground landscape. Over the centuries, sacred hypogea have been frequented for Buddhist and Shinto practices and for burials, caves used as shelters during wars and persecutions, 222tunnels excavated for raw-material exploitation or industrial manufacturing. This study concerns man-made cave sites in central Japan (Kanto region) of diverse age, from the Kofun, about 1,500 years ago, through the Kamakura and Edo periods (13–19th century), until the modern era. They were dug into soft and porous sedimentary rocks, namely volcanic tuffs and tuffaceous mudrocks, which show varied signs of decay, related to salt weathering and water interaction. The first results of the characterization of the textural, mineralogical, petrophysical, and chemical properties of the rock and its weathering products are presented here. The vulnerability of the underground sites is correlated with the relevant environmental conditions, by the monitoring of air temperature and relative humidity. The secondary phases forming crusts and efflorescences on the cave surfaces are mostly sulfates of diverse chemistry. A critical parameter determining their crystallization, stability, or deliquescence is relative humidity, often extremely high, while their composition is controlled most notably by rock mineralogy. The eventual outcomes of this research are expected to support the adoption of countermeasures for preserving and promoting the underground cultural heritage and stone artifacts enshrined therein, and give indications about the influence and safety of visitor traffic.
Keywords: Anthropic cave; Stone decay; Microclimatic monitoring.
Introduction
The landscape of modern Japan is world-renowned for the vertical development and multiform skyline of its megalopolises, shaped by the intense urbanization begun in the post-war era. However, it endured changing fortunes in the course of history, in view of the countless natural catastrophes (earthquakes, tsunami, typhoons, volcanic eruptions) and human-made disasters (wars, fires). Indeed, many social and religious traditions in Japan rather keep a strong cultural bound with the underground world, and with the protection, isolation, and quietness that it offers. Over the centuries, sacred hypogea have been frequented for Buddhist and Shinto practices and for burials, caves used as shelters during wars and persecutions, tunnels excavated for raw-material exploitation or industrial manufacturing. Our interest dwelled on three among the many underground historical sites in the Kanto region, in central Japan (Fig. 1):
— Taya Caves (Yokohama) – excavated and sculpted by Buddhist monks of the Shingon Esoteric sect from the Kamakura until the Edo period (13th to 19th century), and dedicated to ascetic training, rituals, and later pilgrimage. The caves are a maze of halls and galleries extending for about 600 m on three stories,
