Monument Future - Siegfried Siegesmund

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       HISTORICAL MAN-MADE CAVES IN JAPAN: VULNERABILITY OF ROCKS AND CULTURAL ASSETS IN THE UNDERGROUND ENVIRONMENT

      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, decorated with hundreds of rock-cut high and low reliefs, picturing deities and masters of Buddhism, shrines, real and fantastic animals, vegetal motifs, mandala, zodiac signs, and family crests (Ogata 2019).

      — Yoshimi Hundred Caves (Yoshimi) – a composite Kofun, term used for sacred tumulus and megalithic burials of emperors, kings, and aristocrats, widespread in Japan about 1,500 years ago. The Yoshimi Kofun includes 219 hillside-cut tombs dated to the 6–7th century. A part of them were destroyed and tunneled during the Pacific War, making room for a production plant of aircraft parts and munitions, sheltered from the American air raids (Ikegami 2018).

      — Oya quarry district (Utsunomiya) – a complex of underground sites of extraction of Oya stone, a popular building and carving material exploited since the Edo period (17th century) up to the present. Many quarries were abandoned, others converted into geoheritage and tourist attractions (e. g., History Museum, Heiwa Kannon monument, Keikan Park) (Seiki et al. 2017).

      Figure 1: The underground sites under investigation (photo of Taya Caves by S. Sonoda).

       Research outline

      This study is aimed at investigating the vulnerability of the underground sites and its relationship with the relevant environmental and microenvironmental conditions, focusing on the properties and deterioration of the rock those caves are excavated into. The eventual outcomes are expected to support the adoption of countermeasures for preserving and promoting those sites and the stone artifacts enshrined therein, and give indications about the influence and safety of visitor traffic.

      Here we introduce the first results of the characterization 223of the textural, mineralogical, petrophysical, and chemical properties of the rock and its alteration products, reserving particular attention to salt weathering and rock-water interaction. The principal techniques applied were optical microscopy, scanning electron microscope, X-ray powder diffraction, X-ray fluorescence, and mercury intrusion porosimetry. These were combined with the long-term monitoring of air temperature and relative humidity (RH).

       Rock characterization

      The rock of Taya Caves is a Pleistocene tuffaceous marine siltstone, grain-supported and well sorted, composed mainly of quartz, plagioclase, and lithoclasts rich in illite and smectite clay minerals, also distributed in the matrix. Two slightly different siltstone varieties are observable, which can be distinguished by their color and the presence (or not) of calcareous bioclasts.

      Yoshimi Hundred Caves are excavated into a Miocene tuff with dacitic to andesitic composition, which shows a certain lithological variability, ranging from a coarse-grained pumiceous type – with porphyritic texture, plagioclase phenocrysts, abundant lithoclasts, and hypocrystalline groundmass – to a fine grained, mostly glassy type.

      Finally, Oya stone is a Miocene ignimbrite having a rhyolitic to dacitic composition, with fiamme and porphyritic texture, phenocrysts of plagioclase and quartz, and hypocrystalline groundmass. The grain size is highly variable, and the typical clay clusters may reach a size of several centimeters.

      The rocks of all the studied sites share, other than the pyroclastic-related sedimentary origin, also the silicate composition (plagioclase, quartz, and clay minerals) (Fig. 2). Moreover, they are all soft rocks with very high porosity. We determined an average porosity of around 45 %, and a pore-size distribution characterized by the prevalence of capillary pores (> 0.1 µm), those most involved in liquid water absorption.

      Figure 2: Thin-section photomicrographs in plane- and cross-polarized light of the studied rocks.

       Salt weathering and rock-water interaction

      Gypsum is the only secondary phase recurring in crusts or efflorescences on the cave surfaces of all the three studied sites (Fig. 3). The best examples of gypsum crusts, thick and compact, were observed in Taya Caves, where they may jeopardize the readability of the carved decorations; they have a composite stratigraphy, characterized by the

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