Monument Future. Siegfried Siegesmund

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      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 presence of an intermediate layer of calcite between the surface gypsum and the host rock. As 224for the gypsum efflorescences, the most extensive were observed in the WWII tunnels of Yoshimi Hundred Caves, where salt weathering is actually particularly severe, accounting for the crystallization of other sulfates, of Na, Al, Fe, and Mg – jarosite, alunogen, halotrichite, alum-Na, tamarugite, epsomite, and thenardite (Horiguchi et al. 2000; Oguchi et al. 2010). Oya stone also suffers from sulfate-rich efflorescences, constituted of gypsum, mirabilite, and thenardite, each phase preferentially crystallizing in different microenvironments. Low-crystallinity efflorescences were finally found in Taya Caves, composed of chlorides (sylvite, in particular), phosphates, and sulfates.

      Figure 3: Examples of gypsum crusts and efflorescences with the relevant analytical data.

      In addition, we conducted a complementary study of the rock-water interaction in Taya Caves, considering the lack of previous researches and the constant presence of percolating rainfall, rising damp, and extremely high humidity in that environment. We measured an extremely rapid water absorption (~25 %), a more contained yet significant adsorption of hygroscopic water (~5 %), and a very fast decay during the jar slake test (Santi 1998). These findings point out a high susceptibility to clay mineral-promoted swelling and slaking deterioration, which may produce decay patterns like erosion, rounding, scaling, peeling and, in the long term, lead to structural decay and collapses.

       Lithological and environmental constraints

      Gypsum formation in Taya Caves is triggered by the dissolution of the calcareous bioclasts and pyrite crystals in the rock, releasing Ca and S in solution. This is validated by the occurrence of gypsum solely on the fossiliferous rock type, which is also rich in pyrite – diagenetic or secondary, often included inside the shells. Pyrite has been thought to be the precursor also of the sulfate-rich efflorescences in Yoshimi Hundred Caves (Oyama & Chigira 1999), whereas about the gypsum origin 225on Oya stone we need further investigations. The ubiquity of gypsum can be explained in terms of the relevant microenvironmental conditions. The deepest levels of the studied underground sites, the most environmentally isolated, may have a nearly constant RH of about 100 %. This value, under the influence of external airflows, gets significantly lower and more variable closer to the cave entrances, the microclimatic monitoring reveals (Fig. 4). Gypsum has an extremely high deliquescence relative humidity (DRH) – higher than 99 % (Charola et al. 2007) – so that is stable in many environments, even very humid, provided that the substrate is not wet.

      Figure 4: Microclimatic monitoring during summer and early fall (at different distances from the entrances of the underground sites and outdoors).

      With broader fluctuations and lower values of RH, the crystallization of a number of other phases, typically with lower DRH, can occur. Emblematic are the findings on Oya stone, which disclose the exclusive presence of gypsum in the deep, extremely humid quarry levels, whereas the main component of the efflorescences in the middle levels or semi-underground quarries is mirabilite (DRH = 97 % at 15 °C), then replaced by thenardite (DRH = 86 %) in the dryer external environment (Steiger & Asmussen СКАЧАТЬ