Engineering geological research of andesite alteration related to the revitalization of the ?á?ov Castle (central Slovakia)

摘要

An architectural study of the revitalization of the ?á?ov castle in the neovolcanic ?tiavnické vrchy Mts. initiated a revision of the castle buildings. The building stone showed high post-volcanic alteration (hydrothermal and/or by weathering) or total disintegration in some parts of the walls. It must be replaced, because the safety of the ruins determines a sustainable development of this historical site. The castle hill rocks and the main building stone are both Neogene andesites, but not necessarily identical – the building stone came probably from several sources and the original source of the building stone was not the question. Suitable replacement material was searched for in a local quarry. Andesites at different weathering levels were taken to test the impact of alteration upon the rock properties and their suitability for the castle remediation. Expandable clay minerals were considered important for the rock weakening and weathering resistivity. Mineral composition was analysed by X-ray diffractometry. Powdered samples were used for the determination of the particle density and Enslin-Neff tests. The dry bulk density, total porosity, He-effective porosity, and the uniaxial compressive strength (UCS) were determined only on cylindrical rock cores of the quarry andesite. The “sound” building stone consists of feldspars (andesine > orthoclase), amorphous phases and a small amount of augite and quartz. Its alteration is manifested by the falling content of amorphous phases, augite and feldspars, by increasing smectite (up to 28%) and secondary carbonates (up to 22 %), as well as by the color change due to increasing iron oxyhydroxides. Mineralogical composition showed acceptable differences compared to the sound quarry rock. There, higher alteration levels show higher water adsorption and increasing macroporosity, as well, which could be explained by a hypothesis: the basic macroporosity is of syngenetic origin, but, cyclic wetting and drying of the smectite together with freezing water in the pores probably open new cracks and enable rock weathering into weak phases, new-formed smectite included; open porosity resulting from their erosion multiplies the interface area and accelerates the weathering. For increasing porosity leads to a striking drop in UCS, only a sound rock from the deeper parts of the quarry can be recommended for the castle revitalization.