Microstructural analysis of sandstone from Pravcicka brana, the largest rock arch in Europe

Abstract

This study uses a multi-method approach to perform a comprehensive microstructural analysis and to discuss the resistance to various weathering processes of sandstone from Pravčická brána, the largest sandstone arch in Europe. In addition to conventionally used methods of sample testing such as mercury intrusion porosimetry, we focused on experimental and non-destructive sample testing such as pore network modelling and permeability simulation based on post-processed μCT images or saturation flow experiments using non-Newtonian fluids - ANA method. These methods showed consistent pore size distribution (PSD) peaks at 18 μm (ANA), 22 μm (PNM) and 48 μm (MIP). The sandstone exhibited high total porosity (34–45 %) and effective porosity (30–42 %), with a network predominantly composed of macropores (>50 nm) that are readily accessible to water. Laboratory tests yielded a permeability of 3.1 × 10–⁵ m.s-¹, which is close to the simulated value of 2.21 × 10–⁵ m.s-¹. Scanning electron microscopy (SEM) analysis identified features such as quartz grain fractures that were attributed to past frost weathering processes. Analysis of the PSD indicated high resistance of the sandstone studied to freeze-thaw and salt weathering. However, when considering other factors, such as mechanical and hydraulic properties and tectonic predisposition in the arch body, the vulnerability of Pravčická brána becomes more evident. The susceptibility of Pravčická brána sandstone to weathering is increased by its low strength (as evident from previous studies), which is further reduced by ongoing weathering, while the presence of the observed macropores increases the likelihood of fracturing under deformation. This study highlights the need to integrate advanced techniques with traditional methods to provide a comprehensive assessment of sandstone durability.