Response of rock material properties of quartzose sandstones to wildfire: a case study from Bohemian Switzerland National Park, Czechia

In the summer of 2022, a devastating wildfire struck the Bohemian Switzerland National Park in Czechia. It not only had significant ecological and economic impacts but also altered the near-surface properties of the vastly exposed quartzose sandstones of Upper Cretaceous age. This, together with the loss of vegetation and altered composition of slope sediments, may influence the dynamics of sandstone rock pillars and increase their vulnerability to erosion and slope processes in the coming years. This work focuses on describing the rock material affected by the 2022 wildfire, investigating the effects of thermal shock and extreme heat under natural conditions. The research employs a multi-step framework encompassing in situ sampling, laboratory tests and GIS modelling to understand these changes and patterns comprehensively. Samples were collected from outcrops and free-lying blocks at three locations: Dlouhý, Pravčický, and Černý důl. A total of 37 drill cores were extracted, with 185 P-wave velocity measurements conducted to assess variations with depth. Additionally, 78 samples were prepared for Unconfined Compression Tests and Brazilian tensile strength tests, complemented by 156 P- and S-wave velocity measurements. Notably, the Brazilian specimens yielded more pronounced results, revealing significant differences in tensile strength between Dlouhý and Černý důl at varying depths. Given the perspective of recurring monitoring in protected areas such as the study site, the research applied ultrasonic methods to offer a practical tool for assessing post-fire rock quality while adhering to standard laboratory tests. Clear limitations in the heterogeneous material were identified, with values showing limited amplitude in rock strength decay. This work provides a peculiar case study offering insights into changes in the properties of rock materials in the presence of a major wildfire.