Assessing the potential for deterioration of limestones forming Taq-e Bostan monuments under freeze-thaw weathering and karst development

One of the most famous cultural heritage sites in Iran is the Taq-e Bostan world heritage site (northeast of Kermanshah city). This site includes three monuments that all have been carved on limestone. Limestone is sensitive to dissolution and freeze-thaw weathering. Hence, to assess the deterioration of the limestone, field studies and a number of laboratory tests were performed. During the field survey, types of karren forming a group of karstic surface features were observed. These features have made uneven rock surfaces. Formation of the karren near the monuments can be considered as a serious threat. In another part of this article, some index properties, such as weight loss, porosity, Brazilian tensile strength and point load index, were determined for the two series of rock samples in fresh and freeze-thaw conditions. Based on the results, the percentage of weight loss and porosity changes in the tested samples was low. It was found that the index properties of rocks treated with freeze-thaw decrease in varying levels with respect to initial values. The results showed that these variations are due to the deterioration of calcite cement filling microcracks of the rocks in the first place and increase in the porosity of the rocks after recurrent freezing and thawing in the second place. Statistical analyses were developed for predicting the percentage loss values in the index properties after freeze-thaw cycles. There were statistically direct strong relationships between the porosity and the weight loss parameters with the number of freeze-thaw cycles (exponential function with R (2) = 0.94 and linear function with R (2) = 0.97, respectively). In addition, there were inverse linear relationships between Brazilian tensile strength and point load index with the number of freeze-thaw cycles (R (2) = 0.96 and R (2) = 0.93, respectively). On the basis of the obtained results, the monument stones are at risk of deterioration, and it is imperative to take protective treatment measures in this regard. The authors have proposed some physical and chemical preservation methods for the protection of the monuments.