Stability analysis of a large-volume block in the historical rock-cut city of Vardzia (Georgia)

The historical rock-cut city of Vardzia is a cave monastery site in South-western Georgia, excavated from the tuffaceous layers of the Erusheti mountain. Due to its morphology, the archaeological site is continuously affected by rock instabilities. These pose serious constrains to future conservation, as well as to the safety of tourists. In order to improve knowledge about slope stability issues in the Vardzia site and to develop a proper site specific approach, the present research focuses on the conditions of one of the largest potentially unstable rock-block. The geometry of the rock mass as well as the mechanical properties of the joints were investigated by means of in situ surveys and tests. Laboratory tests were carried out to define the strength properties of the volcanic materials, both in dry and wet conditions. Wet conditions yield significantly lower strength values and hence represent the most critical scenario. The block stability was first analysed using the limit equilibrium method. This revealed the sliding mechanism along the base joint as being the most critical, with a safety factor lower than 1. As a consequence, only the presence of rock bridges along the base and the sub-vertical joints has guaranteed the current stable conditions. The stability of the block was further inspected using the Finite Element approach, schematising rock discontinuities using interfaces obeying the Mohr-Coulomb strength criterion. The safety factor computed numerically agrees very well with that computed analytically only if the contribution of the rock bridges along the sub-vertical breccia joint is assumed in the analytical methods as an equivalent cohesive term (i.e. thus increasing the numerator of the safety factor). In conclusion, the stability analysis in Vardzia site cannot disregard a proper investigation and estimation of rock bridges.