Karst deformations due to environmental factors: evidences from the horizontal pendulums of Grotta Gigante, Italy

The aim of this work is to characterize the deformation of a natural cave induced by temperature variations and by the underground water flow. We consider the tilt recorded by two horizontal pendulums installed in the largest cave of the Italian Karst, the Grotta Gigante. The environmental factors we consider are temperature, rainfall and level and flow rate of the river Reka that feeds the underground Karstic waters. The tilt and temperature have a regular annual cycle, with period of 365 days, which is caused primarily by the thermo-elastic deformation. Semiannual periods are present but more than 10 times smaller. For rainfall and river level the semi-annual and annual oscillation have comparable amplitudes. The effect induced by the underground water flow consists in a tilting towards SW, that has a linear relation between maximum tilting and the integrated amount of water entering the Karst during the flood, with tilting coefficient a = 10.7⋅10^-6 nrad/m³. The minimum amount of water giving a tilt signal is V₀ = 5.2⋅10⁶ m³.