ROLE OF ROCK MECHANICAL PROPERTIES IN MAJOR CAVE DEVELOPMENT

Most caves originate along discontinuities, such as fractures or parting along bedding planes. One mechanism that can cause parting between bedding planes is erosion due to river incision, which changes the stress conditions within the valley walls and floor. Master cave passages trend parallel to the valley walls, form downdip within the valleys, and are located near lithologic boundaries within these incised river valleys, indicating a relationship between the physical properties of the rocks and the river incision. In order to determine how the rock properties affect bedding plane parting between lithologic units, we have constructed models based on the physical properties of the formations of four sites in the Appalachian Plateau with caves that are thought to have formed due to valley stress relief.

We performed unconfined compressive strength, triaxial strength and tensile strength experiments to determine the elastic and plastic properties of the cave forming formations (Monteagle and Pickaway Limestones) and their overlying cap formations (Hartselle Sandstone and Union Limestone, respectively). The results of these tests indicate that the physical properties of the cap formations are considerably stronger than the underlying cave-bearing formations. We used these results to construct geomechanical models using RS² software. Valley profiles were used to calculate theoretical displacement of the valley floor and bedding plane partings due to river incision removing 700 m of overburden.

The results of the modelling indicate that the valley floors will be vertically displaced by 0.6 m and bedding planes will part by approximately 600 microns. The vertical displacement and tensile stresses on down-dip sides of valley walls and valley floor indicate that heterogeneous elastic and plastic properties of rocks can increase permeability and allow nucleation sites for speleogenesis. Stress relief on the down-dip valley side may be greater, enhancing the effects of the heterogeneous physical properties.