| Popis: |
Most caves initiate along pathways of increased permeability, which occur along discontinuities such as bedding plane partings or fractures. A primary control on cave form is the nature of that initial rock permeability. One mechanism that can form discontinuities is valley stress relief, which changes the stress conditions within the valley walls and floor. That is the focus of this study. Five Cumberland style caves within the Appalachian Plateau were analyzed to determine if they formed from asymmetric valley stress relief. Graphs indicating overburden thickness above master-passages were constructed using digital cave plots and digital elevation models. These indicate that passage positions (between 27 and 82 m of overburden) are located within stress relief fracture zones and that horizontal passage development is controlled by bedding partings and heterogeneous rheology. Unconfined compressive strength (UCS), triaxial strength, and tensile strength (IDT) experiments were performed to determine the elastic and brittle properties of the cave-capping units (Hartselle Sandstone and Union Limestone) and their underlying cave-forming units (Monteagle and Pickaway Limestones, respectively). Rock mechanical data and valley profiles were then used to construct finite element geomechanical computer models to compute the differential stress, volumetric strain, aperture between lithologic contacts, and vertical displacement of the valley floor from river incision removing over 700 m of overburden to analyze the effect of asymmetric valley stress relief on permeability. The model outputs indicate that differential stress (20-120 MPa) did not exceed the cave-forming unit’s unconfined failure strength (123- 130 MPa), heterogeneous rheology lead to asymmetric differential stress and volumetric strain between lithologic units, bedding planes between lithologic contacts part between 398 µm and 666 µm, and the valley floors will be vertically displaced between 0.37 and 0.79 m at the location of cave initiation. Heterogeneous stress relief does occur from topographic asymmetry, but gently dipping beds do not affect cave development from a geomechanical perspective. Asymmetric valley stress relief can induce permeability from the resulting microcracks, shearing between beds, and bedding partings to create nucleation sites for speleogenesis. |
