Ductile Behavior of Thermally-Fractured Granite Rocks

We investigate the time-dependent deformational properties of fractured rocks in order to evaluate the apparent creep behavior of such rocks and understand the relevant physical mechanisms responsible for such behavior. Rocks surrounding subsurface structures and natural fault zones are densely fractured due to the damage induced by the stress concentrations around each structures. These abundant fractures may serve as independent slip planes within the rock, depending on its frictional properties and hydrothermal conditions, potentially allowing the bulk rock to behave apparently as a ductile medium. Accurate prediction of such behavior may be crucial when assessing the long-term stability of these underground structures and the stress state around faults. We conducted triaxial creep experiments using thermally-fractured granite rocks. Westerly granite samples were heated to a maximum temperature up to 600 degrees Celsius to produce analogues of damaged crystalline rocks with varying degree of fracture densities. Measured index properties of these rock samples and microstructure observations show that fracture densities were successfully varied according to the maximum temperature reached during heating. Laboratory data shows that the samples exhibit substantial amount of time-dependent deformation when constant isotropic or anisotropic stresses are applied even under dry and room temperature conditions.