Microstructures Induced in Porous Limestone by Dynamic Loading, and Fracture Healing: An Experimental Approach

Abstract

Fracturing and healing are crucial processes inducing changes in the permeability and mechanical behavior of fault zones. Fracturing increases the permeability of fault rocks, creating flow-channels for fluid circulation and enhancing the kinetics of such fluid–rock processes as pressure solution or metamorphism. Conversely, healing processes reduce permeability by closing the fractures and lead to rock strengthening. Consequently, the timescales of these two processes are important in determining the strength of fault zones and their ability to rupture during earthquakes. This article reports observations of the microstructure of porous limestone samples subjected to rapid dynamic loading, and long-term healing as a result of fluid percolation. Dynamic loading was performed by impacting the samples with steel bars inside a split Hopkinson pressure bar apparatus. Healing was performed by leaving the samples for three months within a triaxial machine with percolation of supersaturated fluids for five weeks. Two kinds of fracture network were observed in samples damaged at high strain rate: a series of radial and circular macrofractures and an incipient pulverization zone at the center of the sample loaded at the highest strain rate. Fracture density determined microscopically from X-ray images correlates with dissipated energy computed from macro-mechanical data. X-ray images enable good quantification of the damaged state of the samples. Percolation experiments under stress with high-solubility fluid at room temperature show that the main healing processes promoting closure of the fractures in the sample are a combination of mechanical and chemical compaction. Microfracturing networks were found to heal faster than the largest fractures, leading to heterogeneous strengthening of the rock. This feature affects the processes of earthquake nucleation and rupture propagation.