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Collaborating Authors
Results
Abstract: Seismic wave transmission and Digital Image Correlation (DIC) are employed to study slip processes along frictional discontinuities. A series of biaxial compression experiments are performed on gypsum specimens with non-homogeneous contact surfaces. The specimens are composed of two blocks with perfectly mated contact surfaces with a smooth surface with low frictional strength on the upper half and a rough surface with high frictional strength on the lower half. Compressional, P, and shear, S, wave pulses were transmitted through the discontinuity and digital images of the specimen surface were acquired during the test. A distinct peak in the amplitude of transmitted wave occurs prior to the peak shear strength and is considered a "precursor" to the failure. Precursors indicate that slip initiates from the smooth surface and extends to the rough surface as the shear load is increased. From the DIC data, slip is identified as a jump in the displacement field across the discontinuity that initiates from the smooth surface and propagates to the rough surface. Precursors are associated with an increase of the rate of relative displacement across the discontinuity and are a measure of a reduction of the fracture's shear stiffness.
Particle Swarms in Variable Aperture Fractures
Boomsma, E.B. (Purdue University) | Pyrak-Nolte, L.J. (Purdue University)
Abstract: Experiments were performed to determine the effect of a rough walled fracture on the evolution and maintenance of particle swarms, i.e. coherent collections of colloidal size particles. Of particular interest is the response of a swarm to sudden changes in confinement and drag caused by variations in fracture aperture. Two different rough fracture geometries were used and are compared to smooth-walled converging and diverging synthetic fractures. The variation in aperture along a swarmโs transport path exerted a strong influence swarm evolution and, in many cases, enhanced swarm stability and velocity. Converging apertures tend to decelerate a swarm because of increasing confinement and drag from the fracture surfaces, while diverging apertures resulted in the acceleration of a swarm. As long as an unrestricted flow path exists, swarms will maintain their cohesion, even if a fracture contains obstacles to swarm flow (asperities). However, if the aperture of the flow path is too small, swarms will bifurcate around an obstacle and produce smaller sub-swarms that will continue on separate paths.
Dispersive Waves Propagating Along a Surface Fracture
Abell, B.C. (Purdue University) | Pyrak-Nolte, L.J. (Purdue University)
Abstract: Significant work in the past few decades has lead to a well developed understanding of seismic wave propagation in fractured media. However, previous research has focused on fractures within rock as opposed to fractures at the surface of a rock. A theoretical and experimental study was performed to examine seismic wave propagation along a fracture at the surface, i.e., along the intersection of two quarter-spaces. A theoretical model that couples two wedges, using displacement discontinuity boundary conditions, was developed that gives rise to a new guided waveform that is dispersive, depends on fracture specific stiffness at the intersection of a fracture with a free surface and exhibits velocities that range from the single wedge-mode velocity to the Rayleigh wave velocity at a free surface. The existence and behavior of this new guided-mode was verified using a synthetic fracture created between two aluminum blocks. This new guided-mode enables characterization of fracture specific stiffness of fractures, joints and other discontinuities at the surface of an outcrop.
- Research Report > New Finding (0.66)
- Research Report > Experimental Study (0.66)
Abstract: A series of laboratory experiments were performed on synthetic fractures in gypsum and lucite to study the ratio of shear to normal fracture specific stiffness of a single fracture subjected to normal and shear stress. The specimens were made by placing two blocks in contact to form a fracture. The fracture surface was manufactured such that it was either well-mated or non-mated. For well-mated fracture surfaces, asperities were created by casting gypsum against sandpaper. After the first block hardened, the second block was cast against the rough surface of the first block. Non-mated fracture surfaces were fabricated with two lucite blocks that were polished (lucite PL) or sand-blasted (lucite SB) along the contact surface. In the experiments, each specimen was subjected to normal and shear loading while the fracture was probed with transmitted compressional and shear waves. Shear and normal fracture stiffnesses were calculated using the displacement discontinuity theory. The stiffness ratio determined from the experiments was compared to a theoretical ratio that was determined assuming that the transmission of compressional and shear waves was equal. The experimental results show that the fracture roughness of the non-mated fractures affects the stiffness ratio and that the shear fracture specific stiffness for well-mated fractures is sensitive to the applied shear stress.
- Research Report > New Finding (0.49)
- Research Report > Experimental Study (0.34)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Mineral > Sulfate > Gypsum (0.49)
Abstract: Experimental work suggests that the flow-stiffness relationship in single fractures relies on the geometry of the fracture, i.e. the size and spatial distributions of the void and contact areas. It is useful to understand the scaling of these distributions in relation to the hydromechanical properties of fractures to enable the extrapolation of understanding from studies conducted on small laboratory samples to behavior on the field scale. In this study, we uncover the difficulties associated with interpreting measurements taken in the laboratory and show that the scaling is in fact fundamentally different than that of the field. Finite-size scaling methods were used to extract critical thresholds for uncorrelated fractures under load and flow exponents were extracted.