Cemented paste backfill (CPB) is accepted as the optimal backfilling material for many underground mines. However, the lack of in-stope backfill pressure data poses fundamental problems from both operational and research standpoints. In response to the requirement for in situ data, a comprehensive field instrumentation project has been conducted. Results are presented here for two stopes at the Cayeli Mine, where geotechnical instruments were installed at the barricades and throughout the stopes. Measurements from a large (slow rise rate) stope with high binder content CPB demonstrated a rapid departure from hydrostatic loading, resulting in relatively low barricade pressures. Conversely, data from a smaller (fast rise rate) stope with lower binder content CPB demonstrated that when cement hydration is retarded, high barricade pressures occur. These examples illustrate the relationship between CPB rise rate and the moderating effect of cement hydration on in situ pressures, which ultimately control barricade pressures. Once CPB gains shear strength, arching of pressures occurs. In situ pressures were reduced with proximity to stope walls and further, under stope access brows, demonstrating that barricade location influences barricade loads. The application of real-time pressure monitoring of pastefill barricades has been demonstrated as an important tool in optimizing operational backfilling efficiency.

[1] A stick-slip event was induced in a cylindrical sample of Westerly granite containing a preexisting natural fault by loading at constant confining pressure of 150 MPa. Continuously recorded acoustic emission (AE) data and computer tomography (CT)-generated images of the fault plane were combined to provide a detailed examination of microscale processes operating on the fault. The dynamic stick-slip event, considered to be a laboratory analog of an earthquake, generated an ultrasonic signal that was recorded as a large-amplitude AE event. First arrivals of this event were inverted to determine the nucleation site of slip, which is associated with a geometric asperity on the fault surface. CT images and AE locations suggest that a variety of asperities existed in the sample because of the intersection of branch or splay faults with the main fault. This experiment is compared with a stick-slip experiment on a sample prepared with a smooth, artificial saw-cut fault surface. Nearly a thousand times more AE were observed for the natural fault, which has a higher friction coefficient (0.78 compared to 0.53) and larger shear stress drop (140 compared to 68 MPa). However at the measured resolution, the ultrasonic signal emitted during slip initiation does not vary significantly between the two experiments, suggesting a similar dynamic rupture process. We propose that the natural faulted sample under triaxial compression provides a good laboratory analogue for a field-scale fault system in terms of the presence of asperities, fault surface heterogeneity, and interaction of branching faults.

[1] We have deformed basalt from Mount Etna (Italy) in triaxial compression tests under an effective confining pressure representative of conditions under a volcanic edifice (40 MPa), and at a constant strain rate of 5 Â 10 À6 s À1 . Despite containing a high level of pre-existing microcrack damage, Etna basalt retains a high strength of 475 MPa. We have monitored the complete deformation cycle through contemporaneous measurements of axial strain, pore volume change, compressional wave velocity change and acoustic emission (AE) output. We have been able to follow the complete evolution of the throughgoing shear fault without recourse to any artificial means of slowing the deformation. Locations of AE events over time yields an estimate of the fault propagation velocity of between 2 and 4 mm.s À1 . We also find excellent agreement between AE locations and post-test images from X-ray microtomography scanning that delineates deformation zone architecture. Citation: Benson, P. M., B. D. Thompson, P. G. Meredith, S. Vinciguerra, and R. P. Young (2007), Imaging slow failure in triaxially deformed Etna basalt using 3D acoustic-emission location and X-ray computed tomography, Geophys. Res. Lett., 34, L03303,

Theoretically, crack damage results in a decrease of elastic wave velocities and in the development of anisotropy. Using non-interactive crack eective medium theory as a fundamental tool, we calculate dry and wet elastic properties of cracked rocks in terms of a crack density tensor, average crack aspect ratio and mean crack fabric orientation using the solid grains and uid elastic properties.Using this same tool, we show that both the anisotropy and shear wave splitting of elastic waves can be derived. Two simple crack distributions are considered for which the predicted anisotropy depends strongly on the saturation, reaching up to 60% in the dry case. Comparison with experimental data on two granites, a basalt and a marble, shows that the range of validity of our model extends up to total crack density of approximately 0.5, considering symmetries up to orthorhombic. In the isotropic case, Kachanov's [1994] non-interactive eective medium model was used in order to invert elastic wave velocities and infer both crack density and aspect ratio evolutions. Inversions are stable and give coherent results in term of crack density and aperture evolution. In the anisotropic cases -both transverse isotropic and orthorhombic symmetries were considered -anisotropy and saturation patterns were well reproduced by the modelling and mean crack fabric orientations is recovered. Inversion results agree very well with the laboratory data and are consistent with the rock microstructure in the dierent rocks investigated. Our results point out that: (1) it is possible to predict damage, anisotropy and saturation in terms of a crack density tensor and mean crack aspect ratio and orientation; (2) Using well constrained laboratory