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Stellenbosch, South Africa

Sivakumar C.,National Institute of Rock Mechanics | Srinivasan C.,National Institute of Rock Mechanics | Nawani P.C.,National Institute of Rock Mechanics | Lynch R.,Institute of Mine Seismology
45th US Rock Mechanics / Geomechanics Symposium

The underground powerhouse cavern of Tala Hydro Electric Project (THP), Bhutan had many incidents of roof falls and instabilities during construction. The cavern is located very close to the high stress thrust zone known as Main Central Thrust (MCT) of Great Himalayan region. After the project was commissioned there were a number of incidents of rock bolt failures in a violent manner from the side walls of the machine hall cavern. The failures observed in the upstream wall were more compared to the downstream wall. There were about 168 rock bolt failures reported prior to the microseismic monitoring. Hence the need for long-term monitoring of the powerhouse cavern stability was felt necessary in view of safety of the workmen and machinery. Microseismics/ Acoustic Emission technique was found to be the best solution for remote and real time global monitoring of this large underground cavern. This technique can be used without disturbing the power generation and in view of no scope for expansion of the existing conventional instrumentation. In order to assess the applicability, suitability and capability of microseismic monitoring technique for the permanent cavern strata monitoring, an advanced 24 bit digital microseismic instrumentation with twelve geophone stations was installed and monitored on a trial basis. This attempt of microseismic monitoring in an underground hydroelectric powerhouse cavern is the first time in the history of a commissioned hydro electric power project. Six borehole geophones and six surface geophones were installed on the upstream wall as well as in the downstream wall. Approximately 250, 000 microseismic counts were recorded during the monitoring period. Many clear signatures of brittle rock fracturing signals within the walls were recorded. Four prominent microseismic events that were large enough to compute the source parameters were recorded during the monitoring period, including many events detected by only two to three or four sensors. Three of these prominent events were located on the upstream wall, and the fourth event was located in the downstream side. The system responded well for the stress resulted rock cracks in strata throughout the monitoring period. The induced microseismic noise from the cavern walls indicates stress redistribution phenomena in the structure even five years after its commissioning. Micro-cracking was observed very close to the sensors, indicating high seismic attenuation characteristics of the rock mass. Incidentally there was a rock bolt failure on the downstream wall side during the monitoring period, which has been correlated well with the high stress zone identified from the recorded data. This paper discusses the successful demonstration of the microseismic monitoring technique in a highly stressed strata around an underground cavern based on the results obtained during experimentation. © 2011 ARMA, American Rock Mechanics Association. Source

Rebuli D.B.,Institute of Mine Seismology
Journal of the Southern African Institute of Mining and Metallurgy

Seismic networks in deep gold mines of South Africa are often planar owing to the tabular geometry of the orebody and, therefore, the mining infrastructure. Consequently, seismic event locations are poorly constrained in the direction perpendicular to the reef plane, and symmetrical locations above and below the reef plane may fit the observed data equally well. This problem may be resolved in situations where the strata above and below the reef plane have significantly different seismic velocities, which is the case for the Ventersdorp Contact Reef at Kusasalethu mine (previously Elandskraal, the amalgamation of Deelkraal and Elandsrand). A location method using a layered location model that takes into account the velocity contrast between the layers was tested. Observed direct- and head-waves were used to determine the velocity bounds of the P-wave velocities in the two-layer model. The data-set was also used to find the best P- and S-wave velocities for the two layers by grid searching over a range of velocities and minimizing the sum of the travel-time residuals. The results from the grid search and those from the calibration of velocities using direct- and head-waves give similar P-wave velocities for each of the two layers. A second set of seismic events was then relocated with this layered model and locations closer to reef were found. This simple layered location method goes some way to reduce the reefperpendicular location errors for Kusasalethu mine in particular. © The Southern African Institute of Mining and Metallurgy, 2013. Source

Moriya H.,Tohoku University | Naoi M.,University of Tokyo | Naoi M.,Kyoto University | Nakatani M.,University of Tokyo | And 9 more authors.
International Journal of Rock Mechanics and Mining Sciences

We applied advanced mapping techniques to 291 230 acoustic emission (AE) events as small as around M -4 that were recorded over 50 days by an ultra-high resolution network close to the active front of a tabular mining stope being advanced northward at 1. km depth in the Cooke 4 Gold Mine in South Africa. We first applied joint hypocenter determination (JHD) to improve absolute locations, and then applied the double-difference relative location algorithm to the JHD output. These steps resolved the seemingly continuous, dense cloud of AEs that extend about 20. m ahead of the stope front into several discrete, steeply dipping tabular clusters a few meters thick and 10-30. m in dip extent, separated by quiet intervals a few meters thick. The clusters have a strike parallel to the stope face and a dip of about 65°, resembling commonly observed large shear fractures along the plane of maximum shear (Ortlepp shears). In general, the activity of the clusters changed in similar ways as the stope face advanced, but each cluster remained stationary and the gaps between clusters were impressively quiet. This study demonstrates that high-resolution AE mapping can delineate the formation of large structures of localized damage in the highly stressed intact rock mass ahead of the stope face, a process that may culminate in hazardous seismic events. © 2015 Elsevier Ltd. Source

Van Aswegen G.,Institute of Mine Seismology | Stander M.,Open House Management Solutions
Journal of the Southern African Institute of Mining and Metallurgy

The geometry and morphology of a set of low-angle fractures around a stope in a deep Witwatersrand gold mine are explained in terms of extension fractures forming under variable conditions of stress. Primary extension fractures (E1) form some distance ahead of an advancing stope along the σ1, σ2 plane. With stope advance, a couple of these fractures end up in a stress regime conducive to transpressional shear and a secondary set of extension fractures (E2) is formed at a high angle to the primary fractures. i.e. at a low angle to the stope. As the E2 fractures are undermined, they migrate into a stress regime of transtensional shear and a third set of extension fractures (E3) may develop between E2 fractures. These have sigmoidal shapes, being parallel to the E2 fractures at the E2 discontinuity where σ3 is negative, and curved through the unfractured rock between E2 fractures where σ3 is positive at the instant of fracturing. The fractures all display fractographic features characteristic of dynamic extension failure with striae indicative of the direction of rupture propagation and the local, instantaneous orientation of σ1. © The Southern African Institute of Mining and Metallurgy, 2012. Source

Olivier G.,Joseph Fourier University | Olivier G.,Institute of Mine Seismology | Kastner M.,National Institute for Theoretical Physics NITheP | Kastner M.,Stellenbosch University
Journal of Statistical Physics

The anisotropic quantum Heisenberg model with Curie-Weiss-type interactions is studied analytically in several variants of the microcanonical ensemble. (Non)equivalence of microcanonical and canonical ensembles is investigated by studying the concavity properties of entropies. The microcanonical entropy (formula presented) is obtained as a function of the energy (formula presented) and the magnetization vector (formula presented) in the thermodynamic limit. Since, for this model, (formula presented) is uniquely determined by (formula presented), the same information can be encoded either in (formula presented) or (formula presented). Although these two entropies correspond to the same physical setting of fixed (formula presented) and (formula presented), their concavity properties differ. The entropy(formula presented) (formula presented), describing the model at fixed total energy(formula presented) (formula presented) and in a homogeneous external magnetic field(formula presented) (formula presented) of arbitrary direction, is obtained by reduction from the nonconcave entropy(formula presented) (formula presented). In doing so, concavity, and therefore equivalence of ensembles, is restored.(formula presented) (formula presented) has nonanalyticities on surfaces of co-dimension 1 in the(formula presented) (formula presented)-space. Projecting these surfaces into lower-dimensional phase diagrams, we observe that the resulting phase transition lines are situated in the positive-temperature region for some parameter values, and in the negative-temperature region for others. In the canonical setting of a system coupled to a heat bath of positive temperatures, the nonanalyticities in the microcanonical negative-temperature region cannot be observed, and this leads to a situation of effective nonequivalence even when formal equivalence holds. © 2014, Springer Science+Business Media New York. Source

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