Vector analysis of ice-fabric data

PHERIPHERAL SUBJECTS:SNOW & ICE mines are now inaccessible, although the recent investigation of two such mines has included the use of direct mining techniques to gain access to the old workings. Stability at Harford mine has been found to be adequate, due to adequate rock cover, moderate age and a rural setting. Collapses at Earlham Road mine have been found to be well developed, due to inadequate rock cover, great al~e, poor rock quality and disturbance due to the urban locatton. The principal collapse mechanism is identified as localised void migration, following roof collapse, with a possible secondary contribution from pillar over-stressing. -Authors

943411 Contribuicao da geologia para o estudo da subsidencia mineira na mina de carvao de Germunde (NW de Portugal) (A geological contribution to the research on mining subsidence at the Germunde coal mine, NW Portugal) H . I . Chamine & P. Bravo Silva, Cuadernos Laboratorio Xeoloxico de Laxe, 18, 1993, pp 281-287. The aim of this paper is t o p r e s e n t a detailed geological research of the mmmg subsidence induced by exploitation of the Germunde Coal Mine (Douro Coalfield, NW of Portugal). The authors analyse and discuss the different lithelogical and structural aspects that control the location of mining subsidence discontinuities appearing at ground surface. -English summary

943412 Characteristics of a multiple retrogressive failure in a coal mine in southwest T u r k e y R. Ulusay & V. Doyuran, Engineering Geology, 36(1-2), 1993, pp 79-89. This pal~er describes the geotechnical investigations and the stability analysis of a multiple retrogressive circular failure developed through a few benches cut in deposits of a weak transition zone at a strip coal mining area, southwest Turkey. The instability was analyzed to understand the possible causes of failure and to determine mobilized shear strength at the time of failure through the use of back analysis. Based on the results of back analysis performed for various conditions and using field observations, actual failure surfaces were defined by successive slip surfaces developing from the toe and advancing to the crest of the slope. The analysis also indicated that the instability was initiated by the construction of steep and high benches in the transition zone deposits. -from Authors

943413 Correlations between gas-geochemical and geomechanical p a r a m e t e r s of alkali rocks at ore deposits V. A. Nivin & N. I. Belov, Geochemistry International, 30(7), 1 9 9 3 , p p 34-40; translated from: Geokhimiya, 12, 1992, pp 1395-1401. In the ores and rocks of the Khibiny and Lovozero alkali massif apatite and rare-metal deposits, there is a correlation between mechanical parameters (wave velocities, elastic modulus, transverse strain and brittleness coefficients, and strength in compression and tension) on the one hand and the composition and content of the gas phase (hydrogen-hydrocarbon mixture containing a little helium and sometimes carbon dioxide) on the other. The correlations show that gas-geochemical parameters can be used to estimate the geo-mechanical ~pararneters and thus the potential rockburst hazard, which Is an important factor in the safe conduct of mining operations. -from Journal summary

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SUBJECTS PERIPHERAL TO GEOMECHANICS Snow and ice mechanics 943414 Air t e m p e r a t u r e , snow cover, creep of frozen ground, and the time of ice-wedge cracking, western Arctic coast J . R . Mackay, Canadian Journal of Earth Sciences, 30(8), 1993, pp 1720-1729. The time of ice-wedge cracking is examined for several sites with young and old ice wedges along the western Arctic coast. The correlation between sharp air temperature drops and ice-wedge cracking is highest where the snow cover is thin and least where the snow cover is thick. The favoured duration and rate of a temperature drop that results in cracking is about 4 days, at a rate of about 1.8"C/d. Such temperature drops have a minimal effect in cooling the top of permafrost wherever there is an appreciable snow cover. Since short duration temperature drops often result in ice-wedge cracking, the thermal stresses that trigger cracking probably originate more within the frozen active layer than at greater depth in permafrost. Although most ice wedges tend to crack during periods of decreasing air temperatures, about one third of those monitored have cracked during periods of increasing air temperatures. Long-term measurements show that the active layer and top of permafrost move differentially all year in a periodic movement. That is, creep of frozen ground occurs all year, irrespective of whether ice wedges crack or do not crack. -from Author 943415 Vector analysis of ice-fabric data M. G. Ferrick & K. J. Claffey, Journal of Glaciology, 39(132), 1993, pp 292-302. The mechanical properties of ice axe strongly affected by crystal texture and c-axis alignment. In this paper, we develop a general quantitative method for analysis of uniaxial crystal-orientation data. The resulting eigenvalue problem is identical to that obtained by other investigators using different methods. However, here we identify an implicit assumption in the method, and observe that the error measure represents p h y s i c a l distance and quantifies the goodness of fit to the data of idealized structures. Also, a method is developed to transform the data and the results for viewing on Scfunidt nets drawn in the best plane and the predominant basal plane of a sample, in addition to the standard xy-plane. Applications of the analysis to sea-ice samples include both numerical and Schmidt-net presentations of results. -from Authors 943416 Relating the occurrence of crevasses to surface strain rates D. G. Vaughan, Journal of Glaciology, 39(132), 1993, pp 255-266. The presence of crevasses on the surface of ice masses indicates that a fracture criterion has been met. Understanding how crevasses form will provide information about the stress and strain-rate fields in the ice. This study derives a relationship between measurements of strain rate and observations of crevassing on the surface of ice masses. By plotting strain rates (converted to stresses using a creep law) using axes representing the surfaceparallel principal stresses, failure envelopes were derived by enclosing measurements where surface crevassing was absent. The derived failure envelopes were found to conform well to theoretical ones predicted by the Coulomb and the maximum octahedral shear stress (yon Mises) theories of failure. The derived failure envelopes were scaled by the tensile strength, which was found to vary from 90 to 320kPa. There was no systematic variation of tensile strength with either temperature at 10 m depth or .me method used to locate the crevasses. The study also indicates that a temperature rise of a few degrees .throughout the ice column will not result directly in any increase m calving rates from the large Antarctic ice shelves such as theFilchner-Ronne or Ross Ice Shelves. -from Author