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James P. Savely

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Proceedings Papers

Publisher: American Rock Mechanics Association

Paper presented at the The 30th U.S. Symposium on Rock Mechanics (USRMS), June 19–22, 1989

Paper Number: ARMA-89-0537

Abstract

ABSTRACT ABSTRACT: Size distributions of intact rocks, blasted rocks and dumped rocks from the same in-pit location were measured and plotted as log of cumulative number against size. The fractal dimension from the negative exponential plot increased with each energy increment, demonstrating an increase in fine particle fractions which may form a basis for improved rock classification. 1 INTRODUCTION An important need in mining - particularly where leaching processes are involved - is an accurate description of the discontinuity structure of a rock mass, and an understanding of the way in which this structure affects rock fragmentation during excavation. This was specifically identified by the U.S. National Committee for Rock Mechanics (1981) in their study of".. research requirements for resource recovery .... " There have been many attempts to develop models to predict the size distribution of excavated; usually blasted; rock (see forinstance Kuznetzov, 1973; Cunningham, 1983 and Danell and Leung, 1987). Most of these models have failed to allow for the presence of natural or imposed fractures present in the original rock mass. These can often have a dominant role in determining the rock fragment size after excavation (see forinstance Hagan, 1983; Yang and Rustan, 1983). An approach which incorporates the structural properties of the rock mass and their effect on fragmentation is outlined below. 2 BACKGROUND 2.1 Effect of block size on leach recovery Description of rock block size distributions have applications in heap, dump, and in-place leaching. Investigations by Dahlberg (1979) at Inspiration Mine, Arizona, have shown recovery of70% of the copper in 50 days from porphyry ore that was crushed to minus one inch (25mm). The same ore crushed to minus two inches (50mm) gave 60% metal recovery, while ore crushed to minus four inches (100mm) gave 47% recovery in the same period. The results were based on 10 foot (3m) long, 12-inch (0.3m) diameter column tests. In another test at Inspiration Mine a 30 foot (9m) high test pad containing 50,000 tons (44,000 tonnes) of ore crushed to minus four inches (100mm) was constructed. Drilling, sampling, and testing before and after leaching showed a metal recovery of 76%. After leaching, 2-inch (50mm) size rock pieces were examined for solution penetration. On average, a 2-inch (50mm) rock particle showed 0.59 inches (15mm) of leach solution penetration. This indicated that rock fragments larger than about 2 inches (50mm) would have their centers untouched by the leach solution. Both Dahlberg, (1979) and Fountain et al. (1983) have demonstrated relations between extraction rate and recovery of copper during leaching with rock block size, in both oxide and mixed sulfide and oxide ores at Inspiration Mine. Fountain et al. (1983) concluded that even though run-of-mine ore at Inspiration Mine is well broken 30% of fragments are greater than 4 inches (100mm) and there is still a substantial loss of recovery due to the coarse fraction. Thus, block size distribution is a critical factor controlling mineral recovery in leaching operations. It is apparent that if fragmentation could be improved, the percentage recovery would be increased.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper presented at the The 28th U.S. Symposium on Rock Mechanics (USRMS), June 29–July 1, 1987

Paper Number: ARMA-87-0927

Abstract

ABSTRACT ABSTRACT This paper uses shear strength estimates for a boulder conglomerate to predict ripper performance. It is found that ripping test results agree with strength parameters determined from back analysis of slopes. Bimses in sampling and laboratory testing caused by the presence of the large size fractions are believed to cause a wide range in the values determined for shear strength. Probability of ripping is calculated to estimate the percentage of material that can be ripped at a specified ground speed. More accurate predictions of ripper productivity can be made for formations that have highly variable strength. 1 INTRODUCTION Ripping performance and shear strength are closely related, and both are difficult to determine for boulder conglomerate. The presence of large boulders slows the ripping when the shank becomes anchored by a boulder and the tractor loses traction. The occurrence of boulders is usually unpredictable which makes estimates of ripping time and costs inaccurate. Shear strength of boulder conglomerate is difficult to estimate because of sampling and testing biases. Large particle sizes prevent representative sampling, and laboratory test results can be affected significantly when large size fractions are included Shear strength estimates are used in this analysis to predict rippability in a boulder conglomerate. Also considered is the adequacy of methods of shear strength determination for materials that are difficult to representatively sample and test. 2 PROPERTIES OF GILA CONGLOMERATE A test site in Tertiary Gila Conglomerate was selected at the Inspiration Mine near Globe, Arizona. There is extensive exposure of the formation in nearby cut slopes and there is a variety of data from face mapping, core drilling, test pits, and laboratory testing. Large-scale screen analyses and mapping of the boulder conglomerate indicate that as much as 50 percent of the formation has particle sizes greater than 20mm (coarse gravel, cobbles, and boulders), 50 percent has a size between .075mm and 20mm (sand and fine gravel), and 20 percent has a size less than .075mm (silt and clay). Additionally, it is estimated that 10 percent of the material has boulders exceeding 600mm in diameter. Matrix material is calcium cemented sandy clay with particle sizes less than 20mm. The boulders are tightly embedded in the matrix and the formation is firm and consolidated. About 30 percent of the particles larger than 20mm have rock-to-rock contact where a particle larger than 20mm is in contact with an adjacent particle of a size larger than 20mm. Moisture content has been determined to be between 5 and 15 percent and averages about 10 percent. Wet unit weight is approximately 23.4 kN/m 3 . Laboratory large-scale direct shear tests were performed on fine- grained, intact samples of the conglomerate. An effort was made during the sampling to obtain undisturbed samples that excluded particles larger than 50mm. Sample size was approximately 250mm long, 250mm wide, and 200mm thick. Laboratory small-scale direct shear test results were available from previous studies. These samples had been screened of particles larger than 20mm and remolded to assumed field conditions. Sample diameter was a standard 61.5mm (2.42 inches).

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper presented at the 19th U.S. Symposium on Rock Mechanics (USRMS), May 1–3, 1978

Paper Number: ARMA-78-0113

Abstract

ABSTRACT: Regional historical seismicity, regional tectonic processes, and data on local geologic structure are used to assess seismic risk for input into probabilistic slope design methods. Incorporation of earthquake risk into slope design makes economic optimization of open pit mine slopes possible in seismically active areas. In designing open pit mines, the economics of slope design should reflect the trade-off between the benefits and increased risk associated with steeper slopes. The large number of landslides generated by earthquakes indicates that a slope designed to an acceptable level of risk under static loading conditions may prove to be unacceptable when dynamic loading is considered. A Gumbel statistical analysis or similar probabilistic technique may be applied to the maximum yearly earthquake magnitudes occurring within seismically active zones near the site. The results of the Gumbel analysis are used to estimate earthquake risk within specified periods of time. Intensity of ground motion at the site, given that an earthquake of a specified magnitude occurs within a given time period, is derived Based on a study of local faulting and seismic attenuation laws, The resultant ground accelerations and their associated probabilities are then used in a Monte Carlo stability analysis to generate curves which approximate the risk of slope instability throughout the design "life" of the mine. INTRODUCTION Problem solving is often attempted in terms of absolutes; an alternative is either "safe" or "unsafe," a decision is "right" or "wrong." However, the way in which one arrives at such an absolute description of an alternative or decision is obscure at best. Since one can never know absolutely all of the parameters and facts which influence a project, it is more realistic to deal in terms of "risk'' rather than "safety" in approaching complex engineering problems (Wiggins, 1973)¹.

Proceedings Papers

Publisher: American Rock Mechanics Association

Paper presented at the 19th U.S. Symposium on Rock Mechanics (USRMS), May 1–3, 1978

Paper Number: ARMA-78-0048

Abstract

ABSTRACT: The probability of instability for 2-dimensional plane shear failure of a slope is a function of the shear strength, orientation, and length of the fracture forming the potential failure mode. The probability that a fracture has a dip within a given range (PD) and also a sufficient length in that orientation to reach from the toe to the top of the slope (PL), can be calculated from dip and length distributions obtained from field mapping. Using a rigid block analysis, the probability of sliding can be calculated by Monte Carlo sampling of the shear strength and fracture roughness distributions to determine the distribution of safety factors. The area of the safety factor distribution less than 1 is the probability of sliding (Ps). INTRODUCTION Slope instability from rock sliding along a single failure plane can be analyzed to determine the probability of slope failure. Variability in estimates of rock mass properties and rock strength measurements implies the probabilistic nature of geologic phenomenon. A design based on average values or on a single value does not account for this variability. A high safety factor might be calculated by using average values for the geologic parameters, but because of the variability shown by the distributions of these parameters, a high probability of failure may also be present (Höeg and Murarka, 1974). In addition, it is difficult to incorporate the traditional safety factor calculation into an economic analysis. The probability of instability, however, can be used with an economic risk analysis to determine an economic optimum design that considers cost impact of failure (Kim and others, 1976). GEOLOGIC STRUCTURE The simple plane shear failure geometry is analyzed to determine the probability of failure (Figure 1). The geologic structure must occur in an orientation that makes this failure mode viable.