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1.
Natural Hazards - The prediction of open stope hangingwall (HW) stability is a crucial task for underground mines. In this paper, a relatively novel technique, the random forest (RF) algorithm, is... 相似文献
2.
Martin Fahey Matthew Helinski Andy Fourie 《Geotechnical and Geological Engineering》2011,29(5):709-723
Paste backfill used to provide ground support in underground mining is generated from full-stream tailings and is almost always
placed underground with cement. For the backfill, both the rate of strength development and the final strength are important
considerations for design, particularly when the backfill is subsequently exposed in the stope-mining sequence. There is strong
evidence that strengths measured on specimens obtained from coring the in situ cemented backfill are much greater than laboratory-cured
specimens with the same cement content. The paper reviews some of the experimental evidence showing that one of the major
reasons for the different strength is the difference in effective stress acting on the backfill during curing. Laboratory
specimens are (almost) always cured under zero total stress, so no effective stress develops. In contrast, backfill in a stope
may cure under high effective stress, which develops due to either “conventional” consolidation in free-draining backfills,
or to the so-called “self-desiccation” mechanism in fine-grained fills. Evidence is presented showing how the final strength
is affected by applying stress to specimens at different stages of curing and at different rates. It is shown that a fully-coupled
analysis of the filling behaviour is required to determine the appropriate effective stress regime to apply in curing laboratory
specimens, where “fully-coupled” in this context means taking account of the interaction of consolidation/drainage rate, filling
rate and cement hydration rate. Curing protocols for laboratory specimens are proposed, which would ensure that the strengths
obtained are representative of in situ conditions. 相似文献
3.
Pieter H. Fourie Udo Zimmermann Nicolas J. Beukes Thanusha Naidoo Katsuro Kobayashi Jan Kosler Eizo Nakamura Jenny Tait Johannes N. Theron 《International Journal of Earth Sciences》2011,100(2-3):527-541
In order to facilitate the understanding of the geological evolution of the Kalahari Craton and its relation to South America, the provenance of the first large-scale cratonic cover sequence of the craton, namely the Ordovician to Carboniferous Cape Supergroup was studied through geochemical analyses of the siliciclastics, and age determinations of detrital zircon. The Cape Supergroup comprises mainly quartz-arenites and a Hirnantian tillite in the basal Table Mountain Group, subgreywackes and mudrocks in the overlying Bokkeveld Group, while siltstones, interbedded shales and quartz-arenites are typical for the Witteberg Group at the top of the Cape Supergroup. Palaeocurrent analyses indicate transport of sediment mainly from northerly directions, off the interior of the Kalahari Craton with subordinate transport from a westerly source in the southwestern part of the basin near Cape Town. Geochemical provenance data suggest mainly sources from passive to active continental margin settings. The reconnaissance study of detrital zircons reveals a major contribution of Mesoproterozoic sources throughout the basin, reflecting the dominance of the Namaqua-Natal Metamorphic Belt, situated immediately north of the preserved strata of Cape Supergroup, as a source with Archaean-aged zircons being extremely rare. We interpret the Namaqua-Natal Metamorphic Belt to have been a large morphological divide at the time of deposition of the Cape Supergroup that prevented input of detrital zircons from the interior early Archaean Kaapvaal cratonic block of the Kalahari Craton. Neoproterozoic and Cambrian zircons are abundant and reflect the basement geology of the outcrops of Cape strata. Exposures close to Cape Town must have received sediment from a cratonic fragment that was situated off the Kalahari Craton to the west and that has subsequently drifted away. This cratonic fragment predominantly supplied Meso- to Neoproterozoic, and Cambrian-aged zircon grains in addition to minor Silurian to Lower Devonian zircons and very rare Archaean (2.5?Ga) and late Palaeoproterozoic (1.8-2.0?Ga) ones. No Siluro-Devonian source has yet been identified on the Kalahari Craton, but there are indications for such a source in southern Patagonia. Palaeozoic successions in eastern Argentina carry a similar detrital zircon population to that found here, including evidence of a Silurian to Lower Devonian magmatic event. The Kalahari and Río de la Plata Cratons were thus in all likelihood in close proximity until at least the Carboniferous. 相似文献
4.
When designing any earth retaining structure it is necessary to estimate the limiting earth pressures. This is usually achieved by assuming a linear pressure distrigution and by using active and passive pressure coefficients obtained by either limit equilibrium, stress field solutions of limit analysis. These coefficients are approximate in a theoretical sense, do not distinguish between modes of wall movement, and provide no pre-failure information. In practice, wall movements are dependent on the construction method and support conditions provided. Any effect of such movements on earth pressures is therefore of practical interest. In this paper the finite element method is used to investigate the effect of the mode of wall movement on the generation of earth pressure. Both smooth and rough walls are considered. It is shown that the distribution of earth pressure is highly dependent on the assumed mode of deformation. The resultant forces on the wall are also affected, but to a lesser degree. The, effect of soil dilatation, the initial horizontal stress and the distribution of soil stiffness with depth are also examined. 相似文献
5.
B. H. Goff R. Weinberg D. I. Groves N. M. Vielreicher P. J. Fourie 《Mineralogy and Petrology》2004,80(3-4):173-199
Summary The world-class Paleoproterozoic Vergenoeg fluorite deposit in South Africa is hosted in a breccia pipe comprising units with varying proportions of pegmatoidal fayalite, magnetite, fluorite and siderite. The adjacent A-type Bushveld granites also have associated fluorite deposits containing fluorite with similar REE patterns, fluid inclusion and Sr isotope compositions to those at Vergenoeg, leading to the proposal that there is a genetic relationship. This is despite the silica-undersaturated nature (SiO2<30%) of the Pipe and its extreme enrichment in Ca, F, Fe, Nb, P and REE compared to granites. Both liquid immiscibility from a granitic melt and granitic magmato-hydrothermal activity have been proposed as genetic mechanisms to explain this exotic composition. However, the Vergenoeg Pipe shows greater similarities to alkaline rocks, in particular the Phalaborwa carbonatite of similar age, including: i) size and shape, ii) associated maars, iii) mineralogical zoning, iv) geochemical, radiogenic and stable isotope composition, and v) presence of both high-T and low-T fluid inclusions. This suggests an alternative genetic relationship with alkaline magmas, in which some geochemical and radiogenic isotopic similarities to Bushveld granites are the consequence of broadly contemporaneous development in the same tectonic setting within the same lithosphere, whereas others may be due to hydrothermal overprinting. Similarities with Phalaborwa and also with Bayan Obo, Mongolia, indicate that the Vergenoeg pegmatoid pipe could be an extreme carbonatite-associated member of the Fe-oxide Cu–Au (±REE±P) group of deposits.Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1007/s00710-003-0012-6Tables 1-4 available as electronic supplementary material 相似文献
6.
In February/March 2007, an extreme rainfall event occurred in the Jabiru region of the Northern Territory of Australia. Rainfall of 784 mm fell in a 72-h period. This rainfall event resulted in 49 separate landslides occurring in the adjacent, but remote and inaccessible region of Arnhem Land. The landslides were extensively mapped and characterised. A common feature of the landslides was their relatively surficial nature. This paper reports on laboratory and field tests to characterise the material properties of the slide material and the underlying, more competent material. One particular, large and relatively accessible landslide was chosen for detailed investigation. The experimental data are used to carry out seepage and slope stability analyses, taking account of changes in the degree of saturation (and thus the negative pore water pressure or suction) in the slope material during the rainfall event in question. Using a parametric study in which various material parameters were varied around the measured mean values, it is shown that the failure of this particular slope could have been predicted using relatively straightforward seepage and limit equilibrium slope stability analyses, coupled with the relevant rainfall data, as long as the contribution of matric suction to the engineering characteristics of the slope material was accounted for. The work also highlights the importance of in situ conditions at the time a particular rainfall event (particularly an extreme event such as that considered in this paper) occurs. If the slope has a relatively high degree of saturation, manifested as a low initial in situ suction, it is more susceptible to rainfall triggering a slope failure. Although this observation is not novel, the investigation described in this paper confirms the importance of ambient in situ conditions and provides an indication of how the likelihood of landslide occurrence at this particular site may in the future be quantified, i.e. by focussing on antecedent rainfall history. 相似文献
7.
Granophyre dykes in the central part of the Vredefort impact structure are believed to be the remnants of the impact melt sheet, which intruded downwards along the fractures in the crater floor. Little is known about their original penetration depth, dip, structural relationships with the host rocks, and their general geophysical characteristics. This information is critical to understand the emplacement history of the granophyre dykes, as it relates to the formation and modification of large impact structures. We conducted magnetic and resistivity surveys across the Daskop granophyre dyke (DGD), one of the impact melt dykes in the structure's core. The magnetic survey revealed that the DGD gives a strong magnetic response at positions where the dyke outcrop exceeds the surface topography, but a very weak response where the outcrop is nearly at the same elevation as the surrounding topography. The magnetic anomaly is thus predominantly due to the outcrop protruding above ground level, suggesting a limited volume of dyke material in the subsurface and a small penetration depth. The resistivity survey performed on two profiles, set perpendicularly across the DGD, indicated a shallow penetration depth (<3 m), consistent with the magnetic interpretation. Thus, our geophysical study demonstrates that the DGD is currently at the very bottom of its original emplacement. This may either be an erosional coincidence, or it may be controlled by a fundamental process of impact cratering. Further studies are warranted to determine if other granophyre dykes at Vredefort are similarly at their lowermost terminations. 相似文献
8.
F. D. Fourie M. S. Huber E. Kovaleva M. D. Clark 《Meteoritics & planetary science》2023,58(9):1345-1364
Recent observations and geophysical studies at the Vredefort impact structure have indicated that the impact melt dikes in the central uplift of the structure have small depth extents. In this study, we performed magnetic and electrical resistivity tomography (ERT) surveys of the Lesutoskraal granophyre dike (LGD) and trenched to confirm its depth extent. The ERT survey showed that outcrops of the LGD are associated with shallow resistive zones with <3 m depth extent, but such zones do not occur where outcrops are absent. Visual observations in the trench confirmed that the dike has a small depth extent (~0.75 m) at this location. However, the magnetic survey revealed anomalies along the entire strike of the dike, even where no outcrops occur. We suggest that remagnetization of the host rock within a metamorphic contact aureole could explain the presence of magnetic anomalies in the absence of outcrops. Considering the results of the ERT survey, the observations made in the trench, and the surface distribution of outcrops of the LGD, we confirm that this dike has a small depth extent (<3 m) along its entire length and propose that outcrops represent the intersection of the dike terminus with the current erosional surface. 相似文献
9.
Ian Saunders Lars Ottemöller Martin B. C. Brandt Christoffel J. S. Fourie 《Journal of Seismology》2013,17(2):437-451
A relation to determine local magnitude (M L) based on the original Richter definition is empirically derived from synthetic Wood–Anderson seismograms recorded by the South African National Seismograph Network. In total, 263 earthquakes in the distance range 10 to 1,000 km, representing 1,681 trace amplitudes measured in nanometers from synthesized Wood–Anderson records on the vertical channel were considered to derive an attenuation relation appropriate for South Africa through multiple regression analysis. Additionally, station corrections were determined for 26 stations during the regression analysis resulting in values ranging between ?0.31 and 0.50. The most appropriate M L scale for South Africa from this study satisfies the equation: $$ {M_{{{\bf L}}}} = {\text{lo}}{{\text{g}}_{{10}}}(A) + 1.149\;{\text{lo}}{{\text{g}}_{{10}}}(R) + 0.00063R + 2.04 - S $$ The anelastic attenuation term derived from this study indicates that ground motion attenuation is significantly different from Southern California but comparable with stable continental regions. 相似文献
10.
Ming Zhi Wu David A. Reynolds Andy Fourie David G. Thomas 《Ground Water Monitoring & Remediation》2013,33(1):62-74
Numerical simulations were used to identify and evaluate optimum electrode configurations and approaches for electrokinetic in situ chemical oxidation (EK‐ISCO) remediation of low‐permeability sediments. A newly developed groundwater and EK flow and reactive transport numerical model was used to conduct two‐dimensional scenario simulations of the coverage of an injected oxidant, permanganate, and the oxidation of a typical organic contaminant (tetrachloroethene, PCE). For linear configurations of vertical electrodes, the spacing of same‐polarity electrodes is recommended to be about one‐third to one‐quarter of the anode–cathode spacing. Greater coverage could also be achieved by locating additional oxidant injection wells at the divergence of the electric field in linear electrode configurations. Horizontal electrodes allowed greater contact between the injected permanganate and PCE and resulted in faster degradation of PCE compared to vertical electrodes. Pulsed oxidant injection, closer electrode spacing, and electric field reversal also resulted in faster EK‐ISCO remediation. 相似文献