Axial stress–strain relation of encapsulated granular column

The axial stress–strain relations of embedded granular columns encapsulated with flexible reinforcement were evaluated using an analytical procedure based on the cavity expansion method. This proposed method has firstly been verified through an experimental triaxial test on a reinforced sand specimen. A normalized relation was established between the volumetric change and the axial strain of soil to enable the analysis of granular material behavior under a continuous increase in lateral pressure. The analytical results show that the reinforced granular columns embedded in clay behave differently from granular columns subjected only to a constant confining pressure. It is found that reinforcing a column with a sleeve at the top portion will be adequate to prevent the column from bulging and also improve its load carrying capacity. The optimum skirting length that a sleeve can deter a granular column from bulging depends on the characteristics of the in situ soil and the stiffness and yield strength of the sleeve.     

Intelligent prediction of the stress–strain response of intact and jointed rocks

An application of Artificial Neural Networks for predicting the stress–strain response of jointed rocks under different confining pressures is presented in this paper. Rocks of different compressive strength with different joint properties (frequency, orientation and strength of joints) are considered in this study. The database for training the neural network is formed from the results of triaxial compression tests on different intact and jointed rocks with different joint properties tested at different confining pressures reported by various researchers in the literature. The network was trained using a three-layered network with the feed-forward back propagation algorithm. About 85% of the data was used for training and the remaining 15% was used for testing the network. Results from the analyses demonstrated that the neural network approach is effective in capturing the stress–strain behaviour of intact rocks and the complex stress–strain behaviour of jointed rocks. A single neural network is demonstrated to be capable of predicting the stress–strain response of different jointed rocks, whose intact strength varies from 11.32 MPa to 123 MPa, spacing of joints varies from 10 cm to 100 cm, and confining pressures range from 0 to 13.8 MPa.     

The Last Global Transgression of the World Ocean: Glacio-Eustasy or Tectonics?

Geotectonics - The article substantiates the conclusion about the tectonic nature of the last global transgression of the World Ocean. It is based on the results of marine studies in which complete...     

Gold in Egypt: Does the future get worse or better?

Within the Eastern Desert of Egypt, there are more than a hundred vein-type gold deposits and occurrences. Gold was mined in the Dynastic and Roman periods of the Egyptian history and most of the attention was focused entirely on the near-surface high grade parts of these veins. Currently, besides the more problematic sulfide-rich quartz veins, which generally occur in the lower levels of the worked out auriferous quartz veins, some targets including wall-rock alteration, listwaenite, dumps and tailings of ancient mines, by-product gold hosted in volcanogenic massive sulfide deposits and gold-bearing Algoma-type BIF, as well as alluvial gold constitute the new targets for gold in the Eastern Desert of Egypt. In most of these targets, gold displays a bimodal distribution; occurring both as free-milling gold and in the form of sub-microscopic or “invisible” gold in refractory sulfides and sulfoarsenides such as pyrite, chalcopyrite, arsenopyrite, gersdorffite and pentlandite minerals.Characterizing gold in an ore is important in extraction metallurgy. Free-milling gold can be efficiently recovered by crushing, grinding and cyanidation without additional processing. In refractory ores, on the other hand, conventional milling can liberate the sulfides from the gangue allowing a low content of concentrate to be produced by a process such as flotation. However, direct leaching of the concentrate results in poor gold extractions as the cyanide lixiviant is unable to contact the gold locked within the refractory host. Oxidative pretreatment is required for such ores and the methods available for oxidation are roasting (calcination), pressure oxidation, and bacterial oxidation; all oxidize the refractory minerals in the ore to render the gold amenable to cyanidation. While such oxidative methods are capable of achieving reasonable recoveries, they increase the capital and operating costs. An alternative, applicable to the liberation of gold from the refractory host, is to continue the grinding process to reduce the particle size of the host mineral thereby exposing a part of the gold surface for contact with the cyanide solution. A benefit of this technique is that the environmental aspects of the oxidation reaction products are avoided, however the ultra fine grinding also adds more operating costs.Metallurgical studies carried out on some gold deposits, dumps and tailings of ancient gold mines in Egypt, proved that these ores are amenable to percolation leaching (concentration of KCN varies between 0.5 and 1 g/L) for the (− 10 mm) grain size, and consequently heap leaching is a successful technique as the results of these laboratory scale column tests show. In most of these studies, a marked improvement in the percent of gold recovery was gained when the crushed ore is agglomerated by the addition of water, lime and/or cement particularly in ores containing significant quantities of clay minerals and clay-forming minerals (e.g. sericite and muscovite), and by addition of kerosene which passivates the adsorption surfaces on the graphite.Placer gold is recovered by gravity methods utilizing the large difference in specific gravity between gold and commonly associated minerals such as quartz and feldspar. The equipments range from a simple pan to more complex devices used in gold recovery plants and dredges.Modern studies in Egypt point to the presence of different targets of gold; diverse in their gold content, mode of occurrence of gold and the metallurgical treatments required for extracting gold. With the advance of modern techniques such as Electron Microprobe Analyses (EMPA), laser Ablation Microprobe Inductively Coupled Plasma Mass Spectroscopy (LAM-ICP-MS), gold ores in Egypt should be re-characterized precisely to know the specifications of these ores and consequently predict the metallurgical regime required, and with the high world gold price, it has become necessary to take into consideration the presence of refractory gold ores as one of these targets in Egypt. Generally speaking, laboratory experiments conducted on gold processing in Egypt are encouraging to some extent; however the gold extraction rates of the free milling ores with conventional cyanidation and refractory ores with direct and pretreatment techniques can be improved by modifying the conditions of extraction to be consistent with the results obtained from the re-characterization stage. Otherwise any target for gold and whatsoever the mode of occurrence of gold in this target (i.e. free-milling and/or refractory) could be recovered directly by heap leaching technique and we should accept less than ideal gold recovery for any target when weighed against the capital expenditure required for processes such as roasting, and the continuous grinding processes which might improve recoveries by only a small percentage.     

The glacigenic deposits of Western Lleyn,North Wales: Terrestrial or marine?

This paper describes a complex sequence of glacigenic sediments occupying a faultbounded depression at Aberdaron Bay, western Lleyn. The sequence offers an insight into sedimentary environments during deglaciation of the Irish Sea Basin. A lower stratified diamict association (LDA) containing contorted units of fine sand/silt and displaying strong, consistent clast fabrics, is overlain by an upper diamict association (UDA) with weaker fabrics and extensive sand and gravel layers and channel fills. Certain characteristics of the sequence can be explained by a glacimarine depositional model, but there are several problems. In particular, the geometry of the sediments is difficult to explain without recourse to the melting of buried ice. An alternative model that overcomes these problems involves the decay of a terrestrial glacier containing reworked marine sediments. Supraglacial sediment flows released during decay of thinner ice covering the surrounding hills (UDA) would have rapidly buried a thick stagnant ice mass in the Aberdaron depression, facilitating slow melting and release of basal meltout till (LDA). A model is presented that accounts for the stagnation and in situ decay of a debris-rich Irish Sea glacier, and which could explain many of the deposits and landforms surrounding the Irish Sea Basin without recourse to high relative sea-levels.     

The Mazafran river (western Sahel of Algiers): superimposition or antecedence?

At the western Sahel of Algiers, the watercourse of the Mazafran river is perpendicular to the Sahel fold, forming a gorge that has developed in a tectono-eustatic context characteristic of the Sahel region. This gorge has formed in a regime of superimposition which involves the evolution of antecedence in relation to the Sahel uplift. This deduction is the result of multiple analyses that are based on a stratigraphic study which characterizes the succession of continental and marine deposits and the analyses of lineament map of tectonic evolution during the Quaternary which is marked by the Sahel folding. In order to correlate the chronology between the fluvial and marine sedimentary dynamics, a geomorphological study of the Mazafran alluvial terraces associated with the paleoshorelines was realized. The obtained results reveal that the development of the Mazafran river gorge is complex.     

Effect of particle friction and polydispersity on the macroscopic stress–strain relations of granular materials

The macroscopic mechanical behavior of granular materials inherently depends on the properties of particles that compose them. Using the discrete element method, the effect of particle contact friction and polydispersity on the macroscopic stress response of 3D sphere packings is studied. The analytical expressions for the pressure, coordination number and fraction of rattlers proposed for isotropically deformed frictionless systems also hold when the interparticle coefficient of friction is finite; however, the numerical values of the parameters such as the jamming volume fraction change with varying microscopic contact and particle properties. The macroscopic response under deviatoric loading is studied with triaxial test simulations. Concerning the shear strength, our results agree with previous studies showing that the deviatoric stress ratio increases with particle coefficient of friction μ starting from a nonzero value for μ = 0 and saturating for large μ. On the other hand, the volumetric strain does not have a monotonic dependence on the particle contact friction. Most notably, maximum compaction is reached at an intermediate value of the coefficient of friction μ ≈ 0.3. The effect of polydispersity on the macroscopic stress–strain relationship cannot be studied independent of initial packing conditions. The shear strength increases with polydispersity when the initial volume fraction is fixed, but the effect of polydispersity is much less pronounced when the initial pressure of the packings is fixed. Finally, a simple hypoplastic constitutive model is calibrated with numerical test results following an established procedure to ascertain the relation between particle properties and material coefficients of the macroscopic model. The calibrated model is in good qualitative agreement with simulation results.     

Geologic time scales in modern books: a failure of standardization?

This is a response to the Correspondence by Knight (Proceedings of the Geologists’ Association, 2011) entitled ‘Evaluating geological heritage’. In this response I suggest that geodiversity can be evaluated (1) conceptually as a natural treasure within landscape, cultural, and historical contexts and (2) numerically as a number of geosite types (not as a number of geosites). These points of view are not conflicting, but mutually profitable. Moreover, broad context and perception of geodiversity can be involved in its quantification. Geodiversity is viewed as a dynamic idea, which modifications will be reflected in every study of the regional geological heritage.     

The end of the Ediacara biota: Extinction,biotic replacement,or Cheshire Cat?

The Ediacaran–Cambrian transition signals a drastic change in both diversity and ecosystem construction. The Ediacara biota (consisting of various metazoan stem lineages in addition to extinct eukaryotic clades) disappears, and is replaced by more familiar Cambrian and Paleozoic metazoan groups. Although metazoans are present in the Ediacaran, their ecological contribution is dwarfed by Ediacaran-type clades of uncertain phylogenetic affinities, while Ediacaran-type morphologies are virtually non-existent in younger assemblages. Three alternative hypotheses have been advanced to explain this dramatic change at, or near, the Ediacaran–Cambrian boundary: 1) mass extinction of most Ediacaran forms; 2) biotic replacement, with early Cambrian organisms eliminating Ediacaran forms; and 3) a Cheshire Cat model, with Ediacaran forms gradually disappearing from the fossil record (but not necessarily going extinct) as a result of the elimination of unique preservational settings, primarily microbial matgrounds, that dominated the Ediacaran. To evaluate these proposed explanations for the biotic changes observed at the Ediacaran–Cambrian transition, environmental drivers leading to global mass extinction are compared to biological factors such as predation and ecosystem engineering. We explore temporal and biogeographic distributions of Ediacaran taxa combined with evaluations of functional guild ranges throughout the Ediacaran. The paucity of temporally-resolved localities with diverse Ediacaran assemblages, combined with difficulties associated with differences in taphonomic regimes before, during, and after the transition hinders this evaluation. Nonetheless, the demonstration of geographic and niche range changes offers a novel means of assessing the downfall of Ediacara-type taxa at the hands of emerging metazoans, which we hypothesize to be most likely due to the indirect ecological impact metazoans had upon the Ediacarans. Ultimately, the combination of studies on ecosystem construction, biostratigraphy, and biogeography showcases the magnitude of the transition at the Ediacaran–Cambrian boundary.     

The multiple system ADS 9626: A quadruple star or an encounter of two binaries?

The parameters of the quadruple system ADS 9626 (µ Boo) are analyzed. The system consists of two double stars: the Aa pair with an angular separation of ρ = 0.08″ and the BC pair with an angular separation of ρ = 2.2″, separated by ρ = 107″ and having the same parallaxes and proper motions. Position observations with the Pulkovo 26″ refractor have yielded from direct astrometric measurements the difference in the apparent magnitudes and the component-mass ratio for the BC subsystem: Δm = 0.59 ± 0.06, M(B)/M(C) = 1.18 ± 0.02. Spectroscopy with the Russian-Turkish 1.5-m telescope has yielded the radial velocities and physical parameters of the Aa, B, and C components. Speckle-interferometric observations with the 6-m telescope of the Special Astrophysical Observatory have provided the first measurements of the magnitude difference in the close Aa subsystem: Δm = 0.46 ± 0.03 (λ = 5500 Å) and Δm = 0.41 ± 0.03 (λ = 8000 Å). The new observations are consistent with the known orbits, which were used to find the radial velocities for the centers of mass of the inner subsystems. Themotion of the outer pair, Aa-BC, is studied using the apparent motion parameters (AMP) method. It is not possible to derive an elliptical orbit for this pair; the elements of a hyperbolic orbit have been estimated. The difference of the heavy-element abundances for the Aa and BC subsystems of 0.5 dex confirms that these pairs have a different origin. This suggests that we are observing here a close encounter of two binary stars.     

Effect of microstructure on stress–strain and pore-pressure response of sabarmati sand under the influence of mica

ABSTRACT

Micaceous soil is believed to be detrimental for civil engineering constructions due to the effect of high compressibility, low compacted density and low shear strength. Individual mica particle has numerous intact mica flakes foliated over each other making it flexible upon loading and rebound upon unloading due to its low hardness and resilient nature. Hence, micaceous soils with mica content more than 10% are considered undesirable for highway pavements, embankments and railway track constructions. When platy mica particles are sufficiently numerous to interact with spherical sand particles, bridging and ordering phenomena are augmented within the soil mass creating unique sand-mica particle orientation (MS microstructure) unlike sand-sand particle orientation (PS microstructure). The current experimental research was conducted to evaluate the variation in stress–strain, pore pressure and effective stress path response of Sabarmati sand under the influence of mica (sand with 30% mica and pure sand) with MS and PS microstructure respectively. Effect of particle crushing on stress–strain and pore pressure response was also studied on Sabarmati sand with MS and PS microstructure. Distinctive macroscopic response was observed in Sabarmati sand with MS microstructure under the influence of mica as well as mica particle crushing.     

The effect of Dauphiné twinning on plastic strain in quartz

We present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies (T = 300–400°C) granitoid protomylonite from the Arolla unit of the NW Alps. Mechanical Dauphiné twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane {z} almost orthogonal to the compression direction (z-twin orientation). Twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb {r} nearly parallel to the compression direction (r-twin orientation). In contrast, we report the lack of twinning in two porphyroclasts already oriented with one of the {r} planes orthogonal to the compression direction. One twinned porphyroclast has been investigated with more detail. It shows the localization of much of the plastic strain into discrete r-twins as a consequence of the higher amount of elastic strain energy stored by r-twins in comparison to z-twins. The presence of Dauphiné twins induced a switch in the dominant active slip systems during plastic deformation, from basal <a> (regions without twinning) to {π} and {π′} <a> (pervasively twinned regions). Dynamic recrystallization is localized along an r-twin and occurred dominantly by progressive subgrain rotation, with a local component of bulging recrystallization. Part of the recrystallized grains underwent rigid-body rotation, approximately about the bulk vorticity axis, which accounts for the development of large misorientation angles. The recrystallized grain size piezometer for quartz yields differential stress of 100 MPa. The comparison of this palaeostress estimate with literature data suggests that mechanical Dauphiné twinning could have a potential use as palaeopiezometer in quartz-bearing rocks.     

Wet or dry? The difficulty of identifying the presence of water during crustal melting

Partial melting of continental crust and evolution of granitic magmas are inseparably linked to the availability of H₂O. In the absence of a free aqueous fluid, melting takes place at relatively high temperatures by dehydration of hydrous minerals, whereas in its presence, melting temperatures are lowered, and melting need not involve hydrous minerals. With the exception of anatexis in water‐saturated environments where anhydrous peritectic minerals are absent, there is no reliable indicator that clearly identifies the presence of a free aqueous fluid during anatexis. Production of Ab‐rich magmas or changes in LILE ratios, such as an increase in Sr and decrease in Rb indicating increased involvement of plagioclase, are rough guidelines to the presence of aqueous fluids. Nevertheless, all indicators have caveats and cannot be unequivocally applied, allowing for the persistence of a bias in the literature towards dehydration melting. Investigation of mineral equilibria modelling of three metasedimentary protoliths of the Kangaroo Island migmatites in South Australia, shows that the main indicator for the presence of small volumes of excess water under upper amphibolite to lower granulite facies conditions (660–750°C) is the melt volume produced. Melt composition, modal content or chemical composition of peritectic minerals such as cordierite, sillimanite or garnet are relatively insensitive to the presence of free water. However, the mobility of melt during open system behaviour makes it difficult to determine the melt volume produced. We therefore argue that the presence of small volumes of excess water might be much more common than so far inferred, with large impact on the buffering of crustal temperatures and fertility, and therefore rheology of the continental crust.     

The Pliocene Yafo Formation in Israel: Hydrogeologically inert or active?

For several decades the “Saqiye beds” (later renamed Yafo Formation) underlying the Coastal Plain aquifer (Kurkar Group) aquifer of Israel, were regarded as an extremely thick, tectonically undisturbed, and absolutely impervious aquiclude. Following intensive groundwater exploitation from the overlying Kurkar Group aquifer, brackish and saline waters were locally encountered in the lower parts of this aquifer and always at the contact with the underlying Yafo Formation aquiclude. The present study revealed that this aquiclude is not a uniform and impervious rock unit, but rather an alternation of pervious and impervious strata within the Yafo Formation containing highly pressured fluids of different — mostly high — salinities. The permeable beds are at an angular unconformity and in direct contact with the overlying Kurkar Group aquifer. The Yafo Formation and the underlying and overlying rock units are dislocated by numerous fault systems, which facilitate accessibility of brines into the Kurkar Group aquifer. The mobilization of the saline fluids and their injection into the Kurkar Group aquifer could be due either to diffusion of saline fluids occurring in the permeable horizons of the Petah Tiqva Member through the clays of the Yafo Formation or to their upconing following intensive pumping in the Coastal Plain aquifer. It could have also been caused by up-dip movement of saline water as the result of overpressure generated by major accumulation of gas in the permeable horizons. Another possible mechanism could be hydraulic contact with pressurized brines up-flowing along fault zones from deep-seated Jurassic or Cretaceous reservoirs. The squeezing of saline interstitial water from the clays of the Yafo Formation into the overlying Kurkar Group aquifer, is of secondary importance for groundwater salinization (its input is comparable with salt input from rain).     

The monsoon system: Land–sea breeze or the ITCZ?

For well over 300 years, the monsoon has been considered to be a gigantic land–sea breeze driven by the land–ocean contrast in surface temperature. In this paper, this hypothesis and its implications for the variability of the monsoon are discussed and it is shown that the observations of monsoon variability do not support this popular theory of the monsoon. An alternative hypothesis (whose origins can be traced to Blanford’s (1886) remarkably perceptive analysis) in which the basic system responsible for the Indian summer monsoon is considered to be the Intertropical Convergence Zone (ITCZ) or the equatorial trough, is then examined and shown to be consistent with the observations. The implications of considering the monsoon as a manifestation of the seasonal migration of the ITCZ for the variability of the Indian summer monsoon and for identification of the monsoonal regions of the world are briefly discussed.     

The 100-kyr ice-age cycle: internal oscillation or inclinational forcing?

The ice-age hypothesis of Muller and MacDonald (1995, 1997a, 1997b) has two parts: (a) The 100-kyr cycle does not owe its existence to Milankovitch forcing; and (b) variations in inclination of Earth's orbit (i.e., the orbit's angle with the solar system invariable plane) provide the mechanism sought. In support of the first proposition, Muller and MacDonald point to the paradox that the spectrum of oxygen isotope series from deep-sea sediments contains no power for two prominent eccentricity cycles, 125 and 400 kyr. In support of the second proposition, they offer a match between the SPECMAP record (Imbrie et al. 1984) and a plot of the amplitude of orbital inclination, shifted by 33 kyr. The hypothesis of Muller and MacDonald is rejected in both parts, although an influence of inclination forcing is not precluded entirely. The paradox of the missing eccentricity cycles (125 and 400 kyr) is explained by suppression of the two longer cycles, and enhancement of the one near 96 kyr, as a result of internal oscillation. A Muller–MacDonald machine for making the 100-kyr ice-age cycles, however conceived, would have to have a memory near 30 kyr to provide for phase shift between input and output. Precisely this amount of memory is sufficient to produce the needed oscillation in Milankovitch machine here applied; thus, there is no advantage, from the point of view of either necessity or simplicity, in replacing Milankovitch forcing, with its precise phasing (despite the fuzzy physics), with inclination forcing, and with its severe problems in phasing (and, thus far, no physics at all). Received: 10 June 1998 / Accepted: 2 February 1999     

Reassessment of global climate risk: non-compensatory or compensatory?

Natural Hazards - Evidence shows the global climate will continue to change over this century and beyond. A clear understanding of the climate change risk is suggested to be the foundation of the...     

The stress state of the Earth’s lithosphere: Results of statistical processing of the world stress-map data

A method for the statistical processing of the input data on the stress state of the Earth’s lithosphere that takes the initial 3-D position of the principle stress axes into account is elaborated. This approach is based on the calculation of the arithmetic mean value for every six independent tensor components during determination of the average stress in any sampling. When determining the sampling for calculation of the average stress for the current cell of the calculated grid, it is proposed to insert the measurements into the sampling that are spatially located in such a manner where the distance from the measurement point to the cell center is less than some value that is named as a search radius. The latter either was specified as a constant for all cells of the calculated grid or was determined assuming that the dispersion of the average tensor was less than some preset value. The results of the application of this approach are presented based on the example of the processing of the measurements from the World Stress Map (Heidbach et al., 2008). The resultant set of the mean stress-field maps reflects the generalized pattern of the stress distribution in the Earth’s lithosphere.