Empirical evidence has shown that particle breakage affects the mechanical behaviour of granular materials. The source of this mechanism takes place at the particle scale, and the main consequence on the macromechanical behaviour is increasing compressibility. Due to the inverse correlation between particle size and particle crushing strength, coarse rockfill materials are particularly vulnerable to mechanical degradation due to particle breakage. However, such coarse materials do not fit in standard laboratory devices, and the alternative of large sample testing is usually unavailable or too expensive. Alternatively, recent works have proposed multi-scale approaches using the discrete element method (DEM) to carry out numerical testing of coarse crushable materials, although few studies have focused on size effects. This article presents the application of a DEM bonded-cell model to study particle size-strength correlation on angular rock aggregates. Each particle is modelled by a cluster of perfectly rigid polyhedral cells with Mohr–Coulomb contact law. Constant cell density within particles implies that the presence of potential fragmentation planes increases with size. Therefore, particle strength decreases with size. A comprehensive sensitivity analysis was carried out through 1477 particle crushing simulations in a given particle size. Based on published experimental data on calcareous rock aggregates, part of the simulations were used for calibration, and 97 additional simulations of a coarser size fraction were performed for validation. The results show a good agreement with the empirical data in terms of size effect and data scatter through Weibull statistics.
The concentrations of Al, Fe, Mn, Zn, Cu, Pb, Ni, Cr, Ba, V, Sn and As in offshore bottom sediments from the Bacia de Campos oil field, SE Brazil, were measured at the beginning and at 7 months after completion of the drilling operation. Concentrations of Al, Fe, Ba, Cr, Ni and Zn were significantly higher closer to the drilling site compared to stations far from the site. Average concentrations of Al, Cu, and in particular of Ni, were significantly higher at the end of the drilling operation than at the beginning. Comparison between drilling area sediments with control sediments of the continental platform, however, showed no significant difference in trace metal concentrations. Under the operation conditions of this drilling event, the results show that while changes in some trace metal concentrations do occur during drilling operations, they are not significantly large to be distinguished from natural variability of the local background concentrations. 相似文献
Temporal and spatial variability of particulate metal concentrations (Cu, Cr, Zn, Mn and Fe) were investigated in the lower
drainage basin of the Paraíba do Sul River. The results showed that the spatial variability was not important for all the
studied metals, however, temporal variations seems to be considerable. In general, two distinct behaviors were observed for
particulate heavy metals: (1) metal concentration increase together with water flow (Fe and Cu) and (2) concentration decrease
with increasing water flux (Zn, Cr and Mn). The Fe and Cu behavior is probably due to the strong association of these metals
with surface runoff, although their sources seem to be distinct. Iron probably originates from the regional soils rich in
iron oxides, and Cu is possibly associated to the large-scale use of copper fungicides in the sugar cane plantations. The
opposite trend observed for Zn, Cr and Mn probably reflects the importance of the industrial and urban effluents as a secondary
source of these elements for the system. Their behavior is probably associated with the dilution effect caused by the input
of a suspended matter poor in these metals originated from the surface runoff during the rainy season.
Received: 4 March 1998 · Accepted: 30 June 1998 相似文献
It is well known that the compressibility of crushable granular materials increases with the moisture content,due to the decrease of particle strength in a humid environment.An existing approach to take into account the effect of grain breakage in constitutive modeling consists in linking the evolution of the grain size distribution to the plastic work.But how the material humidity can affect this relationship is not clear,and experimental evidence is quite scarce.Based on compression tests on dry and saturated crushable sand recently reported by the present authors,a new non-linear relationship is proposed between the amount of particle breakage and the plastic work.The expression contains two parameters:(1)a material constant dependent on the grain characteristics and(2)a constant depending on the wetting condition(in this study,dry or saturated).A key finding is that the relationship does not depend on the stress path and,for a given wetting condition,only one set of parameters is necessary to reproduce the results of isotropic,oedometric,and triaxial compression tests.The relationship has been introduced into an elastoplastic constitutive model based on the critical state concept with a double yield surface for plastic sliding and compression.The breakage ratio is introduced into the expression of the elastic stiffness,the critical state line and the hardening compression pressure.Incremental stress-strain computations with the model allow the plastic work to be calculated and,therefore,the evolution of particle crushing can be predicted through the proposed non-linear relationship and reintroduced into the constitutive equations.Accurate predictions of the experimental results in terms of both stress-strain relationships and breakage ratio were obtained. 相似文献
Settling particles collected at 1550 m water depth off the São Francisco River, Brazil, between January and May 1995 showed peak fluxes of amino acids, hexosamines, and carbohydrates, which formed the onset of a three-week period of high organic matter (OM) flux, coinciding with the high discharge period of the river. Two phases of OM deposition exist: (1) the fluvial input of nutrients triggering a bloom of non biomineralizing plankton, and (2) suspended sediment mainly derived from shelf erosion increasing the fluxes of refractory OM. This indicates the importance of seasonally varying hydrodynamic conditions and nutrient input from the continent for the production and sedimentation of OM to the continental margin of eastern Brazil. 相似文献
This study addresses the composition of biogenic matter and the metabolic activity of coastal waters of the eastern Brazilian
shelf, bordered by small river–mangrove systems. Oceanic Brazil Current waters induce oligotrophic and near to homogeneous
conditions of chemical constituents along the inner shelf. The impact of small river–mangrove systems upon coastal waters
is minor and of local nature. Bottom topography, coral reef habitats, and local upwelling also induce minor local spatial
variability of dissolved inorganic and organic nutrients and O2 and CO2 saturation levels in the coastal waters. Metabolic activity during the daylight period, inferred from O2 and CO2 saturation levels, varied from slightly autotrophic to heterotrophic. 相似文献
Hydrology and nutrients have been indicated as the main driving factors acting on phytoplankton biomass and composition in estuarine systems, although grazing may occasionally have some influence. In order to identify these factors over temporal and spatial scales, we analyzed physical, chemical, and biological properties of a tropical river-dominated estuary during the dry and rainy seasons. As far as we know, this is the first time that the functional groups approach has been used to analyze the changes in phytoplankton composition in an estuary. This recent framework is based on the tolerances and sensitivities in relation to environmental conditions of groups of species, which are labeled by alpha-numeric codes (Reynolds et al., J. Pl. Res. 24:417–428, 2002). In the estuary of Paraíba do Sul River, all phytoplankton groups were represented by freshwater organisms, indicating the strong influence of the river. However, remarkable shifts in composition and biomass occurred from the low to high flushing seasons, due much more to the river discharge than to nutrient availability. The overall results showed no nitrogen, phosphorus, or silica limitation to phytoplankton growth (mean values: dissolved inorganic nitrogen?=?30.5 µM, soluble reactive phosphorus?=?1.45 µM, and silica?=?208.05 µM). The higher river flow supports a lower phytoplankton biomass composed mainly of nanoplankton (<20 µm) fast-growing functional groups, which are able to maintain biomass even in high flushing conditions (X1), or large heavy organisms, such as some heavy diatoms of group P, which are able to be in suspension in shallow and turbulent systems. The lower river flow led to the coexistence of large organisms (>20 µm) of the groups P and F, which include slow-growing populations typically found in mesotrophic lakes. Although the functional group approach was originally developed for temperate lakes, our data support this approach for a tropical estuarine environment. 相似文献