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1.
This paper investigates the effect of model scale and particle size distribution on the simulated macroscopic mechanical properties, unconfined compressive strength (UCS), Young’s modulus and Poisson’s ratio, using the three-dimensional particle flow code (PFC3D). Four different maximum to minimum particle size (d max/d min) ratios, all having a continuous uniform size distribution, were considered and seven model (specimen) diameter to median particle size ratios (L/d) were studied for each d max/d min ratio. The results indicate that the coefficients of variation (COVs) of the simulated macroscopic mechanical properties using PFC3D decrease significantly as L/d increases. The results also indicate that the simulated mechanical properties using PFC3D show much lower COVs than those in PFC2D at all model scales. The average simulated UCS and Young’s modulus using the default PFC3D procedure keep increasing with larger L/d, although the rate of increase decreases with larger L/d. This is mainly caused by the decrease of model porosity with larger L/d associated with the default PFC3D method and the better balanced contact force chains at larger L/d. After the effect of model porosity is eliminated, the results on the net model scale effect indicate that the average simulated UCS still increases with larger L/d but the rate is much smaller, the average simulated Young’s modulus decreases with larger L/d instead, and the average simulated Poisson’s ratio versus L/d relationship remains about the same. Particle size distribution also affects the simulated macroscopic mechanical properties, larger d max/d min leading to greater average simulated UCS and Young’s modulus and smaller average simulated Poisson’s ratio, and the changing rates become smaller at larger d max/d min. This study shows that it is important to properly consider the effect of model scale and particle size distribution in PFC3D simulations.  相似文献   

2.
Field observations and experimental measurements of the partitioning coefficient (Kd) of7Be in fresh water show that it varies inversely with the solids concentration at typical environmental values (up to 30 mg/1). This behavior is similar to that of many other metals and organic pollutants, which means that 7Be may be useful as a tracer of the movement of these substances in the water column. However, the wide range in the percentage of 7Be adsorbed by solids over this range of concentrations (over 50%) means that in order to use 7Be either to measure total sedimentation rates or to trace lateral sediment movement it will be necessary to monitor changes in sediment concentration over the area and time period of interest. The wide scatter in our data at both high and low solids concentrations suggests that other factors also affect Kd. Until these factors are identified, application of our results to other systems will be risky. At high (greater than 100 mg/1) solids concentrations over 90% of 7Be is associated with the solid phase, so it may be a useful tracer of reworking rates in bottom sediments. 7Be has a slightly greater affinity for the solid phase in fresh water than in seawater.  相似文献   

3.
A set of outcrop samples from the Lokoja and Patti Formations of the southern Bida Basin (Nigeria) was examined for grain size distribution, sedimentary and hydraulic characteristics. Most of the samples are well-sorted with an uniformity coefficient (U) ranging between 1.3 and 6.3. The mean effective grain diameter (d10) is in the order of 0.11 mm, the mean value of d90 was determined as 0.66 mm (both geometric mean) and the median grain size (d50) as 0.23 mm. Based on these values, the sedimentary sequence can be described as ‘fine to medium sized sand’, having minor amounts of either silt, or coarse sand and some gravel. The total porosity of the samples was determined by laboratory methods to be in the range of 9–29%. The hydraulic conductivity (K) was determined according to Hazen.and Beyer, and by Shepherd's formula, the last resulting in a geometric mean of 3.3 m d−1 or 3.3 darcy. The results are used to estimate local groundwater potential. The entire pore space (potential groundwater reservoir) for the area under study is estimated to be in the order of 290–430 km3. Because of higher hydraulic conductivities, it is recommended that the Lokoja Formation is concentrated on as a target for groundwater exploration.  相似文献   

4.
《Applied Geochemistry》1998,13(1):43-47
Although 14C occurs naturally, it is also a waste product of the nuclear industry, and can be important because of its long half-life, high mobility as an anion, and ready incorporation into biota. Some aqueous inorganic species are anionic with migration minimally retarded by most geological and soil materials. Substantial retardation is expected when calcite is present, but there are few data to quantify this effect. The present study measured partition coefficient values, Rd (concentration on solids divided by concentration in liquids), of 8–85 l kg−1 for a series of calcite materials and for a carbonated soil. In contrast, Rd was zero for montmorillonite. The series of calcite materials varied in particle size. In order to investigate the effects of particle size, dissolution and degassing of 14C and 12C were monitored as pH was slowly decreased. The change in pH with addition of acid was strongly affected by particle size, as expected, but there was no systematic effect of particle size on the relative dissolution rates of 14C vs 12C, or on Rd. Apparently, surface area was not a limiting factor in the interaction of 14C with these materials. The 14C in soil behaved most like the very fine calcite, indicating that the specific surface of the soil carbonate was similar to that of the very fine calcite.  相似文献   

5.
 Log-normal size distributions are of the form n=noe−L/α, where n=number density, L=crystal length, and α is a constant. A method for measuring three-dimensional log-normal crystal or grain size distributions (CSDs) in thin section has been deduced from computer experiments, in which 2D sections were cut through assemblages of 3D solids. The size ranges and distributions studied were appropriate for igneous microphenocryst to megacryst populations. Conversion from 2D to 3D is based on an exact correction for spheres of uniform diameter. Cumulate numbers of polygons with length≥L (N2D) are converted to N3D by the equation: ln(N3D)=ln(N2D/[L·S])+ln(γ)−β/[L·S] The number density is then obtained as n=−dN/dL. The parameters S and γ correct the measured lengths and no (no=number density at L=0) respectively, and are functions of crystal shape. The parameter β is a weak function of the degree of spatial orientation of the crystals. Highly symmetrical shapes such as cubes, octahedra, and elongated prisms can be accurately measured when randomly oriented; however, rectangular solids with a≠b≠c cannot be accurately measured because they produce bimodal length distributions in cross section. Strongly oriented textures (trachytic or lineated) can be accurately measured regardless of crystal shape. New CSD data from alkaline rocks and a kimberlite give examples of CSDs modified by megacryst retention, xenocryst addition, phenocryst accumulation, and groundmass nucleation. Received: 25 November 1994 / Accepted: 14 November 1995  相似文献   

6.
The October 8, 2005, Kashmir earthquake (M w 7.6) affected the rough mountainous regions of India and Pakistan with poor accessibility, and thus, no proper comprehensive ground survey was possible. However, due to the ability of remote sensing technology to acquire spectral measurements of damaged areas at various spatial and temporal scales, extraction of damaged areas can be carried out quickly and with great reliability. The fuzzy-based classifiers [Possibilistic c-Means (PCM), noise cluster (NC), and NC with entropy (NCE)] were applied to identify 2005 Kashmir earthquake, induced landslides, as well as built-up damage (BD) areas, as soft computing approaches using supervised classification. Results indicate that for the identification of landslides and BD areas, NCE classifier generated the best outputs, while for the identification of built-up undamaged areas, NC classifier generated the best output. Further, it was found that the proposed Class Based Sensor Independent (CBSI) technique can improve spectral information of specific class for better identification.  相似文献   

7.
The use of inferential particle sizing for wet grinding circuit control is well established in the literature. In this paper an analytical development is presented which yields a general inferential sizing model relating circuit product size to d50 corrected for the circuits' classifier. It is then shown how the different forms of empirical equations reported in the literature can arise, depending on the range of the operating variables and the ratio of feed 50% passing size to d50 corrected. Following this development, data show pilot plant and industrial plant operations to behave as predicted by the general inferential size model.  相似文献   

8.
The thermoelastic analysis of the possible stable stress-free equilibrium states of growth twins is shown to be the basis for a new technique of geobarothermometry. The detailed study of a cyclic cross-shaped twin in low quartz exemplifies the procedure. To explicitly determine the barothermometric function, the simultaneous dependence upon the external pressure and temperature of the lattice-parameter ratios c/c 0 and a/a 0 of quartz has been calculated; we present a five-parameter, temperature-corrected version of the equations proposed by Thurston (1967) for extrapolating to high pressures the behavior of lattice parameters of crystalline solids in isothermal compression.  相似文献   

9.
In dressing plants, the rapid determination of the corrected cut point in hydrocyclone classification is desirable, useful, and important. In this work, classification tests have been carried out on white sand using a 10-cm diameter hydrocyclone to examine closely the relation between the recovery to underflow and the corrected separation size, d50. The study was carried out at wide ranges of: apex diameter, vortex-finder diameter, solid content of feed, and length of vortex finder.A simple, yet reliable, equation relating the solid recovery of underflow to d50, is developed using all the obtained results. The predicted data show good agreement with the experimental.  相似文献   

10.
Shallow slope failure in mountainous regions is a common and emergent hazard in terms of its damage to important traffic routes and local communities. The impact of dry granular flows consisting of rock fragments and other particles resulting from shallow slope failures on retaining structures has yet to be systematically researched and is not covered by current design codes. As a preliminary study of the impact caused by dry granular flows, a series of dry granular impact experiments were carried out for one model of a retaining wall. It was indirectly verified that the total normal force exerted on a retaining wall consists of a drag force (F d), a gravitational and frictional force (F gf), and a passive earth force (F p), and that the calculation of F d can be based on the empirical formula defined in NF EN Eurocode 1990 (Eurocode structuraux. Base de calcul des structures, AFNOR La plaine Saint Denis, 2003). It was also indirectly verified that, for flow with Froude number from 6 to 11, the drag coefficient (C d) can be estimated using the previously proposed empirical parameters.  相似文献   

11.
Volcanic breccia is a representative heterogeneous rock. As a preliminary study for determining its mechanical properties in laboratory and field experiments, its material properties and structural features were described. Three-dimensional clast-size distribution was also estimated for the study site using the theory of stereology. With this distribution determined, a numerical simulation was performed to investigate the relationship between the volumetric proportion of clasts in a sample and the areal proportion and maximum length of clasts on a lateral surface of the sample. The following observations were made: (1) coefficients of variation for volumetric proportion are dependent on the size of the sample, and became large when the size of the sample is small; (2) samples with a small coefficient of variation can be selected at any size if a restriction is placed on the ratio, Dmax/D0, of the maximum length of clasts to the size of the sample; and (3) the volumetric proportion of clasts in each sample can be estimated from the observed areal proportions within an error range that depends on the whole volumetric proportion.  相似文献   

12.
Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the 234U/238U activity ratios of pore water, bulk sediment, and leachates. Site 984A is one of only several sites where closely spaced pore water samples were obtained from the upper 60 meters of the core; the sedimentation rate is high (11-15 cm/ka), hence the sediments in the upper 60 meters are less than 500 ka old. The sediment is clayey silt and composed mostly of detritus derived from Iceland with a significant component of biogenic carbonate (up to 30%).The pore water 234U/238U activity ratios are higher than seawater values, in the range of 1.2 to 1.6, while the bulk sediment 234U/238U activity ratios are close to 1.0. The 234U/238U of the pore water reflects a balance between the mineral dissolution rate and the supply rate of excess 234U to the pore fluid by α-recoil injection of 234Th. The fraction of 238U decays that result in α-recoil injection of 234U to pore fluid is estimated to be 0.10 to 0.20 based on the 234U/238U of insoluble residue fractions. The calculated bulk dissolution rates, in units of g/g/yr are in the range of 4 × 10−7 to 2 × 10−6 yr−1. There is significant down-hole variability in pore water 234U/238U activity ratios (and hence dissolution rates) on a scale of ca. 10 m. The inferred bulk dissolution rate constants are 100 to 104 times slower than laboratory-determined rates, 100 times faster than rates inferred for older sediments based on Sr isotopes, and similar to weathering rates determined for terrestrial soils of similar age. The results of this study suggest that U isotopes can be used to measure in situ dissolution rates in fine-grained clastic materials.The rate estimates for sediments from ODP Site 984 confirm the strong dependence of reactivity on the age of the solid material: the bulk dissolution rate (Rd) of soils and deep-sea sediments can be approximately described by the expression Rd ≈ 0.1 Age−1 for ages spanning 1000 to 5 × 108 yr. The age of the material, which encompasses the grain size, surface area, and other chemical factors that contribute to the rate of dissolution, appears to be a much stronger determinant of dissolution rate than any single physical or chemical property of the system.  相似文献   

13.
Sub-angular-shaped aggregates are used as rail foundation ballasts and must remain sub-angular during their service life time to maintain particle–particle interlocking, in order to ensure the stability of the rail line and prevent accidents by derailment. Here, the screening of dolerite quarry aggregates for use as railway foundation ballasts was investigated by employing simple digital image and chart methods. The average particle size (d 50), flakiness index (FI), Los Angeles abrasion index (LAAI), sphericity (SPH) and roundness (RND) were determined for two batches of dolerite ballasts from the Rooikraal quarry in Johannesburg and Ngagane quarry in Newcastle. Thirty samples from each of the two batches of ballast were analysed. The ballasts were progressively abraded using a Los Angeles abrasion device and were analysed after each cycle of abrasion. A decrease in d 50 and an increase in FI with increased number of abrasion cycles were observed for both batches of dolerite ballast. The difference in the chart and digital image values of RND and SPH were marginal before abrasion; however, these differences increased with each abrasion cycle. The LAAI, d 50, mean RND and mean SPH correlated significantly and were found to have high regression coefficients. Thus, statistical models are proposed for the non-destructive routine screening of in-place ballasts in order to track marginal changes in aggregate shapes, facilitate ballast replacement programmes and avoid rail line instability.  相似文献   

14.
《Applied Geochemistry》2000,15(7):1035-1042
Bacteriogenic Fe oxides (BIOS) and groundwater samples were collected 195 m underground at the Stråssa Mine in central Sweden. Ferrous iron oxidizing bacteria, including stalked Gallionella ferruginea and filamenous Leptothrix sp., were prominent in the BIOS samples. The BIOS samples were found to contain only poorly ordered (amorphous) hydrous ferric oxide, as determined by X-ray diffraction. Inductively coupled plasma mass spectroscopy revealed hydroxylamine-reducible Fe and Mn oxide contents that ranged from 55 to 85% on a dry weight basis. Concentrations of Sr, Cs, Pb and U in filtered groundwater ranged from 0.002 to 1.8 μM. Solid phase concentrations of these heavy metals in the BIOS spanned the 0.04–2.23 mmol/kg range. Distribution coefficients (Kd values), calculated as the ratio between BIOS and dissolved heavy metal concentrations, revealed solid phase enrichments that, depending on the heavy metal and Fe oxide content of the sample, extended from 103.0 to 104.7. At the same time, however, a strong inverse linear relationship was found between log Kd values and the corresponding mass fraction of reducible oxide in the samples, implying that metal uptake was strongly influenced by the relative proportion of bacterial organic matter in the composite solids. Based on the metal accumulation properties of the BIOS, an important role can be inferred for intermixed Fe oxides and bacterial organic matter in the transport and fate of dissolved metals in groundwater systems.  相似文献   

15.
Sediments contained in the river bed do not necessarily contribute to morphological change. The finest part of the sediment mixture often fills the pores between the larger grains and can be removed without causing a drop in bed level. The discrimination between pore‐filling load and bed‐structure load, therefore, is of practical importance for morphological predictions. In this study, a new method is proposed to estimate the cut‐off grain size that forms the boundary between pore‐filling load and bed‐structure load. The method evaluates the pore structure of the river bed geometrically. Only detailed grain‐size distributions of the river bed are required as input to the method. A preliminary validation shows that the calculated porosity and cut‐off size values agree well with experimental data. Application of the new cut‐off size method to the river Rhine demonstrates that the estimated cut‐off size decreases in a downstream direction from about 2 to 0·05 mm, covariant with the downstream fining of bed sediments. Grain size fractions that are pore‐filling load in the upstream part of the river thus gradually become bed‐structure load in the downstream part. The estimated (mass) percentage of pore‐filling load in the river bed ranges from 0% in areas with a unimodal river bed, to about 22% in reaches with a bimodal sand‐gravel bed. The estimated bed porosity varies between 0·15 and 0·35, which is considerably less than the often‐used standard value of 0·40. The predicted cut‐off size between pore‐filling load and bed‐structure load (Dc,p) is fundamentally different from the cut‐off size between wash‐load and bed‐material load (Dc,w), irrespective of the method used to determine Dc,p or Dc,w. Dc,w values are in the order of 10?1 mm and mainly dependent on the flow characteristics, whereas Dc,p values are generally much larger (about 100 mm in gravel‐bed rivers) and dependent on the bed composition. Knowledge of Dc,w is important for the prediction of the total sediment transport in a river (including suspended fines that do not interact with the bed), whereas knowledge of Dc,p helps to improve morphological predictions, especially if spatial variations in Dc,p are taken into account. An alternative to using a spatially variable value of Dc,p in morphological models is to use a spatially variable bed porosity, which can also be predicted with the new method. In addition to the morphological benefits, the new method also has sedimentological applications. The possibility to determine quickly whether a sediment mixture is clast‐supported or matrix‐supported may help to better understand downstream fining trends, sediment entrainment thresholds and variations in hydraulic conductivity.  相似文献   

16.
The rays of enhanced brightness making up the structure of the coronal-streamer belt can be traced to the lowest atmospheric layers in the Sun, with the angular size remaining nearly constant, d ≈ 2.5° ± 0.5°. This suggests that the physical mechanism generating the slow solar wind in the rays of the streamer belt differs from the mechanism giving rise to the fast solar wind from coronal holes. At distances of R < (4–5) R ⊙, the rays of the streamer belt are not radial in the plane of the sky and show deviations toward the corresponding pole. They then become essentially radial at R > (4–5) R ⊙. A transverse cross section of streamers in the corona and its continuation into the heliosphere—a plasma sheet—can be represented as two radially oriented, closely spaced rays (d ≈ 2.0°–2.5°) with enhanced density and an angular size of d. We also show that the ray structure of the streamer belt is involved in the development of coronal mass ejections (CMEs). The motion of a small-scale CME occurs within a magnetic flux tube (ray of enhanced brightness) and leads to an explosive increase in its angular size (rapid expansion of the tube). It seems likely that large-scale CMEs are the result of the simultaneous expansion of several magnetic tubes. We suggest that a small-scale CME corresponds to a “plasmoid” (clump of plasma of limited size with its own magnetic field) ejected into the base of a magnetic tube, which subsequently moves away from the Sun along the tube.  相似文献   

17.
Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dabie Mountain, central China, were measured by inductively coupled plasma-mass spectrometry (ICP-MS). The correlation of δEu ratios between garnet and omphacite indicated that chemical equilibrium of REE distribution between garnet and omphacite could be achieved during ultra-high pressure (UHP) metamorphism. Most of the partition coefficients (Kd=CiOmp/CiGrt) of light rare-earth elements (LREE) are higher than 1. However the partition coefficients of heavy rare-earth elements (HREE) are lower than 1. This indicated that the LREE inclined to occupy site M2 in omphacite, but the HREEs tended to occupy eightfold coordinated site in garnet during the eclogite formation. The REE geochemistry of the eclogites indicated that LREE could be partially lost during the prograde metamorphic process of protolith, but be introduced into the rocks during the symplectite formation. LREE are more active than HREE during the UHP metamorphism. The results are favorable to highlighting the REE behavior and evolution of UHP metamorphic rocks.  相似文献   

18.
Simple models for trace element fractionation during concurrent melting and melt migration in an upwelling steady-state mantle were developed. Based on petrologic considerations, we divided the mantle column into two regions: a single-lithology lower region that consists of partially molten garnet and spinel lherzolites and a double-lithology upper region where high-porosity dunite channels or melt-filled fractures are embedded in a porous lherzolite/harzburgite matrix. Analytical solutions for the case of a constant and uniform relative melting suction rate and a linearly variable relative melt suction rate were obtained. Key parameters and the first order characteristics of melting and melt migration in a 1-D steady-state mantle column were examined through forward calculations and Monte Carlo simulations. Melting in the upwelling single-lithology column is equivalent to non-modal batch melting, whereas melting and melt migration in the double-lithology region can be viewed as a nonlinear combination of batch melting and fractional melting, depending on the amount of melt extracted to the channel. The degree of melting (F), the degree of melting at the depth of melt-channel initiation (Fd) and the relative rate of melt suction (R) are important in controlling the extent of depletion of the incompatible trace element in the matrix. Spatially variable R affects the abundance of an incompatible trace element in the melt and residual solid the most in near fractional melting. There is a strong nonlinear trade off among the three parameters. Given Fd, it is possible to constrain F and R from incompatible trace element abundances in residual peridotite.To explore the dynamics of melt migration in the mantle, we used the two melting models developed in this study and published REE and Y abundances in diopside in abyssal peridotites from the Central Indian Ridge to infer their melting and melt migration history. Overall, the degrees of melting inferred from the trace element data are not sensitive to the value of Fd used in the inversion and ranges from 10% to 15%. The relative rate of melt suction depends slightly on the choice of Fd and ranges from 0.85 to 1.0 for Fd = 0.05 and 0.75 to 0.97 for Fd = 0. Further, the estimated R is inversely correlated with F, a robust feature independent of the choice of Fd. The upward decrease of R in an upwelling mantle column can be understood in terms of melt focusing in the lower part of the double-lithology region. And finally, given F and R, we found that the permeability and porosity of the lherzolite/harzburgite matrix also increase as a function of F in the melting column, with melt fractions ranging from 0.2% to 0.7% for a grain size of 5 mm.  相似文献   

19.
The PG 1316+678 star is classified as a pre-cataclysmic binary, as is evidenced by its photometric and spectroscopic observations. Its orbital period is determined to be P orb = 3.3803d, which coincides with the photometric period. The intensities of the emission HI and HeI lines are shown to vary synchronously with the brightness of the object (Δm V = 0.065 m , Δm R = 0.08 m ). These variations arise as the UV radiation from the DAO white dwarf is reflected from the surface of the cold companion. The parameters of the binary are estimated and the time of its evolution after the common-envelope phase is determined to be t ≈ 240 000 years. Thus, PG 1316+678 is a young pre-cataclysmic NN Ser variable with the smallest known photometric reflection effect.  相似文献   

20.
We used an in situ measurement method to investigate the phase transition of Fe2SiO4 polymorphs under high pressures and temperatures. A multi-anvil high-pressure apparatus combined with synchrotron X-ray radiation was used. The stability of each polymorph was identified by observing the X-ray diffraction data from the sample. In most experiments, the diffraction patterns were collected 10–30 min after reaching the desired pressure and temperature conditions. The transition boundary between the olivine and spinel phase at T = 1,000–1,500 K and P = 2–8 GPa was determined to occur at P (GPa) = 0.5 + 0.0034 × T (K). The transition pressure determined in this study was in general agreement with that observed in previous high-pressure experiments. However, the slope of the transition, dP/dT, determined in our study was significantly higher than that estimated by the previous study combined with the in situ X-ray method.  相似文献   

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