Clayey Silt Research Articles

Swelling potential of volcanic residual soils in Sumatra (Indonesia) in relation to environmental issues

Abstract Swelling potential characterization of clay-silt soil is an essential issue in stabilization, settlement, consolidation, and land suitability studies. This article attempts to explain the swelling characteristics of soils around the area of West Lampung, Lampung Province, Sumatra, Indonesia, in relation to environmental issues. An investigation in relation to the soil swelling potential was carried out using 15 disturbed soil samples collectd in the study area. The methods used were analyses of clay mineral geochemistry, physical characteristics, and the free swell ratio. These results showed that the soil in the study area was Quaternary tropical volcanic residual soil. These soils were formed in a proximal volcanic hydrothermal alteration environment. The soils of the study area have characteristics of high plasticity, a reddish-brown colour, and are clayey silt grained (MH) (USCS). The soils had loose physical characteristics in dry conditions; however, these soils tends to be plastic and sticky in wet conditions. Evidence of groove erosion was found at the soil surface. Based on XRD analysis, kaolinite, halloysite, and montmorillonite were types of clay minerals found in the soil. The soil had a clay content of 11.05–78.9%, a liquid limit value > 50%, a plasticity index value of 16.7–36.9%, a shrinkage value of 14.24–36.89%, a soil activity of 0.38–2.47; and an FSR value of 0.69–0.95. These characteristics have implications for swelling soil potential. The results showed that the soils in the study area had medium to very high swelling potential. These results suggest a risk of erosion in the area, which could cause soil degradation and a change in water quality. These soils are likely to affect land productivity and aquifer replenishment and will cause negative environmental and economic impacts. Thus, soil improvement techniques are needed. It is important to maintaining vegetative cover these soils and revegetation may be required.

WITHDRAWN: Effect of coal ash on strength characteristics of clayey silt soil treated with cement

Coal-based thermal power plants produce coal ash as a by-product and the major concern is for handling and disposal of coal ash without adversely affecting the environment. There is a feasibility to utilize the coal ash in an environmentally safe manner in geotechnical engineering applications. To achieve this objective, an experimental investigation was conducted in which coal ash was used as replacement material in soil mixtures. A clayey silt soil was used in this study and class F type coal ash (CA) was blended in different proportions up to 50% by mass of the soil. The testing program comprised characterization of soil and coal ash, compaction tests and California bearing ratio (CBR) tests. The stress–strain and strength characteristics of compacted soil with coal ash and cement were analyzed. The test results showed that all the mixes achieved a higher CBR value than the control specimen. The optimum coal ash contents for clayey silt–coal ash mixes were found as 20% and 50% which showed higher CBR value, it could be due to the packing effect and imparting cohesion, respectively. The minimum cement content of 1% to the optimum mixes provided a significant improvement in CBR values of 25.19% and 29.14% as compared to the control specimen of 4.72%. These results are acceptable values and hence, the coal ash blended soil is found to be suitable for pavement subgrade and embankments constructions.

MINERALOGY AND PALEONTOLOGY OF THE QUATERNARY SEDIMENTS IN KARMAT ALI AT BASRAH, SOUTHERN IRAQ

The study was carried out on one borehole in Basrah city to describe their texture, mineralogy and fauna contents. The boundary between fluvial recent sediments and the Hammar Formation is hard to determine because both sediments are terrigenous; the present study tries to detect the transitional phase between fluvial and marine environments. The vertical variations in mineralogy and texture of sediments provide an indicator of environmental conversion in the Quaternary period. Quartz is dominated at shallow depth intervals. Thereafter, its abruptly decreases in their abundance after 20 m depth; while the opposite was the case for calcite content, where it increases with the decrease of quartz. halite and gypsum are absent at 3 to 13 m, but they appear at 17 m. Clayey silt exists at 1 to 21 m, and silty sand at 23 to 35 m. Paleontology is enhanced the sedimentological evidence and marked that the Hammar marine transgression started at depths 35 and 30 m with typical sediments at 30 to 27 m; marine regression is restricted at 27-17 m, whereas the fluvial Holocene period is marked at 17 m depth.

Thermal and hydrogeological aquifers characterization by coupling depth-resolved thermal response test with moving line source analysis

In this paper for the first time the Moving Line Source (MLS) model is combined with a depth-resolved Thermal Response Test (TRT). The latter was performed in a heterogeneous and groundwater rich subsoil, composed by a layering of silty sand, medium-fine sand and coarse-medium sand, with a layer of clayey silt separating a shallow aquifer from a deep one.The temperature evolution in the ground along the vertical axis was analysed with both the standard Infinite Line Source (ILS) and the MLS. The two models lead to similar estimates of the thermal conductivity in those regions of the subsoil where conduction prevails, while the MLS performs better where a significant groundwater velocity is expected. In these layers, the MLS analysis allows to derive both the thermal conductivity and the Darcy velocity. The MLS results were validated by comparison with a numerical simulation on a multiple-layers ground model developed in MODFLOW/MT3DMS using a constant energy boundary condition. The combined depth-resolved TRT/MLS approach represents an important method for an accurate design of the Ground Heat Exchangers under the presence of groundwater flow.

GEOENGINEERING INVESTIGATION OF AN EROSION INDUCED HIGHWAY STRUCTURAL FAILURE ALONG IFON – BENIN HIGHWAY, SOUTHWESTERN NIGERIA

Road infrastructural is one of the most important economic indices for development of a country. Therefore in-situ cone penetration test and laboratory soil analysis were performed at two failed segments along Ifon-Benin highway, with the aim of determining cause(s) and extent of the failure. The cone penetration test was carried out to a depth of about 20 m with a lateral spacing of 20 m along the roadway. The laboratory tests conducted were grain size analysis, Atterberg limit test, compaction test, California Bearing Ratio, and undrained unconfined triaxial test. The results revealed that all the soil parameters fell short of the federal ministry of works and housing specification of Nigeria, with plasticity index (>20%), % fines (>35%), CBR values (<80%) recommended, angle of friction and cohesion are less than minimum standard of 30° and 50 Kpa respectively. The CPT revealed predominant sandy silt to clayey silt topsoil and clay substratum with compressive strength of 20 – 40 KN/m2. The water level is higher than 3 m, consequently far below the road foundation baseline. Findings showed that the upper 6 m of the failed segments has been seriously affected by erosion and flooding. Subsequently the process resulted into excessive settlement of the silt/clayey-subgrade soil underneath the pavement structure. This makes the highway to settle largely under traffic load. In addition, incessant heavy flooding around the embankment/shoulder of the highway might have induced the failure, leading to looseness, and less-cohesion of the layers which invariably reduces subgrade support and weakens various pavement layers.

Lake Heimtjønna at Dovre, Mid-Norway, reveals remarkable late-glacial and Holocene sedimentary environments and the early establishment of spruce (Picea abies), alder (Alnus cf. incana), and alpine plants with present centric distributions

Two and a half meters of sediments from Lake Heimtjønna in the Scandes Mountains reveal changes in the vegetation, climate, and sediment environment since deglaciation. The lake was deglaciated in the Late-Glacial (LG), perhaps as early as 16-18 ka cal BP. After deglaciation, the sediment environment at the Heimtjønna coring point was extraordinary and challenging to interpret. The genesis of the 1.7 m basal unsorted sediments including large stones is discussed concluding that the layer was rafted by lake-ice after the LG deglaciation. The Younger Dryas (YD) established a semi-perennial lake-ice that stopped the deposition of the ice-rafted stone-rich sediments. In the early Holocene, the unstratified and well-sorted clayey silt shows fluvial origin. In late Holocene, strong flood activity including the major flood disaster ‘Stor-Ofsen’ in AD 1789, caused a sediment hiatus at the coring point from ca 9.5 ka cal BP to ca 250 a cal BP. The LG interstadial warming initiated the succession from pioneer plants on mineral soils towards local dwarf-shrub heath. July temperatures reached at least 7–8 OC. In the LG and early Holocene, Papaver radicatum, Artemisia norvegica, and Campanula cf. uniflora today occurring at Dovre and with centric Scandinavian distributions, locally established. All have previously been connected to Weichselian survival on nunatak refugia. Continuous pollen curves of spruce (Picea abies) support the much-debated LG and early Holocene presence of spruce in the Scandes mountains. For the first time, it is shown that both grey alder (Alnus incana) and Armeria occurred in the Scandes in the LG and early Holocene. Due to dating errors, LG ages is exclusively based on stratigraphical correlations and considerations. The LG chronology must be thoroughly tested by future studies providing reliable AMS 14C-dates of terrestrial fossils.

Petrophysical properties of the Green Canyon Block 955 hydrate reservoir inferred from reconstituted sediments: Implications for hydrate formation and production

We explore the petrophysical behavior of the two interbedded lithofacies (sandy silt and clayey silt) that constitute the Green Canyon Block 955 hydrate reservoir in the deep-water Gulf of Mexico by performing experiments on reconstituted samples of the reservoir material. Sandy silts reconstituted to the in situ porosity have a permeability of 11.8 md (1.18 × 10−14 m2), which is similar to the intrinsic permeabilities measured in intact cores from hydrate reservoirs of similar grain size offshore Japan (Nankai Trough) and offshore India. Reconstituted clayey silts have a much lower intrinsic permeability of 3.84 × 10−4 md (3.84 × 10−19 m2) at the in situ stress. The reconstituted sandy silt is less compressible than the clayey silt. Mercury injection capillary pressure measurements demonstrate that the largest pores with the clayey silt are still smaller than the pores remaining after 90% hydrate saturation in sandy silt. We interpret that the methane solubility in pores of clayey silt is always less than that necessary to form hydrate, which explains why no hydrate is present in the clayey silt. We upscale the reservoir properties to estimate the behavior of interbedded sandy silt and clayey silt. We find the upscaled intrinsic horizontal and vertical permeabilities for the entire reservoir interval are 8.6 md (8.6 × 10−15 m2) and 1.4 × 10−3 md (1.4 × 10−18 m2). We estimate that during reservoir production, a maximum vertical strain of approximately 12% will result. Ultimately, this study will inform reservoir simulation models with petrophysical properties at scales of both individual lithofacies and reservoir formation.

Sedimentology and stratigraphy of a deep-water gas hydrate reservoir in the northern Gulf of Mexico

We interpret the sedimentologic evolution of a deep-water channel-levee deposit in Green Canyon Block 955 (deep-water Gulf of Mexico) by analyzing hydrate-bearing pressure cores and nonpressure cores collected during The University of Texas-Gulf of Mexico 2-1 (UT-GOM2-1) Hydrate Pressure Coring Expedition that preserve remarkable sedimentary structures. The levee is composed of alternating beds of sandy silt and clayey silt that range from millimeters to meters in thickness. We interpret that each couplet of sandy silt and clayey silt records a single turbidity current flow in which the upper part of the flow overtops the levee and is deposited along its flank. The sandy silt is coarser, its beds are thicker, and the fraction of sandy silt to clayey silt (net-to-gross) is greater near the base of the levee. We interpret that as the levee grew, the channel depth increased and a smaller fraction of the flow overtopped the levee. An increase in net-to-gross, both at the base and near the top of the cored section, may record an increase in the size of turbidity current flows or a decrease in the relative height of the levee. Based on the limited core recovery, we infer that the lithology of the bounding unit immediately above the hydrate-bearing unit is thinner bedded and has lower net-to-gross than the hydrate reservoir. The bounding unit below the hydrate-bearing interval is similarly thinner bedded, yet contains high saturations of hydrate. This study illuminates the lithologic architecture of leveed-channel turbidite reservoirs at core scale and provides insight into how lithology controls hydrate distribution and concentration.

A discrete element simulation considering liquid bridge force to investigate the mechanical behaviors of methane hydrate-bearing clayey silt sediments

For the safe exploration of natural gas hydrate, an understanding of the mechanical behaviors of methane hydrate-bearing sediments is essential. To date, the majority of the related research has focused on the mechanical properties of sandy sediments; research on fine sediments containing clay or silt is very limited, although more than 90% of gas hydrates are found in clay or silt. In this study, a novel discrete element method (DEM) model considering liquid bridge forces caused by the capillary water effect among particles is proposed and is used to simulate methane hydrate-bearing clayey silt sediments with pore-filling distribution. A series of DEM simulations of drained biaxial compression tests under different confining pressures is performed to investigate the mechanical behavior (i.e., the stress-strain relationship, the volumetric response, and strength properties such as friction angle and cohesion) of samples with different hydrate saturations. The numerical results reveal that methane hydrate-bearing clayey silt sediments are characterized by strain-hardening behavior with shear contraction. In addition, the liquid bridge forceincreases with methane hydrate saturation, which considerably enhances cohesion while making no significant contribution to the internal friction angles. The presence of methane hydrates in the pores can increase the failure strength of the sediments, while the initial stiffness is slightly affected by methane hydrate saturation when the latter ranges from 10% to 30%.

Composition and Technological Properties of Clays for Structural Ceramics in Limpopo (South Africa)

This study evaluated the potential of raw clays from the Mukondeni region for structural ceramics and pottery based on traditional firing techniques. Physical properties were identified by particle size distribution, consistency limits, and clay activity. Mineralogical and chemical properties were investigated by X-ray diffraction (XRD) and X-ray fluorescence (XRF). Extruded clay bodies were fired at 900 °C. Technological characteristics were measured by weight loss (WL), bulk density (BD), dry linear shrinkage (DLS), fired linear shrinkage (FLS), water absorption (WA), and flexural strength (FS). The clays were low in <2 µm fractions (≤19%) and of medium to high plasticity with a clayey silt texture. Smectite was the dominant clay mineral while quartz and feldspar were major non clay minerals. The most abundant oxides were SiO2 (63.57–68.73%), Al2O3 (13.9–15.61%), and Fe2O3 (4.86–6.18%), whereas K2O, CaO, MgO, Na2O, TiO2, and P2O5 were depleted. Characterization based on the clay workability chart, Winkler’s diagram, and compositional ternary diagrams revealed acceptable extrusion properties and suitability for structural ceramics and earthenware. The clays showed acceptable WL, BD, LS, and WA, but unsatisfactory FS (≤1.08 MPa). Low mechanical strength was attributed to presence of smectites and inert nature of feldspar at 900 °C. Beneficiation through mixing with carbonate-rich raw materials is recommended.

Effects of Irreducible Fluid Saturation and Gas Entry Pressure on Gas Production from Hydrate-Bearing Clayey Silt Sediments by Depressurization

Gas hydrates in the Shenhu area are mainly hosted in clayey silt sediments, which have the relatively high irreducible fluid saturation and gas entry pressure. And then, they will have an impact on gas production from hydrate-bearing clayey silt sediments, which was evaluated by the numerical simulations of SH2 site in Shenhu area in this paper. The results showed that, with the increase in irreducible water saturation and irreducible gas saturation, the amount of water production and gas production was obviously reduced. When the irreducible water saturation increased from 0.10 to 0.50, the cumulative CH4 production volume decreased from 1668799 m3 to 1536262 m3, and the cumulative water production volume dropped from 620304 m3 to 564797 m3, respectively. When the irreducible gas saturation increased from 0.01 to 0.05, the cumulative CH4 production volume dropped from 1812522 m3 to 1622121 m3, and the cumulative water production volume dropped from 672088 m3 to 600617 m3, respectively. In addition, the capillary pressure increased obviously with the increase in gas entry pressure, but the effect on gas production was small and the effect on water production could be negligible. In conclusion, irreducible water and gas saturation had an important effect on the gas production from gas hydrate, whereas the effects of gas entry pressure could be ignored.

Pore Structure Fractal Characterization and Permeability Simulation of Natural Gas Hydrate Reservoir Based on CT Images

The gas-water two-phase seepage process is complex during the depressurization process of natural gas hydrate in a clayey silt reservoir in the South China Sea, the transport mechanism of which has not been clarified as it is affected by the pore structure. In this study, we select six clayey silt samples formed after the dissociation of natural gas hydrate in the South China Sea, employing CT scanning technology to observe the pore structure of clayey silt porous media directly. The original CT scanning images are further processed to get the binarized images of the samples, which can be used for simulation of the porosity and absolute permeability. Based on the fractal geometry theory, pore structures of the samples are quantitatively characterized from the aspect of pore distribution, heterogeneity, and anisotropy (represented by three main fractal geometric parameters: fractal dimension, lacunarity, and succolarity, respectively). As a comparison, the binarized CT images of two conventional sandstone cores are simulated with the same parameters. The results show that the correlation between porosity and permeability of the hydrate samples is poor, while there is a strong correlation among the succolarity and the permeability. Fractal dimension (represents complexity) of clayey silt samples is higher compared with conventional sandstone cores. Lacunarity explains the difference in permeability among samples from the perspective of pore throat diameter and connectivity. Succolarity indicates the extent to which the fluid in the pore is permeable, which can be used to characterize the anisotropy of pore structures. Therefore, these three fractal parameters clarify the relationship between the microstructure and macroscopic physical properties of clayey silt porous media.

ELEMENTS DISTRIBUTION FOR THE UPPER SANDSTONE MEMBER OF THE ZUBAIR FORMATION IN ZUBAIR OIL FIELD, SOUTHERN IRAQ

The upper sandstone member (Z/2) is one of the most important reservoirs in the Zubair Oilfield, south of Iraq. It belongs to the upper part of the Zubair Formation, the Lower Cretaceous age, and deposited in deltaic environments. The upper sandstone member consists of several sandstone units which are isolated by the shale units. Major trace and rare earth elements (REEs) of the upper sandstone member in Zubair Formation have been analyzed. Six core samples were used to determine the source of sediments, paleo-environment redox condition and elements concentrations. The geochemical analysis data indicate that the source of sediments rock was influenced by a low to moderate degree of weathering conditions in the Hammar and Safwan Domes. The results have shown that all samples were formed in an oxic environment, with humid paleoclimate. Geochemical analysis data indicate that all rock samples are fundamentally derived from rhyolitic and andesitic rocks more a felsic source. The results show that the samples are mainly of marine organic matters input. The values of Chemical Index of Alteration in the Hammar Dome range from 13.05 to 24.37, while from 23.72 to 77.22 in the Safwan Dome. These results may attribute to fine and medium clastic sedimentary rocks in the H sandstone unit, with a low degree of weathering in the Hammar Dome, and high to medium with slightly higher abundance of clayey silts in the Safwan dome.

Geophysical Exploration of a Historical Stamp Mill Dump for the Volume Estimation of Valuable Residues

We present an approach for the estimation of ore processing residue volumes in historical mine waste dumps by the use of different geophysical methods in combination with mineralogical investigations. The stamp mill dump in the Harz mountains, Germany was examined with the methods electrical resistivity tomography (ERT), ground penetrating radar (GPR) and spectral induced polarization (SIP) flanked by mineralogical studies at many drilling points. The mineralogical results were used to calibrate the geophysical results and to distinguish between valuable and non-valuable waste material. With SIP we investigated individual profiles and took lab samples. These lab results emphasize the differences between the fine-grained tailings of clayey silt to silty sand in the top layer and the sandy tailings underneath in both resistivity and phase. From the GPR results we can distinguish between different layers and various backfillings in the first two meters due to the much higher resolution than the other methods. From ERT we achieved an overview about the dimension and inner structure of the dump and the boundary between the sandy residual material and the host rock. To estimate the volume of the residual body we carried out 2D inversion of all ERT profiles followed interpolation between the inverted profiles. From the drilling interpretation, the SIP lab results and the ERT field measurements we defined a resistivity threshold of 350 ohm-m for the ore processing residues to achieve a 3-dimensional body of the dump. The volume of this body was then corrected by a factor due to consideration of uncertainties, e.g., forest areas, inaccessible dump sections, small-scale anomalies (geological or different anthropogenic nature) and inversion coverage. As a result, we were able to calculate the volume of the ore processing residues which can be used further for the determination of the economic potential (remaining metal content).

Grain-Size Distribution of Surface Sediments in the Bohai Sea and the Northern Yellow Sea: Sediment Supply and Hydrodynamics

The grain-size distribution of surface sediments in the Bohai Sea (BS) and the northern Yellow Sea (NYS), and its relationship with sediment supply and hydrodynamic environment were investigated based on grain-size compositions of surface sediments and modern sedimentation rates. The results showed that the surface sediments in the BS and the NYS were primarily composed of silty sand and clayey silt with a dominant size of silt. In addition, the Yellow River delivered high amount of water and sediments to the BS, and they are dominated in surface sediments (mainly silt) in the Bohai Bay, the Yellow River mouth, the center of the BS, and the north coast of Shandong Peninsula. The coarse-grained sediments were mainly deposited at the river mouth due to the estuarine filtration and physical sorting. Meanwhile, there was a significant relationship among the modern sedimentation rate, the surface sediment grain size distribution and sediment transport pattern. The areas with coarser surface sediments generally corresponded low sedimentation rates because of strong erosion; whereas the sedimentation rate was relatively high at the place that the surface sediments were fine-grained. Furthermore, the grain-size trend analysis showed that the areas with fine-grained surface sediments such as the mud area in the central BS and the upper Liaodong Bay were the convergent centers of surface sediments, except for the Bohai Bay and the subaqueous Yellow River Delta where offshore sediment transport was evident.

Theoretical and Experimental Studies on the Signal Propagation in Soil for Wireless Underground Sensor Networks.

Wireless Underground Sensor Networks (WUSNs), an important part of Internet of things (IoT), have many promising applications in various scenarios. Signal transmission in natural soil undergoes path loss due to absorption, radiation, reflection and scattering. The variability and dynamic of soil conditions and complexity of signal attenuation behavior make the accurate estimation of signal path loss challenging. Two existing propagation models for predicting path loss are reviewed and compared. Friis model does not consider the reflection loss and is only applicable in the far field region. The Fresnel model, only applicable in the near field region, has not considered the radiating loss and wavelength change loss. A new two stage model is proposed based on the field characteristics of antenna and considers four sources of path loss. The two stage model has a different coefficient m in the near field and far field regions. The far field distance of small size antenna is determined by three criteria: 2 D2/λ, 5 D, 1.6 λ in the proposed model. The proposed two stage model has a better agreement with the field experiment data compared to Friis and Fresnel models. The coefficient m is dependent on the soil types for the proposed model in near field region. It is observed from experiment data that the m value is in the range of 0~0.20 for sandy soils and 0.433~0.837 for clayey silt.

Effect of permeability anisotropy on depressurization‐induced gas production from hydrate reservoirs in the South China Sea

AbstractThe permeability anisotropy (ie, horizontal, kr, to vertical, kz, permeability ratio) of gas hydrate reservoirs may be a significant factor that affects hydrate dissociation and gas production. Here, site SH2, a candidate for field testing comprising a clayey silt gas hydrate reservoir in the Shenhu area in the South China Sea, was chosen to investigate the effect of permeability anisotropy on gas production behavior through numerical simulations. The spatial distribution of physical fields (ie, pressure, temperature, and saturation) and the evolution of gas and water recovery are comparatively analyzed. The simulation results show that permeability anisotropy has a significant impact on gas extraction, especially when the horizontal permeability is higher than the vertical one. The sensitivity analyses indicate that permeability anisotropy in both medium‐permeability (10‐100 mD) and low‐permeability (1‐10 mD) sediments are conducive to hydrate dissociation and that a higher horizontal permeability is more favorable for gas production. Comparatively, permeability anisotropy in low‐permeability sediments is not beneficial for improving production efficiency. In addition, our work also suggests that heterogeneous hydrate saturation in a reservoir‐scale model should be employed in future predictions because the gas production potential will be overestimated in a homogeneous reservoir under the same conditions. These findings contribute to a clear understanding of the influence of reservoir properties on gas hydrate dissociation processes and provide a reference for future field trials and commercial production.

Enhanced erodibility of deep-sea sediments by presence of calcium carbonate particles

Using an erosion chamber, erodibility experiments on two different deep-sea sediments (mainly clayey silt) acquired in the western Pacific Ocean have been conducted to estimate the erosion rate and its potential controlling factors. Overall, the erosion rates of both deep-sea sediments were in the range of 10−7–10−5 kg m−2 s−1 which was two or three orders of magnitude lower than that of sediments in shallow environments. Such erodibility, however, exhibited two contrasting patterns depending on the carbonate compensation depth (CCD), which determines carbonate contents in sediments. At one site (MC1), above the CCD, the sediments (carbonate content 18.32 ± 1.00 wt%) were easily resuspended by surface erosion, resulting in a high eroded mass (2.49 × 10−2 kg m−2 at shear stress of 0.6 Pa). At another site (MC4), below the CCD, the sediments (carbonate content 0.06 ± 0.04 wt%) were less resuspended by floc erosion, resulting in a low eroded mass (3.70 × 10−3 kg m−2 at shear stress of 0.6 Pa). Over the entire range of bed shear stress, the erosion rate above the CCD was about 1–6 times higher than that below the CCD. The difference in carbonate content between two sites caused different variations in the bed resistance with depth. The erodibility above the CCD was enhanced, probably due to the particle roughness and low bulk density. Once the bed shear stress exceeded about 0.39 Pa, the erosion type of MC1 shifted from depth-limited to transitional erosion. However, the erosion type of MC4 always remained depth-limited over the entire range of bed shear stress. These results underline the effect of calcium carbonate on resuspension and erosion behaviors of the deep-sea sediments.

Fractionation Trends and Variability of Rare Earth Elements and Selected Critical Metals in Pelagic Sediment from Abyssal Basin of NE Pacific (Clarion-Clipperton Fracture Zone)

The geochemical and mineralogical characteristics of pelagic sediments collected from the Interoceanmetal Joint Organization (IOM) claim area, located in the eastern part of the Clarion-Clipperton Fracture Zone (CCFZ; eastern tropical Pacific), are described in this paper. The concentrations of rare earth elements (REE), as well as other selected critical elements contained in 135 sediment samples of siliceous clayey silts, are presented. The vertical and spatial variabilities of elements, with particular emphasis on REE as well as metals of the highest economic interest such as Cu, Ni, and Co, are detailed. The applied methods include grain size analysis by laser diffraction, geochemistry examination using ICP-MS, XRF, AAS, and CNS spectrometry, and XRD analysis of mineral composition (Rietveld method). Additionally, statistical methods such as factor analysis (FA) and principal components analysis (PCA) were applied to the results. Finally, a series of maps was prepared by geostatistical methods (universal kriging). Grain size analysis showed poor sorting of the examined fine-grained silts. ICP-MS indicated that total REE contents varied from 200 to 577 ppm, with a mean of 285 ppm, which is generally low. The contents of critical metals such as Cu, Ni, and Co were also low to moderate, apart from some individual sampling stations where total contents were 0.15% or more. Metal composition in sediments was dominated by Cu, Ni, and Zn. A mineral composition analysis revealed the dominance of amorphous biogenic opaline silica (27–58%), which were mostly remnants of diatoms, radiolarians, and sponges associated with clay minerals (23% to 48%), mostly Fe-smectite and illite, with mixed-layered illite/smectite. The high abundance of diagenetic barite crystals found in SEM−EDX observations explains the high content of Ba (up to 2.4%). The sediments showed complex lateral and horizontal fractionation trends for REE and critical metals, caused mostly by clay components, early diagenetic processes, admixtures of allogenic detrital minerals, or scavenging by micronodules.

Geochemical characterization of uranium mill tailings (Bois Noirs Limouzat, France) highlighting the U and 226Ra retention

As with other metals, the management of tailings from former uranium (U) mines requires a good knowledge of the geochemical mechanisms governing the retention of radioelements of interest: U and 226Ra. This article presents the results of the study of the bearing phases featuring these two radioelements within the Bois Noirs Limouzat tailings storage facility (Loire), the only site in France where the tailings (a sandy silt facies and a clayey silt facies) are currently stored only under water. The aim is to gain a better understanding of their respective mobility under current storage conditions. For this purpose, a multi-scale approach was adopted combining historical research and airborne image analysis to select the core location, chemical and radiological analyses, mineralogical characterizations supplemented by sequential extractions (two specifically developed protocols). The results show that U and 226Ra are mainly found in the clayey silt facies with an average U concentration of 243.3 ppm (132.3 ppm in the sandy silt facies) and an average 226Ra mass activity of 64.7Bq/g (18.0Bq/g in the sandy silt facies). These results are in accordance with the initial U grade of the ore (2‰), the extraction efficiency of the ore processing plant (95%) and the age of mineralization (305 Ma). The approach adopted made it possible to highlight several mineralogical traps available for each radioelement, regardless of the facies type. Thus, a significant part of the U is still trapped within the primary phases, resistant to treatment and therefore relatively immobile under current storage conditions (49.6%–77.8% for the sandy silt facies and 27.2%–36% for the clayey silt facies). Most of the leached U is mainly associated with weakly crystalised iron oxyhydroxides (8.7%–42.4% for the sandy silt facies and 50.9%–71.8% for the clayey silt facies) and to a lesser extent with clay minerals (5%–12.3% for the sandy silt facies and 0.8%–11.5% for the clayey silt facies). For the 226Ra, irrespective of the facies type, a significant part remains trapped within phosphate phases, resistant to the leaching process and therefore also relatively immobile under storage conditions (24.4%–38.9% for the silty sandy silt facies and 39.9%–98.9% for the clayey silt facies). Sequential extractions revealed a different geochemistry of 226Ra depending on the facies. For the silty sandy silt facies, most of the 226Ra is mainly associated with the clay minerals (6.4%–69.2%) and to a lesser extent with iron oxyhydroxides, barite or aluminum phosphate sulphate minerals (APS) (6.4%–33.9%). For the clayey silt facies, most of the 226Ra is mainly associated with iron oxyhydroxides, barite or APS (6.4%–53.3%) and to lesser extent clay minerals (0.4%–6.8%). The leaching process did not allow the differentiation between the contributions of each of these phases to the retention of 226Ra. At last, all the identified bearing phases demonstrate that the U is relatively immobile under the current storage conditions, irrespective of the facies. For the 226Ra, the bearing phases differ according to the facies. Within the sandy silt facies, the 226Ra is mainly borne by clay minerals and can be mobilised more easily. However, the sandy silt facies represents only one third of the tailings currently.