Effect of the Composition of Anodic Purging Solutions on Electroosmosis

The objective of this study is to investigate the effect of the composition of anodic purging solutions on electroosmosis in an electrokinetic (EK) system. The effect of buffering capacity of anodic purging solutions on electroosmosis was first studied. With the increase of buffering capacity, soil pH and electric current increased, but the maximal cumulative electroosmostic flow (EOF) was achieved with 0.010?mol/L Na2CO3/NaHCO3 buffer. Na2CO3/NaHCO3 containing NaCl and Na2SO4, respectively, were used as anodic purging solutions to investigate the effect of cation concentration and anion type on electroosmosis. The increase of cation concentration led to the increase of soil pH and electric current but the decrease of EOF. At the same cation concentration, Na2SO4 resulted in higher electric current and greater EOF than NaCl, but similar distribution of soil pH. The present study provides useful information for the selection of purging solution in the EK remediation of contaminated soils.     

Investigation of Consolidation-Induced Solute Transport. I: Effect of Consolidation on Transport Parameters

This paper presents an experimental investigation of the effect of clay consolidation on parameters that govern the advective-dispersive transport of an inorganic solute. Batch, diffusion, dispersion, and solute transport tests were conducted using kaolinite clay and dilute solutions of potassium bromide (KBr). Batch tests produced the highest levels of K+ sorption and indicated that equilibrium sorption was achieved in approximately 10–30 min. The increase in sorption observed in the batch tests, as compared to the dispersion or solute transport tests, reflects the significantly lower solids-to-solution ratio and more efficient mixing process. By comparison, kaolinite consolidation had little effect on sorption due to the relatively small change in porosity. Values of hydrodynamic dispersion coefficient (Dh), effective diffusion coefficient (D?), and apparent tortuosity factor decreased with decreasing porosity. Values of D? obtained for Br? were generally larger than for K+, whereas Dh values for Br? were significantly smaller than for K+. Values of longitudinal dispersivity (α) were larger for K+ than Br? and showed no clear trend with decreasing void ratio. In general, the experimental results suggest that changes in D? and Dh should be taken into account during clay consolidation whereas the sorption isotherm and α may be considered as unchanged during the consolidation process.     

Mechanisms and effects of intracellular calcium buffering on neuronal survival in organotypic hippocampal cultures exposed to anoxia/aglycemia or to excitotoxins

Neuronal calcium loading attributable to hypoxic/ischemic injury is believed to trigger neurotoxicity. We examined in organotypic hippocampal slice cultures whether artificially and reversibly enhancing the Ca2+ buffering capacity of neurons reduces the neurotoxic sequelae of oxygen-glucose deprivation (OGD), whether such manipulation has neurotoxic potential, and whether the mechanism underlying these effects is pre- or postsynaptic. Neurodegeneration caused over 24 hr by 60 min of OGD was triggered largely by NMDA receptor activation and was attenuated temporarily by pretreating the slices with cell-permeant Ca2+ buffers such as 1, 2 bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA-AM). This pretreatment produced a transient, reversible increase in intracellular buffer content as demonstrated autoradiographically using slices loaded with 14C-BAPTA-AM and by confocal imaging of slices loaded with the BAPTA-AM analog calcium green-acetoxymethyl ester (AM). The time courses of 14C-BAPTA retention and of neuronal survival after OGD were identical, indicating that increased buffer content is necessary for the observed protective effect. Protection by Ca2+ buffering originated presynaptically because BAPTA-AM was ineffective when endogenous transmitter release was bypassed by directly applying NMDA to the cultures, and because pretreatment with the low Ca2+ affinity buffer 2-aminophenol-N,N,O-triacetic acid acetoxymethyl ester, which attenuates excitatory transmitter release, attenuated neurodegeneration. Thus, in cultured hippocampal slices, enhancing neuronal Ca2+ buffering unequivocally attenuates or delays the onset of anoxic neurodegeneration, likely by attenuating the synaptic release of endogenous excitatory neurotransmitters (excitotoxicity).     

Analytical and Numerical Modeling of Soft Soil Stabilized by Prefabricated Vertical Drains Incorporating Vacuum Preloading

This paper describes the analytical formulation of a modified consolidation theory incorporating vacuum pressure, and numerical modeling of soft clay stabilized by prefabricated vertical drains, with a linearly distributed (trapezoidal) vacuum pressure for both axisymmetric and plane strain conditions. The effects of the magnitude and distribution of vacuum pressure on soft clay consolidation are examined through average time-dependent excess pore pressure and consolidation settlement analyses. The plane strain analysis was executed by transforming the actual vertical drains into a system of equivalent parallel drain walls by adjusting the coefficient of permeability of the soil and the applied vacuum pressure. The converted parameters are incorporated in the finite element code ABAQUS, employing the modified Cam-clay theory. Numerical analysis is conducted to study the performance of a full-scale test embankment constructed on soft Bangkok clay. The performance of this selected embankment is predicted on the basis of four different vacuum pressure distributions. The predictions are compared with the available field data. The assumption of distributing the vacuum pressure as a constant over the soil surface and varying it linearly along the drains seems justified in relation to the field data.     

Experimental Study of the Behavior of a Lumpy Fill of Soft Clay

Land reclamation is a major civil engineering activity in Singapore. Due to depletion of suitable local fills and the cost of imported sand, dredged and excavated clay fills, in spite of their poor engineering properties, are being evaluated as a fill material. To reduce double handling, it is desirable for the clay to be used directly in a lump form, instead of the more conventional slurry fill. While the performance of a slurry fill is relatively well understood, the behavior of lumpy fill is not. This paper reports the results of a laboratory study carried out on lumpy fill made of cubical clay lumps of size ranging from 12.5?to?50?mm. The study showed that the interlump voids are substantially closed at a consolidation pressure much lower than the preconsolidation pressure of the lumps. The study also shows that at a consolidation pressure of about 100?kPa, the permeability of a lumpy fill is reduced to an order similar to that for homogeneous clay. However, the shear strength profile obtained using the cone penetration test indicates that the fill is still highly heterogeneous under a pressure of 100?kPa. When the preconsolidation pressure of the lumps is exceeded, the strength profile becomes uniform. The degree of swelling of the lumps plays a significant role. For fully swollen lumps, the consolidation pressure required to close the interlump voids is considerably less than that if the lumps were not allowed to swell. The coefficient of secondary compression of the lumpy fill is comparable to the homogeneous clay indicating that secondary compression is not a serious issue.     

Numerical Analysis of One-Dimensional Consolidation in Layered Clay Using Interface Boundary Relations in Terms of Infinitesimal Strain

The interface boundary relations are derived in this study for the numerical analysis of one-dimensional consolidation in multilayered clay profiles. The finite difference solutions are formulated based on Mikasa’s consolidation equation with infinitesimal strains and constant consolidation parameters under the same fundamental assumptions and limitations of the classic Terzaghi equation. Numerical examples are presented for multilayer clay profiles under single and double drainage conditions that validate the predicted excess pore pressures, strains, settlements, and rates of consolidation using interface boundary relations in terms of infinitesimal strains that are equivalent to those expressed in terms of excess pore pressures.     

Evaluation of Sample Quality of Sensitive Clay Using Intrinsic Compression Concept

A simple index, the degree of sample disturbance, is proposed in this study to quantitatively evaluate the quality of sensitive clay samples based on the concept of void index proposed by Burland in 1990. The degree of sample disturbance is defined as the ratio of the difference between the in situ void index and the void index of the undisturbed sample tested in the laboratory to the difference between the in situ void index and the void index of the completely remolded clay. All these indices are determined at the same effective overburden stress. Theoretically, the degree of sample disturbance varies from 0% (perfect undisturbed sample) to 100% (completely remolded sample). The proposed index is used in this study to evaluate the sensitive Ariake clay in Japan. Oedometer tests on undisturbed samples of natural Ariake clay obtained from the field using the current sampling practice in Japan show the degree of sample disturbance ranging from approximately 5 to 38%. The clay sample with an in situ void index closer to the intrinsic compression line has a lower degree of sample disturbance. In addition, a series of consolidation and unconfined compression tests were conducted on artificially disturbed samples in the laboratory to demonstrate the change of consolidation yield stress, unconfined compressive strength, and compression index with the degree of sample disturbance. A simple method is proposed in this paper to correct the mechanical parameters measured in a laboratory setting considering the degree of sample disturbance.     

Setup Following Installation of Dynamic Anchors in Normally Consolidated Clay

This paper describes a series of centrifuge model tests designed to assess the increase in capacity of dynamic anchors due to setup in normally consolidated clay. The tests involved measurement of the vertical capacity of 1:200 reduced scale model anchors following various periods of postinstallation consolidation. The short-term capacity was shown to be dependent on the anchor impact velocity. Cavity expansion solutions for consolidation around a solid driven pile were found to provide agreement with the experimental results. A simplified capacity calculation technique predicted higher friction ratio values than is typically observed for driven piles; however, these calculations were complicated by the unusual dynamic anchor load–displacement response and uncertainty regarding the true sample shear strength. Dynamic anchor consolidation proceeds at a slower rate than for suction caissons and open-ended piles of similar equivalent diameter. However, the results indicate that depending on the site conditions, dynamically installed anchors remain a viable alternative to conventional deep-water mooring techniques.     

Effect of Microfabric on Mechanical Behavior of Kaolin Clay Using Cubical True Triaxial Testing

Various aspects of the mechanical behavior of kaolin clay are discussed in light of experimental observations from a series of strain controlled true triaxial undrained tests performed on cubical kaolin clay specimens with flocculated and dispersed microfabric, using a fully automated flexible boundary experimental setup with real-time feedback control system. The laboratory procedures used to prepare flocculated and dispersed microfabric specimens are presented. Mercury intrusion porosimetry is used to evaluate the pore structure of these specimens. The influence of microfabric on the consolidation behavior of kaolin clay is evaluated based on the data obtained from K0 consolidation during constant rate of strain tests and the isotropic consolidation during true triaxial tests. Undrained tests on kaolin clay show that the following vary with microfabric of specimen: The shear stiffness, excess pore pressure generated during shear, and strength and strain to failure. For both microfabrics, the observed strength behavior using cubical triaxial testing shows a similar pattern of variation with applied stress anisotropy; hence, only a marginal influence of fabric-induced anisotropy.     

Deep Mixing Induced Property Changes in Surrounding Sensitive Marine Clays

This paper presents a field study of installation effects of deep mixed columns on properties of the sensitive Ariake marine clay. Cone penetration tests were performed in the field to evaluate the change in the strength of the surrounding clay with time. Soil samples were taken before and after column installation to evaluate variations of physical, mechanical, and chemical properties of the surrounding clay. Test results indicated that the water content of the surrounding clay decreased while the concentration of cations increased as sampling locations approached the columns. Shear strength of the surrounding clay decreased during the installation but recovered after a short period of curing. Shear strength continued to increase with time over a period of 70?days. Based on the regression results, the surrounding soil after the installation of the columns took approximately 10?days to recover to the strength value before installation. On average, the shear strength of the surrounding clay increased over the original strength by approximately 23% after 40?days and 50% after 70?days, respectively. Discussion is presented on strength changes and key influence factors including soil disturbance and fracturing, thixotropy, consolidation, and diffusion of cations from deep mixed columns to the surrounding clay.     

Numerical Simulation of Transport of Four Heavy Metals in Kaolinite Clay

A multicomponent reactive solute-transport model was used to study the migration of dissolved heavy metals (Cd2+, Pb2+, Cu2+, and Zn2+) in a clay barrier subject to two leachates having different pH values. This solute-transport model is capable of simulating simultaneous processes of water flow, advective-dispersive-solute transport, and chemical reactions. The migration of these metals was simulated in a kaolinite landfill liner, which was assigned realistic physical and chemical properties and boundary conditions to model one-dimensional contaminant transport. The leachate input properties to the model were those of an actual leachate containing the four heavy metals. The numerical simulations were focused on the concentration profiles of these metals in the simulated clay barrier and leachate pH affects their mobilities. The numerical results indicate that with a nearly neutral leachate, the heavy metals mobility follows: Cd2+ < Pb2+ < Cu2+ < Zn2+. With an acidic leachate, the order changes to Pb2+ < Cu2+ < Zn2+ < Cd2+. Leachate pH has a significant effect on Cd2+ and Pb2+ mobility and a small effect on Cu2+ and Zn2+.     

Consolidation of a Geosynthetic Clay Liner under Isotropic States of Stress

The consolidation behavior of a geosynthetic clay liner (GCL) was evaluated by consolidating duplicate specimens of the GCL in a flexible-wall cell to a final effective stress, σ′, of 241 kPa (35.0 psi). The hydraulic conductivity, k, also was measured at the end of each loading increment. The results indicated that the GCL was normally consolidated for values of σ′ greater than 34.5 kPa (5.0 psi), which correlates well with limited consolidation data reported in the literature for GCLs based on confined compression using oedometers. Values of the coefficient of consolidation, cv, for the GCL ranged from 5.2×10?10?m2/s to 2.1×10?9?m2/s, and generally decreased with increasing σ′, albeit only slightly. Values of the measured k, kmeasured, for the GCL were low ( ? 5.0×10?9?cm/s) due to the sodium bentonite content of the GCL, and were within a factor of about two of the values of k calculated on the basis of classic (Terzaghi) small-strain consolidation theory, ktheory (i.e., 0.5 ? ktheory/kmeasured ? 2.0), suggesting that the theory is appropriate for describing the consolidation behavior of the GCL. The results also are consistent with the results of previous studies based on one-dimensional consolidation of sodium montmorillonite, suggesting that there would be little difference in the consolidation behavior of the GCL under confined compression.     

The Depressing Effect of Clay Minerals on the Floatability of Chalcopyrite

In this work, the depressing effect of clay minerals on the floatability of chalcopyrite using PAX as collector was assessed through induction time and settling-turbidity measurements. The data obtained using these two techniques were correlated with the results of micro-flotation tests carried out to study the effect of kaolinite and smectite particles on the floatability of chalcopyrite at pH 9 and 10. The results indicate that these techniques can be confidently used to examine interactions between clay minerals and chalcopyrite, and their possible consequences on the flotation process. In particular, the use of settling-turbidity measurements to evaluate slime coating seems to be a useful technique that could be applied to study heterocoagulation in other mineral systems. It was found that the depressing effect of clay minerals was stronger at pH 10, which was correlated to a possible effect of Ca²⁺ from lime on the process of coagulation between clay minerals and chalcopyrite. Coating of bubbles with clays was detected in this work by visual observations and it is proposed as a mechanism that might explain the depressing effect of clays on the floatability of chalcopyrite. Further research is needed to propose a mechanism that explains this phenomenon.     

Removal of Hexachlorobenzene from Kaolin by Electrokinetics Coupled with Cu/Fe PRB

This study deals with the remediation of clayed soils contaminated with hydrophobic organic compounds (HOCs) by using electrokinetics (EK) coupled with a permeable reactive barrier (PRB) technique. Hexachlorobenzene (HCB) and kaolin were selected as the typical HOCs and clay, respectively, and microscale Cu/Fe particles were synthesized as the functional materials of PRB. Furthermore, Triton X-100 was used as the solubility-enhanced agent to promote the movement and removal of contaminants. Four bench-scale EK tests were conducted with or without Cu/Fe PRB. The results reveal that coupling EK with Cu/Fe PRB greatly promoted the overall removal of HCB from soil, compared with EK alone. Other than the electroosmotic flow, the adsorption/reductive degradation by Cu/Fe PRB may also be responsible for the HCB removal (over 82% of HCB passing by the PRB was removed). Our investigation suggests that the integration of EK and Cu/Fe PRB is of great promise to promote the application of EK technique in the remediation of HOC-contaminated soils.     

Enhanced Electrokinetic Remediation of Heavy Metals in Glacial Till Soils Using Different Electrolyte Solutions

Previous electrokinetic remediation studies involving the geochemical characterization of heavy metals in high acid buffering soils, such as glacial till soil, revealed significant hexavalent chromium migration towards the anode. The migration of cationic contaminants, such as nickel and cadmium, towards the cathode was insignificant due to their precipitation under the high pH conditions that result when the soil has a high acid buffering capacity. Therefore the present laboratory study was undertaken to investigate the performance of different electrolyte (or purging) solutions, which were introduced to either dissolve the metal precipitates and/or form soluble metal complexes. Tests were conducted on a glacial till soil that was spiked with Cr(VI), Ni(II), and Cd(II) in concentrations of 1,000, 500, and 250 mg/kg, respectively, under the application of a 1.0 VDC/cm voltage gradient. The electrolyte solutions tested were 0.1M EDTA (ethylenediaminetetraacetic acid), 1.0M acetic acid, 1.0M citric acid, 0.1M NaCl/0.1M EDTA, and 0.05M sulfuric acid/0.5M sulfuric acid. The results showed that 46–82% of the Cr(VI) was removed from the soil, depending on the purging solution used. The highest removal of Ni(II) and Cd(II) was 48 and 26%, respectively, and this removal was achieved using 1.0M acetic acid. Although cationic contaminant removal was low, the use of 0.1M NaCl as an anode purging solution and 0.1M EDTA as a cathode purging solution resulted in significant contaminant migration towards the soil regions adjacent to the electrodes. Compared to low buffering capacity soils, such as kaolin, the removal of heavy metals from the glacial till soil was low, and this was attributed to the more complex composition of glacial till. Overall, this study showed that the selection of the purging solutions for the enhanced removal of heavy metals from soils should be primarily based upon the contaminant characteristics and the soil composition.     

Analytical steady-state solution to the rapid buffering approximation near an open Ca2+ channel

We derive an analytical steady-state solution for the Ca2+ profile near an open Ca2+ channel based on a transport equation which describes the buffered diffusion of Ca2+ in the presence of rapid stationary and mobile Ca2+ buffers (Wagner and Keizer, 1994). This steady-state rapid buffering approximation gives an upper bound on local Ca2+ elevations such as Ca2+ puffs or sparks when conditions for the validity of the rapid buffering approximation are met and is an alternative to approximations that assume that mobile buffers are unsaturable. This result also provides an analytical estimate of the cytosolic Ca2+ domain concentration ([Ca2+]d) near a channel pore and shows the dependence of [Ca2+]d on moderate concentrations of endogenous mobile buffer, Ca2+ indicator dye, and bulk cytosolic Ca2+. Assuming a simple relationship between [Ca2+]d and the lumenal depletion domain of an intracellular Ca2+ channel, lumenal and cytosolic Ca2+ profiles are matched to give an implicit analytical expression for the effect of bulk lumenal Ca2+ on [Ca2+]d.     

Online Earthquake Response Test for Stratified Layers of Clay and Sand

Artificial islands often consist of layers of alluvial clay and reclaimed soil of varying order and thickness. Soft clay layers have nonlinear characteristics and can both amplify and attenuate earthquake ground motions. Liquefied ground impedes propagation of shear waves and thus attenuates the earthquake accelerations. Online testing is a method of feeding soil response characteristics directly from soil samples into a modeling algorithm. The effects of the layer thickness, configuration, and degree of consolidation on the earthquake response characteristics of alternating layers of clay and sand have been investigated. The degree of liquefaction and strain generated in sand adjacent to clay layers increased with the degree of consolidation. Clay layers attenuate the motions of sand layers for short period vibrations but amplify the long period motions, increasing the strain in overlying liquefied sand layers. Clay layers which were closer to the ground surface or of greater thickness tended to increase the surface accelerations. Normalized cumulative energy loss was larger in clay than in sand increasing with a decreasing degree of consolidation.     

Modeling Elution Histories of Copper and Lead from Contaminated Soil Treated by Poly(amidoamine) Dendrimers

A dynamic “two-site” model was formulated and tested for simulating the elution histories of copper (Cu2+) and lead (Pb2+) from a contaminated soil treated by poly(amidoamine) dendrimers. In the model, the metal sorption sites of the soil were divided into two compartments: one with a fast desorption rate and the other with a slow desorption rate. The model was tested for simulating and predicting Cu2+ and Pb2+ elution histories obtained from column experiments. Compared to the classical “one-site” model and the modified “gamma distribution” model, the “two-site” model not only provides much improved power for simulating the observed metal elution data, but also can more accurately predict the metal elution histories under various experimental conditions including initial metal concentration in soil, dendrimer concentration, and pH.     

Mechanisms, models, and simulations of metal-coated fiber consolidation

Recent experimental studies of the hot isostatic consolidation of Ti-6Al-4V-coated SiC fibers contained in cylindrical canisters have revealed an unexpectedly high rate of creep densification. A creep consolidation model has been developed to analyze its origin. The initial stage of consolidation has been modeled using the results of contact analyses for perfectly plastic and power-law creeping cylinders that contain an elastic ceramic core. Final stage densification was modeled using a creep potential for a power-law material containing a dilute concentration of cusp-shaped voids with a shape factor similar to that observed in the experiments. Creep rates were microstructure sensitive and so the evolution of matrix grain size and the temperature dependence of the α/β-phase volume fractions were introduced into the model using micromechanics-based creep constitutive relationships for the matrix. To account for load shielding by the deformation resistant canister, the consolidation model was combined with an analysis of the creep collapse of a fully dense pressure vessel. The predicted densification rates were found to agree well with the experimental observations. The high densification rate observed in experiments was the result of the small initial grain size of the vapor-deposited matrix combined with retention of the cusp shape of the interfiber pores.