Waste-Soil Interactions that Alter Hydraulic Conductivity

Hydraulic conductivity tests were conducted in the laboratory and field on geosynthetic clay liners (GCLs) and a sand-bentonite mixture to determine if their hydraulic conductivity is affected by freezing and thawing. In the laboratory, specimens of three GCLs were frozen and thawed 20 times, and no increase in hydraulic conductivity was measured. The hydraulic conductivity of the compacted sand-bentonite also did not increase after freezing and thawing. In the field, two types of GCLs and a sand-bentonite test pad (constructed with the same mixture used in the laboratory) were exposed to one or two winters of freeze-thaw cycling. No large increase in hydraulic conductivity was measured for the field test conducted with the sand-bentonite mixture. An increase in hydraulic conductivity was observed in only one of the field tests with GCLs. Examination of thawed GCLs and specimens of the sand-bentonite mixture showed no evidence of cracking that is commonly found in thawed compacted clays.

Bentonite has been generally used as vertical cutoff barrier material and reported to have several problems regarding its low workability, drying shrinkage cracking by particle cohesion, and ineffective waterproof ability under sea water condition. In this study, the particle sealant, the furnace slag coated by the mixture of bentonite, sepiolite and guargum, was developed to compensate these weak points and the hydraulic conductivity of the particle sealant was evaluated. Drying shrinkage cracking and swelling index was estimated to find the optimal mixing ratio of bentonite, sepiolite and guargum. The hydraulic conductivity of the particle sealants having different amount of sealant (bentonite-sepioliteguargum mixture) coating the furnace slag was estimated using the rigid wall permeameter and flexible wall permeameter. The results showed that drying shrinkage cracking was not found in the bentonite-sepiolite mixture with 20% sepiolite contents and the results from free swelling tests for the sealant having 1 : 0.025, 1 : 0.05 and 1 : 0.075 of weight ratios of bentonite-sepiolite mixture and guargum under simulated sea water condition were higher than those for the bentonitesepiolite mixture without guargum under tap water condition. These three sealants were coated on the furnace slag with 50% and 60% of sealant in the particle sealant and the hydraulic conductivity was estimated. In the cases of the particle sealants having 20% sepiolite in the bentonite-sepiolite mixture and 1 : 0.075 weight ratio of the bentonite-sepiolite mixture and guargum, the hydraulic conductivity from the rigid wall permeameter was below <TEX>$1.0{\times}10^{-7}$</TEX> cm/sec under simulated sea water condition. The hydraulic conductivity of the particle sealant having <TEX>$1.0{\times}10^{-6}$</TEX>~<TEX>$1.0{\times}10^{-7}$</TEX> cm/sec by the rigid wall permeameter was estimated using the flexible wall permeameter and found to be below <TEX>$1.0{\times}10^{-7}$</TEX> cm/sec.

Hydraulic conductivities and effective diffusion coefficients of geosynthetic clay liners with organobentonite amendments

The majority of test data indicate that the hydraulic conductivity of soil-sodium bentonite mixtures increases several folds with many of the chemicals found at waste disposal sites. This investigation is on mixtures consisting of calcium bentonites from four different sources, cement, slag, and fly ash using flexible wall permeameter. The mix with calcium bentonite from Greece yielded hydraulic conductivity values which were equal to or lower than 10-7 cm/s. With water and organic chemicals (aniline 5,000 ppm, phenol 5,000 ppm, toluene 500 ppm) the soil mix showed a decrease in hydraulic conductivity. Over a 40 day period the hydraulic conductivity decreased from about 10-7 cm/s to 10-9 cm/s. The rate of decrease in hydraulic conductivity with phenol was somewhat slower. The hydraulic conductivity did not increase with time with any of the chemicals.

In this study, first, various geosynthetic clay liner (GCL) application areas were briefly defined and afterwards, the details of GCL usage in mine applications were given. GCLs are barrier materials that are preferred in mining facilities such as heap leach pads, tailing dams or waste landfills. In these applications, GCLs might be in contact with aggressive leachates that could deteriorate the hydraulic properties of the GCLs. The hydraulic capability of the GCLs is decreased by an increase in hydraulic conductivity or a decrease in swell index of the bentonite component of the GCL. Due to the biodegradation of the organic substances in a waste containment area, heat is generated and the temperature of the leachates increases. To investigate the effect of temperature, triaxial permeability and free swell tests were conducted on the GCLs that were permeated with 0.5 M MgCl2 solution and deionized water. The temperature of the permeation fluid was chosen as 20 and 40 °C. Furthermore, a cationic polymer having 1 and 2% amounts of mass was added to the bentonite component of the GCL to enhance the hydraulic capability of the GCL. Test results indicated that temperature increase in 0.5 M MgCl2 solution from 20–40 °C caused both an increase in the swell index and the permittivity of the GCLs. The increase in permittivity could be attributed to the lower viscosity of the fluid at higher temperatures. Finally, adding a cationic polymer up to the amount of 2% by mass to the bentonite resulted in almost two orders of magnitude decrease in the permittivity of the GCLs that were permeated with 0.5 M MgCl2 solution. In conclusion, cationic polymer-treated GCLs might be used effectively in waste containment facilities by both decreasing the permittivity and increasing the swell index of the GCLs.

Hydraulic and compaction characteristics of leachate-contaminated lateritic soil

Silicon isotopes record dissolution and re-precipitation of pedogenic clay minerals in a podzolic soil chronosequence

This study addresses the role of nitric oxide (NO) and its downstream mechanism in mediating the shear-induced increase in hydraulic conductivity (L(p)) of bovine aortic endothelial cell monolayers grown on porous polycarbonate filters. Direct exposure of endothelial monolayers to 20-dyne/cm(2) shear stress induced a 4. 70+/-0.20-fold increase in L(p) at the end of 3 hours. Shear stress (20 dyne/cm(2)) also elicited a multiphasic NO production pattern in which a rapid initial production was followed by a less rapid, sustained production. In the absence of shear stress, an exogenous NO donor, S-nitroso-N-acetylpenicillamine, increased endothelial L(p) 2.23+/-0.14-fold (100 micromol/L) and 4.8+/-0.66-fold (500 micromol/L) at the end of 3 hours. In separate experiments, bovine aortic endothelial cells exposed to NO synthase inhibitors, N(G)-monomethyl-L-arginine and N(G)-nitro-L-arginine methyl ester, exhibited significant attenuation of shear-induced increase in L(p) in a dose-dependent manner. Inhibition of guanylate cyclase (GC) with LY-83,583 (1 micromol/L) or protein kinase G (PKG) with KT5823 (1 micromol/L) failed to attenuate the shear-induced increase in L(p). Furthermore, direct addition of a stable cGMP analogue, 8-bromo-cGMP, had no effect in altering baseline L(p), indicating that the GC/cGMP/PKG pathway is not involved in shear stress-NO-L(p) response. Incubation with iodoacetate (IAA), a putative inhibitor of glycolysis, dose-dependently increased L(p). Addition of IAA at levels that did not affect baseline L(p) greatly potentiated the response of L(p) to 20-dyne/cm(2) shear stress. Finally, both shear stress-induced and IAA-induced increases in L(p) could be reversed with the addition of dibutyryl cAMP. However, additional metabolic inhibitors, 2 deoxyglucose (10 mmol/L) and oligomycin (1 micromol/L), or reactive oxygen species scavengers, deferoxamine (1 mmol/L) and ascorbate (10 mmol/L), failed to alter shear-induced increases in L(p). Our results show that neither the NO/cGMP/PKG pathway nor a metabolic pathway mediates the shear stress-L(p) response. An alternate mechanism downstream from NO that is sensitive to IAA must mediate this response.

Predicting hydraulic conductivity and characterizing its size effect in jointed rock masses is fundamental to various geoscience and engineering disciplines. A new empirical method, using drilling data, is established to predict the hydraulic conductivity of a rock mass in the scanline direction. The effect of the scanline unit length on the distribution characteristics of hydraulic conductivity for boreholes in jointed rock masses is studied. In addition, the scanline length of boreholes was used to study the size effect of hydraulic conductivity of jointed rock masses, and a new model was proposed to determine the size effect of hydraulic conductivity. A mathematical method for representative volume element (RVE) determination was proposed based on the proposed size effect model of hydraulic conductivity. Some packer tests were conducted to validate the developed method for predicting hydraulic conductivity. The results show that (a) the predicted hydraulic conductivities are close to those determined from the packer tests; (b) the scanline unit lengths influence the distribution characteristics of hydraulic conductivity; (c) a large but fluctuating increase in hydraulic conductivity with increasing scanline length, and the fluctuation decreases with fewer fractures and joints; (d) the mathematical method for predicting RVE and KRVE is effective to make an initial evaluation on jointed rock masses with high fracture frequency.

Hydraulic conductivity behaviour of soil blended with geofiber inclusions

Hydraulic conductivity is determined in laboratory assays to estimate the flow of water in saturated soils. However, the results of this analysis, when using distilled or deionized water, may not correspond to field conditions in soils with high concentrations of soluble salts. This study therefore set out to determine the hydraulic conductivity in laboratory conditions using solutions of different electrical conductivities in six soils representative of the State of Pernambuco, with the exchangeable sodium percentage adjusted in the range of 5-30%. The results showed an increase in hydraulic conductivity with both decreasing exchangeable sodium percentage and increasing electrical conductivity in the solution. The response to the treatments was more pronounced in soils with higher proportion of more active clays. Determination of hydraulic conductivity in laboratory is routinely performed with deionized or distilled water. However, in salt affected soils, these determinations should be carried out using solutions of electrical conductivity different from 0 dS m-1, with values close to those determined in the saturation extracts.

A comprehensive series of laboratory tests was conducted on an inorganic clayey sediment in order to predict the mechanical behaviour of dredged sediments used in reclamation projects. The soil used was a Louiseville clay, which was mixed with various quantities of lime (0–10%) and had a varied water content (122–650%). Tests were carried out with special large cells and standard oedometers to look at both the compressibility and the hydraulic conductivity. Once pozzolanic reactions are begun, for a given curing time, a linear relationship between preconsolidation pressure and lime concentration is observed. Compressibility results indicate that it is possible to define a separate compression curve for each lime concentration and curing time. Adding lime will influence the hydraulic conductivity both by flocculation and by the formation of secondary minerals. If only flocculation occurs, an increase in hydraulic conductivity follows. If enough lime is added, the resulting secondary minerals will create major changes in the micropore network and this will decrease the hydraulic conductivity by up to one order of magnitude. The development of a secondary micropore network along with the cementation of large flocs induce significant increases in both the liquid and plastic limits. This results in a significant increase in the water sorption potential and higher Atterberg limits. Key words: dredged sediments, compressibility, hydraulic conductivity, lime, stabilization, microstructure.

Hydraulic conductivity and pore fluid chemistry in artificially weathered plastic clay

The changes in hydraulic conductivity and clay dispersivity of a Fallbrook soil (fine‐loamy, mixed, thermic Typic Haploxeralfs) as a function of concentration (⩽ 3 meq/liter) and Na adsorption ratio (⩽ 30) of the percolating solutions were measured. Both phenomena were very sensitive to the level of exchangeable Na and to salt concentration of the soil solution. When the salt concentration is 3.0 meq/liter, the hydraulic conductivity and clay dispersion decrease if the exchangeable sodium percentage (ESP) values exceed 12. Conversely, in distilled water, clay dispersion and hydraulic conductivity decrease at ESP values as low as 1 to 2%. We hypothesize that mineral soils, which readily release salt during leaching with distilled water, will not be susceptible to exchangeable Na &lt; 10%. Conversely, mineral soils with low rates of mineral dissolution, like the Fallbrook soil, should be susceptible even at lower ESP values. Also, surface soils, when leached with rainwater, will be especially susceptible at such low levels of exchangeable Na.

An experimental setup for fluid venting in unconsolidated sediments: New insights to fluid mechanics and structures