The results of a series of gas permeability tests, with monitoring of gravimetric/volumetric moisture content and total suction, on a commercially available needle-punched geosynthetic clay liner (GCL) are presented. GCL specimens were partially hydrated with deionised water under 2 and 20 kPa confinement prior to testing. The tests were conducted at differential pressures ranging from 1 to 10 kPa. Gas permeability was found to decrease with an increase in gravimetric/volumetric moisture content and a decrease of suction. The effect of the preconditioning stress was found to be more pronounced at gravimetric moisture contents greater than 40% (25% apparent degree of saturation, 0·30 m 3 /m 3 volumetric moisture content), and suctions less than 1·6 MPa.
Abstract A series of reduced-charge montmorillonites (RCMs) was prepared from Li-montmorillonite from Jelšový Potok (Slovakia) by heating at various temperatures (105–210°C for 24 h. The amount of fixed Li, 0.09–0.67 per O 20 (OH) 4 , increased with increasing temperature, confirming preparation of a set of samples of variable layer charge from the same parent Li-montmorillonite by varying only the preparation temperature. Infrared spectroscopy revealed that Li was trapped in the hexagonal cavities of the tetrahedral sheet at all temperatures. Partial deprotonation of the samples, reflected in the decrease of the intensities of the OH-bending bands, was observed after treatments above 120°C. Analysis of the OH-stretching region showed Li in the previously vacant octahedra in the samples heated above 150°C. Weak inflections near 660 and 720 cm −1 confirmed development of local trioctahedral character of octahedral cations coordinated with OH groups in the sample heated at 210°C. Gradual decrease of the layer charge due to Li fixation led to a shift of the Si-O stretching band to higher frequencies and to the appearance of new, pyrophyllite-like bands at 1120 and 419 cm −1 .
The fluid loss test has been used by geotechnical engineers for rapid evaluation of the hydraulic barrier properties of bentonites. In this study, a modified fluid loss test, along with viscometric and electrophoretic mobility measurements, was used to assess the colloidal interactions of three powdered sodium bentonites with saline leachates, namely 0–2 N solutions of NaCl and CaCl 2 . The results indicate that the fluid loss test provides robust and reliable information on the water retention of bentonite; moreover, it gives information on the aggregate structure, size, and settling of bentonite flocs and on the high-salt stabilization phenomena (packing and densification of clay aggregates) in saline leachates. Quantifying the water retention characteristics, along with the colloidal (i.e., flocculation) behaviour, using the fluid loss test, provides detailed understanding of barrier properties of bentonites important for various applications (e.g., barrier systems, dewatering).
The crystallochemical structure of reduced Garfield nontronite was studied by X-ray absorption pre-edge and infrared (IR) spectroscopy, powder X-ray diffraction, polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy, and texture goniometry. Untreated and highly reduced (>99% of total Fe as Fe2+) nontronite samples were analyzed to determine the coordination number and the crystallographic site occupation of Fe2+, changes in in-plane and out-of-plane layer structure and mid-range order between Fe centers, and to monitor the changes in structural and adsorbed OH/H2O groups in the structure of reduced nontronite. Contrary to earlier models predicting the formation of fivefold coordinated Fe in the structure of nontronites upon reduction, these new results revealed that Fe maintains sixfold coordination after complete reduction. In-plane PEXAFS evidence indicates that some of the Fe atoms occupy trans-sites in the reduced state, forming small trioctahedral domains within the structure of reduced nontronite. Migration of Fe from cisto trans sites during the reduction process was corroborated by simulations of X-ray diffraction patterns which revealed that about 28% of Fe2+ cations exist in trans sites of the reduced nontronite, rather than fully cis occupied, as in oxidized nontronite. Out-of-plane P-EXAFS results indicated that the reduction of Fe suppressed basal oxygen corrugation typical of dioctahedral smectites, and resulted in a flat basal surface which is characteristic of trioctahedral layer silicates. IR spectra of reduced nontronite revealed that the dioctahedral nature of the nontronite was lost and a band near 3623 cm-1 formed, which is thought to be associated with trioctahedral [Fe2+]3OH stretching vibrations. On the basis of these results, a structural model for the reduction mechanism of Fe3+ to Fe2+ in Garfield nontronite is proposed that satisfies all structural data currently available. The migration of reduced Fe ions from cis-octahedra to adjacent trans-octahedra is accompanied by a dehydroxylation reaction due to the protonation of OH groups initially coordinated to Fe. This structural modification results in the formation of trioctahedral Fe2+ clusters separated by clusters of vacancies in which the oxygen ligands residing at the boundary between trioctahedral and vacancy domains are greatly coordination undersaturated. The charge of these O atoms is compensated by the incorporation of protons, and by the displacement of Fe2+ atoms from their ideal octahedral position toward the edges of trioctahedral clusters, thus accounting for the incoherency of the Fe-Fe1 and Fe-Fe2 distances. From these results, the ideal structural formula of reduced Garfield nontronite is Na1.30[Si7.22Al0.78] [Fe2+3.65Al0.32Mg0.04]O17.93(OH)5 in which the increased layer charge due to reduction of Fe3+ to Fe2+ is satisfied by the incorporation of protons and interlayer Na.
Biocementation processes rely on microbial-induced calcite precipitation (MICP), which is a naturally occurring biochemical process. Biocement materials are a form of environmental cementitious agents used to improve the mechanical properties of granular soils by physically binding soil particles together. Efficient improvement of the macromechanical behavior of coarse sand treated by various amounts of biocement materials requires an in-depth understanding of its microstructure. This paper examined the effect of a number of bacterial suspension and cementation solution flushes on the macromechanical behavior of coarse sand. Also, X-ray computed tomography (XCT), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to investigate changes occurring at microlevels. The results show that compressive strength increased with an increase of biocement materials, and the maximum compressive strength achieved was around 14 MPa. The microscopic investigations were linked to the macromechanical changes, providing unique insight into the causation of the changes. Furthermore, several common soil properties (calcium carbonate content, dry density, void ratio, and porosity) were successfully identified using the XCT technique.
Three different chilled-mirror hygrometer test procedures were developed to investigate the time-dependent unsaturated behaviour of powdered and granular bentonite based needle-punched geosynthetic clay liners (GCLs) on both the wetting and drying paths of the water retention curve (WRC). The GCL structure and bentonite forms governed the effect of measurement time and duration as well as the time-dependent suction changes of the bentonite component at a constant gravimetric water content. A conceptual model is proposed to explain the observed time-dependent unsaturated behaviour of the GCLs. The model suggests that the cross-over points on WRCs correspond to the point where bentonite crystallite separation is maximized within the crystalline swelling regime of smectite, forming a four-layer hydrate state where smectite interlayer spaces are filled with water. At gravimetric water contents below this point, the interlayer space dominated the suction, while at higher water contents, mesopores and macropores played increasingly important roles in determining the suction. The results reported herein provide further proof that the unsaturated behaviour of GCLs is largely controlled by the bentonite component.
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