The Technical Challenges for Applying Unsaturated Soil Sensors to Conduct Laboratory-Scale Seepage Experiments.

Although many unsaturated soil experiments have successfully delivered positive outcomes, most studies just concisely illustrated sensor techniques, because their main objectives focused on bridging research gaps. Inexperienced research fellows might rarely follow up those techniques, so they could encounter very trivial and skill-demanding difficulties, undermining the quality of experimental outcomes. With a motivation to avoid those, this work introduces technical challenges in applying three sensor techniques: high precision tensiometer, spatial time-domain reflectometry (spatial TDR) and digital bench scales, which were utilized to measure three fundamental variables: soil suction, moisture content and accumulative outflow. The technical challenges are comprehensively elaborated from five aspects: the functional mechanism, assembling/manufacturing approaches, installation procedure, simultaneous data-logging configurations and post data/signal processing. The conclusions drawn in this work provide sufficient technical details of three sensors in terms of the aforementioned five aspects. This work aims to facilitate any new research fellows who carry out laboratory-scale soil column tests using the three sensors mentioned above. It is also expected that this work will salvage any experimenters having troubleshooting issues with those sensors and help researchers bypass those issues to focus more on their primary research interests.

Determination of the Porosity Distribution during an Erosion Test Using a Coaxial Line Cell.

The detection of porosity changes within a soil matrix caused by internal erosion is beneficial for a better understanding of the mechanisms that induce and maintain the erosion process. In this paper, an electromagnetic approach using Spatial Time Domain Reflectometry (STDR) and a transmission line model is proposed for this purpose. An original experimental setup consisting of a coaxial cell which acts as an electromagnetic waveguide was developed. It is connected to a transmitter/receiver device both measuring the transmitted and corresponding reflected electromagnetic pulses at the cell entrance. A gradient optimization method based on a computational model for simulating the wave propagation in a transmission line is applied in order to reconstruct the spatial distribution of the soil dielectric permittivity along the cell based on the measured signals and an inversion algorithm. The spatial distribution of the soil porosity is deduced from the dielectric permittivity profile by physically based mixing rules. Experiments were carried out with glass bead mixtures of known dielectric permittivity profiles and subsequently known spatial porosity distributions to validate and to optimize both, the proposed computational model and the inversion algorithm. Erosion experiments were carried out and porosity profiles determined with satisfying spatial resolution were obtained. The RMSE between measured and physically determined porosities varied among less than 3% to 6%. The measurement rate is sufficient to be able to capture the transient process of erosion in the experiments presented here.

Influence of heterogeneity and elevated temperatures on the seismic translational stability of engineered landfills.

This paper proposes a new generalised closed-form solution to estimate the safety factors for translational failure of landfills considering the effects of heterogeneity and temperature variation with the depth of the landfill. The influence of heterogeneity was considered in the form of a shear wave velocity profile, while the influence of temperature was accounted for by considering the temperature-dependent shear strength properties. The proposed method also considered the influence of material damping and the mode change behaviour of the landfill. The safety factor for translational failure of the landfill was estimated using the limit equilibrium-based two-part wedge method. The proposed method was validated by comparing the safety factor obtained from this method with comparable analytical solutions. An extensive parametric study was conducted to demonstrate the influences of the landfill geometry, shear strength properties of municipal solid waste and liner components, parameters of strong ground motion, heterogeneity, and landfill temperature on the calculation of safety factors. The interfacial shear strength properties of liner components played a vital role in the translational stability of landfills compared to the shear strength properties of municipal solid waste. The frequency of base excitation showed a strong influence on landfill stability, while the material damping displayed a nominal effect. For the cases considered in this study, when the frequency of base excitation was close to the fundamental frequency of the landfill, the base acceleration was amplified by 6.7 times which reduced the safety factor by approximately 1.6 times. Heterogeneity and temperature have a significant influence on landfill stability. The increased degree of heterogeneity caused an approximately 27% reduction in the safety factor. For the given set of input parameters, the elevated temperature in the landfill alone decreased the safety factor by a maximum of 18%, while it reduced by approximately 60% under the combined influence of seismic, heterogeneity, and elevated temperature.

Co-solidification of bauxite residue and coal ash into indurated monolith via ambient geopolymerisation for in situ environmental application.

Bauxite residues generated from alumina refineries worldwide have accumulated to more than 4 billion tons, at an annual increment of ~ 0.15 billion tons. It is imperative and urgent for the alumina sector to develop field-operable disposal solutions for rapid and cost-effective stabilisation of alkaline bauxite residues (BR) in the storage facility to minimise/prevent potential environmental risks. Taking advantage of the availability of coal ash (CA) on site, we studied a feasible way to synthesise geopolymer from active (amorphous) aluminosilicate components of BR and CA via the alkaline hydrolysis under ambient conditions. The new geopolymeric binder effectively solidifies BR-CA mixtures into indurated monoliths whose unconstrained compressive strength (UCS) can reach as high as ~ 20 MPa after 8 weeks. The Full Factorial Experimental Design was used to study relative influences of BR:CA ratio, modulus of activating solution, and H2O/Na2O ratio on UCS. Micro-spectroscopic structural analyses using electron-dispersive X-ray spectroscopy and X-ray Photoelectron Spectroscopy suggested a co-occurrence of cement-like calcium aluminosilicate hydrate (C-A-S-H) and Na-rich aluminosilicate 3D-extended network (geopolymer) within the binder phase. The advantage of this ambient geopolymerisation, without resorting to elevated temperature curing, renders a feasible way of valorising BR and CA for environmental management of alkaline wastes at alumina refineries.