Mineral precipitation and the associated reduction of hydraulic conductivity in a PRB

Permeable reactive barriers (PRB) have been used worldwide for the in-situ treatment of con-taminated ground water. One such contamination found in Australia is acidic groundwater enriched with high concentrations of dissolved aluminium and iron produced in acid sulfate soil (ASS) terrains. To treat this acidi-ty in groundwater, a PRB was installed in the Shoalhaven Floodplain and then monitored over a seven year pe-riod. This remediation process was satisfactory but there was a small decrease in efficiency due to a secondary mineral precipitation (aluminium and iron oxy/hydroxides) which reduced the hydraulic conductivity of the PRB. Numerical modelling carried out through MODFLOW and RT3D software showed that this reduction in hydraulic conductivity due to secondary mineral precipitation was 3% at the entrance of the PRB after seven years of installation. This result was satisfactory considering that the predicted longevity of the barrier was 19.5 years, assuming a mean groundwater velocity of 0.05 m/day

Numerical prediction of vadose zone behaviour influenced by vegetation

Bioengineering aspects of native vegetation are currently, and rapidly, being evolved to improve soil stiffness, slope stabilisation, and erosion control. Apart from the reinforcement effect, tree roots establish sufficient matric suction to increase the shear strength and stiffness of the soil. This paper looks at the way, vegetation influences soil matric suction, shrinkage, and ground settlement. A mathematical model for the rate of root water uptake that considers ground conditions, type of vegetation and climatic parameters, has been developed. Based on this proposed model, the distribution of moisture and the matric suction profile adjacent to the tree are numerically analysed. The model formulation is based on the general effective stress theory of unsaturated soils. Field measurements taken from literature published previously are compared with the authors' numerical model. The predicted results calculated using the soil, plant, and atmospheric parameters contained in the numerical model, compared favourably with the measured results, justifying the assumptions upon which the model was developed. Copyright ASCE 2006

Conceptual development and numerical modelling of vegetation induced suction and implications on rail track stabilisation

The effects of tree roots on soil suction and ground settlement are investigated. This paper highlights the inter-related parameters contributing to the development of a conceptual evapo-transpiration and root water uptake equilibrium model that is then incorporated in a comprehensive numerical model. The developed numerical model based on the finite element analysis (ABAQUS) considers fully coupled flow-deformation behaviour of soil. Field measurements obtained by the authors from a field site in western Victoria and from past literature are used to validate the model. The predicted results show acceptable agreement with the field data in spite of the assumptions made for simplifying the effects of soil heterogeneity and anisotropy. The numerical analysis proves that the proposed root water uptake model can reliably predict the region of maximum matric suction away from the tree axis. The paper also compares the natural favourable effect of tree roots with the stabilising mechanisms of geosynthetic vertical drains subjected to vacuum pressure. Although this analogy is only justified for shallow vertical drains, the comparison still emphasises the obvious economical advantages of native vegetation

Rapport général du TC202 Géotechnique pour les infrastructures de transport

Today’s needs of urban transportation including roads, railways, airports and harbours demand significant resources for infrastructure development in view of rapid and efficient public and commercial (freight) services. In most cases, authorities have had difficulties in meeting these service demands due to the rapidly growing public, industrial, mining and agricultural sectors in many parts of the world. In order to maximise efficiency and to reduce the costs of maintenance, sound technical knowledge is required. This general report presents major technical advancements around the globe encompassing 33 articles from 19 countries and it is classified into 6 key categories, namely: compaction and subgrade improvement, laboratory testing, theoretical advancements and contributions to design, applications of geosynthetics, numerical modelling and field performance evaluation.De nos jours, les besoins en transports urbains (routes, chemins de fer, aéroports aériens et maritimes) nécessitent d’importantes ressources pour le développement des infrastructures en vue d’assurer des services commerciaux rapides et efficaces. Dans la plupart des cas, en raison de la croissance rapide des secteurs public, industriel, minier et agricole, les autorités se trouvent confrontées à des difficultés pour atteindre les services escomptés. Un savoir technique est alors nécessaire en vue de maximiser l’efficacité et de réduire le coût d’entretien. Le présent rapport général expose les avancées techniques majeures à travers le monde synthétisant 33 articles émanant de 19 pays ; six thèmes clés sont classés : compactage et amélioration des assises, expérimentation en laboratoire, développements théoriques et contributions au dimensionnement, applications des géosynthétiques, modélisations numériques et évaluation des performances sur le terrain.(undefined

Effect of Smear Due to Vertical Drains on the Behaviour of Two Embankments Constructed on Soft Clays

This paper describes the methodology to include the effect of smear in the prediction of settlements beneath two embankments stabilized with vertical drains. The extent of smearing around the drains was studied using a detailed analytical formulation developed by the authors and used in conjunction with the finite clement code, CRISP. The smear zone propagation around vertical drains was studied in the laboratory using a large radial drainage consolidometer. The case histories selected in this study include (a) embankment stabilized with vertical sand drains at a Naval Dockyard, Thailand, and (b) embankment stabilized with vertical band drains in soft Muar clay, Malaysia. It may be concluded from this study that the inclusion of smearing improves the settlement predictions significantly. The numerical results indicate that the extent of smearing is mainly dependent upon the magnitude of horizontal permeability and the drain geometry

Dynamic Properties of Mixtures of Waste Materials

The stockpiling of waste mining by-products, i.e. steel furnace slag (SFS) and coal wash (CW) has brought significant environmental hazard and attracted research attention to reuse them in a more innovative way. In recent years, SFS+CW mixtures have been successfully applied in geotechnical projects, while the inclusion of rubber crumb (RC, from waste tyres) will extend them into dynamic projects. Thus the investigation of the geotechnical properties of SFS+CW+RC mixtures under dynamic loading is in urgent need. In this paper, the dynamic properties (i.e. shear modulus and damping ratio) have been explored based on extensive drained cyclic triaxial tests. The influences of number of loading cycles, RC contents, shear strain level, and the effective confining pressure have been presented. The dynamic properties of SFS+CW +RC mixtures presented in this paper will be essential for the application of the mixtures in the seismic isolation projects or railway foundation

Key strategies for managing acid sulphate soil (ASS) problems on the southeastern coast of New South Wales, Australia

The acidification of Australian coastal waterways as a result of the oxidation of acid sulphate soil (ASS) containing appreciable quantities of sulphidic material (e.g. pyrite) has well recognised environmental, economic and social effects including the loss of fish, biodiversity and agricultural productivity as well as the corrosion of concrete and steel infrastructure by acidic drainage. Largescale artificial drainage and one-way floodgates in low-lying coastal floodplains has lowered the groundwater table, thus enhancing pyritic oxidation and increasing the distribution, magnitude and frequency of acid generation and release of toxic metals such as aluminium (Al3+) and iron (total Fe) from ASS. Engineering strategies implemented on the Shoalhaven Floodplain, southeast New South Wales, Australia have been designed to remediate ASS. These include: (1) fixed-level v-notch weirs, which raise the groundwater table above the pyritic layer and reduce the rate of discharge of acidic products from the groundwater into the drains; (2) modified two-way floodgates, which allow for tidal buffering of acidic drainage; (3) a subsurface alkaline horizontal impermeable lime-fly ash barrier, which prevents pyrite oxidation and neutralises acidic groundwater and (4) an alkaline permeable reactive barrier (PRB) using recycled materials, which significantly increases groundwater pH and reduces Al and Fe concentrations within and down-gradient of the PRB. A critical review of each of these strategies will outline their role in remediating ASS and their respective benefits and limitations

Effect of cyclic loading frequency on the permanent deformation and degradation of railway ballast

A series of large-scale cyclic triaxial tests were conducted on latite basalt aggregates (ballast) to investigate how the frequency faffects the permanent deformation and degradation of railway ballast. During testing the frequency was varied from 5 Hz to 60 Hz to simulate a range of train speeds from about 40 km/h to 400 km/h. Three categories of permanent deformation mechanisms were observed in response to the applied cyclic loads, namely, the inception of plastic shakedown (f60 Hz). The permanent strain of ballast and particle breakage increased with the frequency and number of load cycles. A cyclic strain ratio was introduced to capture the effect of frequency on the permanent axial and volumetric strains, respectively. An empirical equation was formulated to represent this relationship for latite basalt, and a critical train speed was identified. A good correlation was obtained between particle breakage and volumetric strain under cyclic loading