The geotextile filter design is particularly complex when granular base soils are internally unstable. In these conditions, the design criteria available in literature are not always reliable. This paper deals with a new theoretical method developed to evaluate the internal stability of granular soils. To simulate, theoretically, the filtration process inside these soils, a set of spherical particles and different soil relative densities have been considered. The soil has been represented by means of a sequence of parallel layers, containing constrictions and particles, placed upon each other at a distance, in the direction of hydraulic flow, which is a function of the soil relative density. The movement of the fine particles through the different soil layers has been simulated by means of a mechanism that compares each particle contained in the i layer with the constrictions contained in the next i + 1 layer. The results of the numerical simulations were used to evaluate the internal stability of the analyzed granular soil and the corresponding critical diameter of suffusion, Dc. Finally, the reliability of the proposed theoretical method was evaluated by means of the results of experimental long-term filtration tests performed using a rigid-wall permeameter on different unstable granular soils.
Permeable reactive barriers containing zero valent iron (ZVI-PRBs) represent an in situ technology for groundwater remediation. One of the main design issues of ZVI-PRBs is the determination of the expected lifespan. Therefore, the objective of the present work is to determine, by interpreting column experiments data through a first-order kinetic model, the thickness and duration of a hypothetical PRB constituted respectively by ZVI and by a ZVI/pumice mixture to be built for remediating groundwater contaminated by nickel at two different initial concentrations (40 and 8 mg/L). The kinetic constant values derived from this model are intended solely as an engineering tool for the design of ZVI systems and not as an interpretation of the actual reactions involving ZVI, water, and its constituents. Column test results show a linear decrease over time of the kinetic constant values. The use of the mixture implies an increase of the barrier thickness (from 2·5 to 15·5 times the ZVI thickness) but allows a reduction in the amount of ZVI needed to reach remediation objectives (from 84·5 to 87% considering 4 years of PRB operation) and an increase in the lifespan of the barrier (about 7·2 times for the lowest values of nickel concentration used).
A permeable reactive barrier (PRB) is a consolidated in situ technology for groundwater decontamination, but its long-term effectiveness, and thus the success of any remediation project, depends on the correct design in terms of geotechnical, chemical and hydraulic aspects. This paper examines the performance of a PRB and presents the results of an investigation into its long-term behaviour by using column tests. In particular, the geotechnical aspects involved in the design of a PRB and the effects of hydraulic conditions and contaminant concentration (nickel) are discussed. Moreover, the results of laboratory column tests carried out at various flow velocities and initial contaminant concentrations are discussed. The reactive media used in the tests are zero valent iron (ZVI) and a granular mixture of ZVI and pumice.
Recent experimental studies have shown that it is possible to improve the hydraulic and reactive behaviour of permeable reactive barriers (PRBs) composed of zero-valent iron (ZVI) by admixing this reactive medium with volcanic materials such as lapillus or pumice. In this study, the adsorption behaviour of lapillus and pumice was evaluated through batch tests carried out using nickel- and zinc-contaminated solutions at three different initial concentrations (5, 10 and 50 mg/l). It was observed that the lapillus sorption capacity is four and eight times higher than that of pumice for nickel and zinc, respectively. The comparison of column tests results, carried out using mixtures of ZVI/pumice and ZVI/lapillus with two different contaminated solutions (nickel and zinc at initial concentration of 50 mg/l), has shown that the best performance is by the ZVI/lapillus mixtures. In fact, thanks to the contribution of lapillus to contaminant removal, the use of ZVI/lapillus mixtures would allow reducing PRB thickness compared to the use of ZVI/pumice mixtures when the same mass of ZVI is considered. A column test carried out with lapillus only has also shown that it is possible to use the latter as a reactive medium in PRBs in the presence of low concentrations of contaminants.
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