Liquefaction, Screening, and Remediation of Silty Soils

Assessing liquefaction potential, in situ screening, and remediation of non-plastic silty soils are difficult problems. Presence of silt particles among the sand grains in silty soils affects the soil response in each of the above cases in different ways. First part of this paper addresses the effects of silt content on liquefaction resistance, hydraulic conductivity, and coefficient of consolidation of silty soils compared to clean sand. Secondly it addresses the effects of silt content and consolidation characteristics on cone resistance of silty soils and sand. A relationship between normalized cone resistance, liquefaction resistance, and consolidation characteristics is presented. The third part of the paper focuses on the effects of silt content and soil permeability on the effectiveness of dynamic compaction (DC) and vibratory stone column (SC) supplemented with wick drains to densify silty soils and mitigate liquefaction.

Shear Wave Velocity Relations for Silty and Gravely Soils

Shear wave velocity vs, dynamic shear modulus Gmax, and damping characteristics are important parameters required for both static and dynamic response analyses of earth structures. Traditional indirect methods for estimation of these parameters based on void ratio, relative density, and mean effective stress have been successful for rather narrowly graded soils, but not for the most commonly found silty and gravelly soils. Their direct application to determine the above characteristics for silty and gravely soils are not satisfactory. A primary reason for this is that global void ratio is not a good measure of intergrain contact density for granular mixes. A simple array of two-sized particle system with large size disparity is presented to highlight the relative roles of intercoarser and interfiner grain contacts on mechanical response parameters of such granular mixes. New parameters, namely equivalent intergranular void ratio (ec)eq, and equivalent interfine void ratio (ef)eq are introduced as indices of active intergrain contacts. They are related to shear modulus and vs of silty and gravely soils

Post-Liquefaction Pore Pressure Dissipation and Densification in Silty Soils

Pore pressure generation, and post-liquefaction dissipation and densification characteristics are data essential for detailed analysis of performance of sites containing liquefiable sands during and after earthquakes. These characteristics are also necessary for the design, analysis and choice of appropriate ground modification systems to mitigate liquefaction-induced hazards. Past research has addressed such material characteristics for clean sands. However, there are many sites that comprise non-plastic silts or silty sands have experienced liquefaction-induced damage. This paper presents results from an experimental study on silts and silty sands. Pore pressure generation characteristics are evaluated and compared with that of sands. Pre- and post-liquefaction compressibility and coefficient of consolidation, and densification characteristics are determined from undrained cyclic tests data followed by dissipation. Implications of these findings on the earthquake performance of sites containing non-plastic silts and silty sands are discussed. Their impacts on the choice of ground improvement techniques are also discussed

The influence of fines content and size-ratio on the micro-scale properties of dense bimodal materials

This paper considers factors influencing the fabric of bimodal or gap-graded soils. Discrete element method simulations were carried out in which the volumetric fines content and the size ratio between coarse and fine particles were systematically varied. Frictionless particles were used during isotropic compression to create dense samples; the coefficient of friction was then set to match that of spherical glass beads. The particle-scale data generated in the simulations revealed key size ratios and fines contents at which transitions in soil fabric occur. These transitions are identified from changes in the contact distributions and stress-transfer characteristics of the soils and by changes in the size of the void space between the coarse particles. The results are broadly in agreement with available experimental data on minimum void ratio and contact distributions. The results have implications for engineering applications including assessment of the internal stability of gap-graded soils in embankment dams and flood embankments

Poor quality vital anti-malarials in Africa - an urgent neglected public health priority

BACKGROUND: Plasmodium falciparum malaria remains a major public health problem. A vital component of malaria control rests on the availability of good quality artemisinin-derivative based combination therapy (ACT) at the correct dose. However, there are increasing reports of poor quality anti-malarials in Africa. METHODS: Seven collections of artemisinin derivative monotherapies, ACT and halofantrine anti-malarials of suspicious quality were collected in 2002/10 in eleven African countries and in Asia en route to Africa. Packaging, chemical composition (high performance liquid chromatography, direct ionization mass spectrometry, X-ray diffractometry, stable isotope analysis) and botanical investigations were performed. RESULTS: Counterfeit artesunate containing chloroquine, counterfeit dihydroartemisinin (DHA) containing paracetamol (acetaminophen), counterfeit DHA-piperaquine containing sildenafil, counterfeit artemether-lumefantrine containing pyrimethamine, counterfeit halofantrine containing artemisinin, and substandard/counterfeit or degraded artesunate and artesunate+amodiaquine in eight countries are described. Pollen analysis was consistent with manufacture of counterfeits in eastern Asia. These data do not allow estimation of the frequency of poor quality anti-malarials in Africa. CONCLUSIONS: Criminals are producing diverse harmful anti-malarial counterfeits with important public health consequences. The presence of artesunate monotherapy, substandard and/or degraded and counterfeit medicines containing sub-therapeutic amounts of unexpected anti-malarials will engender drug resistance. With the threatening spread of artemisinin resistance to Africa, much greater investment is required to ensure the quality of ACTs and removal of artemisinin monotherapies. The International Health Regulations may need to be invoked to counter these serious public health problems

Biodiversity inventories in high gear: DNA barcoding facilitates a rapid biotic survey of a temperate nature reserve

Comprehensive biotic surveys, or ‘all taxon biodiversity inventories’ (ATBI), have traditionally been limited in scale or scope due to the complications surrounding specimen sorting and species identification. To circumvent these issues, several ATBI projects have successfully integrated DNA barcoding into their identification procedures and witnessed acceleration in their surveys and subsequent increase in project scope and scale. The Biodiversity Institute of Ontario partnered with the rare Charitable Research Reserve and delegates of the 6th International Barcode of Life Conference to complete its own rapid, barcode-assisted ATBI of an established land trust in Cambridge, Ontario, Canada