Stochastic solution of geotechnical problems in truly discrete media

This research deals with the solution of geotechnical problems on intermediate length scales, i.e. when the length scale of interest is larger than the size of the grains of the soil (or rockfill) but the medium cannot be considered as a continuous body. This is because on such scales, despite the large number of involved grains, the volumetric average stress fluctuates around the mean value and the fluctuation is due to the truly discrete nature of the soil. Then, the smooth stress field that would be predicted by continuum mechanics approaches is replaced by a stochastic system of interparticle forces forming force chains. The forces can be transformed into equivalent stresses by means of homogenization techniques, but the obtained fields are again non-smooth and stochastic. A classical statistical mechanics framework is followed to anticipate the probability distribution functions of equivalent (extensive) stresses according to the macroscopic constraints of the problem. In particular, we get stochastic models for two seminal problems in geotechnics: the at rest lateral earth pressure acting on a retaining wall and the vertical stress at a given point in the soil that is caused by a vertical surface load. The theory is validated through massive numerical simulation with the Discrete Element Method. Mesoscale geotechnical analysis can find its main applications in the case of rockfill or other very coarse granular materials. However, it could be useful as well for laboratory, numerical and theoretical researches that are approached on small length scales. This theoretical framework contributes to fill the gap between micro and macro geotechnics and the resulting stochastic models may be useful for reliability analyses

Statistical mechanics as guidance for particle-based computational methods

Particle-based methods apply some laws of Classical Mechanics to all the particles of a granular system. On the other hand, classical Statistical Mechanics deals with systems consisting of a lot of particles, focusing on the statistical distribution of some intensive properties. Consequently, the macroscopic behaviour and the average properties of the system in equilibrium are based on some microscopic considerations. To do that, statistical tools and mechanical laws are used together. For an implemented particle-based method to be realistic enough, then the obtained simulations should satisfy some basic underlying physical requirements, and Statistical Mechanics is a useful tool to establish such requirements. This paper presents some results based on Statistical Mechanics that are useful for a realistic modelling of granular systems using Particle-based methods. Examples of the discussed issues include that the same protocol makes a disordered granular medium attain always the same packing ratio; that some local arrangements are less probable than others; that there is a well-established limit to the achievable density of granular assemblies of hard spheres; and that the equilibrium of disordered granular systems does not coincide to the state of minimum potential energy (which would be get in a completely ordered arrangement). The consideration of such issues in practical applications could be helpful to save time in computational methods, to avoid mistakes or, at least, to verify that the implementations are realistic

DEM simulation of triaxial tests of railway ballast fouled with desert sand

Some high-speed rail lines go through desert zones where sand particles transported by winds may foul track ballast layers. This fouling can be troublesome since it increases the stiffness of the layer and reduces its capacity to absorb vibrations from the rolling stock. We are studying this phenomenon through both laboratory and numerical experiments. In the laboratory, we performed two kinds of experiments: 9 inches triaxial tests and physical modelling in the CEDEX Track Box testing facility. The latter is a unique 1:1 model of railway track section (of dimensions 21 m ×5 m ×4 m) that has been built to model high-speed rail lines (with passenger and freight trains passing at velocities of up to 400 km/h). The laboratory experiments allowed us to measure the change of stiffness with the fouling level (represented through the void contaminant index, VCI). Numerical simulations are being performed with the Discrete Element Method, reproducing drained triaxial test conditions. Due to the considerable different size of railway ballast and sand grains, we are using idealized packings of spherical particles to study this phenomenon. We are paying particular attention to the sample size effects and are registering the evolution of the stiffness with the fouling level up to high values of VCI. The results obtained from these idealized systems will be contrasted to the laboratory experiments carried out with real railway ballast and sand

The outer membranes of Brucella spp. are not barriers to hydrophobic permeants

The patterns of susceptibility to hydrophobic and hydrophilic drugs and the uptake of the fluorescent probe N-phenyl-naphthylamine in Brucella spp., Haemophilus influenzae, Escherichia coli, and deep rough Salmonella minnesota mutants were compared. The results show that the outer membranes of smooth and naturally rough Brucella spp. do not represent barriers to hydrophobic permeants and that this absence of a barrier relates at least in part to the properties of Brucella lipopolysaccharide

The outer membranes of Brucella spp. are resistant to bactericidal cationic peptides

The actions of polymyxin B, rabbit polymorphonuclear lysosome extracts, 14 polycationic peptides (including defensin NP-2, cecropin P1, lactoferricin B, and active peptides from cationic protein 18 and bactenecin), EDTA, and Tris on Brucella spp. were studied, with other gram-negative bacteria as controls. Brucella spp. were comparatively resistant to all of the agents listed above and bound less polymyxin B, and their outer membranes (OMs) were neither morphologically altered nor permeabilized to lysozyme by polymyxin B concentrations, although both effects were observed for controls. EDTA and peptides increased or accelerated the partition of the hydrophobic probe N-phenyl-naphthylamine into Escherichia coli and Haemophilus influenzae OMs but had no effect on Brucella OMs. Since Brucella and H. influenzae OMs are permeable to hydrophobic compounds (G. Martínez de Tejada and I. Moriyón, J. Bacteriol. 175:5273-5275, 1993), the results show that such unusual permeability is not necessarily related to resistance to polycations. Although rough (R) B. abortus and B. ovis were more resistant than the controls were, there were qualitative and quantitative differences with smooth (S) brucellae; this may explain known host range and virulence differences. Brucella S-lipopolysaccharides (LPSs) had reduced affinities for polycations, and insertion of Brucella and Salmonella montevideo S-LPSs into the OM of a Brucella R-LPS mutant increased and decreased, respectively, its resistance to cationic peptides. The results show that the core lipid A of Brucella LPS plays a major role in polycation resistance and that O-chain density also contributes significantly. It is proposed that the features described above contribute to Brucella resistance to the oxygen-independent systems of phagocytes

Impact of changing oxygenation policies on retinopathy of prematurity in a neonatal unit in Argentina.

AIMS: To assess the impact of different oxygenation policies on the rate and severity of retinopathy of prematurity (ROP). METHODS: Between January 2003 and December 2006, infants of 1500 g birthweight (BW) or less and/or 32 weeks gestational age (GA) or less, and larger, more mature infants with risk factors for ROP were examined through three different time periods: period 1: high target oxygen saturation levels (88-96%) and treatment at threshold ROP; period 2: low target oxygen saturation levels (83-93%) and treatment at threshold ROP; period 3: low target oxygen saturation and treatment at type 1 ROP. RESULTS: Type 1 ROP was detected more frequently in babies of 32 weeks GA or less (50/365, 13.7%) than in more mature babies (15/1167, 1.3%; p<0.001). The rate of type 1 ROP in period 1 was 6.9%; period 2, 3.6% and period 3, 1.8%. Rates of stage 3 ROP declined over time in both BW/GA groups (from 9.0% to 4.1% to 2.0%) as did rates of plus disease (from 7.5% to 3.6% to 1.8%). Mean BW and GA declined from period 1 to period 3, and death rates remained unchanged. 74.4% of babies received all the examinations required; 48.1% of treatments were undertaken after discharge from the neonatal unit. CONCLUSIONS: Lower target oxygen saturation was associated with a lower rate of severe ROP without increasing mortality, and changed the characteristics of affected babies. Screening criteria need to remain wide enough to identify all babies at risk of ROP needing treatment

Lime-based rendering mortars with photocatalytic and hydrophobic agents: assessment of the water repellency and biocide effect

Different rendering mortars were prepared by mixing air lime and air lime-pozzolanic nanosilica with TiO2 and sodium oleate as, respectively, photocatalytic and water repellent agents, added in bulk. The aim of the work was to design and obtain new rendering mortars with improved durability focusing in the reduction of the water absorption of these materials and in their self-cleaning and biocide effect. To achieve a better distribution of the TiO2 particles, which was expected to enhance their efficiency, different dispersing agents were also incorporated to the fresh mixtures. Four diverse polycarboxylate ethers superplasticizers and a poly-naphthalene-sulfonate were tested. Workability and fluidity of the fresh rendering mortars were determined to guarantee the applicability of the final products. Water contact angle was monitored with the aim of assessing the hydrophobicity of the mortars lent by the water repeller. The biocide effect was studied by means of the culture of a strain of Pseudomonas fluorescens. The colonization of the mortars’ surface was analyzed by determining the number of colonies forming units (CFU) after several days subjecting the samples to suitable T and RH conditions. At the same time, the surface of the mortars was irradiated with solar light to activate the photocatalyst. Results showed the efficiency of the sodium oleate in reducing the water uptake of the rendering mortars. Good compatibility between the water repellent agent, the pozzolanic additive and some of the polycarboxylate superplasticizers was observed. The presence of the photocatalyst was found to be very effective in preventing microbiological colonization

Structural features governing the activity of lactoferricin-derived peptides that act in synergy with antibiotics against Pseudomonas aeruginosa in vitro and in vivo

Pseudomonas aeruginosa is naturally resistant to many antibiotics, and infections caused by this organism are a serious threat, especially to hospitalized patients. The intrinsic low permeability of P. aeruginosa to antibiotics results from the coordinated action of several mechanisms, such as the presence of restrictive porins and the expression of multidrug efflux pump systems. Our goal was to develop antimicrobial peptides with an improved bacterial membrane-permeabilizing ability, so that they enhance the antibacterial activity of antibiotics. We carried out a structure activity relationship analysis to investigate the parameters that govern the permeabilizing activity of short (8- to 12-amino-acid) lactoferricin-derived peptides. We used a new class of constitutional and sequence-dependent descriptors called PEDES (peptide descriptors from sequence) that allowed us to predict (Spearman's ρ = 0.74; P < 0.001) the permeabilizing activity of a new peptide generation. To study if peptide-mediated permeabilization could neutralize antibiotic resistance mechanisms, the most potent peptides were combined with antibiotics, and the antimicrobial activities of the combinations were determined on P. aeruginosa strains whose mechanisms of resistance to those antibiotics had been previously characterized. A subinhibitory concentration of compound P2-15 or P2-27 sensitized P. aeruginosa to most classes of antibiotics tested and counteracted several mechanisms of antibiotic resistance, including loss of the OprD porin and overexpression of several multidrug efflux pump systems. Using a mouse model of lethal infection, we demonstrated that whereas P2-15 and erythromycin were unable to protect mice when administered separately, concomitant administration of the compounds afforded long-lasting protection to one-third of the animals