A finite element solution scheme for an elastic–viscoplastic soil model

A Newton–Raphson solution scheme with a stress point algorithm is presented for the implementation of an elastic–viscoplastic soilmodel in a finite element program. Viscoplastic strain rates are calculated using the stress and volumetric states of the soil. Sub-incrementsof time are defined for each iterative calculation of elastic–viscoplastic stress changes so that their sum adds up to the time incrementfor the load step. This carefully defined ‘iterative time’ ensures that the correct amount of viscoplastic straining is accumulated overthe applied load step. The algorithms and assumptions required to implement the solution scheme are provided. Verification of the solutionscheme is achieved by using it to analyze typical boundary value problems.

An improved elastic-viscoplastic soil model

This paper develops an improved and accessible framework for modelling time-dependent behaviour of soils using the concepts of elasticity and viscoplasticity. The mathematical description of viscoplastic straining is formulated based on a purely viscoplastic and measurable phenomenon, namely creep. The resulting expression for the viscoplastic strain rates includes a measure of both effective stress and the corresponding volumetric packing of the soil particles. In this way, the model differs from some earlier viscoplastic models and arguably provides a better conceptual description of time-dependent behaviour. Analytical solutions are developed for the simulation of drained and undrained strain-controlled triaxial compression tests. The model is then used to back-analyze the measured response of normally consolidated to moderately overconsolidated specimens of a soft estuarine soil in undrained triaxial compression. The model captures aspects of soil behaviour that cannot be simulated using time-independent elastic–plastic models. Specifically, it can capture the dependence of stress–strain relationships and undrained shear strength on strain rate, the development of irrecoverable plastic strains at constant stress (creep), and the relaxation of stresses at constant strain

Finite element analysis of an embankment on a soft estuarine deposit using an elastic–viscoplastic soil model

A new elastic–viscoplastic (EVP) soil model has been used to simulate the measured deformation response of a soft estuarine soil loaded by a stage-constructed embankment. The simulation incorporates prefabricated vertical drains installed in the foundation soils and reinforcement installed at the base of the embankment. The numerical simulations closely matched the temporal changes in surface settlement beneath the centerline and shoulder of the embankment. More importantly, the elastic–viscoplastic model simulated the pattern and magnitudes of the lateral deformations beneath the toe of the embankment — a notoriously difficult aspect of modelling the deformation response of soft soils. Simulation of the excess pore-water pressure proved more difficult because of the heterogeneous nature of the estuarine deposit. Excess pore-water pressures were, however, mapped reasonably well at three of the six monitoring locations. The simulations were achieved using a small set of material constants that can easily be obtained from standard laboratory tests. This study validates the use of the EVP model for problems involving soft soil deposits beneath loading from a geotechnical structure.

A commentary on climate change, stone decay dynamics and the ‘greening’ of natural stone buildings: new perspectives on ‘deep wetting’

Environmental controls on stone decay processes are rapidly changing as a result of changing climate. UKCP09 projections for the 2020s (2010–2039) indicate that over much of the UK seasonality of precipitation will increase. Summer dryness and winter wetness are both set to increase, the latter linked to projected precipitation increases in autumn and spring months. If so, this could increase the time that stone structures remain wet and possibly the depth of moisture penetration, and it appears that building stone in Northern Ireland has already responded through an increased incidence of algal ‘greening’.This paper highlights the need for understanding the effects of climate change through a series of studies of largely sandstone structures. Current and projected climatic trends are therefore considered to have aesthetic, physical and chemical implications that are not currently built into our models of sandstone decay, especially with respect to the role played by deep-seated wetness on sandstone deterioration and decay progression and the feedbacks associated with, for example surface algal growth. In particular,it is proposed that algal biofilms will aid moisture retention and further facilitate moisture and dissolved salt penetration to depth. Thus, whilst the outer surface of stone may continue to experience frequent wetting and drying associated with individual precipitation events, the latter is less likely to be complete, and the interiors of building blocks may only experience wetting/drying in response to seasonal cycling. A possible consequence of deeper salt penetration could be a delay in the onset of surface deterioration,but more rapid and effective retreat once it commences as decay mechanisms ‘tap into a reservoir of deep salt’.

Soil resource supply influences faunal size-specific distributions in natural food webs

The large range of body-mass values of soil organisms provides a tool to assess the ecological organization of soil communities. The goal of this paper is to identify graphical and quantitative indicators of soil community composition and ecosystem functioning, and to illustrate their application to real soil food webs. The relationships between log-transformed mass and abundance of soil organisms in 20 Dutch meadows and heathlands were investigated. Using principles of allometry, maximal use can be made of ecological theory to build and explain food webs. The aggregate contribution of small invertebrates such as nematodes to the entire community is high under low soil phosphorus content and causes shifts in the mass-abundance relationships and in the trophic structures. We show for the first time that the average of the trophic link lengths is a reliable predictor for assessing soil fertility responses. Ordered trophic link pairs suggest a self-organizing structure of food webs according to resource availability and can predict environmental shifts in ecologically meaningful ways.

Neutron diffraction, NMR and molecular dynamics study of glucose dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate

beta-D-glucose dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate in a 6 : 1 molar ratio (ionic liquid : glucose) has been studied by neutron scattering, NMR and molecular dynamics simulations. Good agreement was found between simulated neutron scattering profiles generated for isotopically substituted liquid systems and those experimentally determined as well as between simulated and experimental diffusion coefficients obtained by Pulsed Field Gradient NMR spectroscopy. The overriding glucose-ionic liquid interactions in the liquid are hydrogen-bonding between acetate oxygens and sugar hydroxyl groups. The ionic liquid cation was found to play only a minor role in the solvation of the sugar and does not participate in hydrogen-bonding with the sugar to any significant degree. NOESY experiments lend further evidence that there is no direct interaction between sugar hydroxyl groups and acidic hydrogens on the ionic liquid cation.

Contrasting effects of a nonionic surfactant on the biotransformation of polycyclic aromatic hydrocarbons to cis-dihydrodiols by soil bacteria

The biotransformation of the polycyclic aromatic hydrocarbons (PAHs) naphthalene and phenanthrene was investigated by using two dioxygenase-expressing bacteria, Pseudomonas sp. strain 9816/11 and Sphingomonas yanoikuyae B8/36, under conditions which facilitate mass-transfer limited substrate oxidation. Both of these strains are mutants that accumulate cis-dihydrodiol metabolites under the reaction conditions used. The effects of the nonpolar solvent 2,2,4,4,6,8,8-heptamethylnonane (HMN) and the nonionic surfactant Triton X-100 on the rate of accumulation of these metabolites were determined. HMN increased the rate of accumulation of metabolites for both microorganisms, with both substrates. The enhancement effect was most noticeable with phenanthrene, which has a lower aqueous solubility than naphthalene. Triton X-100 increased the rate of oxidation of the PAHs with strain 9816/11 with the effect being most noticeable when phenanthrene was used as a substrate. However, the surfactant inhibited the biotransformation of both naphthalene and phenanthrene with strain B8/36 under the same conditions. The observation that a nonionic surfactant could have such contrasting effects on PAH oxidation by different bacteria, which are known to be important for the degradation of these compounds in the environment, may explain why previous research on the application of the surfactants to PAH bioremediation has yielded inconclusive results. The surfactant inhibited growth of the wild-type strain S. yanoikuyae B1 on aromatic compounds but did not inhibit B8/36 dioxygenase enzyme activity in vitro.

A millennial record of environmental change in peat deposits from the Misten bog (East Belgium)

In this study, palaeoenvironmental changes recorded in the top metre of a peat profile (Misten bog, East Belgium) were investigated using a multiproxy approach. Proxies include bulk density, Ti and Si content, pollen, macrofossils, d13C on specific Sphagnum stems, and d13C–d18O on Sphagnum leaves. A high-resolution chronology was generated using 210Pb measurements and 22 14C AMS dates on carefully selected Sphagnum macrofossils. d13C only records large change in mire surface wetness. This is partly due to the fact that the core was taken from the edge of a hummock, which may make it difficult to track small isotopic changes. The d13C signal seems to be dependent upon the Sphagnum species composition. For example, a change between Sphagnum section Cuspidata towards Sphagnum imbricatum causes a significant drop in the d13C values. On the whole, the C and O isotopes record two shallow pool phases during the 8th–9th and the 13th centuries. Pollen and atmospheric soil dust (ASD) fluxes records increased human occupation in the area. There may be some climatic signals in the ASD flux, but they are difficult to decipher from the increasing human impact (land clearance, agriculture) during the last millennium. The variations in the proxies are not always synchronous, suggesting different triggering factors (temperature, wetness, windiness) for each proxy. This study also emphasizes that, compared to studies dealing with pollution using geochemical proxies, palaeoclimatic inferences from peat bogs need as many proxies as possible, together with highly accurate and precise age-models, in order to better understand climate variability and their consequences during the Holocene.

Stable isotope probing analysis of the influence of liming on root exudate utilization by soil microorganisms

Rhizosphere microorganisms play an important role in soil carbon flow, through turnover of root exudates, but there is little information on which organisms are actively involved or on the influence of environmental conditions on active communities. In this study, a (CO2)-C-13 pulse labelling field experiment was performed in an upland grassland soil, followed by RNA-stable isotope probing (SIP) analysis, to determine the effect of liming on the structure of the rhizosphere microbial community metabolizing root exudates. The lower limit of detection for SIP was determined in soil samples inoculated with a range of concentrations of C-13-labelled Pseudomonas fluorescens and was found to lie between 10(5) and 10(6) cells per gram of soil. The technique was capable of detecting microbial communities actively assimilating root exudates derived from recent photo-assimilate in the field. Denaturing gradient gel electrophoresis (DGGE) profiles of bacteria, archaea and fungi derived from fractions obtained from caesium trifluoroacetate (CsTFA) density gradient ultracentrifugation indicated that active communities in limed soils were more complex than those in unlimed soils and were more active in utilization of recently exuded C-13 compounds. In limed soils, the majority of the community detected by standard RNA-DGGE analysis appeared to be utilizing root exudates. In unlimed soils, DGGE profiles from C-12 and C-13 RNA fractions differed, suggesting that a proportion of the active community was utilizing other sources of organic carbon. These differences may reflect differences in the amount of root exudation under the different conditions.

Carbon flow in an upland grassland: effect of liming on the flux of recently photosynthesized carbon to rhizosphere soil

The effect of liming on the flow of recently photosynthesized carbon to rhizosphere soil was studied using (CO2)-C-13 pulse labelling, in an upland grassland ecosystem in Scotland. The use of C-13 enabled detection, in the field, of the effect of a 4-year liming period of selected soil plots on C allocation from plant biomass to soil, in comparison with unlimed plots. Photosynthetic rates and carbon turnover were higher in plants grown in limed soils than in those from unlimed plots. Higher delta(13)C% values were detected in shoots from limed plants than in those from unlimed plants in samples clipped within 15 days of the end of pulse labelling. Analysis of the aboveground plant production corresponding to the 4-year period of liming indicated that the standing biomass was higher in plots that received lime. Lower delta(13)C% values in limed roots compared with unlimed roots were found, whereas no significant difference was detected between soil samples. Extrapolation of our results indicated that more C has been lost through the soil than has been gained via photosynthetic assimilation because of pasture liming in Scotland during the period 1990-1998. However, the uncertainty associated with such extrapolation based on this single study is high and these estimates are provided only to set our findings in the broader context of national soil carbon emissions.

Arsenic and selenium mobilisation from organic matter treated mine spoil with and without inorganic fertilisation

Organic matter amendments are applied to contaminated soil to provide a better habitat for revegetation and remediation, and olive mill waste compost (OMWC) has been described as a promising material for this aim. We report here the results of an incubation experiment carried out in flooded conditions to study its influence in As and metal solubility in a trace elements contaminated soil. NPK fertilisation and especially organic amendment application resulted in increased As, Se and Cu concentrations in pore water. Independent of the amendment, dimethylarsenic acid (DMA) was the most abundant As species in solution. The application of OMWC increased pore water dissolved organic-carbon (DOC) concentrations, which may explain the observed mobilisation of As, Cu and Se; phosphate added in NPK could also be in part responsible of the mobilisation caused in As. Therefore, the application of soil amendments in mine soils may be particularly problematic in flooded systems. (C) 2012 Elsevier Ltd. All rights reserved.