Stephen Wilkinson

Effect of wind turbulence on gas transport in porous media: experimental method and preliminary results

We demonstrate a novel experimental arrangement for measuring wind turbulence-induced gas transport in dry porous media under controlled conditions. This equipment was applied to assess the effect of wind turbulence on gas transport (quantified as a dispersion coefficient) as a function of distance to the surface of the porous medium exposed to wind. Two different strategies for the measurement of wind-induced gas transport were compared. Experiments were carried out with O2 and CO2 as tracer gases with average vertical wind speeds of 0.02–1.06 m s−1. Oxygen breakthrough curves as a function of distance to the wind-exposed surface of the porous medium were analysed numerically with a finite-difference-based model to assess gas transport. We showed that wind turbulence-induced gas transport is an important transport mechanism that can be 20–70 times larger than molecular diffusion-induced transport. Wind conditions and properties of the porous medium had strong controlling effects on this relationship. Importantly, we show that even though wind-induced gas transport is greatest near to the wind-exposed surface, it can have marked effects on the variation in gas concentration at much greater depths.

Next-generation sequencing showing potential leachate influence on bacterial communities around a landfill in China

The impact of contaminated leachate on groundwater from landfills is well known, but the specific effects on bacterial consortia are less well-studied. Bacterial communities in a landfill and an urban site located in Suzhou, China, were studied using Illumina high-throughput sequencing. A total of 153 944 good-quality reads were produced and sequences assigned to 6388 operational taxonomic units. Bacterial consortia consisted of up to 16 phyla, including Proteobacteria (31.9%-94.9% at landfill, 25.1%-43.3% at urban sites), Actinobacteria (0%-28.7% at landfill, 9.9%-34.3% at urban sites), Bacteroidetes (1.4%-25.6% at landfill, 5.6%-7.8% at urban sites), Chloroflexi (0.4%-26.5% at urban sites only), and unclassified bacteria. Pseudomonas was the dominant (67%-93%) genus in landfill leachate. Arsenic concentrations in landfill raw leachate (RL) (1.11 × 103 μg/L) and fresh leachate (FL2) (1.78 × 103 μg/L) and mercury concentrations in RL (10.9 μg/L) and FL2 (7.37 μg/L) exceeded Chinese State Environmental Protection Administration standards for leachate in landfills. The Shannon diversity index and Chao1 richness estimate showed RL and FL2 lacked richness and diversity when compared with other samples. This is consistent with stresses imposed by elevated arsenic and mercury and has implications for ecological site remediation by bioremediation or natural attenuation.

Active earth pressure acting on retaining wall considering anisotropic seepage effect

This paper presents a general solution for active earth pressure acting on a vertical retaining wall with a drainage system along the soil-structure interface. The backfill has a horizontal surface and is composed of cohesionless and fully saturated sand with anisotropic permeability along the vertical and horizontal directions. The extremely unfavourable seepage flow on the back of the retaining wall due to heavy rainfall or other causes will dramatically increase the active earth pressure acting on the retaining walls, increasing the probability of instability. In this paper, an analytical solution to the Laplace differential governing equation is presented for seepage problems considering anisotropic permeability based on Fourier series expansion method. A good correlation is observed between this and the seepage forces along a planar surface generated via finite element analysis. The active earth pressure is calculated using Coulomb’s earth pressure theory based on the calculated pore water pressures. The obtained solutions can be degenerated into Coulomb’s formula when no seepage exists in the backfill. A parametric study on the influence of the degree of anisotropy in seepage flow on the distribution of active earth pressure behind the wall is conducted by varying ratios of permeability coefficients in the vertical and horizontal directions, showing that anisotropic seepage flow has a prominent impact on active earth pressure distribution. Other factors such as effective internal friction angle of soils and soil/wall friction conditions are also considered.

Stimulation of Indigenous Carbonate Precipitating Bacteria for Ground Improvement

Calcite minerals are precipitated in soil through biomineralisation which can be either organic or inorganic in nature. Biomineralisation can be employed to improve ground conditions in its natural state. Usually, studies of applied biomineralisation are highly interdisciplinary involving expertise from engineers, chemists and microbiologists. In this paper, we study the potential of biomineralisation from indigenous bacteria present in soil. The soil samples were collected from a high permeable zone and the bacteria that inhabit the soil were stimulated at a temperature of 15°C. A cementation solution consisting of 500mM calcium chloride, urea and nutrient broth at a pH of 7.5 was added to the soil samples. Inorganic precipitation was found to be dominant and was more efficient when compared to organic precipitation. Carbonate precipitation data indicated that inorganic precipitation were 1.37 times better at carbonate formation in comparison to organic precipitation. Scanning Electron Microscopy analysis identified cementation bonds formed between soil particles. It was deducted that organic precipitation is dependent on temperature, and may take an extended time at such low temperature. The preliminary data presented in this paper suggests that the implementation of biomineralisation with in-situ microbes is promising but requires further laboratory and field investigation before being considered for engineering application.

MICP and Advances towards Eco-Friendly and Economical Applications

Biomineralization is a natural process aided by living organisms. Due to its applicability in ground improvement and bioremediation, Microbially Induced Calcite Precipitation (MICP) is an interdisciplinary field of study combining engineering, chemistry and microbiology. Bioremediation has been applied widely for contamination containment or removal, in this case it will be containment. MICP can also be applied to improve the efficiency of insitu bioremediation. Urease is an enzyme which can facilitate increased calcite precipitation. However the production of urease by bacteria and thus the resulting carbonate precipitation are inhibited by environmental factors including calcium concentration, bacterial concentration, pH and temperature. Under good conditions MICP can be used for heavy metal and radionuclide immobilization. However technologies such as bioconsolidation and biocementation require improvement such as time and cost. This paper highlights the application of MICP in addition to suggested improvements to make it more eco-friendly and sustainable.

A Case Study on the Microstructure of Fibrous Peat (West Lake, China)

The classification of peat soils involves a very large number of different types. From a descriptive perspective this is useful, however such a system generates too many options for engineering purposes. The behaviour of organic soils varies based on the quantity and type of organic material present within the soil. The effects of fibre content are particularly important. The West Lake in Hangzhou has been dredged many times during its history to maintain its beauty. During the most recent dredging the sludge from the lake was transported via a 4 km pipeline and deposited inside the Jiangyangfan Reservoir. The organic soil situated in Jiangyangfan Ecopark is a particularly interesting peaty material. The organic sludge was mixed and homogenised during the transportation process, and then settled out within the reservoir. This resulted in a more than 20 m thick peat layer deposited with an uneven surface. The Ecopark buildings were then constructed on top of this in 2008. A combined electron microscope and mechanical study of the microstructure and behaviour of the peat has been used to identify the engineering impact of the presence of relatively small number of fibres within the soil matrix. The fibres within the peat modify its behaviour such that it can no longer be understood within the typical critical state framework for soils. The peat starts to deform plastically under very small levels of applied stress. In addition, it does not display a tension cut-off failure and, ultimately, fails in shear.

An Assessment of Particle Characteristics for the Analysis of Wind Turbulence Generated Gas Transport

Gas flow through porous media is an important process for engineers to assess as a part of the design process. It can be modelled physically and numerically. Hazardous gases ranging from flammable vapours to radioactive gases can migrate through soil and are a concern on contaminated sites. Wind turbulence, which can be caused by the presence of man-made structures, can generate 20–70 times the amount of gas dispersion compared to calm conditions. The extent and rate of gas flow through a soil medium is dependent on the properties of the soil, including particle characteristics and the boundary conditions of the soil which includes flow rate and turbulence (gustiness). Such boundary effects may be strongly influenced by local site conditions. The important particle characteristics for gas flow through soils include primarily particle size, with minor contributions from shape and roughness. Many standardised methods for particle size measurement assume that the particles are spherical and so become much less accurate for elongate and irregular particles. Imaging techniques in combination with computational analysis allow more accurate quantification. An analysis of particle shape in two and three dimensions has been undertaken allowing the measurement of particle size and roughness. This can be used to inform models of soil behaviour. The data from this analysis includes roughness measurements from 453 particles. This data is available on request.

An Electron Microscope Study of Biomineralisation for Geotechnical Engineering Purposes

Directed biomineralisation, or using microorganisms to cause the formation of minerals, has been proposed as an effective method for permeability reduction and ground improvement. Where the precipitated mineral is a carbonate, heavy metal carbonates (e.g. otavite, malachite/azurite, cerussite, smithsonite, clearcreekite) can form, locking in heavy metal contamination. Where calcium carbonate forms there is an additional benefit of a high pH which, due to the buffering effect, can greatly reduce the mobility of heavy metal ions. Seven bacteria obtained from the soil & landfill leachate environments in Suzhou China, were induced to precipitate calcium carbonate under laboratory conditions within a medium consisting of a calcium source, urea and nutrient broth in a conical flask. Trials within clean sand columns resulted in a permeability which was 1/5 of that of a non-microbial column in addition to relative increases in strength of X3-5. On this basis, it is suggested that some geotechnical works using biomineralisation may be achieved without requiring external sources bacteria. This may be achieved either by isolating and growing the bacteria for application in the ground, or where growth can be achieved, by stimulating the bacteria in situ. An electron microscope assessment of the mineral structures formed by the bacteria indicates that a variety of different crystal forms are generated by the biomineralisation process. Some crystal structures, especially the open crystal structures, are of less use for engineering purposes. This indicates that not all bacteria that can precipitate carbonates would be of use for achieving geotechnical aims.

Biomineralisation performance of bacteria isolated from a landfill in China

We report an investigation of microbially induced carbonate precipitation by seven indigenous bacteria isolated from a landfill in China. Bacterial strains were cultured in a medium supplemented with 25 mmol/L calcium chloride and 333 mmol/L urea. The experiments were carried out at 30 °C for 7 days with agitation by a shaking table at 130 r/min. Scanning electron microscopic and X-ray diffraction analyses showed variations in calcium carbonate polymorphs and mineral composition induced by all bacterial strains. The amount of carbonate precipitation was quantified by titration. The amount of carbonate precipitated in the medium varied among isolates, with the lowest being Bacillus aerius rawirorabr15 (LC092833) precipitating around 1.5 times more carbonate per unit volume than the abiotic (blank) solution. Pseudomonas nitroreducens szh_asesj15 (LC090854) was found to be the most efficient, precipitating 3.2 times more carbonate than the abiotic solution. Our results indicate that bacterial carbonate precipitation occurred through ureolysis and suggest that variations in carbonate crystal polymorphs and rates of precipitation were driven by strain-specific differences in urease expression and response to the alkaline environment. These results and the method applied provide benchmarking and screening data for assessing the bioremediation potential of indigenous bacteria for containment of contaminants in landfills.

The Influence of Geological History on Preferred Particle Orientation and the Observed Anisotropy of Over Consolidated UK Mudrocks

One of the major aspects of mudrocks which influences their engineering behaviour is anisotropy, especially that of strength and stiffness. Anisotropy is caused by an underlying preferential alignment in particle orientations. Many factors contribute to the production of enhanced preferred particle orientations including: sedimentation process; particle shapes; bioturbation; burial depth; tectonism; weathering; and aging; in short the material’s entire geological history. Initial structure is formed by sedimentation processes during deposition. These are then modified by post-depositional events. With burial, the particles of sediments tend to rearrange to allow for a decrease in void ratio which is combined with an expulsion of water from the soil. Much of the southern UK is underlain by mudrocks of Mesozoic and Cenozoic age displaying some degree of preferred particle orientation and hence anisotropy in their engineering behaviour. The degree of particle orientation is quantified by analysis using environmental scanning electron microscope imagery. Anisotropy in engineering behaviour has been quantified by a range of laboratory and field, static and dynamic, methods that allow the anisotropic elastic behaviour of the mudrocks to be investigated at very small strains. Generally good agreement is observed between four fully independent methods for evaluating the elastic GVH stiffness mode. The results of both the image analysis and the laboratory testing build up a picture of microstructure anisotropy that results from the total geological history of each of the mudrocks investigated.