A three-dimensional parametric study of the use of soil nails for stabilising tunnel faces

Applying an effective nailing system at a tunnel heading, not only improves the stability of the tunnel heading and limits deformation at the tunnel face, but it also reduces volume loss during excavation and hence reduces ground surface settlement. The effectiveness of a soil nail system is affected by many factors such as the diameter and stiffness of the nails. In this paper, a systematic parametric study was conducted to study the axial rigidity of a nail, E_nA_n, for improving the stability of tunnel headings and reducing ground movements in stiff clay. The parametric study involved a series of three-dimensional elasto-plastic coupled-consolidation finite element analyses. The stability of the tunnel face is improved with increasing E_nA_n. For a given nail density applied at the tunnel face, an optimum axial rigidity of the nail (E_nA_n)_opt can be identified. The efficiency of the nailing system diminishes when (E_nA_n)_opt is reached. The use of a soil nailing system reduces the magnitude of stress relief at the tunnel heading during excavation. Thus, this reduction of stress relief minimises the amount of soil yielding and excess pore water pressure generated in the soil around the tunnel heading.     

Morphologic and spectral investigation of exceptionally well-preserved bacterial biofilms from the Oligocene Enspel formation, Germany

The fossilised soft tissues of a tadpole and an associated coprolitic structure from the organic-rich volcanoclastic lacustrine Upper Oligocene Enspel sediments (Germany) were investigated using high-resolution imaging techniques and nondestructive in situ surface analysis. Total organic carbon analysis of the coprolite and the sediment revealed values of 28.9 and 8.9% respectively. The soft tissues from the tadpole and the coprolite were found to be composed of 0.5 to 1 μm-sized spheres and rod shapes. These features are interpreted as the fossil remains of bacterial biofilms consisting probably of heterotrophic bacteria and fossilised extracellular polymeric substances. They became fossilised while in the process of degrading the organic matter of the organism and the coprolite. Comparison with a modern marine biofilm revealed morphologic details identical to those observed in the fossil bacterial biofilms. Although the fossil biofilms on both macrofossils exhibited identical microtextures, their mode of preservation was inhomogeneous and varied between calcium phosphate and an as yet unidentified mineral phase consisting mainly of Si, Ca, Ti, P, and S, but also showing the presence of Mg, Al, and Fe. The coprolite consists purely of fossilised bacterial cells in a densely packed arrangement and associated fossilised extracellular polymeric substances.In addition to preliminary imaging and energy-dispersive X-ray analysis, both the fossil biofilms and the sediment were investigated by nondestructive in situ analysis using time of flight-secondary ion mass spectroscopy (ToF-SIMS). The mass spectra obtained on the coprolite in mass-resolved chemical mapping mode allowed the tentative identification of a number of organic secondary ion species. Some spectra appear to indicate the presence of bacterial hopanoids, but further work using standard techniques such as gas chromatography mass spectroscopy is needed to conclusively verify the presence of these substances. Nevertheless, ToF-SIMS chemical maps were successfully correlated with electron microscopy images, allowing the correlation of molecular spectra, the spatial distribution of individual organic species, and specific morphologic features to demonstrate the potential of this approach in the analysis of microfossils.     

Microbial respiration and diffusive transport of O2, O2, and OO in unsaturated soils: a mesocosm experiment

Although the flux of molecular O₂ between the atmosphere and the subsurface is intrinsically linked to the net soil production of greenhouse gasses, few studies have focused on the controls affecting the isotopic composition of O₂ in the subsurface. Here, we developed and tested a stable oxygen isotope tracer technique and gas transport modeling approach to evaluate O₂ cycling and fluxes from the subsurface that used an environmentally controlled soil mescosm. We measured the O₂ and δ¹⁸O₂ profiles in a model unsaturated soil zone and quantified the O₂ consumption rates and the O₂ isotope fractionation factors resulting from the combined processes of subsurface microbial (including bacteria, fungi, and protozoa) consumption of O₂ and diffusive influx of O₂ from the atmosphere. We found that at high respiration rates in the mesocosm, there appeared to be very little isotope fractionation of O₂ by soil microorganisms. Although the mesocosm respiration rates are not typical of natural soils in northern temperate climes, they may be more representative of soils in warm and moist tropical environments. Our findings caution against the indiscriminate application of laboratory-determined oxygen isotope fractionation factors to field settings. The oxygen isotope tracer and modeling approach demonstrated here may be applied to gain a better understanding of biogenic gas production and O₂ cycling in subsurface systems and soils.     

Mathematical modeling of PCB bioaccumulation in Perna viridis

In the present work, we built a mathematical model of polychlorinated biphenyl (PCB) bioaccumulation in Perna viridis, namely, a one-compartment model with a time dependent incorporation rate R (μg g⁻¹ lipid per ppb water per day), with positive substrate cooperativity as the underlying physical mechanism. The temporal change of the PCB concentration Q (μg g⁻¹ lipid) in the soft tissues of the mussel depends on the competition of the input rate RW and the output rate kQ, where W is the concentration of PCB in water (ppb water) and k is the elimination rate (per day). From our experimental data, k=0.181±0.017 d⁻¹. The critical concentration in water W_c for positive substrate cooperativity was found to be 2.4 ppb. Below W_c, R is a constant. For a water concentration of 0.5 ppb Aroclor 1254, R=24.0±2.4 μg g⁻¹ lipid ppb⁻¹ d⁻¹. Above W_c, positive substrate cooperativity comes into effect and R becomes a function of time and dependent on the concentration Q in a form R=γQ/(Q+δ). This is the case for a water concentration of 5 ppb Aroclor 1254, where γ=15.1 μg g⁻¹ lipid ppb⁻¹ d¹ and δ≈200 μg g⁻¹ lipid. From this model, the uptake is exponentially increasing when the PCB concentration in the mussel is small compared to 200 μg g⁻¹ lipid, and hyperbolically increasing when the concentration is large compared to 200 μg g⁻¹ lipid, which are consistent with the experimental data. The model is useful for understanding the true processes taking place during the bioaccumulation and for risk assessment with higher confidence. Future experimental data which challenge the present model are anticipated and in fact desirable for improvement and perfection of the model.     

Kosa (yellow sand) components in precipitation collected at central Japan

In east Asia, acidic gases derived from fossil fuel combustion have increased in the past decades. On the other hand, the Asian dust, also called Kosa (yellow sand) is transported following windstorms from arid lands in the Asian continent. Many researchers have been interested in the reaction between acidic aerosols and Kosa aerosols as well as the long-range transport of these emissions. To investigate the characteristics of chemical components in precipitation on a long-term basis over Japan, precipitation was sequentially collected from April 1984 to March 1997 at Kanazawa located near the coast of the Sea of Japan. Precipitation samples were collected at 1 mm intervals for the first 5 mm rainfall and all volume of rainwater after 6 mm for all precipitation events with an automatic wet only precipitation collector. According to the analyses of precipitation including Kosa aerosols during Kosa periods, the reaction in the air between Kosa and acidic components during the long-range transport was discussed.     

Climate Suitability: For Stable Malaria Transmission in Zimbabwe Under Different Climate Change Scenarios

As climate is one factor determining the potential range of malaria, climate change may work with or against efforts to bring malaria under control. We developed a model of future climate suitability for stable Plasmodium falciparum malaria transmission in Zimbabwe. Current climate suitability for stable malaria transmission is based on the MARA/ARMA model of climatic constraints on the survival and development of the Anopheles vector and the Plasmodium falciparum malaria parasite. We explored potential future geographic distributions of malaria using sixteen projections of climate in 2100. The results suggest that, assuming no future human-imposed constraints on malaria transmission, changes in temperature and precipitation could alter the geographic distribution of malaria in Zimbabwe, with previously unsuitable areas of dense human population becoming suitable for transmission. Among all scenarios, the highlands become more suitable for transmission, while the lowveld and areas currently limited by precipitation show varying degrees of change, depending on climate sensitivity and greenhouse gas emission stabilization scenarios, and depending on the general circulation model used. The methods employed can be used within or across other African countries. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fine structure of Pn velocity beneath Sichuan-Yunnan region

We use 23298 Pn arrival-time data from Chinese national and provincial earthquake bulletins to invert fine structure of Pn velocity and anisotropy at the top of the mantle beneath the Sichuan-Yunnan and its adjacent region. The results suggest that the Pn velocity in this region shows significant lateral variation; the Pn velocity varies from 7.7 to 8.3 km/s. The Pn-velocity variation correlates well with the tectonic activity and heat flow of the region. Low Pn velocity is observed in southwest Yunnan, Tengchong volcano area, and the Panxi tectonic area. These areas have very active seismicity and tectonic activity with high surface heat flow. On the other hand, high Pn velocity is observed in some stable regions, such as the central region of the Yangtze Platform; the most pronounced high velocity area is located in the Sichuan Basin, south of Chengdu. Pn anisotropy shows a complex pattern of regional deformation. The Pn fast direction shows a prominent clockwise rotation pattern from east of the Tibetan block to the Sichuan-Yunnan diamond block to southwest Yunnan, which may be related to southeastward escape of the Tibetan Plateau material due to the collision of the Indian Plate to the Eurasia Plate. Thus there appears to be strong correlation between the crustal deformation and the upper mantle structure in the region. The delay times of events and stations show that the crust thickness decreases from the Tibetan Plateau to eastern China, which is consistent with the results from deep seismic sounding.