Homogenization framework for three‐dimensional elastoplastic finite element analysis of a grouted pipe‐roofing reinforcement method for tunnelling

This paper deals with the grouted pipe‐roofing reinforcement method that is used in the construction of tunnels through weak grounds. This system consists on installing, prior to the excavation of a length of tunnel, an array of pipes forming a kind of ‘umbrella’ above the area to be excavated. In some cases, these pipes are later used to inject grout to strengthen the ground and ‘connect’ the pipes. This system has proven to be very efficient in reducing tunnel convergence and water inflow when tunnelling through weak grounds. However, due to the geometrical and mechanical complexity of the problem, existing finite element frameworks are inappropriate to simulate tunnelling using this method. In this paper, a mathematical framework based on a homogenization technique to simulate ‘grouted pipe‐roofing reinforced ground’ and its implementation into a 3‐D finite element programme that can consider stage construction situations are presented. The constitutive model developed allows considering the main design parameters of the problem and only requires geometrical and mechanical properties of the constituents. Additionally, the use of a homogenization approach implies that the generation of the finite element mesh can be easily produced and that re‐meshing is not required as basic geometrical parameters such as the orientation of the pipes are changed. The model developed is used to simulate tunnelling with the grouted pipe‐roofing reinforcement method. From the analyses, the effects of the main design parameters on the elastic and the elastoplastic analyses are considered. Copyright © 2004 John Wiley & Sons, Ltd.     

Relative pointing offset analysis of calibration targets with repeated observations with Herschel-SPIRE Fourier-transform spectrometer

We present a method to derive the relative pointing offsets for SPIRE Fourier-Transform Spectrometer (FTS) solar system object (SSO) calibration targets, which were observed regularly throughout the Herschel mission. We construct ratios R _obs(ν) of the spectra for all observations of a given source with respect to a reference. The reference observation is selected iteratively to be the one with the highest observed continuum. Assuming that any pointing offset leads to an overall shift of the continuum level, then these R _obs(ν) represent the relative flux loss due to mispointing. The mispointing effects are more pronounced for a smaller beam, so we consider only the FTS short wavelength array (SSW, 958–1546 GHz) to derive a pointing correction. We obtain the relative pointing offset by comparing R _obs(ν) to a grid of expected losses for a model source at different distances from the centre of the beam, under the assumption that the SSW FTS beam can be well approximated by a Gaussian. In order to avoid dependency on the point source flux conversion, which uses a particular observation of Uranus, we use extended source flux calibrated spectra to construct R _obs(ν) for the SSOs. In order to account for continuum variability, due to the changing distance from the Herschel telescope, the SSO ratios are normalised by the expected model ratios for the corresponding observing epoch. We confirm the accuracy of the derived pointing offset by comparing the results with a number of control observations, where the actual pointing of Herschel is known with good precision. Using the method we derived pointing offsets for repeated observations of Uranus (including observations centred on off-axis detectors), Neptune, Ceres and NGC 7027. The results are used to validate and improve the point-source flux calibration of the FTS.     

Simulating the impact of the large-scale circulation on the 2-m temperature and precipitation climatology

The impact of the simulated large-scale atmospheric circulation on the regional climate is examined using the Weather Research and Forecasting (WRF) model as a regional climate model. The purpose is to understand the potential need for interior grid nudging for dynamical downscaling of global climate model (GCM) output for air quality applications under a changing climate. In this study we downscale the NCEP-Department of Energy Atmospheric Model Intercomparison Project (AMIP-II) Reanalysis using three continuous 20-year WRF simulations: one simulation without interior grid nudging and two using different interior grid nudging methods. The biases in 2-m temperature and precipitation for the simulation without interior grid nudging are unreasonably large with respect to the North American Regional Reanalysis (NARR) over the eastern half of the contiguous United States (CONUS) during the summer when air quality concerns are most relevant. This study examines how these differences arise from errors in predicting the large-scale atmospheric circulation. It is demonstrated that the Bermuda high, which strongly influences the regional climate for much of the eastern half of the CONUS during the summer, is poorly simulated without interior grid nudging. In particular, two summers when the Bermuda high was west (1993) and east (2003) of its climatological position are chosen to illustrate problems in the large-scale atmospheric circulation anomalies. For both summers, WRF without interior grid nudging fails to simulate the placement of the upper-level anticyclonic (1993) and cyclonic (2003) circulation anomalies. The displacement of the large-scale circulation impacts the lower atmosphere moisture transport and precipitable water, affecting the convective environment and precipitation. Using interior grid nudging improves the large-scale circulation aloft and moisture transport/precipitable water anomalies, thereby improving the simulated 2-m temperature and precipitation. The results demonstrate that constraining the RCM to the large-scale features in the driving fields improves the overall accuracy of the simulated regional climate, and suggest that in the absence of such a constraint, the RCM will likely misrepresent important large-scale shifts in the atmospheric circulation under a future climate.     

Evaluation of the Community Land Model 3.5 with carbon and nitrogen cycles (CLM3.5CN) at a Tibetan grassland site

The Tibetan plateau plays an important role in energy and carbon cycles by providing an elevated heat source and by storing a large amount of soil carbon due to low temperature. The main vegetation of the plateau is alpine grassland. This study evaluates performance of Community Land Model 3.5 with carbon and nitrogen cycles (CLM3.5CN) over a alpine grassland in the Tibetan plateau in terms of energy and carbon fluxes in conditions of reasonable phenology and initial carbon pool comparable to observations. Comparison between model and observation shows following features. The model captures the magnitude of maximum leaf area index (LAI) but underestimats leaf mass. Net ecosystem exchange (NEE) is significantly underestimated during the growing season and soil temperature is also underestimated throughout a year with higher negative bias in winter than in other seasons. In order to examine the cause of the model deficiencies, we design four sensitivity tests: seasonal mulch; shallow rooting depth; reduction of critical soil moisture to limit the decomposition rate; smaller specific leaf area (SLA). Considering seasonal mulch improves the negative bias of soil temperature during dormant season has little effect on the NEE during the growing seasson. Underestimation of NEE during the growing season is partly due to underestimated decomposition rate which results from underestimated soil temperature and deep root placement in the soil column. Underestimation of latent heat flux during summer is partly due to use of large SLA in the model. Other deficiencies are also discussed.     

Tidal variability in benthic silicic acid fluxes and microphytobenthos uptake in intertidal sediment

Silicic acid (DSi) benthic fluxes play a major role in the benthic–pelagic coupling of coastal ecosystems. They can sustain microphytobenthos (MPB) development at the water–sediment interface and support pelagic diatoms when river DSi inputs decrease. DSi benthic fluxes have been studied at the seasonal scale but little is known about their dial variations. This study measured the amplitude of such variations in an intertidal area over an entire tidal cycle by following the alteration of DSi pore water concentrations at regular intervals over the flood/ebb period. Furthermore we independently estimated the potential DSi uptake by benthic diatoms and compared it to the variations of DSi pore water concentrations and fluxes. The microphytobenthos DSi demand was estimated from primary production measurements on cells extracted from the sediment. There were large changes in DSi pore water concentration and a prominent effect of tidal pumping: the DSi flushed out from the sediment at rising tide, occurs in a very short period of time, but plays a far more important role in fueling the ecosystem (800 μmol-Si m⁻² d⁻¹), than diffusive fluxes occurring throughout the rest of the tidal cycle (2 μmol-Si m⁻² d⁻¹). This process is not, to our knowledge, currently considered when describing the DSi cycling of intertidal sediments. Moreover, there was a large potential MPB requirement for DSi (812 μmol-Si m⁻² d⁻¹), similar to the advective flow periodically pumped by the incoming tide, and largely exceeded benthic diffusive fluxes. However, this DSi uptake by benthic diatoms is almost undetectable given the variation of DSi concentration profiles within the sediment.     

Permafrost-associated natural gas hydrate occurrences on the Alaska North Slope

In the 1960s Russian scientists made what was then a bold assertion that gas hydrates should occur in abundance in nature. Since this early start, the scientific foundation has been built for the realization that gas hydrates are a global phenomenon, occurring in permafrost regions of the arctic and in deep water portions of most continental margins worldwide. In 1995, the U.S. Geological Survey made the first systematic assessment of the in-place natural gas hydrate resources of the United States. That study suggested that the amount of gas in the gas hydrate accumulations of northern Alaska probably exceeds the volume of known conventional gas resources on the North Slope. Researchers have long speculated that gas hydrates could eventually become a producible energy resource, yet technical and economic hurdles have historically made gas hydrate development a distant goal. This view began to change in recent years with the realization that this unconventional resource could be developed with existing conventional oil and gas production technology. One of the most significant developments was the completion of the BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well on the Alaska North Slope, which along with the Mallik project in Canada, have for the first time allowed the rational assessment of gas hydrate production technology and concepts. Almost 40 years of gas hydrate research in northern Alaska has confirmed the occurrence of at least two large gas hydrate accumulations on the North Slope. We have also seen in Alaska the first ever assessment of how much gas could be technically recovered from gas hydrates. However, significant technical concerns need to be further resolved in order to assess the ultimate impact of gas hydrate energy resource development in northern Alaska.     

Thermal infrared (8-13 μm) spectra of 29 asteroids: the Cornell Mid-Infrared Asteroid Spectroscopy (MIDAS) Survey

We report the results of the Cornell Mid-IR Asteroid Spectroscopy (MIDAS) survey, a program of ground-based observations designed to characterize the 8-13 μm spectral properties of a statistically significant sample of asteroids from a wide variety of visible to near-IR spectral classes. MIDAS is conducted at Palomar Observatory using the Spectrocam-10 (SC-10) spectrograph on the 200-in Hale telescope. We have measured the mid-infrared spectra of twenty-nine asteroids and have derived temperature estimates from our data that are largely consistent with the predictions of the standard thermal model. We have also generated relative emissivity spectra for the target asteroids. On only one asteroid, 1 Ceres, have we found emissivity features with spectral contrast greater than 5%. Our spectrum of 4 Vesta suggests emissivity variation at the 2-3% level. Published spectra of several of the small number of asteroids observed with ISO (six of which are also included in our survey), which appeared to exhibit much stronger emissivity features, are difficult to reconcile with our measurements. Laboratory work on mineral and meteorite samples has shown that the contrast of mid-IR spectral features is greatly reduced at fine grain sizes. Moreover, the NEAR mission found that 433 Eros is covered by a relatively thick fine-grained regolith. If small bodies in general possess such regoliths, their mid-IR spectral features may be quite subtle. This may explain the evident absence of strong emissivity variation in the majority of the MIDAS spectra.     

ISO Observations of Classical Novae

We describe the ISO programme to observe classical novae. The programme includes observations of novae during and shortly after eruption, and old novae. ISO observations of far infrared fine structure lines are providing us with information about physical conditions in nova ejecta which complement and extend knowledge obtained from ground-based observations. Surprisingly, we are getting little information about dust in nova systems, despite the fact that many novae are prolific dust-producers. This revised version was published online in August 2006 with corrections to the Cover Date.     

Greenhouse gases mitigation against climate change: United States-Mexico border study case

Solar radiation is one of the most important energy resources of our planet. The interest in its use as a renewable and clean energy to mitigate the greenhouse gases (GHG) effects has increased significantly. This paper evaluates the measurements of global solar radiation and its energy potential and presents a comparison between both of them, as an example of the effort to reduce GHG emissions. The measurements were made with pyranometers installed in the city of Mexicali, Baja California, located in northwestern Mexico, and the city of Yuma, Arizona, located in the southwestern United States. Separated by a distance of 96 km, both cities have a sustained development and are climatically similar, since they present numerous sunny days, extreme hot temperatures and little precipitation. The results presented show differences in their behavior and in the solar radiation measurement values, especially for the critical spring and summer seasons, with values 15.73% (0.042 kW/m²) higher in Mexicali with respect to Yuma. Energy power is estimated, and it is discussed with some variables as global solar radiation, rainfall, air temperature, relative humidity and climatology of clear, partly cloudy, and cloudy days. With this estimation, the solar energy used and GHG avoided is projected for Mexicali. It is assessed that 291 tons of GHG are prevented. The Mexicali values of potential energy are higher than those of Yuma; therefore, this solar and energy comparative study provides reasons to develop these technologies in Mexico, but solar technologies should be deployed also in Yuma. The measured data at the regional level demonstrate their importance, and the relevance of the proposed mitigation strategy for climate change.     

Application of the chemometric approach to evaluate the spatial variation of water chemistry and the identification of the sources of pollution in Langat River, Malaysia

The current study presents the application of selected chemometric techniques—hierarchical cluster analysis (HCA) and principal component analysis (PCA)—to evaluate the spatial variation of the water chemistry and to classify the pollution sources in the Langat River. The HCA rendered the sampling stations into two clusters (group 1 and group 2) and identified the vulnerable stations that are under threat. Group1 (LY 1 to LY 14) is associated with seawater intrusion, while group 2 (LY 15 to LY 30) is associated with agricultural and industrial pollution. PCA analysis was applied to the water datasets for group 1 resulting in four components, which explained 85 % of the total variance while group 2 extracted six components, explaining 88 % of the variance. The components obtained from PCA indicated that seawater intrusion, agricultural and industrial pollution, and geological weathering were potential sources of pollution to the study area. This study demonstrated the usefulness of the chemometric techniques on the interpretation of large complex datasets for the effective management of water resources.