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51.
The paper presents a closed-form solution for the convergence curve of a circular tunnel in an elasto-brittle-plastic rock
mass with both the Hoek–Brown and generalized Hoek–Brown failure criteria, and a linear flow rule, i.e., the ratio between
the minor and major plastic strain increments is constant. The improvement over the original solution of Brown et al. (J Geotech
Eng ASCE 109(1):15–39, 1983) consists of taking into account the elastic strain variation in the plastic annulus, which was assumed to be fixed in the
original solution by Brown et al. The improvement over Carranza-Torres’ solution (Int J Rock Mech Min Sci 41(Suppl 1):629–639,
2004) consists of providing a closed-form solution, rather than resorting to numerical integration of an ordinary differential
equation. The presented solution, by rigorously following the theory of plasticity, takes into account that the elastic strain
components change with radial and circumferential stress changes within the plastic annulus. For the original Hoek–Brown failure
criterion, disregarding the elastic strain change leads to underestimate the convergence by up to 55%. For a rock mass failing
according to the generalized Hoek–Brown failure criterion, using the original failure criterion leads to a high probability
(97%) of underestimating the convergence by up to 100%. As a consequence, the onset or degree of squeezing may be underestimated,
and the loading on the support/reinforcement calculated with the convergence/confinement method may be largely underestimated. 相似文献
52.
The role of land use and environmental factors on microbial pollution of mountainous limestone aquifers 总被引：1，自引：0，他引：1
Limestone aquifers in Southern Italy are often affected by bacterial contamination produced by pasture and agriculture. The
main goals of this study were (1) to analyze the role of land use and environmental factors on microbial contamination and,
(2) to identify, at field scale, the most suitable indicator of fecal pollution, by comparing fecal coliforms and fecal enterococci.
Analyzing surface and spring water, it was noted that both fecal indicators showed a significant decrease during the period
characterized by freezing and/or freeze-thaw intervals. The data analysis shows that fecal coliforms are characterized by
a significant decrease in population (3 orders of magnitude, at least) during the freezing period, while fecal enterococci
are temporarily inhibited. A taxonomic classification of fecal enterococci detected in spring water samples was performed
by the API 20 Strep system and by sequencing of the ribosomal 16S DNA genes. The results showed that freezing conditions did
not cause any significant change on the set of enterococcal species. 相似文献
53.
Gennaro A. Stefania Marco Rotiroti Letizia Fumagalli Fulvio Simonetto Pietro Capodaglio Chiara Zanotti Tullia Bonomi 《Hydrogeology Journal》2018,26(1):147-162
A groundwater flow model of the Alpine valley aquifer in the Aosta Plain (NW Italy) showed that well pumping can induce river streamflow depletions as a function of well location. Analysis of the water budget showed that ～80% of the water pumped during 2 years by a selected well in the downstream area comes from the baseflow of the main river discharge. Alluvial aquifers hosted in Alpine valleys fall within a particular hydrogeological context where groundwater/surface-water relationships change from upstream to downstream as well as seasonally. A transient groundwater model using MODFLOW2005 and the Streamflow-Routing (SFR2) Package is here presented, aimed at investigating water exchanges between the main regional river (Dora Baltea River, a left-hand tributary of the Po River), its tributaries and the underlying shallow aquifer, which is affected by seasonal oscillations. The three-dimensional distribution of the hydraulic conductivity of the aquifer was obtained by means of a specific coding system within the database TANGRAM. Both head and flux targets were used to perform the model calibration using PEST. Results showed that the fluctuations of the water table play an important role in groundwater/surface-water interconnections. In upstream areas, groundwater is recharged by water leaking through the riverbed and the well abstraction component of the water budget changes as a function of the hydraulic conditions of the aquifer. In downstream areas, groundwater is drained by the river and most of the water pumped by wells comes from the base flow component of the river discharge. 相似文献
54.
In order to analyze the stability of a 3D rock block using a limiting equilibrium method, some simplifications must be introduced, such as neglecting the effects of deformability of the rock block, fractures, and the surrounding rock mass, the progressive failure and the mobilization of shear strength of fractures, and changes in in situ stresses with block displacement. These limitations may cause some errors when calculating the factor of safety of the block. This paper quantifies the error caused by these simplifications comparing the results against those obtained with a numerical method (BS3D). In addition, the effects of the normal stiffness of the fractures, dilatancy, the tunnel radius, and the block size on stability of the tetrahedron are investigated using the numerical tool to demonstrate the importance of the effects eliminated by the limiting equilibrium approach. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
55.
Majd Abdelqader Fulvio Melia † 《Monthly notices of the Royal Astronomical Society》2008,388(4):1869-1878
The hierarchical clustering inherent in Λcold dark matter cosmology seems to produce many of the observed characteristics of large-scale structure. But some glaring problems still remain, including the overprediction (by a factor of 10) of the number of dwarf galaxies within the virialized population of the local group. Several secondary effects have already been proposed to resolve this problem. It is still not clear, however, whether the principal solution rests with astrophysical processes, such as early feedback from supernovae, or possibly with as yet undetermined properties of the dark matter itself. In this paper, we carry out a detailed calculation of the dwarf halo evolution incorporating the effects of a hypothesized dark matter decay, D → D'+ l , where D is the unstable particle, D ' is the more massive daughter particle and l is the other, lighter (or possibly massless) daughter particle. This process preferentially heats the smaller haloes, expanding them during their evolution and reducing their present-day circular velocity. We find that this mechanism can account very well for the factor of 4 deficit in the observed number of systems with velocity 10–20 km s−1 compared to those predicted by the numerical simulations, if , where Δ m is the mass difference between the initial and final states. The corresponding lifetime τ cannot be longer than ∼30 Gyr, but may be as short as just a few Gyr. 相似文献
56.
The interactions between the stream and the geomorphologic units that compose the stream channel result in an exchange of water, heat, and chemicals that is an important component of the flows of energy and nutrients in the river ecosystem. This exchange is characterized by complex spatial and temporal dynamics that depend on the characteristics of the stream flow and morphology. At present, many studies have addressed the development of spatial patterns of hyporheic exchange that are induced by many geomorphological factors at different scales. However, much less is known about the temporal evolution of the surface–subsurface exchange in response to the dynamics of the stream discharge. In order to investigate this problem, the present work analyzes the influence of streamflow variability on the hyporheic exchange induced by fluvial bedforms. A stochastic approach is employed to generate streamflow series whose statistical properties are representative of streams with different hydrological regimes. The resulting exchange fluxes and travel times are then computed, and the relationships between the streamflow regime and the dynamics of the exchange flux and travel times are investigated. The results show that the mean stream discharge can be used to estimate the average features of the temporal dynamics of hyporheic exchange. Moreover, exchange fluxes and residence times distributions exhibit significant fluctuations, which are tightly related to the coefficient of variation of the streamflow hydrograph. 相似文献
57.
The interaction between surface and subsurface water has a crucial influence on the biochemistry of stream environments. Even though the river discharge and the flow conditions can seldom be considered to be steady, the influence of this unsteadiness on the hyporheic exchange has often been neglected. In this work, a model for the study of hyporheic exchange during unsteady conditions has been developed. The model provides a sound analytical framework for the analysis of the effects of a varying stream discharge on the exchange between a stream and the hyporheic zone. The effects of the unsteadiness on the water exchange flux, the residence time of the solutes in the bed, and the stored mass are quantified. A synthetic example shows the substantial influence of a flood on the hyporheic exchange, and that the application of a steady model can lead to an underestimation of the exchanged mass, even after the flood has ended. 相似文献
58.
Fulvio Franchi Barbara Cavalazzi Catherine Pierre Roberto Barbieri 《Geological Journal》2015,50(5):634-650
New evidences based on a combination of field and laboratory investigations reinforce the hypotheses that the circulation of warm fluids has remarkably contributed to the origin and development of the Devonian Kess Kess mounds of the Hamar Laghdad Ridge (eastern Anti‐Atlas, Morocco). The limestones of the Hamar Laghdad Ridge were deposited above a structural high generated by calc‐alkaline volcanic activity that has probably fuelled the circulation of warm fluids throughout the overlying carbonate units. The geological and palaeontological attributes described throughout the succession of the Hamar Laghdad Ridge (from the Lochkovian to Frasnian intervals) are interpreted as the result of hydrothermal processes related to a volcanic system. In particular, these attributes seem consistent with a chemo‐physical environment fuelled by the circulation of warm and late magmatic fluids. These attributes include a very low oxygen stable isotope signature (δ18O ~ −10‰) for carbonates. Evidences for a late magmatic fluid circulation consist of volcanic glass and pyroclasts replacement with hydrothermal minerals such as quartz, anatase and clinochlore. Fluids circulating through veins and pores into sediments, and venting to the seafloor, probably induced the formation of cavities where monospecific trilobite communities were detected. The partially silicified trilobite remains are associated with traces of goethite. This iron‐bearing oxide mineral is also present in the upper part of the Hamar Laghdad Ridge. All these attributes are here interpreted as possible evidences for a low‐temperature hydrothermal venting system active during the Lochkovian–Frasnian time span. This study combines an updated revision with new petrographic, geological and geochemical results aimed at providing an overall framework on the origin and early diagenesis of the Devonian succession of Hamar Laghdad. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
59.
Theoretical and numerical models of chorus waves are reviewed in this paper. Specifically, we focus on the nonlinear wave particle interactions and the current understanding of the frequency chirping of rising tone chorus waves. Various other related topics, such as the optimal excitation condition of chorus, the formation of subpackets, and the non-adiabaticity of the nonlinear interaction are also discussed. We end this review paper with a short list of questions of chorus waves that are still under research and debate. 相似文献