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581.
582.
The paper investigates aspects of the localization analysis of frictional materials. We derive closed formulas and diagrams for the inclination angle of critical discontinuity surfaces which develop in homogeneous compression and biaxial loading tests. The localization analysis is based on a Drucker–Prager‐type elastoplastic hardening model for non‐associated plastic flow at small strains, which we represent in spectral form. For this type of constitutive model, general analytical formulas for the so‐called critical hardening modulus and the inclination angle of critical discontinuity surfaces are derived for the plane strain case. The subsequent treatment then specializes these formulas for the analysis of compression and biaxial loading modes. The key contribution here is a detailed analysis of plane strain deformation modes where the localized failure occurs after subsequent plastic flow. The derived formulas and diagrams can be applied to the checking of an accompanying localization analysis of frictional materials in finite‐element computations. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
583.
Kuno M. Strassmann Matthias S. Brennwald Frank Peeters 《Geochimica et cosmochimica acta》2005,69(7):1665-1674
This paper presents results from the numerical modelling of the transport of atmospheric noble gases (He, Ne, Ar, Kr, Xe), tritiated water and 3He produced by radioactive decay of 3H, in unconsolidated lacustrine sediment. Two case studies are discussed: (1) the evolution of 3H and 3He concentrations in the sediment porewater of Lake Zug (Switzerland) from 1953 up to the present; and (2) the response of dissolved atmospheric noble gas concentrations in the sediment porewater of a subtropical lake to an abrupt climatic change that occurred some 10 kyr before the present. (1) Modelled 3H and 3He porewater concentrations are compared with recent data from Lake Zug. An estimate of the effective diffusion coefficients in the sediment porewater is derived using an original approach which is also applicable also to lakes for which the historical 3H and 3He concentrations in the water column are unknown. (2) The air/water partitioning of atmospheric noble gases is sensitive to water temperature and salinity, and thus provides a mechanism by which these environmental variables are recorded in the concentrations of atmospheric noble gases in lakes. We investigate the feasibility of using noble gas concentrations in the porewater of lacustrine sediments as a proxy for palaeoenvironmental conditions in lakes. Numerical modelling shows that heavy noble gases in sediment porewater, because of their comparatively small diffusion coefficients and the strong temperature sensitivity of their equilibrium concentrations, can preserve concentrations corresponding to past lake temperatures over times on the order of 10 kyr. Noble gas analysis of sediment porewaters therefore promises to yield valuable quantitative information on the past environmental states of lakes. 相似文献
584.
Abstract Since 1973 new data have been obtained on the maximum extent of glaciation in High Asia. Evidence for an ice sheet covering Tibet during the last glacial period means a radical rethinking about glaciation in the northern hemisphere. The ice sheet's subtropical latitude, vast size (2.4 million km2 ) and high elevation (6000 m a.s.l.) are supposed to have resulted in a substantial, albedo-induced cooling of the Earth's atmosphere and the disruption of summer monsoon circulation. Moraines were found to reach down to 460 m a.s.l. on the southern flank of the Himalayas and to 2300 m a.s.l. on the northern slope of the Tibetan Plateau, in the Qilian Shan region. On the northern slopes of the Karakoram, Aghil and Kuen-Lun Mountains, moraines occur as far down as 1900 m a.s.l. In southern Tibet, radiographic analyses of erratics suggest a former ice thickness of at least 1200 m. Glacial polish and roches moutonnées in the Himalayas and Karakoram suggest former glaciers as thick as 1200–2700 m. On the basis of this evidence, a 1100–1600-m lower equilibrium line has been reconstructed, resulting in an ice sheet of 2.4 million km2 , covering almost all of Tibet. Radiometric ages, obtained by different methods, classify this glaciation as isotope stage 3–2 in age (Würmian, the last glacial period, ca 60 000–18 000 years ago). 相似文献
585.
Assessment of Discharge through a Dike Breach and Simulation of Flood Wave Propagation 总被引:2,自引:0,他引:2
This paper presents a simple and fast method to calculate flow through a dike breach. The approach was based on two-dimensional
numerical simulations of idealized dike breakages at straight river-sections. As a result, computation of discharge through
a breach can be achieved by use of the new developed formula (denoted as dike break formula). Furthermore, a methodology that combines one-dimensional hydrodynamic modelling, the dike break formula and a simple GIS-based
method to estimate inundation areas is described. This fast and easy-to-handle tool can be utilized for near real-time forecasting
or evacuation decisions. Detailed predictions were made for a number of flood and dike break scenarios at the River Rhine
to prove the accuracy of the new method compared with two-dimensional numerical models. 相似文献
586.
Earthquake activity related to seismic cycles in a model for a heterogeneous strike-slip fault 总被引:1,自引:0,他引:1
We investigate the evolution of seismicity within large earthquake cycles in a model of a discrete strike-slip fault in elastic solid. The model dynamics is governed by realistic boundary conditions consisting of constant velocity motion of regions around the fault, static/kinetic friction and dislocation creep along the fault, and 3D elastic stress transfer. The fault consists of brittle parts which fail during earthquakes and undergo small creep deformation between events, and aseismic creep cells which are characterized by high ongoing creep motion. This mixture of brittle and creep cells is found to generate realistic aftershock sequences which follow the modified Omori law and scale with the mainshock size. Furthermore, we find that the distribution of interevent times of the simulated earthquakes is in good agreement with observations. The temporal occurrence, however, is magnitude-dependent; in particular, the small events are clustered in time, whereas the largest earthquakes occur quasiperiodically. Averaging the seismicity before several large earthquakes, we observe an increase of activity and a broadening scaling range of magnitudes when the time of the next mainshock is approached. These results are characteristics of a critical point behavior. The presence of critical point dynamics is further supported by the evolution of the stress field in the model, which is compatible with the observation of accelerating moment release in natural fault systems. 相似文献
587.
Anders Svensson Katrine K. Andersen Matthias Bigler Henrik B. Clausen Dorthe Dahl-Jensen Siwan M. Davies Sigfus J. Johnsen Raimund Muscheler Sune O. Rasmussen Regine Rthlisberger Jrgen Peder Steffensen Bo.M. Vinther 《Quaternary Science Reviews》2006,25(23-24):3258
A new Greenland Ice Core Chronology (GICC05) based on multi-parameter counting of annual layers has been obtained for the last 42 ka. Here we compare the glacial part of the new time scale, which is based entirely on records from the NorthGRIP ice core, to existing time scales and reference horizons covering the same period. These include the GRIP and NorthGRIP modelled time scales, the Meese-Sowers GISP2 counted time scale, the Shackleton–Fairbanks GRIP time scale (SFCP04) based on 14C calibration of a marine core, the Hulu Cave record, three volcanic reference horizons, and the Laschamp geomagnetic excursion event occurring around Greenland Interstadial 10. GICC05 is generally in good long-term agreement with the existing Greenland ice core chronologies and with the Hulu Cave record, but on shorter time scales there are significant discrepancies. Around the Last Glacial Maximum there is a more than 1 ka age difference between GICC05 and SFCP04 and a more than 0.5 ka discrepancy in the same direction between GICC05 and the age of a recently identified tephra layer in the NorthGRIP ice core. Both SFCP04 and the tephra age are based on 14C-dated marine cores and fixed marine reservoir ages. For the Laschamp event, GICC05 agrees with a recent independent dating within the uncertainties. 相似文献
588.
Dr. Matthias Selg 《International Journal of Earth Sciences》1986,75(3):693-702
Archaeocythan limestones of the Lower Cambrian Nebida Fm. (SW-Sardinia) form bioherms (lower Nebida Fm. = Matoppa Mbr.) as well as biostromes (upper Nebida Fm. = Punta Manna Mbr.). In both cases algae are the main rock forming organisms; they act as sediment binding and sediment baffling organisms and thus are responsible for the growth of these carbonates.Both bioherms and biostromes derive from a low energy environment of shallow water depth. While the bioherms developed in an open marine setting, the biostromes belong to a restricted lagoonal area. Apparently, this is the reason for the growth of different algae:Renalcis andEpiphyton in the bioherms,Girvanella in the lagoonal biostromes. Moreover, facies analysis and occurence of archaeocyathans show that these organisms could flourish well above the water depth of 20–50 m mentioned byHill (1972).
Zusammenfassung Archaeocyathen führende Kalke der unterkambrischen Nebida Fm. (SW-Sardinien) treten sowohl als Bioherme (tiefere Nebida Fm. = Matoppa Mbr.) auf, als auch in Form von Biostromen (höhere Nebida Fm. = Punta Manna Mbr.). Als Hauptgesteinsbildner treten in beiden Fällen Algen auf, die als Sediment bindende und Sediment fangende Organismen für die Entstehung dieser Karonate verantwortlich sind. Sowohl Bioherme wie auch Biostrome entstammen Bereichen geringer Wassertiefe und nur mä\iger Wasserbewegung. Während aber die Bioherme wohl in einem offen-marinen Bereich entstanden sind, lassen sich die Biostrome einem geschützten Lagunenbereich zuordnen. Hierin scheint der Grund für das Auftreten unterschiedlicher Algen zu liegen:Renalcis undEpiphyton in den Biohermen,Girvanella in den lagunären Biostromen. Faziesanalyse und Auftreten von Archaeocyathen legen ferner nahe, da\ diese auch oberhalb der vonHill (1972) als bevorzugt angegebenen Tiefe von 20–50 m recht gut existieren konnten.
Résumé Les calcaires à Archéocyathes de la formation du Nedida au Cambrien inférieur (Sardaigne du Sud-Ouest) forment non seulement des bioherms (formation du Nebida inférieure = membre de Matoppa) mais encore des biostromes (formation du Nebida supérieure = membre de Punta Manna). On considère que les algues sont les organismes responsables de l'édification de ces calcaires, tant comme »cimenteurs« que comme capteurs des sédiments. Non seulement les biostromes, mais encores les-faible profondeur. Tandis que les bioherms se formaient dans un milieu marin ouvert, on peut attribuer les biostromes à un milieu confiné. Ceci semle Être la raison de la présence d'algues différentes:Renalcis etEpiphyton dans les bioherms,Girvanella dans les biostromes lagunaires. L'analyse du facies et la présence des Archéocythes indiquent que ceux-ci peuvent aussi exister au-dessus de la profondeur de 20 à 50 m proposée parHill (1972).
- - . - . : 1) , , , ; 2) ( ); 3) ; 4) , . Aguacate , , . , . - - , , ( ). , , . / /. , Guacimal'e , (mobile belt) . , - -, , . - 4 : - - ( ); - ( ); ( Valle Central E-W ); - ( ).相似文献
589.
590.
Matthias Radke Helmut Willsch Detlev Leythaeuser Marlies Teichmüller 《Geochimica et cosmochimica acta》1982,46(10):1831-1848
Aromatic distribution patterns have been evaluated for a series of twenty-four German high volatile bituminous B to low volatile bituminous coals of Upper Carboniferous Westphalian C-, B- and A-ages using high performance liquid chromatography and glass capillary gas chromatography. The study concentrates on dicyclic and tricyclic aromatic hydrocarbons most of which have been identified by combined gas chromatography-mass spectrometry. The distribution patterns of the methylhomologs of naphthalene and phenanthrene are strongly controlled by rank. A sudden increase of individual isomer ratios at 0.9% Rm coincides with an abrupt shift of the sporinite fluorescence colour from yellow towards red. A recently developed aromatics-derived maturity parameter, the Methylphenanthrene Index (MPI), correlated well with the vitrinite reflectance data over the whole rank range. Deviations have been attributed to variations of maceral composition or migration phenomena. The influence of artificial thermal alteration (350 and 400dgC) on the aromatic distribution pattern of coal is described. 相似文献