A constitutive model for sand in triaxial compression     

R. Nova  D. M. Wood 《国际地质力学数值与分析法杂志》1979,3(3):255-278

Although there is a large number of constitutive models for sand available in the literature it is believed that a fresh approach, striking a balance between complexity and theoretical rigour, is desirable. The approach here has certain conceptual links with the Cam Clay series of elastic–plastic models, but includes the more general starting assumption that the yield function, plastic potential and failure locus should be given quite distinct mathematical expressions. Possible physical bases for the proposed forms are discussed. Ways in which the parameters required to define the model may be determined are suggested and the use of the model is then demonstrated. Firstly, it is shown that, where a limited set of experimental data is available, the model is flexible enough to be able to match the test results. Secondly, it is shown that, where a wide range of test results has been produced, it is possible to determine the model constitutive parameters from a small number of tests and proceed to make satisfactory predictionsfor other, quite different, types of test. The model is developed for sand at a single initial density, but the way in which the constitutive parameters might be expected to vary with density is discussed. The model is described for conditions of triaxial compression, and extension to more general stress states will be needed before it can be put to the test of incorporation in, for example, a finite element program.  相似文献   

Moon: Origin and evolution of multi-ring basins   总被引：2，自引：0，他引：2  

W. K. Hartmann  C. A. Wood 《Earth, Moon, and Planets》1971,3(1):3-78

This paper summarizes current data and new observations on lunar basin systems. Parts 1–4 review earlier literature and give new crater-counts used to reconstruct basin histories. Among the results are: basin rings are defined by faults, hills, craters, and/or wrinkle ridges; all of these are inter-related; 2 plays a special role in the ratios of ring diameters; flooding occurred in many basins prior to the formation of the familiar front-side maria; 3 km is a typical depth of lava flooding in basins. Parts 5–11 interpret these results in terms of origin and evolution of basins. Polar concentrations of basins and old, large craters are found (Figures 28 and 29). Basins originated by impacts of very early planetesimals left over from or created during formation of the Moon (6). Concentric fractures were produced by the impacts. Concentric rings developed along fractures during subsequent sagging of the basin into partially melted substrata, along the lines of theory and experiments by Lance and Onat (1962) (Figures 36 and 37). There is marginal empirical evidence that some rings formed significantly after their basins (8). The structure of specific rings depended on the nature of volcanic products extruded. Wrinkle ridges, peak-rings, rings of craters, concentric graben, and central peaks are all consequences of basin-forming evolutionary processes (9, Figure 41), Flooding by lava was a final stage in basin evolution. Lava extruded from concentric ring-faults, wrinkle ridges, and crater and basin rims (10). Mascons are directly correlated with the amount of mare lava, but not correlated with basin age or morphology (11). Section 12 summarizes the results and compares them to those of other authors.  相似文献   

Polycyclic aromatic hydrocarbons (PAHs) in small island coastal environments: a case study from harbours in Guam, Micronesia     

Denton GR  Concepcion LP  Wood HR  Morrison RJ 《Marine pollution bulletin》2006,52(9):1090-1098

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Light-toned layered deposits in Juventae Chasma, Mars     

David C. Catling  Stephen E. Wood  David R. Montgomery  Christopher R. Glein 《Icarus》2006,181(1):26-51

We examine hypotheses for the formation of light-toned layered deposits in Juventae Chasma using a combination of data from Mars Global Surveyor's Mars Orbiter Camera (MOC), Mars Orbiter Laser Altimeter (MOLA), and Thermal Emission Spectrometer (TES), as well as Mars Odyssey's Thermal Emission Imaging System (THEMIS). We divide Juventae Chasma into geomorphic units of (i) chasm wall rock, (ii) heavily cratered hummocky terrain, (iii) a mobile and largely crater-free sand sheet on the chasm floor, (iv) light-toned layered outcrop (LLO) material, and (v) chaotic terrain. Using surface temperatures derived from THEMIS infrared data and slopes from MOLA, we derive maps of thermal inertia, which are consistent with the geomorphic units that we identify. LLO thermal inertias range from ∼400 to 850 J m⁻² K⁻¹ s^−1/2. Light-toned layered outcrops are distributed over a remarkably wide elevation range () from the chasm floor to the adjacent plateau surface. Geomorphic features, the absence of small craters, and high thermal inertia show that the LLOs are composed of sedimentary rock that is eroding relatively rapidly in the present epoch. We also present evidence for exhumation of LLO material from the west wall of the chasm, within chaotic and hummocky terrains, and within a small depression in the adjacent plateau. The data imply that at least some of the LLO material was deposited long before the adjacent Hesperian plateau basalts, and that Juventae Chasma underwent, and may still be undergoing, enlargement along its west wall due to wall rock collapse, chaotic terrain evolution, and exposure and removal of LLO material. The new data allow us to reassess possible origins of the LLOs. Gypsum, one of the minerals reported elsewhere as found in Juventae Chasma LLO material, forms only at low temperatures () and thus excludes a volcanic origin. Instead, the data are consistent with either multiple occurrences of lacustrine or airfall deposition over an extended period of time prior to emplacement of Hesperian lava flows on the plateau above the chasm.  相似文献   

Observed changes in pan-arctic cold-season minimum monthly river discharge     

Asa K. Rennermalm  Eric F. Wood  Tara J. Troy 《Climate Dynamics》2010,35(6):923-939

Recent changes in pan-arctic land-surface hydrology may significantly affect ecosystems and the built environment. While spatial and temporal patterns in land-surface hydrology trends can be identified using in situ observations of river discharge, the existing studies of cold-season river discharge are often inconsistent and incomparable because they use different methodologies, time periods and geographic regions. By using a comprehensive dataset of river discharge and a trend analysis applying one algorithm over a range of temporal and spatial scales, a synthesis of pan-arctic cold-season low-flow was performed. Cold-season low-flow is increasing over most of the pan-arctic, with decreasing flow in eastern North America and unchanged flow in small basins in the late twentieth century in eastern Eurasia as the main exceptions. This study provides the first synthesis of spatially distributed cold-season low-flow trends in the pan-arctic and indicates that widespread changes in pan-arctic subsurface hydrology are occurring.  相似文献   

Hotspots and a clumpy disc: variability of brown dwarfs and stars in the young σ Ori cluster     

A. Scholz  X. Xu  R. Jayawardhana  K. Wood  J. Eislöffel  C. Quinn 《Monthly notices of the Royal Astronomical Society》2009,398(2):873-886

The properties of accretion discs around stars and brown dwarfs in the σ Ori cluster (age 3 Myr) are studied based on near-infrared (IR) time series photometry supported by mid-IR spectral energy distributions (SEDs). We monitor ∼30 young low-mass sources over eight nights in the J and K band using the duPont telescope at Las Campanas. We find three objects showing variability with J -band amplitudes  ≥0.5 mag  ; five additional objects exhibit low-level variations. All three highly variable sources have been previously identified as highly variable; thus, we establish the long-term nature of their flux changes. The light curves contain periodic components with time-scales of  ∼0.5–8 d  , but have additional irregular variations superimposed – the characteristic behaviour for classical T Tauri stars. Based on the colour variability, we conclude that hotspots are the dominant cause of variations in two objects (#19 and #33), including one likely brown dwarf, with spot temperatures in the range of 6000–7000 K. For the third one (#2), a brown dwarf or very low-mass star, inhomogeneities at the inner edge of the disc are the likely origin of variability. Based on mid-IR data from Spitzer , we confirm that the three highly variable sources are surrounded by circum-(sub)-stellar discs. They show typical SEDs for T Tauri-like objects. Using SED models, we infer an enhanced scaleheight in the disc for the object #2, which favours the detection of disc inhomogeneities in light curves and is thus consistent with the information from variability. In the σ Ori cluster, about every fifth accreting low-mass object shows persistent high-level photometric variability. We demonstrate that estimates for fundamental parameters in such objects can be significantly improved by determining the extent and origin of the variations.  相似文献   

Reconstructing the 3D Morphology of the 17 May 2008 CME     

B. E. Wood  R. A. Howard  A. Thernisien  S. P. Plunkett  D. G. Socker 《Solar physics》2009,259(1-2):163-178

We model the kinematics and three-dimensional distribution of mass in a coronal mass ejection (CME) observed on 17 May 2008, using a comprehensive analysis of STEREO images of the CME. The CME is a surprisingly fast one for solar minimum, reaching velocities of up to 1120 km?s^?1. It can be followed continuously from inception all the way out to 1 AU. We find that the appearance of the CME can be modeled reasonably well as a combination of two distinct fronts that expand outward in a self-similar fashion. The model implies that STEREO-B is struck by the weaker of these two fronts on 19 May, and the in situ instruments on STEREO-B do see a weak density and magnetic field enhancement at the expected time.  相似文献   

金川铜镍硫化物矿床F1断裂系统演化及其意义     

李生栋  杨永春  艾启兴  达瑞 《大地构造与成矿学》2022,46(1):63-76

金川铜镍硫化物矿区构造活动频繁且复杂,查明矿区构造规律,是实现深部找矿突破的必经途径之一.文章在充分挖掘区域及以往勘查资料的基础上,通过野外实地调查,对矿区内成矿后断裂系统——F1断裂系统分析研究,发现F1断层角砾岩呈棱角状,深部产状北倾,断层发育在龙首山岩群一侧,无法仅靠现今的叠置关系解释F1的演化.结合潮水盆地研究...  相似文献   

Signal crayfish burrowing,bank retreat and sediment supply to rivers: A biophysical sediment budget     

H. Sanders  S.P. Rice  P.J. Wood 《地球表面变化过程与地形》2021,46(4):837-852

Burrowing into riverbanks by animals transfers sediment directly into river channels and has been hypothesised to accelerate bank erosion and promote mass failure. A field monitoring study on two UK rivers invaded by signal crayfish (Pacifastacus leniusculus) assessed the impact of burrowing on bank erosion processes. Erosion pins were installed in 17 riverbanks across a gradient of crayfish burrow densities and monitored for 22 months. Bank retreat increased significantly with crayfish burrow density. At the bank scale (<6 m river length), high crayfish burrow densities were associated with accelerated bank retreat of up to 253% and more than a doubling of the area of bank collapse compared with banks without burrows. Direct sediment supply by burrowing activity contributed 0.2% and 0.6% of total sediment at the reach (1.1 km) and local bank (<6 m) scales. However, accelerated bank retreat caused by burrows contributed 12.2% and 29.8% of the total sediment supply at the reach and bank scales. Together, burrowing and the associated acceleration of retreat and collapse supplied an additional 25.4 t km⁻¹ a⁻¹ of floodplain sediments at one site, demonstrating the substantial impact that signal crayfish can have on fine sediment supply. For the first time, an empirical relation linking animal burrow characteristics to riverbank retreat is presented. The study adds to a small number of sediment budget studies that compare sediment fluxes driven by biotic and abiotic energy but is unique in isolating and measuring the substantial interactive effect of the acceleration of abiotic bank erosion facilitated by biotic activity. Biotic energy expended through burrowing represents an energy surcharge to the river system that can augment sediment erosion by geophysical mechanisms.  相似文献   

The pulsating DA white dwarf star EC 14012−1446: results from four epochs of time-resolved photometry     

G. Handler  E. Romero-Colmenero  J. L. Provencal  K. Sanchawala  M. A. Wood  I. Silver  W.-P. Chen 《Monthly notices of the Royal Astronomical Society》2008,388(3):1444-1456

The pulsating DA white dwarfs are the coolest degenerate stars that undergo self-driven oscillations. Understanding their interior structure will help us to understand the previous evolution of the star. To this end, we report the analysis of more than 200 h of time-resolved CCD photometry of the pulsating DA white dwarf star EC 14012−1446 acquired during four observing epochs in three different years, including a coordinated three-site campaign. A total of 19 independent frequencies in the star's light variations together with 148 combination signals up to fifth order could be detected. We are unable to obtain the period spacing of the normal modes and therefore a mass estimate of the star, but we infer a fairly short rotation period of  0.61 ±0.03 d  , assuming the rotationally split modes are  ℓ= 1  . The pulsation modes of the star undergo amplitude and frequency variations, in the sense that modes with higher radial overtone show more pronounced variability and that amplitude changes are always accompanied by frequency variations. Most of the second-order combination frequencies detected have amplitudes that are a function of their parent mode amplitudes, but we found a few cases of possible resonantly excited modes. We point out the complications in the analysis and interpretation of data sets of pulsating white dwarfs that are affected by combination frequencies of the form   f _A + f _B − f _C   intruding into the frequency range of the independent modes.  相似文献