822 research outputs found
Microstructural Controls on the Uniaxial Compressive Strength of Porous Rocks Through the Granular to NonâGranular Transition
Under uniaxial compression, a porous rock fails by coalescence of stressâinduced microcracks. The micromechanical models developed to analyze uniaxial compressive strength data consider a single mechanism for the initiation and propagation of microcracks and a fixed starting microstructure. Because the microstructure of clastic porous rock transitions from granular to nonâgranular as porosity decreases during diagenesis, their strength cannot be captured by a single model. Using synthetic samples with independently controlled porosity and initial grain radius we show that highâporosity granular samples, where microcracks grow at grainâtoâgrain contacts, are best described by a grainâbased model. Lowâporosity nonâgranular samples, where microcracks grow from pores, are best described by a poreâbased model. The switch from one model to the other depends on porosity and grain radius. We propose a regime plot that indicates which micromechanical model may be more suitable to predict strength for a given porosity and grain radius
The Influence of Grain Size Distribution on Mechanical Compaction and Compaction Localization in Porous Rocks
The modes of formation of clastic rocks result in a wide variety of microstructures, from poorly-sorted heterogeneous rocks to well-sorted and nominally homogeneous rocks. The mechanical behavior and failure mode of clastic rocks is known to vary with microstructural attributes such as porosity and grain size. However, the influence of the grain size distribution, in particular the degree of polydispersivity or modality of the distribution, is not yet fully understood, because it is difficult to study experimentally using natural rocks. To better understand the influence of grain size distribution on the mechanical behavior of porous rocks, we prepared suites of synthetic samples consisting of sintered glass beads with polydisperse grain size distributions. We performed hydrostatic compression experiments and found that, all else being equal, the onset of grain crushing occurs much more progressively and at lower pressure in polydisperse synthetic samples than in monodisperse samples. We conducted triaxial experiments in the regime of shear-enhanced compaction and found that the stress required to reach inelastic compaction was lower in polydisperse samples compared to monodisperse samples. Further, our microstructural observations show that compaction bands developed in monomodal polydisperse samples while delocalized cataclasis developed in bimodal polydisperse samples, where small grains were systematically crushed while largest grains remained intact. In detail, as the polydispersivity increases, microstructural deformation features appear to transition from localized to delocalized through a hybrid stage where a compaction front with diffuse bands propagates from both ends of the sample toward its center with increasing bulk strain
Land resource survey of Rottnest Island - an aid to land use planning
This report provides a description of the landforms, soils and vegetation of Rottnest Island. In addition, it provides a discussion of the relative capability of each of the landforms of the island to sustain various forms of development. The primary objective of this work is to direct development into areas of high capability (low risk of land degradation) in preference to areas of lower capability (higher risk of land degradation).
Rottnest Island is located approximately 30 km west of Perth. The proximity of the island to the capital city of Western Australia, its warm Mediterranean climate and the subsequent likelihood of further development, mean that there will be increasing pressure on the natural resources of the island. This report details the nature and inherent stability of those natural resources, and provides a guide for their management.https://researchlibrary.agric.wa.gov.au/bulletins/1062/thumbnail.jp
A Candidate Substellar Companion to HR 7329
We present the discovery of a candidate substellar companion from a survey of
nearby, young stars with the NICMOS coronagraph on the Hubble Space Telescope.
The H ~ 12 mag object was discovered approximately 4" from the young A0V star
HR 7329. Using follow-up spectroscopy from STIS, we derive a spectral type
between M7V and M8V with an effective temperature of ~ 2600 K. We estimate that
the probability of a chance alignment with a foreground dwarf star of this
nature is ~ 10^(-8) and therefore suggest the object (HR 7329B) is physically
associated with HR 7329 with a projected separation of 200 AU. Current brown
dwarf cooling models indicate a mass of less than 50 Jupiter masses for HR
7329B based on age estimates of < 30 Myr for HR7329A.Comment: 8 pages LATEX, 5 ps figures, accepted for Ap
The Heavy Element Enrichment of Lyman alpha Clouds in the Virgo Supercluster
Using high S/N STIS echelle spectra (FWHM=7 km/s) of 3C 273, we constrain the
metallicities of two Lya clouds in the vicinity of the Virgo cluster. We detect
C II, Si II, and Si III absorption lines in the Lya absorber at z = 0.00530.
Previous observations with FUSE have revealed Ly beta - Ly theta lines at this
redshift, thereby accurately constraining N(H I). We model the ionization of
the gas and derive [C/H] = -1.2^{+0.3}_{-0.2}, [Si/C] = 0.2+/-0.1, and log
n_{H} = -2.8+/-0.3. The model implies a small absorber thickness, ~70 pc, and
thermal pressure p/k ~ 40 cm^{-3} K. It is most likely that the absorber is
pressure confined by an external medium because gravitational confinement would
require a very high ratio of dark matter to baryonic matter. Based on Milky Way
sight lines in which carbon and silicon abundances have been reliably measured
in the same interstellar cloud (including new measurements presented herein),
we argue that the overabundance of Si relative to C is not due to dust
depletion. Instead, this probably indicates that the gas has been predominately
enriched by Type II supernovae. Such enrichment is most plausibly provided by
an unbound galactic wind, given the absence of galaxies within a projected
distance of 100 kpc and the presence of galaxies capable of driving a wind at
larger distances. We also constrain the metallicity and physical conditions of
the Virgo absorber at z = 0.00337 based on detections of O VI and H I and an
upper limit on C IV. If this absorber is collisionally ionized, the O VI/C IV
limit requires T > 10^{5.3} K. For either collisional ionization or
photoionization, we find that [O/H] > -2.0 at z = 0.00337.Comment: Final Ap.J. versio
Paraphrases and summaries: A means of clarification or a vehicle for articulating a preferred version of student accounts?
The use of group discussions as a means to facilitate learning from experiences is well documented in adventure education literature. Priest and Naismith (1993) assert that the use of the circular discussion method, where the leader poses questions to the participants, is the most common form of facilitation in adventure education. This paper draws on transcripts of facilitation sessions to argue that the widely advocated practice of leader summaries or paraphrases of student responses in these sessions functions as a potential mechanism to control and sponsor particular knowledge(s). Using transcripts from recorded facilitation sessions the analysis focuses on how the leader paraphrases the studentsâ responses and how these paraphrases or âformulationsâ function to modify or exclude particular aspects of the studentsâ responses. I assert that paraphrasing is not simply a neutral activity that merely functions to clarify a student response, it is a subtle means by which the leader of the session can, often inadvertently or unknowingly, alter the studentâs reply with the consequence of favouring particular knowledge(s). Revealing the subtle work that leader paraphrases perform is of importance for educators who claim to provide genuine opportunities for students to learn from their experience
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