Microstructural Controls on the Uniaxial Compressive Strength of Porous Rocks Through the Granular to Non‐Granular Transition

Under uniaxial compression, a porous rock fails by coalescence of stress‐induced 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 non‐granular 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 high‐porosity granular samples, where microcracks grow at grain‐to‐grain contacts, are best described by a grain‐based model. Low‐porosity non‐granular samples, where microcracks grow from pores, are best described by a pore‐based 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

Mass fluxes for O stars

The theory of moving reversing layers for hot stars is updated to include an extensive line list, a radiative boundary condition from static model atmospheres, line transfer by scattering, and continuation to supersonic velocities. A Monte Carlo technique determines the theory's eigenvalue J, the mass flux, and the derived J's are in good agreement with the wind models of Pauldrach et al. (2001). The solutions' sensitivity to the photospheric microturbulent velocity reveals that this parameter has a throttling effect on J: turbulent line-broadening in the quasi-static layers reduces the radiation force available to accelerate matter through the sonic point. If photospheric turbulence approaches sonic velocities, this mechanism reduces mass loss rates by factors > 3, which would partly account for the reduced rates found observationally for clumpy winds.Comment: Accepted by A&A; 9 pages, 4 figure

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