title: Relationships between the geomechanics and petrography of ignimbrite

authors: Moon, Vicki G.

abstract: Fundamental petrographic controls on ignimbrite strength are examined, and those aspects of the texture and fabric of the rocks which are primarily responsible for the strength of ignimbrite identified. The range of strength exhibited by ignimbrite is quantified, and the value of common, non-destructive indices as predictors of ignimbrite strength assessed, with predictive equations presented where appropriate. Models for the strength, jointing, and geomorphic development of an ignimbrite sheet are derived.

Compressive, shear, and tensile strengths, measured by using standard techniques, provide the basic geomechanical data; strength indices (density, porosity, rebound hardness, slake durability, and ultrasonic wave velocity) and petrographic measurements (bulk rock chemistry, mineralogy, texture, fabric, and the extent of post-depositional alteration) are compared with these data using regression analysis. Textural and fabric parameters are determined using image analysis, optical microscopy, and scanning electron microscopy to quantify the size, shape, and three-dimensional arrangement of the components making up the ignimbrite.

Ignimbrites have highly variable strength, but in all forms of loading are classified as weak rocks. The strength variation ranges over at least two orders of magnitude for the compressive strength, and one order of magnitude for the tensile strength. Such variation may occur within a single vertical section through an ignimbrite sheet. Marked softening upon saturation occurs for all forms of strength, and ignimbrites show considerable plastic deformation prior to failure - rarely is a brittle failure mechanism exhibited.

Bulk rock properties and index measurements reflect the low strength values; ignimbrites have low bulk densities, high porosities, low rebound hardnesses, and low velocities of ultrasonic wave transmission. They display a highly variable response to slaking, with some being almost completely broken down by gentle agitation, and others remaining unaffected.

The porosity provides the best indicator of compressive strength, and predictive equations relating dry and saturated compressive strength to porosity are presented. Dry bulk density can also be used as a predictor of compressive strength, but the relationship is not as strong as that for porosity. Predictive equations relating the compressive and tensile strength to the Schmidt hammer rebound provide good indicators of major variations in rock strength, but are insensitive to subtle strength changes.

Ultrasonic wave velocity is a useful laboratory indicator of the tensile strength of ignimbrite. Unlike those for other index properties, the relationship between ultrasonic wave velocity and tensile strength is insensitive to changes in the moisture content of the rock. This means it should also, with appropriate calibration, be applicable under field conditions.

The groundmass fabric exerts the primary control over the compressive and tensile strengths of ignimbrite. The influence of groundmass fabric is modified by the size of the crystals and clasts included within the ignimbrite, but these two components are of less significance than the groundmass fabric. Shear strength is not readily related to petrographic parameters.

Compressive strength is controlled by the packing of the groundmass shards (their alignment and the density of pore filling), and the extent to which the shards are welded at their points of contact. These factors control the distribution of stresses in the material, and the ease with which microfractures can propagate through the groundmass. The size of crystals and pumice clasts controls the concentration of stresses around such inhomogeneities, and hence the initiation of microfractures. However, the proportion of such clasts is not important, so long as a sufficient number are present to initiate microfractures. Tensile strength is controlled by the degree of alignment of the shards, and the hardness of the pumice clasts; aligned shards and hard pumice clasts result in an ignimbrite with high tensile strength. Only a small anisotropy exists in terms of the direction of shard alignment with respect to the direction of principal stress.

Idealised models of joint development, hillslope form, and geomorphic development are based upon predictable variations in the primary petrographic features recognised as influencing the strength of the ignimbrites - the groundmass fabric and the nature of shard welding. Variations in these properties caused by emplacement conditions result in predictable strength variations, and in characteristic jointing patterns associated with each strength zone. It is these strength and jointing patterns which dictate the means by which erosion can affect an ignimbrite, and hence the resulting landform patterns.

text: Relationships between the geomechanics and petrography of ignimbrite Fundamental petrographic controls on ignimbrite strength are examined, and those aspects of the texture and fabric of the rocks which are primarily responsible for the strength of ignimbrite identified. The range of strength exhibited by ignimbrite is quantified, and the value of common, non-destructive indices as predictors of ignimbrite strength assessed, with predictive equations presented where appropriate. Models for the strength, jointing, and geomorphic development of an ignimbrite sheet are derived.

Compressive, shear, and tensile strengths, measured by using standard techniques, provide the basic geomechanical data; strength indices (density, porosity, rebound hardness, slake durability, and ultrasonic wave velocity) and petrographic measurements (bulk rock chemistry, mineralogy, texture, fabric, and the extent of post-depositional alteration) are compared with these data using regression analysis. Textural and fabric parameters are determined using image analysis, optical microscopy, and scanning electron microscopy to quantify the size, shape, and three-dimensional arrangement of the components making up the ignimbrite.

Ignimbrites have highly variable strength, but in all forms of loading are classified as weak rocks. The strength variation ranges over at least two orders of magnitude for the compressive strength, and one order of magnitude for the tensile strength. Such variation may occur within a single vertical section through an ignimbrite sheet. Marked softening upon saturation occurs for all forms of strength, and ignimbrites show considerable plastic deformation prior to failure - rarely is a brittle failure mechanism exhibited.

Bulk rock properties and index measurements reflect the low strength values; ignimbrites have low bulk densities, high porosities, low rebound hardnesses, and low velocities of ultrasonic wave transmission. They display a highly variable response to slaking, with some being almost completely broken down by gentle agitation, and others remaining unaffected.

The porosity provides the best indicator of compressive strength, and predictive equations relating dry and saturated compressive strength to porosity are presented. Dry bulk density can also be used as a predictor of compressive strength, but the relationship is not as strong as that for porosity. Predictive equations relating the compressive and tensile strength to the Schmidt hammer rebound provide good indicators of major variations in rock strength, but are insensitive to subtle strength changes.

Ultrasonic wave velocity is a useful laboratory indicator of the tensile strength of ignimbrite. Unlike those for other index properties, the relationship between ultrasonic wave velocity and tensile strength is insensitive to changes in the moisture content of the rock. This means it should also, with appropriate calibration, be applicable under field conditions.

The groundmass fabric exerts the primary control over the compressive and tensile strengths of ignimbrite. The influence of groundmass fabric is modified by the size of the crystals and clasts included within the ignimbrite, but these two components are of less significance than the groundmass fabric. Shear strength is not readily related to petrographic parameters.

Compressive strength is controlled by the packing of the groundmass shards (their alignment and the density of pore filling), and the extent to which the shards are welded at their points of contact. These factors control the distribution of stresses in the material, and the ease with which microfractures can propagate through the groundmass. The size of crystals and pumice clasts controls the concentration of stresses around such inhomogeneities, and hence the initiation of microfractures. However, the proportion of such clasts is not important, so long as a sufficient number are present to initiate microfractures. Tensile strength is controlled by the degree of alignment of the shards, and the hardness of the pumice clasts; aligned shards and hard pumice clasts result in an ignimbrite with high tensile strength. Only a small anisotropy exists in terms of the direction of shard alignment with respect to the direction of principal stress.

Idealised models of joint development, hillslope form, and geomorphic development are based upon predictable variations in the primary petrographic features recognised as influencing the strength of the ignimbrites - the groundmass fabric and the nature of shard welding. Variations in these properties caused by emplacement conditions result in predictable strength variations, and in characteristic jointing patterns associated with each strength zone. It is these strength and jointing patterns which dictate the means by which erosion can affect an ignimbrite, and hence the resulting landform patterns.

year: 1989

title: Perceived and actual wildfire danger: An economic and spatial analysis study in Colorado (USA)

authors: Kaval, Pamela

subjects: GIS

abstract: Over the last 20 years, costs for wildfire initial attack in the U.S. have increased significantly. The increased cost relates to wildfire suppression practices, as well as the growing number of homes in the wildland urban interface. Requiring wildland urban interface residents to pay an annual tax for their wildfire risk could lower costs to the general taxpayer. Willingness-to-pay for wildfire prevention, in relation to both perceived and actual wildfire danger, was the focus of this study. Surveyed Colorado wildland urban interface residents were found to have a high awareness of wildfire risk and were willing-to-pay over $400 annually to reduce this risk. Respondents' beliefs about wildfire frequency were comparable to the wildfire regimes of their areas' pre-European settlement.

text: Perceived and actual wildfire danger: An economic and spatial analysis study in Colorado (USA) Over the last 20 years, costs for wildfire initial attack in the U.S. have increased significantly. The increased cost relates to wildfire suppression practices, as well as the growing number of homes in the wildland urban interface. Requiring wildland urban interface residents to pay an annual tax for their wildfire risk could lower costs to the general taxpayer. Willingness-to-pay for wildfire prevention, in relation to both perceived and actual wildfire danger, was the focus of this study. Surveyed Colorado wildland urban interface residents were found to have a high awareness of wildfire risk and were willing-to-pay over $400 annually to reduce this risk. Respondents' beliefs about wildfire frequency were comparable to the wildfire regimes of their areas' pre-European settlement.

year: 2009

title: Kaupapa Māori framework and literature review of key prinicples

authors: Kennedy, Nathan

subjects: New Zealand

abstract: The literature review in this report was the starting point for the development of a Māori research strand within the Planning Under Co-operative Mandates (PUCM) research programme. The original purpose of this report Kaupapa Māori Framework and Literature Review of Key Principles was to establish definitions of environmentally significant concepts of kaupapa and tikanga Māori. In  addition,  the  review  sought  to  identify  and  briefly  describe  significant  variations between  understandings of the key concepts without attempting to reconcile these. As the purpose of the review in 2005 was to inform the development of a kaupapa Māori methodology for the identification and development of Māori environmental outcomes and indicators, we paid particular regard to Māori perceptions of the environment and the relevance of each concept in environmental terms.

text: Kaupapa Māori framework and literature review of key prinicples The literature review in this report was the starting point for the development of a Māori research strand within the Planning Under Co-operative Mandates (PUCM) research programme. The original purpose of this report Kaupapa Māori Framework and Literature Review of Key Principles was to establish definitions of environmentally significant concepts of kaupapa and tikanga Māori. In  addition,  the  review  sought  to  identify  and  briefly  describe  significant  variations between  understandings of the key concepts without attempting to reconcile these. As the purpose of the review in 2005 was to inform the development of a kaupapa Māori methodology for the identification and development of Māori environmental outcomes and indicators, we paid particular regard to Māori perceptions of the environment and the relevance of each concept in environmental terms.

year: 2009-06-30