title: Sensitivity and clay mineralogy of weathered tephra-derived soil materials in the Tauranga region

authors: Wyatt, Justin Burns

subjects: Sensitive soils

abstract: Soil sensitivity is defined as the ratio of peak to remoulded shear strength. Problem soil materials are those that show large strength losses on disruption, resulting in catastrophic failure, liquefaction and long run-out distances. This study focussed on sensitive, weathered, mainly tephra-fall derived soils of mid-Pleistocene age in the Tauranga region. The liquefiable character of these soils is well known, but little detailed study has been directed towards the reasons for sensitivity. The objective of this work was to examine soil sensitivity by investigating geomechanical properties, clay mineralogy, and microfabric, and to determine how these factors combine to develop sensitivity. To achieve these objectives a combination of both field and laboratory investigations was undertaken. 

Field investigations indicated that sensitive soils are common in the Tauranga region. Sampling was undertaken at sites in Tauriko and Otumoetai. Selected samples ranged across high (76) and low (≈8) field sensitivity. Stratigraphically, samples from Otumoetai lie below the Rangitawa Tephra (ca. 0.34 Ma), and those from Tauriko underlie the Te Ranga Ignimbrite (ca. 0.27 Ma). 

Geomechanical investigation revealed that the sensitive soils had high moisture contents (gt; 60 %), low dry bulk density (lt; 966kg m-3), and high porosity (gt; 60 %). Liquidity index values ranged between 0.27 and 2.41. Plasticity index values ranged from 13.2 % to 42.7 %, with all samples plotting below the A line. Strength tests indicated effective friction angles from 25.7   to 38.5  , effective cohesion from 4.7 kPa to 34.5 kPa, residual friction angles of 19.34   to 33.18  , and cohesions of 0 kPa to 4.87 kPa. Remoulded vane shear strengths ranged between 1 kPa and 36 kPa. 

Clay minerals were dominantly hydrated halloysite. Scanning electron microscopy indicated that clay morphology was in the form of hollow tubes, spheres, plates, and platy vermiforms ('books'). Tubes and spheres represent characteristic forms of halloysite in soils, plates are less common, and books have never previously been observed. Hence, these books represent a new morphology for halloysite. Individual plates in each of the books appear to show structural Fe enrichment (~5.2 %). This enrichment indicates that Fe had replaced Al in octahedral positions reducing the mismatch with the tetrahedral sheet, lessening layer curvature and thus generating flat plates.    

All microfabrics were continuous with larger sand and silt grains supported in a background of clay minerals. Microfabrics ranged from extremely open with components being loosely packed to those which were dense and tightly packed. A feature common to all structural types was an abundance of extremely small pores (lt; 20 μm) which are capable of tightly retaining water. The loosely packed microfabrics had void ratios that allowed moisture content to exceed liquid limits, producing a liquidity index gt; 1. These open microfabrics are probably a result of quick burial by subsequent pyroclastic beds; hence weathering to clays occurred as a process of subsurface diagenesis. Dense microfabrics with low void ratios and high liquid limits did not have liquidity indexes gt; 1. These dense microfabrics arose as a result of the deposits being at, or near, the land surface for a considerable time, thus allowing strong pedogenic processes to occur, which promoted clay formation and clay migration (illuviation) and reduced void ratios.    

Liquidity index was a major control on remoulded strength and sensitivity. Liquidity index is controlled by clay type and content, void ratio, and natural moisture content. When remoulded, structures with natural moisture contents exceeding the liquid limit release a large amount of water, which both dilutes the plasticity of binding clays and supports grains and broken aggregates of clay, allowing the material to flow. The development of sensitivity with low remoulded strength requires a number of factors. These include: a void ratio that is sufficiently high to allow natural moisture content to exceed the liquid limit; the presence of halloysite, which encourages samples to retain a coherent structure when saturated and to ensure the liquid limit remains sufficiently low so that it can be exceeded by natural moisture content; and a saturated environment, which ensures the liquid limit is exceeded.

text: Sensitivity and clay mineralogy of weathered tephra-derived soil materials in the Tauranga region Soil sensitivity is defined as the ratio of peak to remoulded shear strength. Problem soil materials are those that show large strength losses on disruption, resulting in catastrophic failure, liquefaction and long run-out distances. This study focussed on sensitive, weathered, mainly tephra-fall derived soils of mid-Pleistocene age in the Tauranga region. The liquefiable character of these soils is well known, but little detailed study has been directed towards the reasons for sensitivity. The objective of this work was to examine soil sensitivity by investigating geomechanical properties, clay mineralogy, and microfabric, and to determine how these factors combine to develop sensitivity. To achieve these objectives a combination of both field and laboratory investigations was undertaken. 

Field investigations indicated that sensitive soils are common in the Tauranga region. Sampling was undertaken at sites in Tauriko and Otumoetai. Selected samples ranged across high (76) and low (≈8) field sensitivity. Stratigraphically, samples from Otumoetai lie below the Rangitawa Tephra (ca. 0.34 Ma), and those from Tauriko underlie the Te Ranga Ignimbrite (ca. 0.27 Ma). 

Geomechanical investigation revealed that the sensitive soils had high moisture contents (gt; 60 %), low dry bulk density (lt; 966kg m-3), and high porosity (gt; 60 %). Liquidity index values ranged between 0.27 and 2.41. Plasticity index values ranged from 13.2 % to 42.7 %, with all samples plotting below the A line. Strength tests indicated effective friction angles from 25.7   to 38.5  , effective cohesion from 4.7 kPa to 34.5 kPa, residual friction angles of 19.34   to 33.18  , and cohesions of 0 kPa to 4.87 kPa. Remoulded vane shear strengths ranged between 1 kPa and 36 kPa. 

Clay minerals were dominantly hydrated halloysite. Scanning electron microscopy indicated that clay morphology was in the form of hollow tubes, spheres, plates, and platy vermiforms ('books'). Tubes and spheres represent characteristic forms of halloysite in soils, plates are less common, and books have never previously been observed. Hence, these books represent a new morphology for halloysite. Individual plates in each of the books appear to show structural Fe enrichment (~5.2 %). This enrichment indicates that Fe had replaced Al in octahedral positions reducing the mismatch with the tetrahedral sheet, lessening layer curvature and thus generating flat plates.    

All microfabrics were continuous with larger sand and silt grains supported in a background of clay minerals. Microfabrics ranged from extremely open with components being loosely packed to those which were dense and tightly packed. A feature common to all structural types was an abundance of extremely small pores (lt; 20 μm) which are capable of tightly retaining water. The loosely packed microfabrics had void ratios that allowed moisture content to exceed liquid limits, producing a liquidity index gt; 1. These open microfabrics are probably a result of quick burial by subsequent pyroclastic beds; hence weathering to clays occurred as a process of subsurface diagenesis. Dense microfabrics with low void ratios and high liquid limits did not have liquidity indexes gt; 1. These dense microfabrics arose as a result of the deposits being at, or near, the land surface for a considerable time, thus allowing strong pedogenic processes to occur, which promoted clay formation and clay migration (illuviation) and reduced void ratios.    

Liquidity index was a major control on remoulded strength and sensitivity. Liquidity index is controlled by clay type and content, void ratio, and natural moisture content. When remoulded, structures with natural moisture contents exceeding the liquid limit release a large amount of water, which both dilutes the plasticity of binding clays and supports grains and broken aggregates of clay, allowing the material to flow. The development of sensitivity with low remoulded strength requires a number of factors. These include: a void ratio that is sufficiently high to allow natural moisture content to exceed the liquid limit; the presence of halloysite, which encourages samples to retain a coherent structure when saturated and to ensure the liquid limit remains sufficiently low so that it can be exceeded by natural moisture content; and a saturated environment, which ensures the liquid limit is exceeded.

year: 2009

title: Ecology of common bully (Gobiomorphus cotidianus) in the Tarawera and Rangitaiki rivers: isolation by inland distance or anthropogenic discharge?

authors: Bleackley, Natalie Anne

subjects: diadromy

abstract: Previous research has identified distinct genetic, life-history and reproductive differences between populations of common bully (Gobiomorphus cotidianus) upstream and downstream of a pulp and paper mill outfall on the Tarawera River in the Bay of Plenty, New Zealand. This study investigated the distribution of common bully in the Tarawera River by examining fish collected from upstream (37 km inland) and downstream (20 km inland) locations and comparing them to fish from similar inland locations (40 km and 17 km inland, respectively) in the nearby Rangitaiki River. Reproductive divergence was observed between upstream and downstream sites of both rivers by differing annual trends in gonadosomatic index. Stable carbon and nitrogen isotopes confirmed residency at each sampling site and otolith microchemistry demonstrated different life-history strategies between upstream and downstream populations. Diadromous recruits dominated in both downstream river populations, with a general disappearance of diadromy upstream. A mixture of diadromous and non-diadromous fish were found in the upstream Rangitaiki River, whereas diadromous recruits were absent in the upstream Tarawera River. A reduction in oculoscapular canal structures also coincided with loss of diadromy in fish from both rivers. A behavioural study to determine whether pulp and paper mill effluent may deter fish migration within the Tarawera River demonstrated a strong avoidance of effluent, but only at concentrations (>25%) greater than those that naturally occur in the river (<15%). The results of this study suggest that combinations of influences coupled with inland distance are likely to be responsible for the isolation of common bully subpopulations within the Tarawera River.

text: Ecology of common bully (Gobiomorphus cotidianus) in the Tarawera and Rangitaiki rivers: isolation by inland distance or anthropogenic discharge? Previous research has identified distinct genetic, life-history and reproductive differences between populations of common bully (Gobiomorphus cotidianus) upstream and downstream of a pulp and paper mill outfall on the Tarawera River in the Bay of Plenty, New Zealand. This study investigated the distribution of common bully in the Tarawera River by examining fish collected from upstream (37 km inland) and downstream (20 km inland) locations and comparing them to fish from similar inland locations (40 km and 17 km inland, respectively) in the nearby Rangitaiki River. Reproductive divergence was observed between upstream and downstream sites of both rivers by differing annual trends in gonadosomatic index. Stable carbon and nitrogen isotopes confirmed residency at each sampling site and otolith microchemistry demonstrated different life-history strategies between upstream and downstream populations. Diadromous recruits dominated in both downstream river populations, with a general disappearance of diadromy upstream. A mixture of diadromous and non-diadromous fish were found in the upstream Rangitaiki River, whereas diadromous recruits were absent in the upstream Tarawera River. A reduction in oculoscapular canal structures also coincided with loss of diadromy in fish from both rivers. A behavioural study to determine whether pulp and paper mill effluent may deter fish migration within the Tarawera River demonstrated a strong avoidance of effluent, but only at concentrations (>25%) greater than those that naturally occur in the river (<15%). The results of this study suggest that combinations of influences coupled with inland distance are likely to be responsible for the isolation of common bully subpopulations within the Tarawera River.

year: 2009

title: New Zealand English and Te Reo Māori: A critical perspective on the loanword narrative using data from translingual picturebooks

authors: Barbour, Julie Renee

subjects: language contact

abstract: Aims/Purpose: New Zealand English has been characterised by the use of vocabulary ‘borrowed’ from the Indigenous Māori language since early contact between English and Māori speakers. In this paper, we consider the relevance of the ‘loanword narrative’ to New Zealand English, examining data from translingual picturebooks published in Aotearoa New Zealand.
Design/Data: We built a small corpus of 10 translingual picturebooks spanning a 30-year period beginning in the early years of Māori language revitalisation.
Analysis: We applied techniques of language analysis from descriptive linguistics, including studies of loanwords.
Findings: We find some evidence of linguistic ‘borrowing’.
Originality: However, in texts created by Māori content creators, with Māori cultural themes, a language practice akin to pedagogical code-switching or pedagogical translanguaging is observed. Māori content creators alternate between languages, treating Māori lexemes as distinct from NZE, preserving Māori orthographic traditions, and providing translation, explanation, or contextual support of Māori lexemes for NZE reader comprehension. Our observations find parallels in translanguaging studies in bilingual early childhood settings, and they are supported by new psycholinguistic research that has identified the development of substantial body of linguistic knowledge about the Māori language by non-Māori speakers through societal exposure.
Significance: We argue that the prevailing loanword narrative does not account for Māori language usage in translingual picturebooks developed by Māori content creators. Rather, we find a clear pattern of differentiation between Māori and NZE.

text: New Zealand English and Te Reo Māori: A critical perspective on the loanword narrative using data from translingual picturebooks Aims/Purpose: New Zealand English has been characterised by the use of vocabulary ‘borrowed’ from the Indigenous Māori language since early contact between English and Māori speakers. In this paper, we consider the relevance of the ‘loanword narrative’ to New Zealand English, examining data from translingual picturebooks published in Aotearoa New Zealand.
Design/Data: We built a small corpus of 10 translingual picturebooks spanning a 30-year period beginning in the early years of Māori language revitalisation.
Analysis: We applied techniques of language analysis from descriptive linguistics, including studies of loanwords.
Findings: We find some evidence of linguistic ‘borrowing’.
Originality: However, in texts created by Māori content creators, with Māori cultural themes, a language practice akin to pedagogical code-switching or pedagogical translanguaging is observed. Māori content creators alternate between languages, treating Māori lexemes as distinct from NZE, preserving Māori orthographic traditions, and providing translation, explanation, or contextual support of Māori lexemes for NZE reader comprehension. Our observations find parallels in translanguaging studies in bilingual early childhood settings, and they are supported by new psycholinguistic research that has identified the development of substantial body of linguistic knowledge about the Māori language by non-Māori speakers through societal exposure.
Significance: We argue that the prevailing loanword narrative does not account for Māori language usage in translingual picturebooks developed by Māori content creators. Rather, we find a clear pattern of differentiation between Māori and NZE.

year: 2025