title: Failure mechanisms in sensitive volcanic soils in the Tauranga Region, New Zealand

authors: Mills, Philippa

abstract: Sensitive soils derived from weathered pyroclastic materials have contributed to major landslides in the Bay of Plenty. Sensitive soils have a high ratio of peak to remoulded undrained strength. While it is known that (a) sensitive soils flow once failed, causing long runout distances, and (b) these failures often occur following heavy rainfall, the mechanisms that lead to failure are less understood. The aim of this thesis is to determine static and cyclic failure mechanisms of sensitive soils sampled from the failure scarps of two recent landslides in the Tauranga Region. Revelations about how these soils fail will allow slope stability models to more accurately capture geomechanical behaviour.
Recent publications on sensitive soils derived from glacial till materials have indicated that these soils are brittle materials displaying undrained strain softening behaviour, where deviator stress drops significantly following peak stress. Failure is governed by rate dependant, excess pore pressure gradients accumulating during undrained, consolidated triaxial compression (Gylland et al. 2013c; 2014; Thakur et al. 2014). These publications provided a methodological backbone for this thesis. Field methods included geomorphological and stratigraphic site characterisation, and sampling of extra sensitive soil suspected of contributing to failure. Laboratory methods included geotechnical tests (Atterberg Limits, moisture content, bulk and particle density, particle size distribution, and static and cyclic undrained, consolidated triaxial tests). Static triaxial testing was undertaken at a high compression rate of 0.5mm/min to model rapid undrained during slope failure. Different combinations of average and cyclic shear stresses allowed replication of Anderson’s (2015) cyclic contour plot. Shear zone microstructure of failed triaxial samples was analysed using thin section and micro-CT techniques.
Two coastal cliff landslide sites were characterised and sampled: (1) a significant landslide at Bramley Drive, Omokoroa, which initially occurred in 1979, with reactivations in 2011 and 2012, and (2) a landslide on the south side of Matua Peninsula, which occurred in 2012. The bowl-shaped landslide crater at Bramley Drive and long runout component of sensitive material are likely due to failure within an over-thickened sequence pyroclastic material (Pahoia Tephras), which initially accumulated in a paleovalley. At Matua, the failure surface was long, slightly rotational, and comprised a sequence of variable sandy lenses and silty clays. Landslide debris comprised remoulded sensitive material underlying intact overlying blocks, indicating failure of a sensitive soil layer at depth. Material sampled at Omokoroa (OM1) was an extra sensitive (St = 15 ± 3) silty CLAY, 19 m from top of profile within Pahoia Tephras. Material at Matua (M1) was an extra sensitive (St = 10 ± 1) silty CLAY, 16 m from the top, within the Matua Subgroup. Clay mineralogy of these soils is known to be various morphologies of hydrated halloysite.
Samples from both sites have dominant clay fractions (OM1: clay: 62.6%, silt: 37.3%, sand: 0.1%, M1: clay: 40.1%, silt: 22.3 %, sand: 37.6%). High porosity (OM1: 70% M1:66%), void ratio (OM1: 2.3 M1: 1.8), and moisture content (OM1: 72%, M1: 64%), together with low wet and dry bulk densities (wet b.d: OM1:1320 kgm-3, M1: 1690 kgm-3, dry b.d: OM1: 760 kgm-3, M1: 980 kgm-3), are in keeping with previously published values of halloysite-rich clays derived from pyroclastic material. Atterberg Limits are high for both materials (Liquid limit: OM1: 66 M1: 52, Plastic limit: OM1: 41, M1: 37, Plasticity index: OM1: 25 M1: 15, Liquidity index: OM1: 2.9 M1: 1.8). M1 and OM1 both plot below the A-line in the range of high compressibility silts (MH). M1 and OM1 both have low activity, reflecting the hydrated halloysite composition (OM1: 0.4, M1: 0.4). 
Static undrained, consolidated triaxial tests show that failure occurs at less than or near to 5% strain for all tests, indicating brittle failure. Two main types of failure mechanisms were recognised from triaxial results. Post failure, type A was characterised by significant strain softening, contractive, left trending p’q’ stress paths, and a rise in global pore pressure after failure. Type B response post-peak deviator stress showed minor to no strain softening, dilative, right trending p’q’ paths, and a drop in global pore water pressure. In general, test rate, confining pressure and material affect the type of failure: higher compression rates and confining pressures correlate with type A failure, whereas the opposite is true for type B failure.  Failure modes observed in failed triaxial samples were either wedge or shear, with the exception of M1a (tested at 75 kPa confining pressure) which failed by barrel deformation. Strain softening increased with effective confining pressure (R2= 0.58). Average effective cohesion and friction remain essentially consistent between peak and residual states (OM1: c’f = 26, c’r = 24, φ’f = 31, φ’r=26, M1: c’f = 17, c’r = 17 φ’f = 32, φ’r = 29). 
Thin sections captured shear zones tested at 240 kPa and 340 kPa (OM1), and 150 kPa and 255 kPa (M1) confining stress. Riedel shears (R and R’) and P shears were observed in all thin sections. Evidence for progressive failure, most notably changes in the abundance and spacing of shears along the same shear zone, was found in both materials. Clay mineral realignment was observed in shear zones. Micro-CT results showed clay matrix material to be denser in shear zones, implying localised contraction of microstructure.
I infer that type A failure mechanism is comparable to sensitive soils that Gylland et al. (2013c; 2014) studied. During compression, pore pressure does not have time to dissipate, leading to excess pore pressure gradients, which initiate brittle failure where a release of potential energy results in R shear fractures and R’ fractures which become linked by P shears. Microstructural collapse within these fractures induces further excess pore pressure, liquefying material in shear zones, and resulting in a loss of material resistance as evidenced by the strain softening behaviour observed. Realigned material in shear zones provides a pathway for excess pore pressure to dissipate, finally registering as a rise in pore pressure in the post-peak region. Integrity of cohesive bonds and asperity interaction is preserved during shearing, resulting in little to reduction of c’ and φ’. For type B failure, lower confining pressures and/or test rates mean that pore pressure has ample time to dissipate during compression, so that when the critical state line in p’q’ diagrams is reached, grains interlock, causing pore pressures to drop (dilation). 
Boulanger & Idriss (2007) conclude that for sensitive materials, it is difficult to assess the strain or ground displacement that will reduce the clay from peak to residual strength during cyclic loading. In this study, I utilised a new geotechnical tool, a cyclic contour plot (Anderson, 2015), that predicts the cycles to failure, and the average shear strain and cyclic shear strain at failure, for combinations of applied average and cyclic shear stresses. Seven samples were tested at different combinations of average and cyclic shear stresses. Tests with high average and low cyclic shear stress applications resulted in progressive, positive strain accumulation. Tests with no average but high cyclic shear stresses resulted in progressive accumulation of strain in both positive and negative directions. In comparison to Drammen Clay (Anderson, 2015), in general, for the same application of average and cyclic shear stress, failure occurs after a lesser number of cycles, but both average and cyclic strain accumulation is lower. Although limited microstructural evidence was analysed, observations tests show similar mechanisms as described above are responsible for failure under cyclic stresses; post-failure strain softening occurs, and excess pore pressure increases. One micro-CT sample of an entire failed sample tested at high (60 kPa) cyclic shear stress and zero average shear stress shows intense contraction in the shear zone. 
It is likely that following heavy rainfall events, excess pore pressure gradients develop in sensitive material at Bramley Drive and Matua, resulting in localised fracture development. Collapse of the disturbed sensitive soil in developing shear zones releases additional pore water, enhancing pore water pressure gradients and leading to progressive fracture. Ultimately, breakdown of the sensitive material results in liquefaction along a macroscopic failure surface and rafting away overlying material.

text: Failure mechanisms in sensitive volcanic soils in the Tauranga Region, New Zealand Sensitive soils derived from weathered pyroclastic materials have contributed to major landslides in the Bay of Plenty. Sensitive soils have a high ratio of peak to remoulded undrained strength. While it is known that (a) sensitive soils flow once failed, causing long runout distances, and (b) these failures often occur following heavy rainfall, the mechanisms that lead to failure are less understood. The aim of this thesis is to determine static and cyclic failure mechanisms of sensitive soils sampled from the failure scarps of two recent landslides in the Tauranga Region. Revelations about how these soils fail will allow slope stability models to more accurately capture geomechanical behaviour.
Recent publications on sensitive soils derived from glacial till materials have indicated that these soils are brittle materials displaying undrained strain softening behaviour, where deviator stress drops significantly following peak stress. Failure is governed by rate dependant, excess pore pressure gradients accumulating during undrained, consolidated triaxial compression (Gylland et al. 2013c; 2014; Thakur et al. 2014). These publications provided a methodological backbone for this thesis. Field methods included geomorphological and stratigraphic site characterisation, and sampling of extra sensitive soil suspected of contributing to failure. Laboratory methods included geotechnical tests (Atterberg Limits, moisture content, bulk and particle density, particle size distribution, and static and cyclic undrained, consolidated triaxial tests). Static triaxial testing was undertaken at a high compression rate of 0.5mm/min to model rapid undrained during slope failure. Different combinations of average and cyclic shear stresses allowed replication of Anderson’s (2015) cyclic contour plot. Shear zone microstructure of failed triaxial samples was analysed using thin section and micro-CT techniques.
Two coastal cliff landslide sites were characterised and sampled: (1) a significant landslide at Bramley Drive, Omokoroa, which initially occurred in 1979, with reactivations in 2011 and 2012, and (2) a landslide on the south side of Matua Peninsula, which occurred in 2012. The bowl-shaped landslide crater at Bramley Drive and long runout component of sensitive material are likely due to failure within an over-thickened sequence pyroclastic material (Pahoia Tephras), which initially accumulated in a paleovalley. At Matua, the failure surface was long, slightly rotational, and comprised a sequence of variable sandy lenses and silty clays. Landslide debris comprised remoulded sensitive material underlying intact overlying blocks, indicating failure of a sensitive soil layer at depth. Material sampled at Omokoroa (OM1) was an extra sensitive (St = 15 ± 3) silty CLAY, 19 m from top of profile within Pahoia Tephras. Material at Matua (M1) was an extra sensitive (St = 10 ± 1) silty CLAY, 16 m from the top, within the Matua Subgroup. Clay mineralogy of these soils is known to be various morphologies of hydrated halloysite.
Samples from both sites have dominant clay fractions (OM1: clay: 62.6%, silt: 37.3%, sand: 0.1%, M1: clay: 40.1%, silt: 22.3 %, sand: 37.6%). High porosity (OM1: 70% M1:66%), void ratio (OM1: 2.3 M1: 1.8), and moisture content (OM1: 72%, M1: 64%), together with low wet and dry bulk densities (wet b.d: OM1:1320 kgm-3, M1: 1690 kgm-3, dry b.d: OM1: 760 kgm-3, M1: 980 kgm-3), are in keeping with previously published values of halloysite-rich clays derived from pyroclastic material. Atterberg Limits are high for both materials (Liquid limit: OM1: 66 M1: 52, Plastic limit: OM1: 41, M1: 37, Plasticity index: OM1: 25 M1: 15, Liquidity index: OM1: 2.9 M1: 1.8). M1 and OM1 both plot below the A-line in the range of high compressibility silts (MH). M1 and OM1 both have low activity, reflecting the hydrated halloysite composition (OM1: 0.4, M1: 0.4). 
Static undrained, consolidated triaxial tests show that failure occurs at less than or near to 5% strain for all tests, indicating brittle failure. Two main types of failure mechanisms were recognised from triaxial results. Post failure, type A was characterised by significant strain softening, contractive, left trending p’q’ stress paths, and a rise in global pore pressure after failure. Type B response post-peak deviator stress showed minor to no strain softening, dilative, right trending p’q’ paths, and a drop in global pore water pressure. In general, test rate, confining pressure and material affect the type of failure: higher compression rates and confining pressures correlate with type A failure, whereas the opposite is true for type B failure.  Failure modes observed in failed triaxial samples were either wedge or shear, with the exception of M1a (tested at 75 kPa confining pressure) which failed by barrel deformation. Strain softening increased with effective confining pressure (R2= 0.58). Average effective cohesion and friction remain essentially consistent between peak and residual states (OM1: c’f = 26, c’r = 24, φ’f = 31, φ’r=26, M1: c’f = 17, c’r = 17 φ’f = 32, φ’r = 29). 
Thin sections captured shear zones tested at 240 kPa and 340 kPa (OM1), and 150 kPa and 255 kPa (M1) confining stress. Riedel shears (R and R’) and P shears were observed in all thin sections. Evidence for progressive failure, most notably changes in the abundance and spacing of shears along the same shear zone, was found in both materials. Clay mineral realignment was observed in shear zones. Micro-CT results showed clay matrix material to be denser in shear zones, implying localised contraction of microstructure.
I infer that type A failure mechanism is comparable to sensitive soils that Gylland et al. (2013c; 2014) studied. During compression, pore pressure does not have time to dissipate, leading to excess pore pressure gradients, which initiate brittle failure where a release of potential energy results in R shear fractures and R’ fractures which become linked by P shears. Microstructural collapse within these fractures induces further excess pore pressure, liquefying material in shear zones, and resulting in a loss of material resistance as evidenced by the strain softening behaviour observed. Realigned material in shear zones provides a pathway for excess pore pressure to dissipate, finally registering as a rise in pore pressure in the post-peak region. Integrity of cohesive bonds and asperity interaction is preserved during shearing, resulting in little to reduction of c’ and φ’. For type B failure, lower confining pressures and/or test rates mean that pore pressure has ample time to dissipate during compression, so that when the critical state line in p’q’ diagrams is reached, grains interlock, causing pore pressures to drop (dilation). 
Boulanger & Idriss (2007) conclude that for sensitive materials, it is difficult to assess the strain or ground displacement that will reduce the clay from peak to residual strength during cyclic loading. In this study, I utilised a new geotechnical tool, a cyclic contour plot (Anderson, 2015), that predicts the cycles to failure, and the average shear strain and cyclic shear strain at failure, for combinations of applied average and cyclic shear stresses. Seven samples were tested at different combinations of average and cyclic shear stresses. Tests with high average and low cyclic shear stress applications resulted in progressive, positive strain accumulation. Tests with no average but high cyclic shear stresses resulted in progressive accumulation of strain in both positive and negative directions. In comparison to Drammen Clay (Anderson, 2015), in general, for the same application of average and cyclic shear stress, failure occurs after a lesser number of cycles, but both average and cyclic strain accumulation is lower. Although limited microstructural evidence was analysed, observations tests show similar mechanisms as described above are responsible for failure under cyclic stresses; post-failure strain softening occurs, and excess pore pressure increases. One micro-CT sample of an entire failed sample tested at high (60 kPa) cyclic shear stress and zero average shear stress shows intense contraction in the shear zone. 
It is likely that following heavy rainfall events, excess pore pressure gradients develop in sensitive material at Bramley Drive and Matua, resulting in localised fracture development. Collapse of the disturbed sensitive soil in developing shear zones releases additional pore water, enhancing pore water pressure gradients and leading to progressive fracture. Ultimately, breakdown of the sensitive material results in liquefaction along a macroscopic failure surface and rafting away overlying material.

year: 2016

title: Boat electrofishing survey of the Waimapu and Kopurererua streams, Bay of Plenty, Waitara River, and a pond at Mokoia, Taranaki

authors: Hicks, Brendan J.

subjects: New Zealand

abstract: We conducted boat electrofishing surveys at four sites to establish the presence of, and to remove, koi carp (Cyprinus carpio). We fished the lower Waimapu Stream, Bay of Plenty, on 15 April and 14 June 2005, and the adjacent Kopurererua Stream on 15 June 2005. We caught and removed two large koi carp ( a 7.9 kg male and a 6.9 kg female) from the Waimapu Stream in 15 April, but saw no other koi carp in 3.2-km reaches of both streams on 14-15 June. We did, however, catch a wide diversity of native fish species, including common smelt, shortfin and longfin eels, yelloweye mullet, brown and rainbow trout, giant bullies, and black flounder. We also caught a lamprey macrophthalmia in the Waimapu Stream.
No koi carp were found in a 2.4-km tidal reach of the Waitara River, northern Taranaki, on 8 June 2005, but we did catch a sparse native fish assemblage of adult inanga, shortfin eels, a few common smelt, black flounder, yelloweye mullet, and a 200-mm giant bully.
We caught and killed 16 large koi carp (total weight 58.5 kg) from a pond on private farmland near Mokoia, southern Taranaki, on 9 June 2005. This pond had a perimeter of 215 m, a surface area of 951 m², and maximum depth of 2.0 m. These fish were removed by depletion fishing in 7 circuits of the pond. We caught 9, 3, 1, 1, 1, 1, and 0 carp in successive circuits. The Zippin method of population estimation (±95% confidence interval) suggests that there were 16±1 koi carp in the pond, so it is possible that we eradicated all of the koi. All the carp were the same age and no juveniles were found, so it appears that the carp had not breed successfully in this pond.

text: Boat electrofishing survey of the Waimapu and Kopurererua streams, Bay of Plenty, Waitara River, and a pond at Mokoia, Taranaki We conducted boat electrofishing surveys at four sites to establish the presence of, and to remove, koi carp (Cyprinus carpio). We fished the lower Waimapu Stream, Bay of Plenty, on 15 April and 14 June 2005, and the adjacent Kopurererua Stream on 15 June 2005. We caught and removed two large koi carp ( a 7.9 kg male and a 6.9 kg female) from the Waimapu Stream in 15 April, but saw no other koi carp in 3.2-km reaches of both streams on 14-15 June. We did, however, catch a wide diversity of native fish species, including common smelt, shortfin and longfin eels, yelloweye mullet, brown and rainbow trout, giant bullies, and black flounder. We also caught a lamprey macrophthalmia in the Waimapu Stream.
No koi carp were found in a 2.4-km tidal reach of the Waitara River, northern Taranaki, on 8 June 2005, but we did catch a sparse native fish assemblage of adult inanga, shortfin eels, a few common smelt, black flounder, yelloweye mullet, and a 200-mm giant bully.
We caught and killed 16 large koi carp (total weight 58.5 kg) from a pond on private farmland near Mokoia, southern Taranaki, on 9 June 2005. This pond had a perimeter of 215 m, a surface area of 951 m², and maximum depth of 2.0 m. These fish were removed by depletion fishing in 7 circuits of the pond. We caught 9, 3, 1, 1, 1, 1, and 0 carp in successive circuits. The Zippin method of population estimation (±95% confidence interval) suggests that there were 16±1 koi carp in the pond, so it is possible that we eradicated all of the koi. All the carp were the same age and no juveniles were found, so it appears that the carp had not breed successfully in this pond.

year: 2005

title: Kei hea te tangi a te Tūī? An exploration of Kaitiakitanga in urban spaces

authors: Walker, Erana

subjects: Kaitiakitanga

abstract: Indigenous people’s connection to the natural world differs from place to place. In Aotearoa, New Zealand, Māori connection to nature is often articulated through the concept of kaitiakitanga whilst intertwining concepts of whakapapa, wairua, mana, mauri and place. Kaitiakitanga captures the relationships, narratives and practices that Māori utilise to protect kin of the natural environment as well as Māori communities in general. Over recent decades increase use of kaitiakitanga with respect to resource management has been linked to ideas of guardianship and stewardship, departing from important aspects such as place, whakapapa, intergenerational knowledge, spirituality and resource use. Calls to recognise the diverse ways kaitiakitanga has and can be expressed by Māori have echoed through academic literature, encouraging wider perspectives and application of the concept to develop. This has reinvigorated not only the need for natures protection but also the recognition of cultural knowledges and concepts in practicing kaitiakitanga. 
 
As with most indigenous people, Māori have experienced urbanisation that has spanned across generations of whānau and hapū. Urbanisation has the ability to challenge health outcomes, cultural practices and cultural knowledge, and more importantly, relationships to nature. Growing urbanisation and environmental degradation continue to challenge people and our relationships to nature. There is now a need to understand how urban spaces may also challenge the concepts that encourage connections to nature like kaitiakitanga. Moreover, there is a need to understand if such cultural concepts can become challenged when transient Māori reside in another tribal group’s boundaries. This research provides a deeper analysis of kaitiakitanga by understanding its application within the urban space of Kirikiriroa/Hamilton. From data gathered through a survey, focus groups and interviews, this research project illustrates the experiences of Māori in the urban space who know and practice kaitiakitanga in Kirikiriroa/Hamilton. More importantly, this study posits the integral role of place connection and mana in supporting and shaping kaitiakitanga practices in urban spaces.

The data from participants shows a clear narrative of kaitiakitanga from both local and transient Māori exist and indicates that kaitiakitanga is practiced in accordance with their location and whakapapa to their area of residence. The study has found that kaitiakitanga practices have adapted to suit urban spaces allowing for both transient and local knowledges to exist within the urban space. The key findings of this research show many influences that impact kaitiakitanga practices like the recognition of childhood spaces and experiences, ideas of kinship, cultural knowledges, spirituality, mobility, nature as well as modernisation. Through exploring participants use and knowledge of kaitiakitanga, this research provides a new lens in which to view this concept and bring to the fore, the various experiences and ways that participants connect to place, people, culture and most importantly, nature.

text: Kei hea te tangi a te Tūī? An exploration of Kaitiakitanga in urban spaces Indigenous people’s connection to the natural world differs from place to place. In Aotearoa, New Zealand, Māori connection to nature is often articulated through the concept of kaitiakitanga whilst intertwining concepts of whakapapa, wairua, mana, mauri and place. Kaitiakitanga captures the relationships, narratives and practices that Māori utilise to protect kin of the natural environment as well as Māori communities in general. Over recent decades increase use of kaitiakitanga with respect to resource management has been linked to ideas of guardianship and stewardship, departing from important aspects such as place, whakapapa, intergenerational knowledge, spirituality and resource use. Calls to recognise the diverse ways kaitiakitanga has and can be expressed by Māori have echoed through academic literature, encouraging wider perspectives and application of the concept to develop. This has reinvigorated not only the need for natures protection but also the recognition of cultural knowledges and concepts in practicing kaitiakitanga. 
 
As with most indigenous people, Māori have experienced urbanisation that has spanned across generations of whānau and hapū. Urbanisation has the ability to challenge health outcomes, cultural practices and cultural knowledge, and more importantly, relationships to nature. Growing urbanisation and environmental degradation continue to challenge people and our relationships to nature. There is now a need to understand how urban spaces may also challenge the concepts that encourage connections to nature like kaitiakitanga. Moreover, there is a need to understand if such cultural concepts can become challenged when transient Māori reside in another tribal group’s boundaries. This research provides a deeper analysis of kaitiakitanga by understanding its application within the urban space of Kirikiriroa/Hamilton. From data gathered through a survey, focus groups and interviews, this research project illustrates the experiences of Māori in the urban space who know and practice kaitiakitanga in Kirikiriroa/Hamilton. More importantly, this study posits the integral role of place connection and mana in supporting and shaping kaitiakitanga practices in urban spaces.

The data from participants shows a clear narrative of kaitiakitanga from both local and transient Māori exist and indicates that kaitiakitanga is practiced in accordance with their location and whakapapa to their area of residence. The study has found that kaitiakitanga practices have adapted to suit urban spaces allowing for both transient and local knowledges to exist within the urban space. The key findings of this research show many influences that impact kaitiakitanga practices like the recognition of childhood spaces and experiences, ideas of kinship, cultural knowledges, spirituality, mobility, nature as well as modernisation. Through exploring participants use and knowledge of kaitiakitanga, this research provides a new lens in which to view this concept and bring to the fore, the various experiences and ways that participants connect to place, people, culture and most importantly, nature.

year: 2021