TY - JOUR

T1 - Hydrothermally induced edifice destabilisation

T2 - The mechanical behaviour of rock mass surrounding a shallow intrusion in andesitic lavas, Pinnacle Ridge, Ruapehu, New Zealand

AU - Mordensky, S. P.

AU - Villeneuve, Marlene

AU - Kennedy, B. M.

AU - Struthers, J. D.

N1 - Funding Information: Thank you to Hollei Gabrielsen, Harry Keys, and Blake McDavitt from the New Zealand Department of Conservation, and we would like to acknowledge the people of Ngāti Tūwharetoa, Uenuku and Ngāti Rangi for their support of our work at Ruapehu. This work has also benefitted from discussions with Jim Cole and D. M. Gravley. The authors of this study acknowledge the support of the University of Canterbury Doctoral Scholarship and the University of Canterbury Mason Trust Fund. Ben Kennedy was supported by the NSC Resilience to nature's challenges: “ Āhea riri ai ngā maunga puia? When will our volcanoes become angry? Ministry of Business, Innovation & Employment. “. Funding Information: Thank you to Hollei Gabrielsen, Harry Keys, and Blake McDavitt from the New Zealand Department of Conservation, and we would like to acknowledge the people of Ngāti Tūwharetoa, Uenuku and Ngāti Rangi for their support of our work at Ruapehu. This work has also benefitted from discussions with Jim Cole and D. M. Gravley. The authors of this study acknowledge the support of the University of Canterbury Doctoral Scholarship and the University of Canterbury Mason Trust Fund . Ben Kennedy was supported by the NSC Resilience to nature's challenges: “ Āhea riri ai ngā maunga puia? When will our volcanoes become angry? Ministry of Business, Innovation & Employment. “. Publisher Copyright: © 2022 Elsevier B.V.

PY - 2022/8

Y1 - 2022/8

N2 - Volcanic edifices are composed of an assortment of materials with varying mechanical behaviours. The range in mechanical behaviours of these rock materials presents a critical unknown element to volcanic stability assessments. In this study, we explore the deformation behaviour (e.g. uniaxial, triaxial, and tensile deformation behaviour) of volcanic materials from seven geotechnical units associated with a small, shallow intrusion at Pinnacle Ridge, Ruapehu, New Zealand. First, we provide a complete characterisation of the rock masses using field (e.g. rock mass characterisation) and lab (compressive and tensile strength analyses) data for each geotechnical unit. The new data show a range of tensile and triaxial compressive strengths dependent largely on porosity, which is further modified by hydrothermal alteration. More altered rocks are generally weaker (with the exception of the brecciated lava margins) under triaxial conditions. Altered rocks generally transition from brittle to ductile behaviour at lower confining pressure, although this depends on the porosity. Using the stratigraphy and geomorphology of Pinnacle Ridge in conjunction with the rock property data we collect, we then use finite element modelling to depict the deformation behaviour of these units under different pore pressure and seismic conditions. The modelling shows that a relatively minor increase in pore pressure (e.g. < 3 MPa above hydrostatic conditions) is sufficient to change the slope failure type from deep-seated rotational sliding to extensional rupture. The presence of a low-permeability, low-rock mass strength material, even if very localised like a hydrothermal vein, can reduce slope stability by locally increasing pore pressure and providing a weakness plane that facilitates failure. The modelling also shows that a relatively minor increase in pore pressure (e.g. < 6 MPa above hydrostatic conditions) in a shallow hydrothermal system is sufficient to cause localised failure in the material, especially in the hydrothermal vein material. Additionally, the low-permeability hydrothermal vein material can cause pore pressure compartmentalisation, decreasing the strength factor in all materials in the compartment. While Pinnacle Ridge and its associated hydrothermal veins are >1 km from the currently active hydrothermal system, similar veins may be present near active parts of the hydrothermal system at Ruapehu and are an important consideration for future stability models at Ruapehu and other volcanoes.

AB - Volcanic edifices are composed of an assortment of materials with varying mechanical behaviours. The range in mechanical behaviours of these rock materials presents a critical unknown element to volcanic stability assessments. In this study, we explore the deformation behaviour (e.g. uniaxial, triaxial, and tensile deformation behaviour) of volcanic materials from seven geotechnical units associated with a small, shallow intrusion at Pinnacle Ridge, Ruapehu, New Zealand. First, we provide a complete characterisation of the rock masses using field (e.g. rock mass characterisation) and lab (compressive and tensile strength analyses) data for each geotechnical unit. The new data show a range of tensile and triaxial compressive strengths dependent largely on porosity, which is further modified by hydrothermal alteration. More altered rocks are generally weaker (with the exception of the brecciated lava margins) under triaxial conditions. Altered rocks generally transition from brittle to ductile behaviour at lower confining pressure, although this depends on the porosity. Using the stratigraphy and geomorphology of Pinnacle Ridge in conjunction with the rock property data we collect, we then use finite element modelling to depict the deformation behaviour of these units under different pore pressure and seismic conditions. The modelling shows that a relatively minor increase in pore pressure (e.g. < 3 MPa above hydrostatic conditions) is sufficient to change the slope failure type from deep-seated rotational sliding to extensional rupture. The presence of a low-permeability, low-rock mass strength material, even if very localised like a hydrothermal vein, can reduce slope stability by locally increasing pore pressure and providing a weakness plane that facilitates failure. The modelling also shows that a relatively minor increase in pore pressure (e.g. < 6 MPa above hydrostatic conditions) in a shallow hydrothermal system is sufficient to cause localised failure in the material, especially in the hydrothermal vein material. Additionally, the low-permeability hydrothermal vein material can cause pore pressure compartmentalisation, decreasing the strength factor in all materials in the compartment. While Pinnacle Ridge and its associated hydrothermal veins are >1 km from the currently active hydrothermal system, similar veins may be present near active parts of the hydrothermal system at Ruapehu and are an important consideration for future stability models at Ruapehu and other volcanoes.

UR - http://www.scopus.com/inward/record.url?scp=85130194992&partnerID=8YFLogxK

U2 - 10.1016/j.enggeo.2022.106696

DO - 10.1016/j.enggeo.2022.106696

M3 - Article

AN - SCOPUS:85130194992

VL - 305.2022

JO - Engineering Geology

JF - Engineering Geology

SN - 0013-7952

IS - August

M1 - 106696

ER -