Collapsing Cones, Slippery Slopes, Hot Ground, Titanic Topples, Dams and Development

Engineering Geological Challenges and Models for the Active New Zealand Environment

Dr. Warwick Prebble

New Zealand is a highly varied landmass with many different terrains displaying defective and difficult rock and soil masses crafted by a collection of hazardous geologic processes. This presents a unique challenge to engineering development. This talk offers a personal view of the contribution engineering geology has made to our understanding of this active New Zealand environment drawing on 35 years of experience in field-based research on engineering projects. Examples were presented from a wide range of hazardous terrain as introduced below.

Very large clay matrix debris flows from hydrothermally altered and collapsing active andesite volcanoes are a potential geotechnical hazard, but their deposits have provided very useful dam foundations and reservoir containment. Thermally weakened fault scarps release large debris flows every 1200 years in the centre of the Taupo Volcanic Zone. Geothermal fields spontaneously erupt, including those in the suburbs of Rotorua City and others can be overrrun by landslides, promoting further instability. Clay seams give rise to slides in weak tabular rock masses throughout the Auckland region and elsewhere in the North Island and highly sensitive rhyolitic silts are being found as basal ruptures in terrace deposits well outside the volcanic zone in the northern North Island. Toppling of very large masses of greywacke and schist is widespread in the South Island, in dip slopes as well as scarp slopes. Deep-seated bending surfaces in dip slopes has given rise to catastrophic rock and debris avalanches. Overtoppling of this type is not widely recorded but is turning out to be a significant rock mass failure mode, especially in the rapidly uplifting steeplands.

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