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      Risk analysis associated with flank failure from Putauaki, Bay of Plenty, New Zealand

      Hewitt, Dolan
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      Hewitt, D. (2007). Risk analysis associated with flank failure from Putauaki, Bay of Plenty, New Zealand (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/2337
      Permanent Research Commons link: https://hdl.handle.net/10289/2337
      Abstract
      Volcanoes are dynamic evolving structures, with life cycles that are punctuated by episodes of

      flank instability. Putauaki (Mount Edgecumbe) is a stratovolcano located onshore in the Bay of

      Plenty, New Zealand. The aim of this study was to assess the stability of Putauaki and analyse the risk associated with volcanic collapse. To achieve this objective, a multidisciplinary approach was used, incorporating geomorphological and geological mapping, rock mass classification,

      laboratory testing to identify geotechnical properties of materials representative of the volcano,

      stability modelling, and analysis of landslide run-out zones.

      Putauaki comprises two predominant features including the larger and younger Main Cone (the

      summit lying 820 m a.s.l., slope angles up to 36 ), and smaller and older Main Dome (the summit

      lying 420 m a.s.l., slope angle of 24 ). Both features show little evidence of erosion or surface

      water. Rock mass description defined six lithotechnical units including indurated andesite,

      indurated dacite, scoriaceous andesite, altered andesite (all categorised as hard rocks), and block

      and ash flow and Matahina Ignimbrite (both categorised as soft rocks). The uniaxial compressive

      strength (UCS) of indurated andesite and indurated dacite was 60 4 MPa and 44.7 0.9 MPa

      respectively, correlating with moderately strong rock. Discontinuities of the indurated units were

      widely spaced, showed medium persistence and wide aperture, and were slightly weathered. Infill

      comprised predominantly loosely packed, very strong, coarse gravel. UCS of scoriaceous andesite

      and altered andesite was 25 5 MPa and 15 1 MPa respectively, allowing categorisation as very

      weak rock. Discontinuities of scoriaceous andesite were widely spaced, showed high persistence

      and wide aperture, and were moderately weathered. Discontinuities of the altered andesite were

      moderately spaced, showed low persistence and wide aperture, and were highly weathered. Infill

      of scoriaceous and altered andesite was loosely packed, moist, weak to very weak medium gravel.

      The block and ash flow was a poorly sorted, loosely packed, sandy, gravely and cobble rich matrix

      supported deposit. The Matahina Ignimbrite was a very weak, discontinuity-poor deposit. Shear

      box testing indicated cohesion and friction angle of 0 MPa and 42.1 (block and ash flow) and 1.4

      x 10-3 MPa and 41.7 (Matahina Ignimbrite) respectively. These values are similar to published

      values. Correlation of each lithotechnical unit to its respective rock mass description site allowed

      approximate boundaries of each unit to be mapped.

      Each unit's mass strength was combined with measured bulk densities and incorporated into two

      dimensional slope profiles using the stability modelling package GalenaTM. Ten slope profiles of

      Putauaki were constructed. Failure surfaces for each slope profile were defined using the Bishop

      simplified multiple analysis method. Four slope profiles showed the potential for small scale

      failure (less than 0.1 km2 of material). The remaining six slope profiles showed the potential for large

      scale failure (greater than 0.1 km2 of material). Stability of these six slope profiles was investigated further

      in relation to earthquake force, watertable elevation, and a disturbance factor of the rock mass (D).

      Conditions of failure graphs for profile 6a showed that at low D (less than 0.4), earthquake forces and

      watertable elevation must be unrealistically high for the region (greater than 0.33 g; greater than 15% watertable

      elevation) in order produce a factor of safety less than 1. The remaining five slope profiles showed

      potential to be unstable under realistic earthquake forces and watertable elevations. Two of these

      profiles were unable to achieve stability at D greater than 0.8 (profile 4) and D greater than 0.9 (profile 5). A D value

      of 0.6 (intermediate between 0.4 and 0.8) is argued to most realistically represent Putauaki. The

      fact that Putauaki has not undergone large scale failure to date supports the conclusion that the

      constructed models overestimate the influence of those factors which promote slope instability.

      Maximum and minimum landslide run-out zones were constructed for the slope profiles exhibiting

      the potential for large scale failure. Definition of the position and extent of maximum and

      minimum run-out zones assumed H/L (fall height to run-out length) ratios of 0.09 and 0.18

      respectively, as well as the 'credible flow path' concept. Identified impacts of landslides sourced

      from Putauaki include inundation of Kawerau Township, Tarawera River, forestry operations, road

      networks, and power supplies. Based on these impacts, the risk posed by landslides from each

      slope profile was categorised as ranging from relatively low to relatively high. Landslides sourced from the south-west flanks pose a relatively low risk due to their prerequisite of unrealistically

      high watertable elevations and earthquake forces. Landslides sourced from the north-west flanks pose a relatively high risk as minimum run-out will inundate north-east parts of Kawerau

      Township. Landslides sourced from the eastern flanks pose a moderate risk due to their run-out

      zones avoiding Kawerau Township.
      Date
      2007
      Type
      Thesis
      Degree Name
      Master of Science (MSc)
      Publisher
      The University of Waikato
      Rights
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