Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
9537912 | Engineering Geology | 2005 | 33 Pages |
Abstract
Geotechnical characterisation is undertaken for 3 broad units comprising the bulk of the stratigraphy identified on White Island Volcano, Bay of Plenty, New Zealand, an active island stratovolcano. Field and laboratory measurements were used to describe rock mass characteristics for jointed lava flow units, and ring shear tests were undertaken to derive residual strength parameters for joint infilling materials within the lavas. Rock Mass Rating (RMR) and Geological Strength Index (GSI) values were calculated and converted to Mohr-Coulomb strength parameters using the Hoek-Brown criterion. Backanalysis of known landslide scarps was used to derive strength parameters for brecciated rock masses and hydrothermally altered rock masses. Andesite lava flows have high intact strength (Ïci = 184 ± 50 MN mâ 2; γ = 24.7 ± 0.3 kN mâ 3) and typically 3 wide, infilled joint sets, one parallel to flow direction and two steeply inclined, with spacings of 0.3-1.7 m. Joints are rough, with estimated friction angles for clean joints of Ïj = 42-47°. Joint infill materials are clayey silts derived from weathering of wall rocks and primary volcanic sources; they have low plastic (54%) and liquid (84%) limits and residual strength values of cr = 0 kN mâ 2 and Ïr = 23.9 ± 3.1°. RMR values range from 70 to 73, giving calculated strength parameters of câ²Â = 1161-3391 kN mâ 2 and Ïâ²Â = 50.5-62.3°. Backanalysis suggests brecciated rock masses have câ²Â = 0 kN mâ 2 and Ïâ²Â = 35.4°, whereas GSI observations in the field suggest higher cohesion (câ²Â = 306-719 kN mâ 2) and a range of friction angles bracketing the backanalysed result (Ïâ²Â = 30.6-41.7°). Hydrothermally altered rock masses have câ²Â = 369 kN mâ 2 and Ïâ²Â = 14.9°, indicating considerable loss of strength, especially frictional resistance, compared with the fresh lava units. Values measured at outcrop scale in this study are in keeping with other published values for similar volcanic edifices; backanalysed data suggest weaker rock mass properties than those determined at outcrop. This is interpreted as a scale issue, whereby rock mass characteristics of a large rock mass (crater wall scale) are weaker than those of small outcrops, due in part to the overestimation of friction angle from measurements on small exposures.
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geotechnical Engineering and Engineering Geology
Authors
Vicki Moon, Jennifer Bradshaw, Richard Smith, Willem de Lange,