The engineering geology of Frankton Arm (1999)
AuthorsStossel, Deborah Louiseshow all
Seven bedrock landslides situated within quartzofeldspathic schist exist up slope of the residential area along the Frankton Arm of Lake Wakatipu, South Island, New Zealand. Engineering geological and geotechnical failure models for these landslides have been established using engineering geological mapping at scales of 1:5000 and 1:10000, geotechnical testing, and the development of limit-equilibriurn sensitivity models. Geotechnical testing of artificially fractured schist bedrock obtained shear strength values of Φ = 24°-36° and zero cohesion, and point load strength indexes of 0.6-3.83MPa for rock tested perpendicular to foliation, and 0.11-0.92 for rock tested parallel to foliation. Testing of shear zone material gave values of Φ 60°-110° and zero cohesion. The largest failure is the Queenstown Hill Landslide, with an estimated volume of 240M m³ which is interpreted as a retrogressive translational landslide with the toe forming a compressional bulge in the mid-slope area of Queenstown Hill. Three phases of movement have taken place, the earliest phase probably being initiated in the southeastern area of the slide mass by ice scouring and the over steepening of slopes during the final stages of the Last Glaciation. On retreat of the glacial ice, lateral support was removed and increased pore water pressures may have acted to reduce the shear strength of the slope along critical failure or shear surfaces. Movement is inferred to have been by translational planar sliding by slow rock mass creep, not from buckling in the toe, partly along foliation shear zones and a stepped failure surface in fractured schist bedrock immediately following glacial retreat. The second and third phases of movement were initiated as a result of the removal of support by the previous phase, with the second phase forming small translational- slides and. retrogressive features, and the third phase forming the toe bulging by gravitational creep down slope. Six smaller bedrock failures (up to 2.8M m³ each in volume) are situated further east along Frankton Arm. These landslides are interpreted as shallow retrogressive translational failures, with their slide bases orientated sub-parallel to the schist foliation. These failures may have initially occurred following glacial retreat (similar to the Queenstown Hill Landslide), with the slides situated at lower elevations activated by seismic events at a much later stage following deposition of lake beaches as the enlarged Lake Wakatipu was lowering. The only evidence for continual movement for within the last 100 years is on Slide No.3 and Slide No.4. Minor wedge failures have occurred from the head scarp, but the high frictional interlock between the displaced blocks creates minimal risk to the residential areas below and if further development was to occur in these areas, prudent engineering geological practices should be implemented. Future sub-surface work needs to be completed to accurately locate the depth, shape and angle of the failure surfaces for each landslide. This work would also determine the parts of the failure surfaces that occur through foliation shear zones, or fractured schist.