Mechanisms and Remediation of Cut Batter Failures Along the Queens Road at Wainigasau and Namuka I Lau, Viti Levu, Fiji Islands.
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The first cut batter failures along the Queens Road, Viti Levu occurred in 1979 following a prolonged high precipitation event. This has continued to occur through to 2009 and this thesis research is centred on understanding the mechanisms of recurring cut batter failures occurring at Namuka i lau and Wainigasau. The Namuka i lau and Wainigasau cut batters are cut into highly weathered polymict conglomerates belonging to the Veisari Sandstone. Each site comprises three benches at approximately 9 m in height. The Namuka i lau benches are 1 m wide with 50ºslope faces and the Wainigasau benches are 3 m wide with 60ºslope faces. Field investigations revealed previous landslide activity at the Namuka i lau site in contrast to the Wainigasau site which showed a lack of evidence of previous landslide activity. XRD analysis determined the clast and matrix material to be composed of quartz, hematite and kaolinite with quartz compositions ranging from 65% - 100%, hematite compositions from 1% - 25% and kaolinite composition percentages ranging between 1% and 25%. Other laboratory studies determined the material to have a high water content (Namuka i lau – 47.2% and Wainigasau – 44.9%), low unit weight (Namuka i lau – 14.5 kN/m³ and Wainigasau – 15.8 kN/m³), low permeability (K for Namuka i lau – 2.78 x 10⁻⁷m/s and Wainigasau – 6.66 x 10⁻⁷m/s), high plasticity, low cohesion (cr = 3.9 kPa) and a residual friction angle of 15.0 º. Factors of safety calculated for the cut batters are low (FS = 0.4) due to the low values of cohesion and friction angles used in the modelling process. Sensitivity analysis determined that the highest factors of safety were achieved when both the cohesion value was increased to 20 kPa and the friction angle increased to 30º using the Bishop Simplified Method of slope stability analysis for Wainigasau (FS = 1.28) and Namuka i lau (FS = 1.23).