Abstract:
This thesis is an engineering geological investigation of the Big Moma landslide, located in Port Waikato, New Zealand. The landslide is a noticeable scar on the landscape, featuring a wide head scarp, a secondary scarp, hummocky terrain below, and erosional features, including slumps, gulley’s, and concave features. The head scarp imposes on the landscape, reaching a peak elevation of 140 metres above sea level relative to the ~45-metre elevation in the hummocky terrain area below. Towards the west, along the cliffs, there are numerous secondary landslides that vary in classification. These secondary landslides are indicative of the ongoing erosion and continual mobilisation of the study site, past the initial landslide event, as the landslide is still in a state of dis-equilibrium. This dis-equilibrium is further shown in the limit equilibrium tests conducted on the landslide to assess the stability of the slope. In these tests, Factor of Safety values below the minimum threshold for New Zealand regulations was calculated in both hydrological extremes of low- and high-water tables, which means that the slope will likely remobilise again in the future under both dry and saturated conditions.
The two geologic groups of the Quaternary aged Kaihu Group sands and the mid-Oligocene aged Whaingaroa siltstone Formation provided the right conditions necessary for the slope to become mobilised. The Kaihu Group consists primarily of sand, has a loose to medium dense density, and is well graded, showing low amounts of clay and silt grains. This unit is an approximately 90-metres thick covering most of the site, up towards the head scarp. The Whaingaroa siltstone lies underneath, containing calcium carbonate of varying polymorphs and pyrophyllite clays within its structure. The cliffs along the study site consist of this Formation, making it around 30-metres thick as it extends underneath the study site towards the head scarp with a gently angled dip. This Formation serves as an impermeable barrier to water flow within the overlying Kaihu Group sands, causing conditions of high pore water pressures that have decreased the sands shear strength. This then set the right conditions for rainfall or seismic activity to act upon the slope to remobilise it.
The landslide is classified as a sandy translational slide over a gentle angled bedding plane. This landslide classification is unique because of the scarceness in literature and from within the Port Waikato region. Meaning that this study serves provides insight into a unique type of translational slide under unique conditions not commonly found elsewhere.