Engineering geological investigations of the Wharanui earthflow, northeast Marlborough.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The Wharanui Earthflow, located 67 km south of Blenheim on the Kaikoura Coast, is of concern because of its proximity to State Highway 1. This thesis provides a detailed engineering geological assessment of its nature and movement to assist in determination of appropriate remedial measures. Field methods used in this study included engineering geological mapping, geophysical profiling, trench excavation, hand augering, and the installation of a comprehensive surface monitoring network.
The Earthflow descends 210 metres over a total length of 1 km, it ranges in width from 30 to 100 metres, and it has five distinct terminal lobes, only one of which is currently active. The lobes result from episodic movement of accumulated materials, which are derived from adjacent feeder earthflows and from other types of slope movement occurring on the valley sides. These deposits consist of fine grained cohesive soils with a significant proportion of smectite clays. In-situ bedrock underlies the Wharanui Earthflow at depths of approximately 2 to 7 metres, and a number of shear surfaces have been identified in the earthflow deposits.
Localized zones of continuing movement are bounded by clearly defined basal and lateral shears, although monitoring showed little movement in the period since installation of the survey network, with displacements commonly less than 100 mm. Past movements have apparently exceeded 4•5 metres/year , and periods related to cycles (5 year plus) of displacement are higher than average rainfall. Larger scale reactivation than is currently observed must be anticipated given the nature and quantities of debris in storage within the Earthflow, particularly in the upper portion.
Past remedial measures including drainage and planting have not successfully stabilized the Wharanui Earthflow. It is necessary that monitoring of the active toe be continued on a regular basis, and recommended that a simple warning system be installed to indicate any sudden acceleration towards State Highway 1. Potential long-term solutions include drainage and planting, but a full geotechnical investigation and analysis is an essential pre-requisite to the design of any such remedial measures.