A wide range of techniques is available for investigating landslide movements.
Additionally, rainfall forecasts and real-time rainfall measurements have been the basis for landslide warning systems in several parts of the world. However, these techniques suffer from shortcomings in terms of spatial and temporal resolution, and as a result many monitoring programs have been unable to link specific landslides to the triggering factors. There are other shortcomings regarding collection, processing and display of monitoring data.
But a team from GNS Science, a New Zealand government-owned research organisation, claims to have found solutions to these problems by installing a near real-time landslide monitoring network in Taihape, in New Zealand's central North Island.
The project is to build and operate a modern geological hazard monitoring system. When complete, it will comprise a network of geophysical instruments, automated software applications and skilled staff.
GNS team leader Chris Massey says although he knows about near real-time systems in Europe and the US, they are not as detailed as this. "The increased spatial and temporal resolution of the network is providing a better understanding of landslide movement patterns, and due to the near real-time monitoring system periods of movement can now be linked to the triggering factors," says Massey. "The system allows improved definition of movement triggering thresholds, such as rainfall intensity and duration, which can be used as part of a landslide warning system."
By comparing monitoring data from multiple similar landslides it will be possible to assess the movement patterns of such landslides. It will also help determine whether relationships exist between these patterns, triggering factors and triggering thresholds (whether initiated by periods of prolonged rainfall or ground-shaking due to earthquakes) and properties of the materials forming the landslide slip plane.
Information gathered from this project could be used as the scientific basis for developing alert criteria, which could be used for landslide warning systems, similar to volcano monitoring alert criteria also carried out by GNS.
The Taihape project involves a combination of field mapping, sub-surface investigation and near real-time monitoring of rainfall, ground-shaking intensity, groundwater levels and surface movement. A key component of the project is a laser survey network that automatically tracks, at defined hourly intervals, the positions of reflectors placed on the landslide.
Movement data and information from piezometers and rain gauges installed on the landslide, are transmitted by radio and internet to GNS, which can be viewed in near real-time on the internet.
Power for all equipment is generated from photovoltaic cells, allowing the system to be remotely operated without any need for mains power.
There is an approximate site-to-office data transfer delay of one hour. Wireless data transfer from the robotic total station, rain gauges, piezometers and strong motion sensors is achieved via radios to the Taihape Town Hall and via the internet to GNS Science buildings located near the capital Wellington, about 250km south of Taihape. Data is automatically processed, formatted, checked and made available in both human and machine-readable formats. The results are presented in an interactive web-based chart, which is updated at 15min intervals and can be viewed at www.geonet.org.nz.
l To read this article in full, see December's issue of Ground Engineering, out now.