About the Landslide

Detailed information regarding the history and movement of the Taihape Landslide.

Survey prism installed on the landslide.

The Taihape landslide first moved between 1,800 and 11,000 years ago. It is situated towards the west of Taihape township and includes approximately 45 hectares of land containing approximately 209 houses and a school. Several previous studies of it have been carried out, the earliest in 1971 when significant instability was first noticed.

The Taihape landslide has been classified as a deep-seated translational block-slide. This classification refers to the characteristics of the landslide. Deep-seated refers to the depth of movement (depth to the landslide slip-plane); the slip plane of the Taihape landslide has been recorded at 25 m below ground level at the toe (bottom) and increasing to 35 m towards the back scarp (top). The term translational refers to the movement style of the landslide, where it moves as a relatively intact mass (raft) of material, along a planar zone of weakness. In this case, the zone of weakness corresponds to a thin clay layer thought to represent a bedding plane within the local Taihape sandstone. The term block-slide describes the structure within the landslide, which for the Taihape landslide is a series of large blocks (tens to hundreds of metres in size).

Historical information from ground surveys (dating back to 1985) indicate that landslide movement is triggered by rainfall, and to a lesser extent erosion caused by Otaihape Valley Stream, located along the toe of the landslide. Periods of prolonged rainfall cause groundwater levels to rise, which lead to a reduction in strength of the materials forming the landslide slip plane (clay layer). It is this reduction in strength that leads to movement of the landslide. Movement of the landslide can therefore be linked to rainfall events. As more data is collected it will be possible to determine movement triggering thresholds, e.g. how much rainfall does it take to trigger movement and by how much does the landslide move. In the past it has been impossible to correlate the triggering factors to the different periods of landslide movement due to the large time intervals (typically greater than yearly) between the historical surveys. To overcome these problems GNS Science installed new monitoring equipment on the landslide in June/July 2006.

The robotic survey station.

Information recorded from the landslide is being used to calculate the landslide velocity, identify areas of movement and assess the movement patterns of the landslide to aid RDC (Rangitikei District Council) and EQC to plan potential mitigation strategies.

Monitoring Equipment

The equipment installed on the landslide is designed to monitor two triggering factors (factors that trigger movement) and two response factors:

• Rainfall (trigger);
• Ground shaking intensity (trigger);
• Groundwater levels (response);
• Surface movement (response).

Rainfall is being monitored by two rain gauges installed on the landslide. One rain gauge is installed near the back scarp of the landslide, the other near the toe of the landslide and so the variation in rainfall across the landslide can be monitored. These rain gauges record rainfall as and when it occurs, therefore allowing the precise time of peak rainfall to be determined. Equipment to record ground motion in response to earthquakes is installed at the nearby Taihape Rural Hospital. Groundwater levels within the landslide are being monitored using piezometers installed in 4 boreholes. The piezometers are recording groundwater levels acting along the slip plane of the landslide.

The surface movement of the landslide is being monitored using a robotic survey station. The survey station automatically tracks the position of survey prisms installed on the landslide and the collected data is managed and displayed through custom software, which allows the entire system to run remotely. The survey station has been installed on the hillside opposite (south) the landslide. The survey error of the total station installed at Taihape varies for each prism and is dependent on several factors, mainly temperature and pressure, caused by changes in the weather. The prisms installed on the landslide have been strategically located based on engineering geological mapping and the results from historical surface movement surveys. Several prisms have also been installed off the landslide (on stable ground) to act as back sites for the monitoring. These stable prisms are then used to calculate the movement of the prisms installed on the landslide. The distance from the survey station to each prism varies, and typically ranges from 700 to 1000 m.

Typical rain gauge and piezometer installation.

The monitoring network

At present the prisms installed on the landslide are monitored at hourly intervals, the piezometers record a groundwater measurement every five minutes and the rain gauges record rainfall as and when rainfall occurs. The data from the instruments on the landslide is transferred to GNS Science via a communications network. The data is then checked and the results are calculated. The photographs show what the different types of equipment actually look like. The process of retrieving the data takes approximately one hour from site (Taihape) to the office (GNS Science, Avalon).

Results from the Monitoring

Recent monitoring data can be viewed here (Graphs A, B & C).

To view the data for the different prisms, piezometers and rain gauges please select the item from the left hand side of this window and select refresh or reload on your browser. A map showing the locations of the various instruments can also be found on this page. The different instruments can also be selected from the map, and the data for the chosen instruments will be shown once the refresh button has been clicked. The results shown on this web page are updated every 15 minutes.

The data shown on Graph A is the horizontal movement (displacement) calculated for each prism installed on the landslide. The main direction of landslide movement is approximately 160 degrees (towards the south-east) and so the displacement of each prism has been calculated in this direction. The data plotted on Graph A represents the average movement (of each prism) over a 24 hour period (daily average), with the data plotted cumulatively to give an indication of the total movement since monitoring began. The spikes in the data (shown on Graph A) are a result of the pressure and temperature fluctuations across the area. These are typically in the range of 1 to 3 mm and are essentially errors in the measurement process. It is therefore important when assessing movement of the landslide to look at the trends in the data rather than individual measurements. The data shown on Graph B is from the piezometers and has been shown as a water level relative to the ground surface in each of the 4 locations, and the data in Graph C shows the daily rainfall.

The information contained on this website is designed to give the viewer an overview of the monitoring being carried out by GNS Science. Much more detailed information is actually being gathered for the landslide, and this information is being used to assess the movement patterns of the landslide to aid in the implementation of mitigation works currently being assessed by EQC and Rangitikei District Council.

Data Web services

All data is made available via web services.

Equipment Locations

The NZMG locations of prisms, tipping bucket rain gauges, and borehole piezometers.

Processed Data

The processed data is available as XML and compressed (gzipped) XML web services. The compressed web services offer a considerable download bandwidth saving and should be used where possible.