The coastal town of Kaikoura is located on New Zeland’s South Island in the North Canterbury region. It is a popular tourist destination and renowned for its scenic views, nature, and sperm whale population. In November 2016, the region was badly affected by a 7.8 magnitude earthquake. The event caused landslides and rockfall along the coastal State Highway (SH1), which is an important roadway not only for tourists between Christchurch, Kaikoura, and Blenheim, but for transporting essential resources from Picton.
Restoring SH1 after the earthquake was paramount. Bunds, rockfall barriers, and hybrid barriers utilizing geosynthetics have played an important role in responding to the disaster. Geofabrics and Maccaferri share their rockfall bund case study here, and in it one can find a highly useful approach that translates easily to other world regions and geohazard zones.
A 488M ROCKFALL BUND
Not long after the earthquake, the NZ Transport Agency and KiwiRail set up the North Canterbury Transport Infrastructure Recovery (NCTIR) alliance with four major contractors. NCTIR is tasked with restoration of the transport infrastructure networks within a given time frame to leave it safer and more resilient than it was before the earthquake. Various solutions and rockfall protection structures of different energy capacities were considered along the coastal road, depending on the site’s condition.
Among the solutions designed into the area: Green Terramesh bunds, high energy rockfall barriers, and hybrid barriers.
Site 7 of the reconstruction works provides an exemplary case.
Located about 22 km north of the Kaikoura township, it was the scene of a significant rockfall event during the earthquake. The NCTIR designers decided a rockfall bund was the preferred protection solution. The structure would offer the capability to absorb multiple future rock fall events up to a high energy level.
Furthermore, by using a reinforced gabion system (Green Terramesh) in the design, the embankment slopes could be steepened and the footprint reduced to form a stable and robust bund with high energy absorption characteristics.
This type of structure is typically filled with compacted granular material or engineered soil fill with a horizontal soil reinforcement. The front face can be vegetated or finished with a rock veneer.
The final bund design was 488 m long, including two parts and a 10 m gap to allow access for clearing debris. The southern section is 185 m with an exposed height of 5.4 m above the existing ground level. The northern section is 293 m long and stands 3.6 m high. Both bunds have a minimum crest width of 1.5 m.
INTERNATIONAL APPLICABILITY OF THE DESIGN
Actual rock impacts in excess of 4,000 kJ into the rockfall bund system used at Kaikura have been tested, back analysed, and the design methodology verified using FEM numerical modeling techniques. This research completed in Italy has resulted in the development of simplified design charts that designers around the world may use with a simplified design method based on rock penetration depth.
Geofabrics assisted the designers with this information and looked at other aspects of rockfall embankment design, such as:
- The projection of fragments over the embankment after impact;
• The over topping of the upper proportion of the embankment by blocks;
• Penetration depth estimation and determination of serviceability and ultimate limit state;
• The internal stability of the embankment;
• Development of instability within the soil foundation as a result of the dynamic forces such as an earthquake.
Learn more about Geofabrics’ applications, engineering, and geosynthetic materials at www.geofabrics.co.