This primary route across the island is built along the ridge tops where the ground is predominantly firmer, more stable, and less susceptible to flooding. In the vicinity of Kasavu following an exceptionally wet rainy season, a deep-seated landslide activated, taking with it an entire lane. The landslide extended some 130m downslope with approximately a 40m length of roadway affected by the failure.
The road was immediately closed to a single lane only; however, continued rain saw the regression of the headscarp back into the live lane and cracking in the pavement on the opposite side of the road, indicating imminent failure of the western slope also. With no room to widen the road, it was closed with only foot traffic allowed to cross the slip site. Disruption to the local community had disastrous effects on the local economy, with a key transport route closed.
GeoStabilization’s personnel inspected the site some six months following the original failure and proposed a design-build lump sum solution. After an international competitive tender process, our company was awarded the design/build project with an innovative solution, which included stabilisation of the headscarp, the rebuilding of the roadway, and stabilisation of the western slope.
A temporary Bailey bridge was built across the headscarp before commencing stabilisation work to enable single lane traffic to pass over the slip. GSI’s solution involved the stabilisation of the headscarp using our proprietary low carbon steel hollow bar soil nails and slope stabilisation mesh. The road rebuild included a 50m long 7m high Geosynthetically Confined Soil® (GCS®) retaining wall supported by an A-Frame micropile wall & reinforced concrete capping beam. The western slope was stabilised using soil nails and slope reinforcing steel wire mesh.
The site was classified a “Class D –Deep or Soft soil site” as defined in the New Zealand Standard NZS 1170.5:2004, with soils comprising Hinuera Sands and Silts up to 10m thick overlying Puketoka Clays. The upper sands and silts were deemed liquefiable and the new bridge structure needed suitable ground improvement measures incorporated into its foundation design.