Kaikōura Earthquake Recovery, Design of a 13m high geogrid reinforced, no fines concrete gravity seawall in a high seismicity environmentYoung Geotechnical Professionals Conference, Hobart 2018 - Winner

The North Canterbury Transport Infrastructure Recovery (NCTIR) Alliance was formed to deliver the repairs to the national road and rail transportation corridors after the Mw 7.8, Kaikōura earthquake which occurred on 14 November 2016. At Ōhau Point, located approximately 26 km north east of the Kaikōura CBD via road, approximately 240,000 m3 of landslide debris buried the rail, road and adjacent coastline.

Between February 2017 and July 2018, a new 900m long seawall was designed and constructed as part of the coastal realignment of State Highway 1 (SH1) around Ōhau Point. This structure incorporates mechanically stabilised earth comprising mainly cement stabilised backfill with geogrid reinforcing, and five-ton concrete block facing.

The most complex section of this seawall is around a rock outcrop known locally as Shag Rock where the seawall is up to 13m high. The constraints and challenges in this area include maintaining access along the existing SH1 above the wall and ecological constraints. A special complex no fines concrete gravity wall (NFC G-Wall) was designed and constructed to buttress the slope and older seawall. This structure, and the wider fill and earth platform which supports the widened roadway, is designed to slide as a block under 0.76g peak horizontal ground acceleration.

This paper presents the results of the two-dimensional FLAC modelling which was completed to analyse and design the 13m high geogrid reinforced NFC G-Wall at NCTIR site 6. It also describes the pragmatic observational approach which was taken for the seawall design, highlighting the seismic sliding mechanism and issues that arose during design and construction of the seawall.