New Zealand: the first train viaduct featuring innovative base isolation as anti-seismic technology

Its slender piers soar above the Rangitikei River, one of the longest rivers on the North Island of New Zealand. It is one of the tallest and longest viaducts along the North Island Main Trunk (NIMT), the railway that connects the cities of Auckland and Wellington.

Its structure is the first in the world to have featured an an innovative system called “base isolation” to withstand seismic tremors, inspiring countless research papers by engineers and academics alike. And it is considered part of the country’s heritage.

Its name is the South Rangitikei Viaduct, and it was built more than 40 years ago by Cogefar-Codelfa, an Italian civil engineering partnership. Cogefar would later become part of the Webuild Group, one of the world’s leading construction groups, which in almost 120 years of engineering applied to over 3,200 completed projects boasts a track record of 1,020 km of bridges and viaducts.

“(It has) cultural heritage significance, due to its rarity, innovative design, and overall technical merit,” reads a 2012 report by state-owned rail operator KiwiRail on the bridges and viaducts of the NIMT. “Its social value as an impressive design of modern engineering also contributes to its overall heritage significance.”

The last notable recognition of its importance came six years later when the viaduct was given the Enduring Concrete award by the New Zealand Concrete Society, later to be known as the NZ Concrete – Learned Society. “This award is a tribute to those who built and designed the bridge, and to those who have maintained it over the years,” declared KiwiRail at the time.

The South Rangitikei Viaduct is indeed part of the legacy left by Cogefar-Codelfa in New Zealand. The partnership was involved in several projects that contributed to the country’s development, including the Tongariro Power Scheme, a hydropower project also located on North Island.

Completed in 1981, the South Rangitikei Viaduct is 315 metres long, with a deck consisting of six spans, whose varying lengths reach up to 56 metres. The deck rests on seven twin-shafted, or H-shaped piers, which rise 76 metres above the riverbed. The entire structure is made of pre-stressed concrete.

The viaduct’s innovative feature is located at the bottom part of the piers. Known as base isolation, it was developed by Ivan Skinner, a leading expert in earthquake engineering. A New Zealander, he was often seconded to the United Nations Educational, Scientific and Cultural Organisation (UNESCO).

As described in 2014 in Wellington’s main newspaper, The Dominion Post, his invention was first applied to this viaduct. Each pier has two sections, and between them steel hysteretic dampers. Not only do these seismic dampers carry the weight of the structure, but they also allow it to move, absorbing the effect of a tremor. The stress on the viaduct caused by a tremor is transmitted through the piers, whose rocking motion helps attenuate the seismic impact.

“We want to give… a soft ride,” was Skinner’s oft-quoted comment regarding the trains crossing the viaduct.

Ever since its construction, the South Rangitikei Viaduct has drawn interest – and admiration – from engineers the world over. “(The viaduct) represents the first and one of the very few major modern structures with rocking isolation,” reads a 2008 conference paper from the Department of Civil & Environmental Engineering at the University of Auckland. “Since the design and construction of the (viaduct), researchers have gained new insights into the rocking phenomena.”

The South Rangitikei Viaduct belongs to the Mangaweka Deviation, a seven-kilometre track that diverts from the NIMT between the towns of Mangaweka and Utiku. It is located two kilometres north of Mangaweka.

Built between 1973 and 1981, the deviation crosses the Rangitikei River twice and the Kawhatau River once. This required the construction of three viaducts, one of them being the South Rangitikei Viaduct.

The deviation was built because the steep and hilly land between the two towns, through which the NIMT crosses, has always carried the risk of erosion and eventual collapse.