In 2003 BBO were engaged by Transit New Zealand as consultants for the strengthening and raising of Whangaehu River Bridge to withstand future lahar surges from Mount Ruapehu of worst case scenario predictions in relation to size and velocity. The Whangaehu River Bridge is located on SH49 near the site of the Tangiwai disaster of 1953, New Zealand’s worst ever rail disaster. The bridge is a vital link for communities, emergency authorities and other agencies during the event of a lahar.
BBO undertook detailed survey, obtained resource consent for the project, developed the detailed design, and undertook construction observation of the project. Transit engaged with local authority, government organisations and emergency services over several years to plan for the hazard, ensuring that motorists and infrastructure would be protected and that plans would flow smoothly during the predicted lahar event. The bridge was raised by two metres, foundations deepened to 28 metres, and extensions made to strengthen existing piers to provide clearance and structural resistance.
The programme imposed by government was extremely demanding, being geared to earliest possible completion of physical works ahead of the expected lahar event in February 2005. BBO identified the current world steel shortage as a risk to the achievement of the programme and arranged the procurement of large diameter pile casings, to facilitate the earliest possible start to construction. BBO also developed and obtained Transit approval for special contractual conditions to incentivise contractor performance.
The lahar event occurred on 18 March 2007. The value of the project was demonstrated as the strengthened bridge stood firm while millions of cubic metres of water, mud and debris surged beneath it.