The Design and Construction of Major Bridges on the Mangaweka Rail Deviation

The original route of the North Island Main Trunk from Mangaweka to Utiku followed a tortuous route along narrow benches some 100m above the Rangitikei River. Steep gradients, tight curves and narrow tunnels made it a difficult and costly piece of line to operate and maintain in this geologically unstable and seismically active region. With these costs in mind the New Zealand Railways decided to reroute rather than upgrade this stretch of line. The route chosen crosses the Rangitikei River plains, and to achieve a suitable alignment, required the bridging of three major river valleys. These bridges, which are major structures by any standard, presented a challenge to the engineers to establish a high level of earthquake resistance at an acceptable cost. The North Rangitikei and Kawhatau gorges called for a column free span in excess of 100m, while the South Rangitikei Bridge whose piers tower 76m above the river, has a total length of 315m.