The bridge was originally constructed at the beginning of the 20th century and was one of the last links required to form the railway between the east and west coasts of New Zealand. Six of the original mass piers were made in concrete with the remaining four piers/abutments constructed in timber. The replacement of the existing timber with new concrete piles and pile caps was a necessary upgrade in order to maintain the safety and reliability of the Midland Railway Line.
"We were required to permanently case piles - 4 x 1200mm in diameter and 4 x 1050mm in diameter - with casing depths to 14m," said Jaron McMillan, Company Director of McMillan Drilling. "We also used temporary driven UC piles and steel truss supports to hold the end spans of the viaduct in position while the permanent concrete pile caps were constructed by Downer. The conditions were pretty tough as the crews were subjected to heavy frosts and snow at times during the five months of work."
"The site is extremely isolated as it is about a two hour drive from our Darfield plant in Christchurch," said Dominic Sutton, Acting Chief Operating Officer - Southern for Firth. "We were also required to pump the concrete over 100m across a live rail line as there was no vehicle access to the south side of the bridge."
"A key characteristic required of the concrete on this job was that it had to have early high strength," explains David Jackways, Project Manager for Downer. "The design required a concrete strength of 40MPa and the concrete was required to achieve a minimum strength of 30MPa before loading."
Independent concrete testing carried out by Downer's IANZ accredited laboratory in Christchurch showed the concrete achieved the required strength and showed that the strength was developed very quickly, with a minimum seven day test result of 41MPa and a maximum 28 day strength test result of 60.5MPa.
"Firth did a good job as the site provided many challenges for concrete work, including the cart distance from the Darfield plant, difficult access track conditions, pumping over 100m across the bridge and the cold weather," adds David. "The concrete was generally of a high quality and we achieved an excellent end product."
