The piles on Christchurch’s Ferrymead Bridge would never survive a decent earthquake, the experts said – and then one arrived and proved them right. Hugh de Lacy reports on the re-build.
Work was about a third complete on re-piling the Ferrymead Bridge, linking the Sumner and Port Hills suburbs to Christchurch, when the scenario that prompted the work in the first place came to pass on February 20, 2011.
The previous August, national construction firm HEB Construction had begun strengthening and widening the existing Ferrymead Bridge in response to engineers’ concerns from as early as 1994 that the existing piles weren’t founded on bedrock, and the overburden on which they were sitting would liquefy in the event of a decent quake.
In 2004 Christchurch City Council announced plans for the strengthening as part of a wider city bridge upgrade plan, though it took a further five years of complex design work before tenders could be called.
As predicted, the quakes severely damaged the bridge and stopped HEB’s strengthening and widening work a third of the way down the track, as well as disrupting the 30,000 vehicle movements the bridge catered for every day.
Traffic was still able to cross the old bridge thanks to temporary emergency strengthening work, but HEB was directed to upgrade and reinstall the two damaged temporary bridges, one upstream and one downstream of the existing one, for it to be diverted over.
The council then had to work out what it was going to do: continue with the upgrade HEB had already started, or demolish it and start again.
In the desperate atmosphere that followed the main quakes, it understandably took the besieged council until the end of the following year to opt for demolition and replacement, and HEB was able to start work in earnest in February of 2013.
While the traffic flowed past on the temporary structures on either side, HEB tackled the demolition of the old bridge in between, and, with extensive use of concrete nibblers and some highly specialised equipment supplied by subcontractor Ceres, had the job completed by April of that year. Notably the bridge was demolished around the existing telecommunications cable feeding the eastern peninsula communities, a critical element to the port of Lyttelton operations.
Then the fun began in earnest.
At 70 metres long, the new bridge was not a particularly challenging build in itself, but the extraordinary geology beneath it presented immense challenges to the designers.
For a start the bedrock was volcanic, the detritus of the two eruptions that formed Banks Peninsula about six million years ago, and it could vary hugely within a short distance.
“In some cases you could drill an investigation borehole where you were standing, and two metres from it you could drill another that showed completely different geotechnical layer depths,” HEB project manager Werner du Plessis told Contractor.
“It required extensive geotechnical investigations up front, and then close monitoring during pile driving to make sure the piles were founded on the right layers as required by design.”
A total of 10 steel-cased piles had to be driven, the four supporting the pier in the middle of the river being 2.4 metres in diameter, 25 metres long and the casings alone weighing approximately 50 tonnes each.
These four had to be driven first under a particular work sequence to minimise the danger of lateral spreading under the impact of the continuing aftershocks.
Once driven through the 13 metres of alluvial silt at the bottom of the river, and approximately seven metres into the bedrock, the piles were concrete filled using approximately 100 cubic metres of special concrete. Then a series of six 150mm diameter holes had to be drilled down the outside of each pile to the toe of the casing so that grout could be inserted to fill the void between the outside face of the casing and the rock to complete the rock socket.
Once complete, the four pier piles were used to provide propping to each of the six abutment piles through means of installing a temporary steel propping arrangement. Thereafter it was bridge-building as usual, though with a few twists unique to the Ferrymead project.
Four 1.6 metre diameter columns were built on top of the four central piles, topped by a highly reinforced pier-cap.
The bridge was also to resume its service as a conduit for water, sewerage, telecommunications and power to the 4450 households in the Sumner suburbs.
These services were incorporated into the bridge’s 26 main Super T beams, pre-cast by HEB at the nearby Ferrymead Heritage Park and weighing approximately 50 tonnes each.
By Christmas last year they were all in place with the services connected, and work was underway on the construction of the stormwater reticulation for the new road layout.
It was a notable challenge to du Plessis and HEB that all this had to be coordinated with work being done in the near vicinity by other major contracting teams from the Stronger Christchurch Infrastructure Rebuild Team (SCIRT).
For example while HEB was building the bridge, other contractors were working on projects directly adjacent to the Ferrymead Bridge project site boundary. This was particularly a challenge when coordinating traffic management between sites and ensuring all commuter traffic was catered for.
On Monday, March 13 this year, the great day arrived and the bridge was opened to traffic for the first time since 2012. Four years down the track, it’s difficult to see how the project could have been completed any sooner, given the immense design difficulties posed by the piling – not to mention the massive disruption to the city’s response capacity from the earthquakes themselves.
Costing $34.87 million, with $22.12 million contributed by the New Zealand Transport Agency, the Ferrymead Bridge is now functioning on all four lanes, and commuting times have settled back to what they were before the quakes.
Permanent approach work, including lighting, footpaths and retaining walls, will continue through this winter, along with removal of the site offices.
Spring will see the start of the last phase of the project, landscaping and planting around the new bridge.
For all the complexities and challenges it faced, HEB is approaching the completion of the project “on time and looking to meet budget”, du Plessis said, and the rattled and shaken citizens of Christchurch can tick off another major landmark in the reconstruction of their stricken city.