Repairing and widening the roads around Christchurch’s iconic Avon-Heathcote Estuary posed many interesting challenges, not all of them anticipated, as CHRIS MACANN explains.
FOR MANY, CHRISTCHURCH’S McCormacks Bay Causeway is a vital commuter route; for others, a well
beaten path to the city’s south-eastern beaches of Sumner and Taylors Mistake. The major road link carrying more than 20,000 vehicles a day was due to be widened but the February 22 earthquake in 2011 hastened all that.
The job of fixing the vital connection was given to the Stronger Christchurch Infrastructure Rebuild Team (SCIRT)’s Fulton Hogan delivery team. SCIRT is rebuilding the city’s earthquake damaged horizontal infrastructure.
After the causeway work, it was a logical step to incorporate a planned widening involving three-laning plus the second part of the Coastal Pathway into the repairs.
Project engineer Ash Mitchell is happy to report that despite quite a few unforeseen things that cropped up, the project team managed to deliver on time.
Stage one of the three-laning project involved widening the existing seawall on up to 10 metres of reclaimed land to enable the roads to be widened and a four metre cycle path to be included. Essentially a new 600-metre wall was built which involved a five-metre thick rock wall being placed with up to five metres of bulk fill behind it. As well, a swale to filter stormwater and road run-off was incorporated between the road and cycle track.
Working in the estuary during the three-laning project provided a few challenges. The digger could only work at low tide so working hours varied.
Pouring concrete for five sets of public access steps to the estuary had to be done with care. The steps had to be made in situ, and because the alkalinity of the mortar affects the water, the concrete had to have additives to allow it to set early.
While the original seawall was mostly made from old concrete, the new wall had to be a more attractive and shapely feature. Originally it was thought Port Hills rock released by the earthquake would suffice as aggregate but it didn’t pass the 15 or so rigorous tests for strength and abrasion resistance.
“This involved a fair bit of work at the start, going around a lot of quarries seeing what suited,” says Ash.
In the end the rock came from Oxford – a 70-minute, 70-kilometre one-way trip.
In all, 340 truck and trailer movements were needed to move 3500 cubic metres of rock.
“With a decent truck and trailer unit you’d get three loads a day,” says Ash.
The final rock choice was important as stonemason David Packman explains.
“Locally sourced Port Hills stone [basalt] would suffer from exposure to the seawater because it was not of consistent strength because it contains ash and impurities.
“The basalt from Oxford was very impervious and very strong. It has the strength and ability to last because the water was not going to get through it and cause damage.”
Five sets of access steps to the estuary were built to match the stone in the seawall. They were built in andecite found on top of the basalt at the Oxford quarry. It is still igneous and hard wearing but splits cleaner and easier making for a better walking platform.
“It was cut with hammers not diamond cutters from one piece of stone so that it looked good,” David says.
As well, the team had to replace the original earthquake-damaged culverts which had sunk, including a large replacement boxed culvert, built from prefabricated sections and placed as the tide allowed.
History too had to be preserved. It’s not the first time the causeway has had a major makeover. The causeway was originally built between 1903 and 1907 for the electric tram. It was widened to include a road during the Depression years, beginning in 1932 and opening in 1941.
Ash said the team had to make sure not to disturb any old history.
“We thought we might find old tram tracks which we haven’t actually come across.
“Archaeologists were involved in all discoveries, to inspect. We found a few wooden drains and bits and pieces but none predated 1900 – the cut-off date archaeologists were keen about,” Ash said.
A lone macrocarpa also had unusual status where anywhere else it would be just another tree. Its location made it a vital nesting tree for a handful of shags, therefore it was protected.
“After an arborist’s inspection we knew, fortunately, the roots inclined away from any required excavation areas therefore eliminating any major arborist works.
“You can see by the marks on all the rocks underneath that it’s not a good place to park your ute or have your lunch,” says Ash.
Fish also had rights on this project. All culverts had to be set at levels which allow for the passage of fish, so stones were put over their bottom so they looked like a riverbed.
SCIRT’s Fulton Hogan team started repair work on the damaged culverts, seawall and road surface in late April 2013.
The causeway was closed to all traffic for six months and despite the heavy rain weather conditions in June that year and a design change during construction the causeway opened to all traffic in November 2013 within the deadline.
That was the easy part.
Not so for the second stage of the project – the three-laning of the section of road between the Christchurch end of the causeway and the newly completed Ferrymead Bridge.
As Ash Mitchell explains, traffic management then became a major issue because there was no alternative route between the short section of road between the causeway and the new Ferrymead Bridge.
“It was a balance between causing major disruption and working quickly or causing less disruption but taking longer. We had to build the road piecemeal and join it all together,” he said.
The team used around 10 traffic management stages and had to build two temporary lanes. “All that traffic driving on it helped compact the ground – and it’s free,” says Ash.
As well as road widening, this stage required realignment of old rock walls and “massive” dips to repair.
Being a major artery and coastal route meant that foam-stabilised bitumen was used for the pavements, a process whereby foam bitumen is mixed through the aggregate.
“On a main road like this, it future-proofs the pavements, and adds strength and life to them. It also reduces the pavement’s susceptibility to water which prolongs the life.
“If you keep pavements dry you’re off to a good start,” Ash says.
The work also included 600 metres of wastewater lines, a new pump station and 300 metres of new stormwater lines including outfalls to the estuary.
Ash said communication with the locals was “really good” but communicating with the wider group of commuters from further afield was challenging.
“People know you’re there for a year at a time but if they don’t – that’s where frustration comes, particularly the morning traffic when it banks up.”
Because of other earthquake repairs there were a lot of other road works in the area which meant having to liaise with workers on other projects.
“Traditionally you’d be the only road works so you’d just get on with it,”Ash says.
To further complicate matters, some of the work needed to be carried out in an area under a slope which was potentially unstable. A GNS Science report found this area was where potential mass movement presented an intolerable risk to life.
“It did have [shipping] containers [on the road side] like a lot of other hills, to stop falling rocks from hitting pedestrians or drivers.
“The council purchased the property and we removed a lot of excess soil at the top and reduced the overburden. This enabled us to move the containers,” says Ash.
This package of work was known as the mass movement project.
“It was an interesting experience because these large earthworks in an area of earthquakes means it could fall in on you, and large machinery tends to vibrate like earthquakes. We had to get the methodology right on how to attack it. We used long reach diggers and sat back and worked from a distance,” Ash says.
Biodegradable coconut mats, hydro grass and deliberately placed logs were used to prevent run-off and to slow and redirect water flows.
“Unless we did the work we couldn’t complete the three-laning job below. It was a bit of an unknown how long it was going to take and what we were going to find. The majority of that work was done in three weeks. If there were forecasts of rain we didn’t go near it because of the increased risk.”
This three-laning stage took 18 months. The works started February 2014 and were completed by May this year within schedule.
Fulton Hogan received an environmental award for work on the causeway and donated $250,000 worth of work to form that section of Christchurch City Council’s Coastal Pathway.