Building a 180m bridge across a major flood plain in a seismically sensitive area like Kāpiti is no mean feat. The team designing and building it faces challenges every step of the way, and we sat down with Zone Design Manager Doug Stirrat to learn more about what's involved.
Why do we need a new bridge?
Well it's really two separate bridges, each carrying two lanes of traffic that will be travelling at 100km/h. Since the Expressway is a whole new road, the new alignment meant we had to cross the Waikanae River in a completely different place. The new bridge stretches 180m across the river, as well as the flood plain, at a height of 5 to 6m. There's a really steep catchment, and the river can rise incredibly fast, so we had to make sure that our bridge was well above the flood plain and strong enough to withstand any seismic events.
And how will it compare to the current bridge?
It's obviously a much bigger structure—180m long rather than the current bridge length of 80m, and four traffic lanes rather than the current two. It will have five spans and four piers. Our urban design team was tasked with designing our "family" of bridges to respect the landscape, so this one will look very similar to the 10 other bridges that are also being built along the Expressway. There's a pedestrian walkway and cycleway on the bridge with links down to the paths on either side of the river. And as I said, we're actually building two bridges that separate the lanes of traffic, so that's very different to the current situation.
But beyond that, we're also building it to an exceptionally high seismic standard. This bridge has been designed to withstand a 1 in 2500 year event, and this is over three times larger than earthquakes that bridges in Auckland would be designed for (because we are a much more earthquake prone area). That's pretty daunting.
So what have you had to do to meet this standard?
This area is prone to liquefaction, so for a start our main bridge piles had to be exceptionally strong. At 3m diameter and sunk to a depth of over 30m they are some of the largest piles ever constructed in NZ. We also had to use a number of different foundation techniques, to keep the bridge safe in case of a major earthquake. One of these is stone columns, which are particularly good for the sandy soils we have to deal with here in Kāpiti. They're also one of the most environmentally friendly ways to improve the ground, as they make use of natural materials (water and stone). We use a powerful vibrator to add stone and compact the ground to a firm foundation.
Another technique is to use steel H-piles under the ends of the bridge, which are more flexible than other types of piles and will give the bridge support that can bend and move with the ground. We start with a vibrating hammer, which vibrates them into the ground, and then a hydraulic hammer drives them down to a depth of up to 15m.
It all adds up to a very strong foundation for the bridge that will be able to withstand a major seismic event and prevent liquefaction from affecting it.
That does sound pretty challenging. Would you say that this is the most challenging project you've worked on?
The ground engineering presents a huge set of challenges here, because no matter how much investigation you do in the lead-up to construction, you really don't know what you're going to hit until you get in. But then over the course of my 30 years in engineering, I've found that every job has seemed to be the most challenging when you're in the midst of it. I will say that working on M2PP is very exciting. I haven't worked in an Alliance set-up before, and it's amazing to see so much construction going on right on your doorstep. Things are happening all over the route and the momentum is really building. It's like we're designing things one day and building them the next. The Expressway is the biggest construction project that the region has ever seen, and I'm really proud of how things are taking shape.