Measuring and Mitigating Turbidity for the Kaitoke Gorge Bridge Refurbishment
The bridge replacement presents significant construction challenges for the contractors and engineers working on it, including limited access, use of heavy machinery in difficult terrain, steep rock walls and rapid changes in river levels.
An essential component of the project is to monitor the impact of bridge construction on the Te Awa Kairangi/Hutt River flowing under the work site.
So, Adroit was engaged by construction contractor Brian Perry Civil to develop an Environmental Monitoring solution to measure any turbidity created by construction work in real-time, to ensure that the impact of construction on river quality is monitored and mitigated in order to meet the Greater Wellington regional consent conditions.
The new bridge construction at Kaitoke Gorge will make Wellington’s water supply more resilient to possible earthquakes. The four-year project is challenging, requiring a lot of ground improvement, involving scaling, and removal of topsoil, trees, and some of the loose rock – then drilling new foundation piles to support the abutments into the rock face on either side of the river and grouting in anchors.
The existing bridge will stay in position while the new bridge is being built. A 96 Tonne lifting jib tower-crane has been brought in to efficiently install the new bridge and assist in demolition of the existing bridge.
Brian Perry Civil criteria was for a 24/7 monitoring system that provided numerical environmental data to show the baseline for the water quality in the gorge and allowed them to see when they were working, whether the water turbidity has changed due to construction activity.
“It also helped to have data to understand how the river itself has behaved over time. We’ve seen spikes in turbidity when there’s been heavy rainfall, but we’ve not been working,” the project team says.
“So, there are natural changes in the river, which do actually increase the turbidity levels within the river, beyond our own limits for our work. But it’s just a natural event.
“So, it’s good to know that information, because it means you can have that conversation with your client or stakeholders and say: “Look, that’s just the way the river is.” But we can also confirm that when we are working, we’re not making any changes to it.”
A Challenging Installation
One of the challenges was the fact that the site is deep in the Kaitoke Regional Park, with limited access to the site and no simple access to the chosen downstream location for the sensor package.
Adroit Delivery Manager Greg McNaughton says that the initial site survey of the geographical location of where they actually wanted the sensor had very steep banks and limited daylight during the winter months. The sun would only appear between the two ridges for approximately an hour a day, not enough to maintain an effective solar charge for the remote sensor.
“That led us to thinking about how we could power the solution – not just from solar but using a wind turbine as well. That was factored into our design of powering the whole water sensor unit. That was one really quite significant factor that we had to use in our design.
The second challenge was actually getting to the location. Hillside faces were at an approximately, 45-degree angle in most cases, vertical in some locations. And the Adroit team needed to work out how to get the gear from the bridge entrance, to where the sensor needed to be. And then, how to mount the equipment once they were out there?
“That included being led in by a trained abseiling team that helped us carry a lot of our gear in, but also kept us safe by having proper harnesses and proper PPE, to be able to traverse that terrain,” McNaughton says.
The first installation was successful, but soon after the installation was hit by a significant weather event, where the gully went from around 20 cubic metres per second of water flow to more than 200 cubic metres per second – damaging the installation.
“The whole valley floor just fills up with water in flood conditions. When we were presented with one of those significant weather events, it just ripped our sensor cable apart like it was a piece of cobweb,” says McNaughton.
“So, the Adroit team re-approached the installation, this time encasing the sensor cable in resilient tubing, reinforced by drilling into the hard rock surfaces and anchored every metre using stainless steel screws.
“Since we’ve installed it, we’ve had a couple of weather events and it has held up to those,” he says.
The Adroit Solution
For the Kaitoke Gorge installation, Adroit installed an In-Situ Aqua Troll 500 Multi-parameter Sonde calibrated to measure depth, temperature, conductivity, pH and turbidity.
The sensor is backed via a data logger in an IP-rated enclosure placed further up the bank, out of harm’s way, with a solar panel and a wind turbine connected, plus two backup batteries.
The Libelium data logger transmits data to the cloud using the Cat-M1 Spark network – despite the remoteness of the location, the flow of environmental data “hasn’t missed a beat”.
Data is uploaded to the Adroit Cloud Platform and visualised on the Adroit Dashboard, which shows key metrics in real-time, from any connected device.
“Physically, this installation in particular was quite challenging,” says McNaughton. “We’ve taken a lot of learnings out of this about our technology and how it should be more resiliently deployed in the future. If we ever get presented with something like this again, we know all the right questions to ask and get answered,” he says.
The Kaitoke Project team says that monitoring sediment in waterways is becoming integral to the construction business.
Brian Perry Civil, works on all kinds of projects, from the massive scale of Kaikoura earthquake remediation right down to the large stormwater drains in the middle of cities. So, are they anticipating that this system could be utilised in those types of environments in the future?
“I think there’s certainly an opportunity for it, especially when you’re working with anything to do with water. Where that waterway meets interaction with the environment or public, you want to make sure that you’re trying to control your impact as best as possible.
“Clients these days, especially councils and any major company really, are conscious of the environmental impact of our work. There’s no 100% perfect solution, unfortunately, but at least we’re mitigating it to the best of our ability,” the project team commented.
“An Adroit system certainly can help you create a baseline. That’s the important bit. It’s a bit late to put it in when you’ve already started the job. You need a baseline, and then you can see the impact of your work, or the effectiveness of your environmental control systems.
“It’s quite impressive”