Engineering Insight: Building Resilience into the Christchurch Sewer System
The most costly disruptions always happen when something we take for granted stops working. Rebuilding Christchurch’s sewers involves balancing the demands of time and money against future-proofing the system.
Before the Canterbury earthquakes, 1,700 kilometres of gravity-fed sewerage pipes carrying up to 160 million litres of wastewater a day underwrote Christchurch’s primary wastewater infrastructure. That’s a lot of pipe – laid out in a straight line, it would cover the same distance as State Highway One from Auckland to Bluff. It weaves its way underground to Christchurch’s eastern suburb of Bromley and its landmark wastewater treatment plant.
“A good day post-February 2011 meant the treatment plant working at just 30 per cent capacity.”
Then came the 2010 and 2011 earthquake series: eighteen months of extreme shaking, including the 22 February event – the most intense quake ever recorded to hit a modern city. There was differential settlement, lateral spreading and liquefaction silt at a scale the city had never experienced before. Something had to give.
The extent of the damage
And give it did. The city’s sewerage system almost ground to a halt. John Mackie, an experienced three-waters engineer, head of Christchurch City Council’s infrastructure rebuild team and board member of the Stronger Christchurch Infrastructure Rebuild Team (SCIRT), takes up the story: “It was pretty much the same after all the major quakes. We could see some physical damage, the obvious above-ground stuff. Manholes had popped
out of the ground or the ground around them had settled, and pumping stations – the city has over 170 – were blinded with silt. At the treatment plant, screens and sedimentation tanks were filling up with sand; clarifiers were badly damaged; trickling filters had tilted; and oxidation ponds – the earth bunds – were damaged and slumped.” A good day post-February 2011 meant the treatment plant working at just 30 per cent capacity. Untreated wastewater was being released into the oxidation ponds. The plant was literally choking. By early April, it had sucked 1,000 tonnes of sand out of the sewerage network.
But what was the extent of damage to the underground pipe hierarchy? “That was the harder question to answer,” John says. “We knew there was cracking and collapse, joint breakage and gradient loss but how much, and where?” They hoped the closed-circuit television (CCTV) inspection programme would provide the answers. “A team of at least 20 CCTV crews were on the job but even then many pipes were too clogged with sand and debris to allow inspection. At one point, I think at least 90 trucks were out and about flushing sand.” Meanwhile, up to 60 million litres of wastewater a day was leaking into backyards, waterways and the ocean, rendering beaches and rivers unsafe for recreational use. For many residents in the city’s eastern suburbs, the convenience of a flushing toilet quickly became a thing of the past.
Solving the cost conundrum
The CCTV programme was creating issues of its own: an estimated four years and $125 million to inspect and analyse footage of all gravity-fed pipes in the network. It simply wasn’t going to work. It was this conundrum – the need to evaluate the damage in a timely and cost effective manner – that led to some out-of-the-box thinking at SCIRT. John explains: “My colleague, Christchurch City Council’s unit manager of water and wastewater rebuild, John Moore, headed up SCIRT’s wastewater asset assessment team at the time. He and his team discovered a set of relationships between observed damage and other parameters that could be seen and measured and could potentially predict the level of damage to pipes for which we didn’t have CCTV footage.” Those other parameters included geotechnical data, pipe asset data, network performance data and earthquake data. So the multi-parameter Pipe Defect Assessment Tool (PDAT) – a desktop application that predicts earthquake damage to gravity-fed pipe networks with an astounding 95 per cent accuracy was born. “You might, if you’re lucky, achieve 60 per cent accuracy with a single-parameter approach, so this was a big improvement,” John says. Using PDAT, the city’s entire wastewater and stormwater networks – 50,000 and 60,000 pipes respectively – were analysed. The results were used to target CCTV resources to inform the design of rebuild works, scoping and budgeting, prioritising and estimating. It was a huge step towards recovery.
In all, nearly a third of the wastewater pipe network – almost 500 kilometres – needed some form of repair or remediation by SCIRT, under the cost-share agreement (60:40) between the Crown and the City. An original cost estimate was $3.4 billion – reduced to $2.9 billion with efficiencies from the likes of funding of renewals, economies of scale and application of innovations – to fix the underground infrastructure as a whole. Due to the size and scale of damage, the wastewater network has proved the most costly element.
So, are repair costs still in line with those original figures? According to John, they are, but not without some design guideline revisions along the way. Repairs, particularly in the eastern suburbs, began on a damage-based approach: if it was broken, it got fixed. However, it soon became apparent there was more work than money to pay for it. “We needed to rationalise the spend to get the best return on investment. We had to think a little smarter, apply some lateral thinking. Were there any damaged assets that, if they were left for five, 10 or even 15 years, we could still get some useful life out of while critical damage was fixed?” The result was revised design guidelines. Using a level-of-service approach, the goal now was for the network as a whole to operate at a similar service level as it did pre-quakes, meaning in some areas it might be better and in others, it might be worse. John says: “That worked ok for a time but we were still trending more damage than money to fix.” This time the decision was made to take some non-critical damage in non-critical locations out of the repair programme altogether and put them into the business-as-usual repair and renewal programme. “So, while costs haven’t moved much, the scope of works has. Of course, this has consequences. We’re still going to have infiltration. Cracked pipes mean water is going to get into the network and sewage is going to leak out. It’s not ideal but in the end it boils down to cost optimisation.”
“An absolute priority”
John believes while building back better and building in resilience are laudable goals, without adequate fiscal resource, it’s quite simply unachievable in a whole-of-network context.
“It’s an absolute priority for us to take resilience opportunities where we can. ”
“Nevertheless, it’s an absolute priority for us to take resilience opportunities where we can,” he says. Older, less durable and damaged asbestos cement, earthenware, concrete and brick arch pipes are routinely replaced, where practicable, with inherently more resilient and more durable materials like polyethylene. Bedding materials and wrappings are being used to prevent floating and newly constructed reservoir-top ring beams are designed to hold together when the next quake hits. Three wastewater technologies – vacuum, pressure and enhanced gravity – are being installed in areas where grade damage is so bad gravity systems are no longer feasible or affordable. Vacuum systems don’t rely on gravity for wastewater flow, and that’s significant when land levels are shifting around in an earthquake. Then there’s the advantage of a pressure system in which wastewater is collected in a 900-litre below-ground holding tank, often located on the residential property itself, before being pumped into a pressurised wastewater main and heading to the nearest gravity outlet or pump station and on to the treatment plant. “In terms of resilience, you’ve got 900 litres of on-site wastewater storage. In an emergency and with careful management, it’ll be several days before the tank needs pumping out, and that could be done by a sucker truck if the power was still off.” Surely a favourable proposition, when you consider the deployment and use of around 2,000 port-a-loos and 40,000 chemical toilets post-quake.
For many affected residents, though, the idea of sewage storage on their land was unappealing. “We didn’t anticipate the level of opposition, that not everyone would be a fan. It’s a system that’s been installed in other parts of the country – Allenton (near Dunedin), Auckland Region and parts of Rotorua – with little or no fuss. And in Australia and America, pressure systems have been routinely used for over 30 years,” says John. In fact, opposition was such that following High Court and District Court judgements, additional community consultation and a comprehensive reassessment of repair options, the decision has been made at city council level to not extend the pressure wastewater network in those particular areas. Instead, the gravity wastewater system will be repaired. “It’s been a long process but finally, after almost three years, we’re about to highlight the residual schemes to be finished, and those areas will run a mix of pressure and gravity systems.”
“Resilience is not that easy to achieve, especially when pressure on the project is grossly compounded by post-event demands for return of service combined with severe capital constraints.”
As the Christchurch example shows, resilience is not that easy to achieve, especially when pressure on the project is grossly compounded by post-event demands for return of service combined with severe capital constraints. But the message is clear: not only is the pursuit of resilience for such critical infrastructure essential, but also the pre-damage costs of such investment must show acceptable return when compared to post-event costs for any community located in