On 2 July 2019 a burst water main on Lake Terrace created a washout, collapsing part of a footpath and taking out the 675mm concrete trunk sewer. This resulted in approximately 800 cubic metres of sewage entering the lake and head of the Waikato River. Our Taupō 3 Waters Maintenance team responded to the incident quickly. Starting with a team of 12 onsite they proceeded to shut down the water main, blocked off the overflowing sewer and initiated over pumping to divert wastewater from the damaged pipe overland back to the treatment plant.
Our construction team mobilised on 4 July 2019. This team included reinforcements from our Auckland and Tauranga teams, as well as contractors.
We reinstated the damaged services as far as possible from the lakefront. This included the following:
Installation of a new 120 metre, 675mm concrete sewer main through the centre of Lake Terrace at a grade of 4%. This included an 2500mm man hole, cut out and installed over a live sew main to enable a fixed start point of the new line. This was later reinforced and cut into the sewer main. Two 1500mm manholes and a 1050 riser put onto the existing sewer main. All of the man holes were grouted and and fitted with risers and access ladders etc. The new main was air tested in sections prior to going live.
Throughout this installation we were over pumping 300l per second of sewer in a 24/7 fully manned operation.
Two new stormwater manholes were installed one 1500 and the other a 1050. These were connectected into the existing storm water network with a 40m installation of 600mm RCRRJ pipe and 15m of 400mm RCRRJ pipe. This new ‘T’ connection was into a new downstream defender unit with a 710mm PE pipe installed under the live sewer main. The new Downstream Defence Unit consisted of a 10m deep by 3000mm weir chamber and a Hynds DN2550 Defender chamber connection. There was also a 30m 600mm RCRRJ pipeline installed to a new 1050mm man hole outfall chamber.
While the storm water was being installed the catchment area was blocked off and we had a manned over pumping set up to deal with 10,000l per second potential storm water flow.
We installed a 650m-long 350mm PE water main. This was installed using 600m worth of Horizontal Directional Drilling and 50m of open cut. This included installing connection sluce valves DN300, lateral mechanical connections, air valves and a final connection into an existing magflow chamber.
The washout had additionally caused a major slip on the nearby cliff face. We carried out works to make the area safe and stabilise and rebuild the 4,500m3 MSE wall. Finally, our team successfully reinstated the road back to its original layout, resurfacing 1200 m2 of road.
This project was completed as an emergency project and the design and construction was completed concurrently. This enabled the design and engineer sign off of works as it was completed.
The project was awarded practical completion on 14 November 2019 with the key milestone of road opening on 24 September 2019, only three months after the start of construction.
"Downer have responded to the incident with significant resourcing that has enabled a quick response to the initial incident and the subsequent reinstatement works. They have worked effectively and efficiently with the design team to ensure the completed works is fit for a long term solution." - Denis Lewis, Infrastructure Manager, Taupō District Council