D1270 - Southern Sewer Renewals

This project saw us working with multiple teams for two consultants across Wellington City. Working in a suburban environment on Bruce Ave, Coromandel St, Camperdown Rd and Northland Rd was straightforward. We self delivered all aspects of the work including: Traffic Management, PE Welding, Open Trenching & Dynamic Pipe Bursting.

Work in Victoria St and Waterloo Quay was more complex and each location presented it’s own unique set of challenges.

Victoria St Sewer Renewal

The Victoria St Wastewater renewal was intended to address seawater infiltration into the pipeline between Hunter St and Willeston St where it was having a significant impact on the nearby pump station and the whole of the downstream network. The existing asset had been constructed in the 1980s but the quality of the product was not of the required standard. The proposed solution was an online renewal with a 500OD PE100 pipe constructed between 2m and 3m deep on the centreline of the carriageway.

There were three key risks on the project that were effectively managed to make this project a success:

Significant Design Change due to unexpected operational issues – part of the main was found to be a storage main and therefore not suitable for an online replacement. The design solution was changed to offline and the project team collaborated well to mitigate the affects of this significant design change and get the project underway.

Traffic Management and working with the RCA – The project team worked closely with the RCA to plan and execute a permanent lane closure – rather than having to work under restricted hours and having to open the carriageway for peak traffic. This well planned and executed traffic management created an environment that was safe and where the project could work productively.

Dewatering under a dewatering consent – Effective dewatering was key to the successful delivery of the Victoria St Sewer renewal project. Dewatering presented a significant risk not only to the environment but also to project delivery. Failure to control the water would have resulted in serious productivity issues, lost welds or other quality defects. The project had a dewatering plan, and the consultants supported the site team to execute that plan successfully. The dewatering system worked for approximately 352 hours over a three month period. 15,410,000 litres of water was extracted. On the biggest shift 1,010,000 litres of water was extracted. The average flow rate was 12.16ltr/sec and the on the biggest day it was 28.06ltr/sec.

Waterloo Quay Sewer Renewal

The Waterloo Quay Wastewater renewal was intended to address a poor condition asset that was at the end of it’s design life. As part of the work the main was upsized to cope with high flows from the upstream catchment. The proposed solution was to upsize the existing sewer from a 150mm diameter earthenware main to a 250mm OD PE100 main by pipe bursting. The existing main was approximately 2.5m deep in the carriageway of Waterloo Quay with a section under the rail crossing into Centerport.

The project had a number of design and consent issues that we had to react to on site, it was hampered in the early stages by onerous traffic management constraints and it suffered from a series of unforeseen underground obstructions. These issues compromised our plan and caused us to have to change the way we delivered the project.

Significant Design and Consenting issues – Some errors in the original design meant that changes were required at the start of the project. These had an impact on the methodology used on site and made the traffic management solution more complex. The phasing of the work was also changed as the section fo the project under the rail corridor was not consented. By collaborating with the Engineer, the project team was able to minimise the impact of these issues on the outcome of the project.

Unforeseen Ground Conditions – During the work an unforeseen underground obstruction resulted in the pipe bursting tool becoming stuck in the ground. Faced with a compromised sewer and an immobile pipebursting tool stuck in the ground under Waterloo Quay the project had to react quickly to excavate, recover the tool and complete the renewal. In the light of this new obstruction the project team reviewed the CCTV footage and identified that the obstruction was a repair on the pipe and that there were a number of other locations that were probably also repairs. One of these locations was under the rail corridor.  The project approached Kiwirail and negotiated access to an existing block of line. This repair was excavated during the block of line while working 24/7.

Traffic Management and working with the RCA – All the issues described above were exacerbated by and themselves contributed to a very complex TM environment. At the outset of the project it was identified that the traffic management presented a high risk and would be critical to delivery of a successful project. Waterloo Quay is a high flow, high speed road that acts as the main corridor for traffic entering and exiting Wellington’s CBD. Initially the RCA applied a 9am to 4pm restriction on the work but the project team struggled to deliver under that constraint and conflict began to develop with the RCA. GPFL suspended the work and re-engaged with the RCA.

Together the project team and the RCA worked through the constraints to develop a better traffic management solution. It was identified that the morning peak would be the only affected flow and would need two lanes to manage disruption. For the remainder of the day the flow was reduced to a single lane. To secure this concession GPFL committed to working two shifts per day to maximise use of the available working time and minimise the overall project duration. The result was that the work was delivered in a safe and high production environment with working space and working time maximised.

Our track record for successfully delivering complex underground infrastructure in challenging CBD environments clearly demonstrates that our team leads the industry in this field. We collaborate with our Customer and the Engineer to produce value for money solutions that deliver the best outcome for the City.

Feedback from the GHD Project Manager clearly describes that our customer was satisfied with the collaborative approach that we took to problem solving, identifying risk and successfully implementing solutions.

“The success of these projects was a result of the collaborative working environment created and encouraged by GPFL, GHD, and the customer. All parties worked together to deliver a common outcome and effectively manage the project risks.”

The project was recognized with a best project award from CCNZ Wellington & Wairarapa Branch in 2019.