Christchurch water treatment pond pipe replacement Hero

Christchurch water treatment pond pipe replacement: undoing the damage caused by the earthquakes

The Christchurch earthquakes badly damaged the Christchurch Wastewater Treatment Plant, its oxidation ponds and its pipes. The earthquakes cracked pipe walls, allowing other liquids and obstructions such as groundwater and liquefaction silt in. Some wastewater pipe sections completely collapsed and all joints leaked, which meant wastewater was released into the surrounding area.

As a result new pipes connecting oxidation ponds 2 and 3 at the water treatment pond needed to be installed.

Christchurch City Council’s aim:

  • To reverse damage caused by the earthquakes to the Christchurch water treatment pond pipes by installing new pipes connecting oxidation ponds 2 and 3.

What we did:

This was a multi-disciplined, fast-track project involving vibro-compaction works with significant scope changes to cater for the current and future operational needs of Christchurch City Council as part of the Christchurch earthquake repairs.

We were the main contractor responsible for completing the measure and value contract. We managed and coordinated a team of five subcontractors to carry out all works including:

  • Installing 112 metres of 1660mm internal diameter PE pipe under Dyers Road (SH74), including all traffic management
  • Connecting this pipework with the existing pond 2 outlet structure
  • Constructing a new inlet structure into pond 3
  • Decommissioning, removing and reinstating the existing 1350mm diameter and 1800mm diameter pipeline
  • Constructing 445 stone columns totalling over 4.5km in length for the ground stabilisation works.

Site materials were used to construct the vibro platforms in both ponds. Using treated water from adjacent ponds for the jetting process created cost savings and had a positive environmental impact because we did not have to access fresh water from Christchurch City Council’s town supply.

Traffic management was a critical component of health and safety management during construction as the site spanned SH74. The TMP was set out in 2 stages:

  1. Constructing a temporary road to divert traffic control flow, while works started beneath the original alignment
  2. Removing the temporary road once the original alignment was reinstated and traffic returned to normal flows.

Unique challenges:

  • The prominent soil for the site was insitu silty sands. Layers of un-compactable soils encountered 2m and 5m below invert, meant extra effort was required in identifying these ‘thin layers’ and ensuring that the compacted effort being applied would be sufficient to cancel out lack of compaction in the thin layers. This involved suspending a large vibrating probe from a 110T crawler crane for vibro compaction. A total of 445 stone columns were installed over 2350m². Water is pumped into the probe (at a rate of 1,000L/min) and this jetting effect helps the probe enter the ground and keeps the hole open enough to allow the stone to travel to the bottom. Each column required approximately 10,000 litres of water for the jetting process.
  • A large increase in ground area for stabilisation, unexpected deep concrete haunching around redundant pipe work and a large increase in imported material were just some of the changes that meant we needed to work together with our subcontractors and the client to successfully manage the programme, quality and safety.

The outcome:

  • Our goal on this project was to inconvenience the public as little as possible. We completed the upgrades without issue – ensuring the public wastewater system remained fully operational during construction.
  • The pipes, which carry Christchurch’s wastewater, will withstand any future seismic activity and, should another earthquake occur, groundwater and liquefaction silt will not enter the pipes and wastewater will not contaminate the surrounding area.