The Northern Pipeline Interconnector (NPI) – Stage 2 involves the construction of the 48 km water pipeline from the Noosa Water Treatment Plant to the NPI – Stage 1 at Eudlo, located in South East Queensland.

The pipe used for the project is 1,200 mm in diameter, mild steel cement lined and includes cathodic protection anodes, air valves, drain down valves and pigging pits along its length.

The Stage 2 construction is extremely important for providing a stable water supply to the residents of South East Queensland during a time of unprecedented population growth.

Northern Network Alliance Manager John Palmer said “The completed NPI (Stage 1 and Stage 2) will supply a target volume of 65 mega litres per day of potable fresh water to existing storage facilities at Elimbah and Morayfield. Successful completion of Stage 2 includes a number of integration works with Stage 1 in order to operate the project as a whole.”

Going the distance

“One of the most difficult components of the project has been the planning and construction of the Pringle Hill tunnel—the longest single pipe-jacked microtunnel of its kind in the southern hemisphere,” the Mr Palmer said.

The Pringle Hill tunnel is 1,032 m long, and travels under the established suburban areas of Nambour.

The tunnel boring machine (TBM) used for the tunnel was the Herrenknecht AVN2000 slurry machine, rented for the duration of construction.

“Usually a microtunnel of comparable length would be divided into sections with one or more intermediate shafts. However, the steep slope and topography of the area made this option impossible, so the Pringle Hill tunnel had to be one long, continuous tunnel.

“This created a number of challenges for the alliance, including long walks for the crew when the cutter heads needed to be changed,” the Mr Palmer said.

The launch shaft

In addition to the length of the microtunnel, the launch shaft itself provided its own challenges.

A single microtunnel of this length generates a great deal of thrust force on the back of the launch shaft as the TBM progresses. Thrust force is absorbed by thick concrete walls at the back of the launch shaft. This concrete is reinforced by soil which has usually not been disturbed before and as a result is very compacted and solid.

This, however, was not the case at Pringle Hill. Up until approximately ten years ago, the launch site had been used as a dumping ground for cars, building supplies and general waste.

As a result, the Queensland Government’s Department of Environment and Resource Management declared the site contaminated.

The entire property had to be excavated up to a depth of 7 m, the soil tested and then backfilled before the alliance could commence construction works.

“This meant the soil was significantly less compacted than anticipated and as such could not absorb as much of the thrust force as intended, causing the need for a late redesign for the launch shaft,” the NPI representative said.

Degrees of difficulty

A significant challenge for the Pringle Hill tunnel was the vertical difference between the launch and reception shafts, which is approximately 60 m.

This presents not only the logistical difficulties of creating an upward curving tunnel, but also impacts the operation of the pipeline once complete.

Works at the launch site were required to operate on a twenty four-hour, seven-day basis to minimise the possibility of elements of the TBM binding with the rock in the hill.

The TBM completed the tunnel on 1 November 2010, after three months of drilling.

The project is being constructed by the Northern Network Alliance which is made up of Abigroup, McConnell Dowell, KBR and LinkWater Projects as proponent.

Construction on the NPI – Stage 2 began on 15 February 2010 and is due for completion in mid-2011. Commissioning is expected to be completed by December 2011.

Once commissioned, ownership of the completed pipeline will ultimately be transferred to LinkWater, the Queensland Government’s Bulk Water Transport Authority.