The Northern Pipeline Interconnector (NPI) – Stage 2 project involves the construction of a 1,200 mm diameter, 48 km mild-steel cement-lined reverse-flow water pipeline from the Noosa Water Treatment Plant to the NPI – Stage 1 pipeline at Eudlo, South East Queensland. The project is one of the final components of the South East Queensland Water Grid.

The South East Queensland Water Grid is an integrated water infrastructure network, proposed by the Queensland Government following the state’s worst drought on record in 2006. The grid will also provide a stable water supply for South East Queensland, which has been experiencing unprecedented population growth.

The project footprint is approximately 148 hectares, comprising a 30 m-wide pipeline corridor plus associated facilities including five pump stations, one balance tank and two water quality treatment facilities. The pipeline is being constructed with cathodic protection anodes, air valves, drain down valves and pigging pits along its length.

The pipeline alignment principally overlaps an existing electricity corridor through the rural and urban locales of the Sunshine Coast hinterland. The project is being constructed by the Northern Network Alliance (NNA), a joint venture between LinkWater Projects, McConnell Dowell, Abigroup and KBR.

“The completed NPI (Stage 1 and Stage 2) will supply a target volume of 65 ML/d 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,” says NNA Alliance Manager John Palmer.

Construction on NPI – Stage 2 began on 15 February 2010 and is due for completion in December 2011. Once commissioned, ownership of the completed pipeline will be transferred to LinkWater, the Queensland Government’s bulk water transport authority.

A monumental tunnel

Mr Palmer says that one of the most challenging 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 1,032 m Pringle Hill tunnel travels under established suburban areas of Nambour.

Herrenknecht’s AVN2000 tunnel boring machine (TBM) was used to construct the tunnel.

“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,” says Mr Palmer.

The launch shaft

In addition to the sheer 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 that has not been disturbed before and as a result is compacted and solid.

However, this was not the case at Pringle Hill as the launch site had previously 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 had the property listed on its Environmental Management Register.

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,” Mr Palmer says.

Degrees of difficulty

Another 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.

Mr Palmer says “For operational purposes, the pipeline cannot have a gradient of greater than 10 per cent. Considerable work was done by the design team to ensure this could be achieved without the need to install additional pumps or pumping stations.”

Works at the launch site were required to operate on a 24-hour-a-day, 7-day-a-week basis to minimise the possibility of elements of the TBM binding with the rock in the hill.

In addition, the site for the tunnelling was located close to residential areas, with the closest home being less than 50 m from the launch shaft. To reduce the potential impacts on residents, the alliance employed a range of measures including erecting noise walls, enclosing noisy machines and generators, and designing the site layout to absorb the majority of noise.

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

Project challenges

Mr Palmer says that the critical safety challenge on the pipeline project was working under powerlines.

Approximately 80 per cent of the pipeline easement followed an existing electricity easement, containing high-voltage, alternating current, overhead transmission lines.

To mitigate this safety risk, the NNA has been working closely with the state electricity authority, ENERGEX, to facilitate de-electrification of lines as construction advanced. The NNA provided an onsite safety representative who liaised with construction teams and ENERGEX, and facilitated pole-to-pole risk assessments identifying areas to be de-energised.

Daily procedures were put in place to ensure workers were fully briefed on how to identify and deal with risks and were well equipped to handle emergency situations should they arise.

Other safety measures included fitting excavators with auditory sensors that were set to emit an alarm when the boom of the excavator moved to within the space of a nominated exclusion zone set around the transmission lines.

The project also employed spotters, trained and authorised by ENERGEX, to warn the machinery operator if the boom of their machine moved too close to the powerlines.

Land rehabilitation

Mr Palmer says “The methods we employ to best manage and rehabilitate the environment along the pipeline corridor are critically important during the construction of the NPI – Stage 2. The NNA is committed to rehabilitating land disturbed during construction.”

The NNA mitigates the risk of soil erosion caused by earthworks by working within small sections of the corridor at a time – only opening ground that is required for the following days of pipelaying.

Prior to entering an area, the NNA erects temporary sediment control measures in the areas where construction is set to occur, including rock channels, sediment fences and ridges.

As soon as pipelaying is completed in a corridor, the rehabilitation process begins, involving respreading the subsoil and topsoil, contouring the land and revegetating the area.

In planning this rehabilitation, the NNA consulted with landowners prior to construction via the LCRP process with a view to developing site-specific rehabilitation plans.

Mr Palmer says “The NNA has an entire work crew devoted to completing rehabilitation activities such as seeding, planting tube stock, installation of erosion controls and hydro-mulching.”

Mr Palmer says that the NNA will continue to monitor the pipeline corridor for 12 months after construction and will undertake additional works where necessary to ensure rehabilitation is successful.