A critical section of the 80 km Chichester Trunk Gravity Main has been rehabilitated by the trenchless installation of dual PE100 1,200 mm pipelines under the Hunter River in NSW. The project featured the installation of one of Australia’s largest diameter HDPE pipes by HDD.
The Chichester Dam in New South Wales provides potable drinking water for the people of Newcastle, Lake Macquarie and the lower Hunter region. The dam is connected to the distribution network by the Chichester Trunk Gravity Main (CTGM), an 80 km long pipeline that was installed nearly 100 years ago.
The pipeline is operated by Hunter Water and passes beneath the Hunter River near the rural town of Osterly. The CTGM comprises two DN 900 pipes contained in a concrete lined tunnel with sufficient space to allow personnel access.
Originally manufactured from cast iron, the pipes have been upgraded over the years to mild steel except for the 90° bends at the base of the vertical shafts. Recent condition inspection of the tunnel revealed spalling of the concrete lining, infiltration of ground water and advanced corrosion of the pipelines.
Investigators concluded there was a high risk of failure of the cast iron bends within the next 5 to 10 years and advised urgent rehabilitation of the pipeline river crossing.
The unsafe condition of the tunnel meant that personnel access was prohibited, rendering horizontal directional drilling (HDD) the only viable solution to replace the two pipelines.
Independent consultant engineers Hunter H2O designed the project, which involved two directional drills to install DN 1200 high-density polyethylene (HDPE) PE100 pipelines connecting to the CTGM by trenching sections on each shore of the Hunter River.
Comdain Infrastructure was awarded the contract, which then entrusted the drilling to HDD specialist Coe Drilling and the manufacture of the pipe to Iplex Pipelines. The scale of the pipe, difficult ground conditions and proximity to existing residential properties represented a challenge to the team.
The right pipe
In order to resist the tensile forces experienced during the pull-through, and to minimise the risk of buckling, a PN16 rated PE100 pipe was specified. The pipe wall thickness measured 109 mm, pushing the processing limits of HDPE pipe manufacturing by vacuum sizing.
During extrusion of large bore and thick walled pipes, conventional HDPE pipe resins have a tendency to flow to the bottom of the pipe under gravity, before the molten material is fully solidified. This effect is known in the industry as slumping or sagging and results in non-uniform wall thickness and ovality of the pipes, which cannot be tolerated in pressure applications.
Iplex Pipelines’ extensive experience in manufacturing large bore PE100 pipes of up to 2 m in diameter predicted Qenos’ Alkadyne® HDF145B PE100 HDPE pipe resin would be required for the project.
Alkadyne HDF145B is manufactured in Altona, Victoria using the latest technology single reactor gas phase bimodal PE100 production process that allows the material to be extruded at high throughput and offers exceptional melt strength to resist slumping.
Through the combination of Alkadyne resin, Iplex’s experience and its state-of-the-art pipe manufacturing equipment at its Albury plant, the Poliplex® 1,200 mm PE100 pipeline was produced efficiently within tight dimensional tolerances.
Over a period of three months, two 320 m channels were drilled beneath the Hunter River. Concurrently, Iplex Pipelines delivered the Poliplex PE100 pipes to the site in 12 m lengths for welding into two pipe strings by GEM Industrial Services.
Due to the diameter and wall thickness of the pipe, the welding task was far from routine. Welding parameters were optimised at GEM’s facilities in Leongatha, Victoria before 49 welds were executed in the field using the latest in high pressure welding machines.
Tight control of the dimensional tolerances of the pipe simplified alignment of the pipe segments, resulting in highly efficient welding. The skilled welding team was able to complete each joint being in less than two hours. The integrity of the joints was critical both during the pull-through and normal operation.
The pull-through of each pipeline was completed within six hours. For this exercise to be successful, high precision handling equipment using a long series of rollers and cranes was used to manoeuvre the large diameter pipe string – which weighed more than 100 t – into the borehole while passing meters from residential properties on the northern shore.
Once connected to the drill shaft, the pipe was dragged back through the borehole on its underground journey to the other side of the river. The tensile forces working on the pipe during a pull-through were as high as 150 t, a level well within the 300 t tensile capability of the pipe.
When the first pipe emerged from the borehole on the southern shore of the Hunter River, it was met with a round of applause from the team. The pipe manufacture, delivery, welding and installation were delivered on time and budget, including 195 t of PE resin, 12 truck deliveries to site and 49 welds.
The Hunter River Crossing project marks a breakthrough for the pipeline industry by creating a new benchmark on the scale of Australian manufactured HDPE pipelines used in HDD. For Hunter Water, the use of PE pipe on the Hunter River Crossing represents a key component in a critical water mains rehabilitation project.
The team and all parties involved were highly satisfied with the execution of the project, with Hunter Water already planning the rehabilitation of another section of the CTGM 12 km downstream from the Osterly site.
This article was featured in the September edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
If you have a project you would like featured in Trenchless Australasia contact Managing Editor Nick Lovering at firstname.lastname@example.org