In Porirua, New Zealand the Paremata Wastewater Rising Main was a vital part of the wastewater infrastructure serving the northern suburbs of Porirua and satellite towns up to Pukerua Bay. However, after significant rainfall events, the existing pipe burst several times and investigations showed that its condition was poor. This led to the decision that the fragile AC wastewater rising main urgently needed to be replaced.
The client for the project was Wellington Water, with Connect Water (Beca) acting as the project’s consulting engineer. The design of the work largely utilised open cut techniques for the rising main replacement, with E Carson & Sons Ltd (ECS) being appointed as the main contractor. However, where the pipeline route passed beneath a main highway, there was a requirement for the installation of two pipe sleeves, so as to eliminate the need to excavate the highway itself.
This trenchless installation was awarded to contractor G P Friel Ltd which was to utilise the pipe ramming method for both the sleeve installations. This option also meant that the works could be undertaken during normal working hours irrespective of the traffic demands on the highway above.
Ground conditions on the project site comprised a combination of fill material over a weathered greywacke.
To complete the pipe ram installations GP Friel utilised a Grundoram Koloss pipe ramming hammer supplied by Tracto which was powered by two 367 cfm (10.4 m3/min) compressors.
The sleeves being installed comprised 867 mm o.d. steel pipe with a wall thickness of 20 mm. The first sleeve was installed under the Whitford Brown on/off ramps from the highway over a length of approximately 36 m. The second installation will be under the northbound lanes of the highway over a length of approximately 42 mm.
Prior to the ramming operations a series of boreholes and trial pits were excavated along the routes, including at the launch and receive locations of the rams.
INSTALLATION
The first pipe ram took approximately 4 weeks to complete including excavation and preparation of launch and receive pits, pipe ramming, welding and flushing to clean out the casing. At the time of writing the second pipe ram was due to commence and is expected to take a similar amount of time.
The first pipe ram was not however, as straight forward as expected. Ground conditions were significantly harder than the boreholes and trial pits had suggested. The pipe ram slowed down in the second half of the installation. David Philipson, General Manager of GP Friel Ltd said: “We focussed our efforts on cleaning out the inside of the casing to keep the skin friction to a minimum. The ram however never stopped so we persevered and completed successfully.
The flushing operation was difficult as the casing was filled with shattered rock rather than granular material as expected. This slowed the operation down but did not affect the final outcome.”
Given the change in expected ground conditions and the change in spoil this created the contractor utilised flusher/recycler trucks provided by JB Environmental to clean out the casings. This was an efficient operation as the trucks do not have to fill up or tip off as frequently as a normal flusher and sucker trucks. It also minimised run off that had to be controlled to protect the environment. Settlement tanks had to be used to control the runoff generated by the flushing operation prior to it being released to the environment.
The ramming operation noise was another unexpected occurrence. The contractor believed that the location, in the state highway corridor rather than a residential or commercial district, would mitigate any noise nuisance caused. However, the topography and soft landscaping in the area seemed to exaggerate the noise from the operation. GP Friel undertook noise monitoring at the workface and at the nearest property boundaries. This showed that noise was within allowable limits, however, a decision was made to limit the ramming operation to strictly within normal working hours to reduce any nuisance caused.
David went on to say: “Every day is a good day when we are ramming pipes. Pipe ramming is a fast and effective way to carry out crossings of linear obstructions without causing disruption. On this project we were able to ram casings under State Highway One without affecting the normal operation of the road. This reduced indirect costs due to congestion and direct costs associated with planning and executing complex traffic management and roadwork solutions. Pipe ramming is dynamic meaning that little or no temporary work is required to transfer or contain reaction forces generated by the system. This significantly reduces the set-up costs in comparison to some other trenchless methods. The trade off being that pipe ramming is limited in the length that can be achieved and can only be used on a straight grade or alignment. But where that suits the project, it often cannot be beaten.”
Referring to the equipment used he said: “The Grundoram Koloss is powerful enough to cope with almost any ground conditions. On this project we encountered rock and the tool kept going, albeit at a slower rate of advance. In this case the rock was unexpected but where it is expected we can include different cutting shoes or lubrication to help us keep moving.
For Connect Water (BECA) Richard Hickman, Principal – Project Management said: “The plan to go trenchless under the existing state highway was a real no-brainer for the project team. A traditional approach would have been far slower and created a great deal more disruption for road users. The trenchless approach allowed for a much easier conversation with Waka Kotahi regarding traffic management and stakeholder communications, particularly once they were assured that we would be compliant with their depth requirements. Despite the ground investigations undertaken by the project, we knew there remained some risk that a large obstruction might require a change in approach mid-thrust. However, the team was able to push through the material we encountered and accurately land the pipe into the receiving pit. In terms of downsides, they were minimal. We had one nearby resident who lodged a complaint regarding the noise, however she was satisfied with some personalised communication. The main difficulty was actually sourcing the pipe in the first place to meet our urgent programme. Logistics delays due to the ongoing pandemic was a challenge GP Friel was fortunately able to overcome. One highlight was arranging a site visit for a group of recent graduates from Wellington Water and their key supplier panel.”
Ultimately, the wastewater rising main being installed is to be sliplined through the pipe rammed casings by ECS. The new main will comprise a 630 mm o.d. PE100 SDR17 pipe. Most of the pipe has been butt welded into strings, including the sliplining sections. However, in trench, jointing is being achieved by Electrofusion Welding of the individual strings.
www.gpfl.co.nz
