MWH New Zealand was commissioned by Hutt City Council to investigate, design and monitor the construction of a new pipeline between two major pump stations in the Hutt Valley Trunk Wastewater Network. The pipeline consisted of approximately 1,200 m of 630 mm OD SDR17 HDPE pipework.
While the pump stations are located only 1,200 m apart, they are situated on opposite sides of the Hutt River. The route seemed clear at the concept design stage, but the complexity of the project soon became apparent when it involved crossing the busy railway line embankment and the construction of a pipebridge across the Hutt River on the existing Ava Rail Bridge.
The possibility of damaging the busy railway line was not attractive. The New Zealand Railways Corporation, ONTRACK, advised that trains would be crossing every 20 minutes during daylight hours and any remedial works would be difficult and expensive.
The success of the project relied on having the flexibility within the contract documentation to change the trenchless techniques used to suit the ground conditions encountered on the site. It also required a team, including the contractor and sub contractor, with the technical ability to implement the most appropriate project techniques.
Initial investigations indicated that the ground conditions at the site were variable but potentially suitable for either pipe jacking or ramming. These investigations revealed the presence of occasional buried logs in the flood plain adjacent to the railway embankment. An old landfill site had also been encountered nearby during recent stopbank upgrading works. This added the possibility of finding obstructions during the embankment crossing to an already complex job.
For this reason the contract proposed that either a 1,050 mm diameter RCRRJ pipe be jacked beneath the embankment or a 750 mm diameter steel sleeve be rammed through. It was expected that pipe jacking would be preferred as it would allow some ability to remove obstructions if any were encountered.
Local contractor, E Carsons & Sons, had initially planned to take the pipe jack option but after further site excavations within the embankment it became apparent that this would not be possible due to inadequate ground conditions and pipe ramming became the preferred option.
E Carsons & Sons had previously performed underground drilling work with Blackley Construction, a contractor based in Palmerston North, and called on them to assist. Blackley used a Gigant Grundo-Ram Collossus model 500 mm air powered ram, capable of installing pipework up to 800 mm in diameter. As the crossing length was only 34 m, it was possible to ram a total of 6 pipes, each 6m long, of open-ended NB 700 mm diameter 16mm WT casing. Once installed, this was augered out and the 630 mm OD SDR17 HDPE rising main pipework inserted with the annulus grouted.
A 10 m launch pit was set to grade on a washed stone bedding, with guide rails set into the stone. A strengthening ring and stepped cone between the rammer distributed the force to prevent the casing from crumpling under pressure. Lugs were then welded to chain winch the 6 m casing lengths to the ram and additional casings were weld-jointed in situ. The lead casing had a forcing ring tapered to guide the cutting outwards.
No significant physical obstructions were encountered in the job and the full operation, including establishment and disestablishment of site, was completed in less than four days.
The ramming speed achieved averaged about 1 m per 10 minutes at 180psi at 350 cfm, giving about 250 tonnes per strike. This was well within the equipment capability, providing an adequate margin within which to work.
As part of the construction monitoring throughout the job, the contractor was required to survey track levels twice daily to detect any track settlement. Given that it had been decided to ram the casing through prior to augering, no track settlement was detected. For ONTRACK, and its many commuters, this led to business as usual with no delays encountered or speed restrictions required on the line.
For MWH New Zealand a combination of specialist Trenchless Technology skills and close working relationships across the project team ensured the right decisions were made to allow the work to be done in good time, within budget and with no disruption to the public, the client or railway operations.
Project manager David Hogg said “The key to the success of the embankment crossing was ensuring all affected parties were consulted early in the design phase particularly ONTRACK. We also ensured that the contractor selected had the specialist skills and track record required to complete this high risk and high profile portion of work.”
Wellington Water Management was also pleased with the completion of the project. Capacity Program Management manager Prithi Gajanayaka said “Trenchless Technology is becoming more and more widely used and it is definitely a technique I will be happy using again.”