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Drilling, dewatering and project delivery

In March 2016, Dunstans Construction Group was contracted by John Holland to complete the design and installation of a horizontal dewatering system as part of the AU$6 billion Level Crossing Removal Project (LCRP) in Melbourne, Australia.

The Victorian Government is in the process of removing 50 dangerous and congested level crossings throughout the greater area of Melbourne as part of the LCRP.

The removal of the train crossings will significantly improve safety for drivers and pedestrians, and generally result in better travel around the city for public transport users, cyclists and drivers alike.

While construction is underway on some sites, it has already been completed at others, and planning and early consultation has commenced for the delivery of the entire project.

Dunstans was engaged to provide an alternative dewatering system to quickly and efficiently lower the water table, allowing for bulk excavation works beneath the McKinnon Road and Centre Road sites, both situated on the Frankston line, which services Melbourne’s south eastern suburbs.

Due the to the long, narrow and active rail corridor, conventional spear type dewatering was not suitable for the job.

As a result, Dunstans redesigned an innovative dewatering system which incorporated horizontal directional drilling (HDD) and challenged industry standards.

The redesign included the utilisation of an inner pipe, which was surrounded by a filtering membrane, to prevent intrusion from the silty sands.

The inner pipe was contained in an outer pipe which protected the filter membrane from damage during installation.

The design of the dewatering system included the installation of four separate perforated pipelines, totalling 2.8 km in length.

With 250 mm high density polyethylene pipe (HDPE) on the outside of the pipe, and 160 mm of HDPE on
the inside.

A unique machine was designed and manufactured at Dunstans’ workshop in Wangaratta, Victoria, to drill over one million holes into the HDPE to be used for the project.

The holes were drilled radially to create a diamond pattern, which allowed for the maximum perforations in the pipe, without compromising the integrity of the material.

Working within the confined rail corridor required specialised training, and only allowed for a small area for setting up the rig and pipe stringing.

Dunstans Engineering Manager Michael Porter says the company utilised its Maxi Rig Spread, which is quiet, mobile, and compact, and allows for set up and operating in a constrictive urban setting.

“Access was a major issue for the HDD works on the rig site and on the pipe side. The set up of the Maxi Rig Spread was in a typical inner city side street that runs parallel to an urban railway line.

“The very confined area was in among overhead powerlines and a web of underground power, water, signalling, sewer and gas services.

“Welding, stringing and inserting the 160 mm HDPE pipe into the 250 mm HDPE pipe had to be completed within a 5 m wide corridor on the pipe side and although one of three railway lines had been removed from service, two were still active.

“For each location there was approximately 250 m from the exit point to a major road, which required welding of pipe strings in 250 m lengths and joining them with Dunstans designed and constructed quick connectors during installation.”

Mr Porter says Trenchless Technology methods were the only option for dewatering in this specific environment. He says the project required the Dunstans team to innovate every angle of design and construction.

“Typical dewatering of this environment would have required the installation of 4,000 spears to dewater the required area.

“This conventional method of dewatering was not feasible due to the costs involved and the exceptionally short timeframe to complete the works.

“Using HDD to install the perforated pipe accurately below the water table allowed for the train services to continue running during the preparation works.

“After only a short period of time dewatering the corridor, the bulk excavation works were able to be carried out in an incredibly dry environment, providing huge savings in both time and cost and removing the difficult task of handling wet material.

“There were a number of innovations required due to the complexity and constraints of working in the railway environment. Specialised connections, installing the 160 mm pipe inside the 250 mm pipe with geo-sock, manufacture of a machine to drill holes in both the 160 mm and 250 mm HDPE pipes.”

Dunstans Project Engineer Janelle Scott-Hand says a specifically designed fluid was utilised to combat the geology of the environment, with super-fine sands and silts making for challenging HDD.

“The geology encountered was running sands and silts that were super fine and not the most favoured geology for HDD purposes.

“Specifically designed drilling fluid was utilised to ensure no frac-outs would occur underneath the live railway lines while still removing cuttings and creating a filter cake to keep the hole open during drilling and insertion.

“The drilling fluid also had to be broken down and flushed from the borehole after installation of the pipe to allow water ingress and the dewatering to occur.”

Ms Scott-Hard says careful planning was required to find a suitable entry location for each bore, as a result of a congestion of utility services in the area.

“Conventional wireline tracking systems utilising magnetic fields were not suitable due to the interference from a myriad of sources including the sheet pile walls, railway lines, underground high voltage power and the trains themselves.

“To overcome this, a gyro tracking system was shipped from Europe to be utilised for this project, which required no surface tracking system. This allowed for the completion of the project in the short timeframe required.”

The AU$2.5 million project is the first horizontal dewatering system, on this scale, installed in Australia.

The successful project exceeded the client’s expectations for installation time and effectiveness. It is the largest dewatering system that Dunstans has designed and installed.

For more information visit the Dunstans website.

This article was featured in the December 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 covered in Trenchless Australasia contact Assistant Editor Nick Lovering at nlovering@gs-press.com.au

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