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Installing the Yarra sewer siphon

Originally speaking with Great Southern Press publication Trenchless Australasia, with Yarra Valley Water Project Manager David Fox discusses the utility’s $A2.7 million project to replace a 100-year-old sewer siphon that crosses underneath Melbourne’s Yarra River in Abbotsford by using horizontal directional drilling.

The siphon connects the sewer network on either side of the Yarra River and is located near the Capital City Trail Bridge. Mr Fox says the siphon was built in 1911, at a time when Melbourne was shrugging off its “˜Smellbourne’ nickname due to sewerage flowing into the Yarra River.

The tunnel was constructed by a team of Welsh miners, who used hand tunnelling with simple shovels through rock, constantly pumping water as they worked. The thorough craftsmanship of the original siphon is evidenced by the detailed, hand-drawn plans – shown as backdrop to this article.

Siphon as a last resort

A sewer siphon is a pipe that sits below the sewer’s hydraulic grade line, and therefore is always filled with sewage. As sewage enters the upstream end of a siphon the same volume is displaced from the downstream end.

Mr Fox explains that siphons are generally only built in difficult locations, such as crossing under waterways, drains, roads, rail lines, utility services or other obstacles where the sewer cannot be continued on-grade for practical, environmental or economic reasons.

Siphons, generally, have high maintenance requirements and are only considered where the alternative is a severe and unacceptable deepening of the downstream sewer or environmental damage.

Because of the nature of the installation, horizontal directional drilling (HDD) was selected as the ideal technique.

Existing condition

The 100-year-old single siphon consisted of DN300 CICL housed in a brick tunnel. Prior to the upgrade, the last inspection of the tunnel was in the 1980s. The inspection revealed leaks in the tunnel in addition to the CICL pipe being in poor condition. The project would replace the old siphon with new, dual siphons of DN180 and DN160 high density polyethylene (HDPE).

Speaking on the upgrades at the time of installation, Yarra Valley Water (YVW) Managing Director Tony Kelly said “The siphon has certainly serviced the community well but is now approaching the end of its useful life and is being replaced. We build two so that if one siphon blocks or fails, the other can still do the job.”

Installation access

Neither the upstream manhole, located in a park under a foot bridge, nor the downstream manhole, located in a cul de sac 50 m from a busy intersection in Richmond, were conveniently placed for the project.

YVW knew there would not be enough space for the drill to emerge normally, and if left unaltered would emerge in the middle of an intersection in Richmond.

To overcome this issue, YVW decided to install a new downstream manhole for the drill to emerge from. The new manhole would be 3 m diameter, 18 m deep, and constructed with a glass-reinforced pipe liner.

A challenging bore

The pipe path passed through varied geotechnical conditions, including basal, sandstone, siltstone, sand, gravel, clay and silt. There was not enough space for the large drill machine to have access, and it was difficult to get the drill rods in and out.

For the installation, crews utilised a Vermeer 100×120 Series II, which was small enough to fit the tight location, and powerful enough to drill through the tough rock.

There was only access on one side of the river, and in order to keep track of the bore path, crews had to track via boat on the river. The pilot bore was required to hit a 250 x 250 mm target at the bottom of the newly installed manhole.

It was planned the pilot would be tracked with psonde by boat, forward reamed with two pipes attached to the back of the reamer, then the borehead removed for pulling back of the rods.

A sinking sensation

When the pilot drilling began, the drill head kept sinking in the silt near the back of the river. To overcome this, the contractor suggested going in at a deeper angle to get to the harder ground faster.

The next challenge the project encountered was that the bore missed the target manhole due to the fact that the HDD pipe was not in the location expected from the psonde. Mr Fox said the important lesson they learnt from this was not to build the location of the manhole based on the psonde readings, and instead would recommend putting a wire through the pipe to locate it before constructing the manhole.

Further works in planning

According to Mr Kelly, the project forms part of YVW’s wider $A19 million annual Sewer Mains Renewal Program. The program targets the replacement of sewer mains based on factors such as the number of customers it supplies, its location, age, construction material, and reliability.

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