From the magazine

Engineering away risk

The story of the M5 Motorway crossing began in September 2014, when Pezzimenti Tunnelbore received an enquiry from a Sydney Water-accredited designer asking for feedback on a project design they were working on.

John M Daly & Associates Engineering Manager Terry Hams contacted Pezzimenti Tunnelbore about a crossing required under the M5 Motorway at Moorebank, a southwestern suburb of Sydney.

Land at the New Brighton Golf Club was being redeveloped and, in order to drain the sewer reticulation, a crossing under the busy arterial motorway was needed.

The crossing required a 225 mm diameter PVC pipe for 92 m at 1.43 per cent negative grade.

The ground was expected to be original clay ground transitioning into embankment fill.

A concern for Mr Hams, as the designer, was the limited clearance available to pass the proposed microtunnel over the top of two 1,200 mm culverts.

Because these two culverts ran square under the M5, the microtunnel crossed over the two pipes at an oblique angle.

Traditionally, Sydney Water would require the 225 mm PVC pipe to be housed within a nominal 450 mm diameter steel casing.

However, following consultation, Pezzimenti Tunnelbore NSW Manager Jim Shooter suggested to Mr Hams that a larger steel casing be used to allow room for a retractable microtunnelling head to be used.

Proposing the M5 Motorway project

Glenmore Park Plumbing (GPP) was awarded the work to lay the reticulation sewer and then looked to subcontract the M5 Crossing to specialist drillers.

Initially, GPP sought quotes from auger boring companies.

However, Pezzimenti Tunnelbore was able to provide a compelling proposal for laser-guided microtunnelling, citing the technique’s high accuracy, spoil capture without the need for an excavator and smaller site footprint as key advantages.

“For the M5 Motorway Project, the proposal included a fully retractable laser-guided microtunnelling head in case obstructions were encountered,” continued Mr Shooter.

“Further, the head was to be housed inside a steel casing specifically rolled to an outside diameter of 560 mm.”

GPP accepted the proposal and engaged Pezzimenti Tunnelbore to complete the works using laser-guided microtunnelling.

Retractable vs. non-retractable microtunnelling heads

A retractable microtunnelling head allows the head to be withdrawn back through the jacked pipe at any time during microtunnelling.

If an obstruction is encountered, then the head is able to be withdrawn, the obstruction can be removed and then microtunnelling recommenced.

Thus, retractable heads are a major advantage over non-retractable in fill ground where the risk of encountering obstructions is high.

Because microtunnelling heads are usually not fabricated to be retractable when in pipejacking mode, there is no practical way to pull the head back into the drilling shaft once pipejacking has begun.

This is because the diameter of the microtunnelling head is larger than the internal diameter of the jacking pipe.

When using a non-retractable microtunnelling head, if an obstruction is encountered and progress is halted, a serious intervention is required.

Examples of such interventions can include a dig-down on the head, microtunnelling from the opposite end, open trenching the remaining section, or abandoning the head altogether if recovery is impossible.

“We’ve not had to abandon a microtunnelling head yet, touch wood,” commented Mr Shooter.

“But it is not unknown in the industry. Clients are rightfully becoming increasingly aware of risks, and so fallback plans are a critical aspect of project planning.”

The distinction between retractable and non-retractable heads was important because ground conditions can account for up to 90 per cent of the risk of any installation project, continued Mr Shooter.

“Drilling through fill is a “÷Pandora’s Box’ as almost anything can be found in backfill material,” he said.

“While an embankment may be classified as “÷engineered fill’, Pezzimenti Tunnelbore has found old bottles, lumps of concrete, steel fragments and all and sundry in what was supposed to be carefully selected backfill.”

With the composition of the material in the M5 Motorway crossing’s road embankment unknown – the road authority was unable to provide any details of the embankment fill and would not permit and destructive investigation of the subsurface materials beneath the pavement – the deployment of retractable tunnelling heads provided crucial redundancy for the project.

Project progress

At the 75 m mark, the ground changed and hard gravels and rail ballast-like material came back inside the vacuum lines.

The rig was established in early May 2015 and the 92 m length was completed in six days in rainy conditions.

The microtunnelling head maintained line and level; however, progress was slowed drastically as the vacuum line inside the microtunnel became blocked on numerous occasions.

Investigations of the site’s recent history, which included the use of NearMap and the studying of aerial photographs, later concluded that earthworks completed in 2013 had included a layer of gravel, recycled material, or gabions, a likely explanation for the ground change.

Once the exit pit was reached, the head was retracted back along the steel outer casing and into the drilling shaft, as access to the exit pit was not possible at the time.

The 225 mm PVC pipes were strapped with timber centralising spacers and sliplined into the steel casing.

The standard grout-retaining bulkhead was then formed at the downstream end and grout poured under gravity from the upstream end due to accessibility issues.

Grout grumbles

The calculated grout volume was 18 m3.

However, after 19 m3 was poured there was still no show of the grout at the top end and there was no show of grout leakage observed on the downstream embankment or into the two stormwater culverts.

“Instead, grout had made its way out of the steel casing and had escaped into surrounding ground – a very rare event,” said Mr Shooter.

In order to stop the flow of grout, Fibermesh 150 Synthetic – a very fine diameter fibre used in concrete to inhibit and control the formation of intrinsic cracking – was added to the top-up grout batch, said Mr Shooter.

“The intended use for Fibermesh 150 on our site was to act as a plug in the 1 to 2 mm gaps in the joints in the steel casing pipes to enable the grout to be retained inside the steel casing.

“The mix was tested on-site and flowed readily through the 10 mm openings on the grout screens, ensuring its ability to flow down the casing.”

Another 10 m3 was poured before grout at last showed at the upstream end.

The level was monitored and no loss of grout was observed.

Project success and lessons learned

The project was completed to the client’s satisfaction.

While the identified risk was initially thought to be ground conditions hindering the microtunnelling phase of the works, it was actually the ground conditions at the grouting phase that caused more hiccups.

The retractable microtunnelling head performed well throughout the installation despite the unexpected ground change.

“After 900 microtunnels you think you’ve seen it all,” concluded Mr Shooter. “Nevertheless, the grouting phase provided a new learning experience in the use of Fibermesh in grout mixes.”

Mr Hams attributed the early input and collaboration on the project’s trenchless design with Pezzimenti Tunnelbore as a key factor in the project’s success.

“Expert advice from knowledgeable, experienced contractors was sought early in the design process and aided in the identification of issues of concern,” Mr Hams told Trenchless Australasia.

“Design manuals only provide the basic concept information – there is no substitute for practical experience.”

For Mr Hams, the recommendation to increase the diameter of the bore casing to provide additional room for excavation of any unexpected larger-diameter material during the bore was a crucial eureka moment.

“This advice was provided by the contractor during the early collaboration due to the unknown nature of the road fill material,” he continued.

“The advice was accepted and a larger bore casing adopted for the design. This proved beneficial when ballast rock was encountered part way through the bore. I recommend that all designers listen to the advice of a competent, experienced contractor.”

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