Project details

Timeline: 2017-2024
Application: Metro
Length: 15.5km twin tunnels
Contractor: John Holland CPB Ghella
Client: Sydney Metro
Reference design: WSP Parsons Brinkerhof, Aecom, Cox, Hassall
Designers: Hochtief Engineering, Noma Consulting, Pells Sullivan Meynink, McMillan Jacobs, Arcadis BG&E
Lining: Precast concrete segmental lining
Geology: Hawkesbury Sandstone, clay, silt, sand

Machines:

1x Herrenknecht Mixshield
Diameter: 7,040mm
Lining method: Segmental lining
Cutterhead power: 1,440 kW
Torque: 5,037 kNm

4x Herrenknecht Double Shield TBMs
Diameter: 7,010mm
Lining method: Segmental lining
Cutterhead power: 2,800 kW
Torque: 4,910 kNm

Sydney Metro Is Australia’s Biggest public transport system. The Metro North West Line opened in May 2019. The extension into the city and beyond to Bankstown will open in 2024. Major construction will start in 2020 on the Sydney Metro West project, linking the greater Parramatta and Sydney central business districts.

Currently, tunnelling is underway on the City & Southwest section. And to think of tunnelling in Sydney is usually to picture fleets of roadheaders in operation. On Sydney Metro, these are in use for the complicated geometry of the station caverns, where 13 are in use: eight Mitsui SLB 300s, two Misui S220s and one each of Mitsui S200, Mitsui S125 and Mitsui S65.

However, the majority of tunnelling is undertaken by tunnel boring machines (TBMs), and for the first time in Australian history, five TBMs are underway on a project.

The project comprises 15.5km of twin tunnels excavated through Sydney’s characteristic sandstone and shale by four double-shield hard rock TBMs. There is also one slurry machine for a 900m section that passes under the harbour. The clayey sands (and sandy clays) as well as the marine sediments in this section constitute the riskiest section of the TBM running tunnels.

Excavation

When matching the TBMs to the ground conditions, the contractor was able to draw on its experience from the North West section as well as other multiple projects around Australia. These were tendered separately and had separate funding packages, but technically there were some similarities. North West was 75% Hawkesbury Sandstone, while City & Southwest is 85%, so comparisons have been easy to make.

At least it was for the hard rock machines.

The decision was made to go for 7.01m diameter Herrenknecht double shield machines, but the contractor made the decision to increase from 10 to 12 backup gantries as certain areas of the machine were more cramped than necessary, for example the ventilation and generator areas. This is a boost for safety and makes the machine a more ergonomic and pleasant working environment. In terms of length, this represents an increase from 125m to 145m. Improved articulation was also needed at the front of the machine, to ensure the tail can did not rub against the segments, as the TBMs navigated two 250m radius curves along the southern section of the alignment.

To keep the machines on this alignment, VMT navigation was employed on the TBMs (and also the project’s 13 roadheaders and six rockbolters), with the contractor opting for the TUnIS Navigation Double Shield supplemented by the TUnIS.mono cam which was released at Bauma earlier this year. Basically, a camera is installed in the gripper shield, while special markers are mounted in the front shield. During operation, the system recognises the markers in the camera image and uses them to calculate the 3D position of the front shield in relation to the known position of the gripper shield.

The contractor also subtly tweaked the spacing of the gauge and centre cutters based on throughput analysis of the previous job. Cutters are from Herrenknecht with refurbishment done at Herrenknecht Australia’s Sydney facility. “The machines have performed exceptionally well,” says Sleiman from John Holland CPB Ghella. “Our best day was 85m, our best week was 330m. All of the TBMs operate a day shift and a night shift, (under-harbour crews have 12-hour shifts, 5.5 days per week, while hardrock TBM crews work 11-hour shifts, five days a week) so we have absolute, fixed times for maintenance between the shifts and at the weekend. The only way we can perform at these levels of production is if the machines are very well maintained. It takes an exceptional team to operate them too, fortunately we’d done enough on the previous job to get these people to come back.”

The tool consumption has been about one change for every 800m3, with any complete changes to the entire head undertaken at the weekend. Settlement has been negligible, with an average depth of 35m and a maximum of 60m.

Other points of note, mucking is done by an H+E continuous conveyor with a 200t per hour capacity which is considered a relatively high capacity for the region. Also, the TBMs were configured to be asymmetrically opposite in the twin hard rock drives to allow the pipes, cables and other services to be installed on the non-cross passage side of the tunnel.

But in truth, the sandstone work makes up the easy tunnelling.

Harbour Crossing

At the time of Tunnels and Tunnelling’s visit, the slurry machine had almost completed its first pass under the harbour. It would be any project director’s pleasure to speculate on what could go wrong while passing under what is arguably the most iconic harbour in the world, but Sleiman says the team’s tactic has been to keep the drive off the critical path and to take the drive slowly and carefully. The crossing is also one of the primary reasons that Ghella was brought into the joint venture with its international experience in this area.

The length of the harbour crossing is 900m and geologically is basically a valley–an ancient paleo river channel–with 200m of Hawkesbury Sandstone on both sides, then a transition zone of about 40m with clayey sands and sandy clays, then a challenging 120m central section of marine sediments. This incorporates organic material, shells and any seabed detritus. The client performed 50 borelogs/CPTs to determine the strength of the material prior to tender, as the harbour area is less well-known than the remainder of the drive. The contractor then carried out additional CPT investigation after contract award to confirm the geology under the harbour.

The first question asked by JHCPBG was whether they could alter the gradient to avoid this strata, but the client turned the idea down. The harbour crossing hits the 4.4% maximum gradient at both ends.

With these restrictions, the solution to run the machine sedately has been successful so far, despite the challenges of the geology.

“We are not putting the machine under load. I have significantly cut back the productivity that I would normally like. Purely and simply, I want to understand the geology and how the machine is reacting to it. We encountered a clay band that unfortunately followed us from the moment we passed through the southern transition zone. We got through the marine sediments really well, but this clay has followed the machine almost perfectly, and slurry machines do not like clay. We thought we’d intersect it again and get through it as we moved upwards, but it’s basically just followed us. My daily report always reads: ‘we’re still in the clay’.”

The solution to the clay was to retard the advance rate further to give the flushing operations more time to work. No extra flushing lines have been added.

As for the slurry treatment plant, it has had to deal with far more clay than expected due to this wandering band of the material. “With a slurry treatment plant, you design it based on what is expected then make adjustments based on actual conditions,” says Sleiman. “With the significant amounts of clay we were getting, we were concerned that we would have to start putting in some serious treatment to break it down. But this would cause problems at the water treatment plant – what chemicals would we have to put into that to counter the earlier chemicals?

“So I put a stop to that and concentrated on flushing and flushing. We have clay clumps and balls coming out, but we can deal with them more easily than a chemical soup that we have no way of disposing.”

The material coming out of the system is consequently very wet, but as the contractor is simultaneously excavating a station box, they are able to mix the TBM-excavated material with the dry, roadheader-excavated material and truck it away to be used for road construction.

Environmental controls in Australia are very strong, so a heavily-dosed chemical soup would not be allowed to landfill and could not be used in construction. The Sydney Metro has a target of 100% reuse of material, which is another reason to stick with the flushing approach.

Ground treatment hasn’t been extensive yet, but Sleiman is most worried for the first cross-passage. The TBM will be extracted and shipped back to the south bank before beginning the second drive.

This first cross-passage may be a water ingress risk, but it is also an area where the team has learned from experience on the North West project. Cross passages were significantly bigger on North West and took up the space of three segments. For City & Southwest, they will take up two segments, which the contractor sees as a safer option for the workforce and leaves them less exposed. Sump sizing and other design features for the passages depend on inflows and other conditions. The spacing of cross-passages under the harbour was enlarged from 240m to 500m as a safety precaution, so there is not one in the alluvial sediments.

Cross-passages are typically built 1-1.5km behind the advance on this project. The precast units are reinforced with steel rebar and the methodology is to put in a sill, a jam, two vertical pieces.

“They’re all fairly bulky structural steel. We bolt them in first, we run the saw cuts in, we hammer out the segments. Then it’s a remote-controlled electrical mini excavator, a Brokk. Our tolerance is 1mm of movement once we’ve excavated,” adds Sleiman.

As for other risky parts of the crossing, hyperbaric interventions are the concern and one of these was needed during the first crossing to replace a spacer block on the cutterhead which had broken off and was found on the primary shaker of the slurry treatment plant. The maximum pressure needed during an intervention would be 4.5 bar, at the deepest point of the harbour.

The largest item on the risk register would be the need to replace a main bearing – particularly under the harbour. Sleiman refers to this as being at the top of his “catastroscope”, but there is a plan for this and there is a bearing on standby at Herrenknecht’s factory in Nansha, China.

“Given the length of traverse under the harbour, the likelihood is very, very small. And given the material and the way we are operating the machine – not putting it under load – this is particularly unlikely,” says Sleiman.

Lining

The main tunnels are lined with two main types of precast concrete segment (although there have been slight adjustments to the mix according to the geology, resulting in a total of 12 mix designs).

The sandstone tunnels house a six-segment rhomboid and trapezoidal lining. Each segment is 1.7m long, 260mm thick and weighs about 3.75t. The thickness is so the segments can tolerate the push of the TBMs thrust rams, for situations where the TBM is not just pushing off the grippers.

The lining under the harbour crossing is 280mm thick for increased ground support and also incorporates a keystone, for seven segments in total. This is because higher loads on the under-harbour sections required straight edges for the segments and so a traditional segment design with a keystone was selected.

Predominantly the segments are reinforced with Dramix steel fibres from Bekaert Maccaferri, but conventional rebar from the Australian Reinforcing Company is used for cross passages and might be considered if unusual geology is encountered. All of the segments incorporate cast-in Datwyler gaskets and fireproofing is with poly fibres from Propex.

The approximately 100,000 segments needed were cast by JHCPBG at a facility in Marrickville, with production beginning four months after the contract was awarded.

The SDS production/management system was in use on the project to manage segment production and storage.

Varied station design

From north to south, the stations were designed as follows:

  • Crow’s Nest will be an open box excavation with the station and an over station development. The idea is to integrate the station into this development, which will involve a high-rise building that has the station in its basement.
  • Victoria Cross is an extremely large single cavern (270m long x 25m high x 26m wide) between two shafts. The contractor will present the client with a fully-lined cavern and the follow-on contractors will fit it with escalators and other operational equipment and facilities. The station will feature a single, central platform
  • Martin Place is two separate caverns, with a platform on one side of each. Again, the fit-out will be done later. This station is located in the CBD.
  • Pitt Street is a binocular cavern, like Martin Place, and also underneath the CBD.
  • Waterloo is similar to Crows Nest, an open station box. It is assumed that there will be an over station development by a contractor who will integrate the station within a building basement.

Final thoughts

Key to the success of the project so far, for the contractor, has been close links with the community and working together with the client, which is also committed to making the effort here. Sleiman says that keeping close to the community has paid dividends and they end up being an advocate for the project.

Regarding the project itself, he concludes, “From my perspective a successful project is one where you deliver what you tender, with minimal changes needed. With a few exceptions to station design, where we have made them structurally thicker for development reasons, it looks like we will do that.”

At the moment the contractor (on a fixed sum contract) is due to finish construction at the end of 2021, where it will hand over to the fit-out contractors. Sydney Metro’s Lawson says that they have seen how long fit-out can take on other projects, and have been careful to leave plenty of time.