Sydney’s very obvious and unique selling points are its harbour and bridge which, together with the opera house beside them, are iconic. The harbour does, however, pose an obstacle to travel. Routes across it are few.
Yet Sydney is Australia’s largest city, and its most congested. Its suburbs, on both sides of the harbour, are growing rapidly; the population is projected to increase by 60-80% within the next 30 years. The city’s rail network, of ten lines constructed piecemeal over the past 160 years, is already straining at the limits of its capacity.
Sydney Metro is hosting Australia’s largest public transport project. It has major new lines planned for construction over the next decade, to the west and to the planned new airport and the developing region around it. In May 2019, it opened the first stage of its expansion, the Sydney Metro North West line, which for the first time connected the growing outer suburbs with interchanges to the central business district. Usage on the line quickly exceeded forecasts.
The current stage of Sydney Metro’s expansion is the City & Southwest line. At 66km long, it will extend the North West line and increase the number of trains on the network by 60% in peak periods. One component of it, and perhaps the most challenging, is the Chatswood to Sydenham section, which is currently nearing completion. It will serve six new stations, cover the central business district (CBD), and it will tunnel rail lines under the harbour.
The ultimate capacity will be for a metro train every two minutes in each direction under Sydney, a level of service never before seen in the city. Sydney’s current system can reliably carry only 24,000 people per hour per line; the target capacity is around 40,000 customers per hour, similar to other metro services around the world. Trains entering the CBD will increase from around 120 an hour currently to 200 after 2024.
The Chatswood to Sydenham component of Sydney Metro City & Southwest project involves the construction and operation of twin 15.5km railway tunnels from Chatswood on the lower north shore, under the harbour, and thence to Sydenham and Bankstown. John Holland CPB Ghella Joint Venture (JHCPBG) was awarded a US$2bn contract in January 2017 to build the line and excavate six new Sydney Metro stations to serve it. The same consortium had competed the Sydney Metro North West project six months ahead of schedule.
Excavation Details
Terry Sleiman is project director for the joint venture. “We used five TBMs,” he says. “All of the machines were Herrenknecht, excavating a 7.09m external diameter for a 6.2m internal diameter tunnel. Four were double shield; the fifth was a mixedshield (slurry) machine.
Two permanent dive structures were built, one at Chatswood and one at Marrickville. “Each was used to launch two doubleshield TBMs, and the excavated Barangaroo rail crossover cavern, near the southern end of Sydney Harbour Bridge, was used as the launch site for the under-harbour TBM, as well as for tunnelling support infrastructure. A slurry treatment plant, a spoil shed, a grout plant and barging facility were all sited there. “The TBMs launched approximately one month apart from 17 October 2018 and, ultimately, all five were operating simultaneously, which is a first in Australian history.”
Obvious challenges to tunnelling were the congested city centre and the harbour. Before tunnelling under the harbour, Sydney Metro conducted a geophysical study and targeted borehole investigation. A boat towed sonar gear and other equipment to carry out the geophysical study, which defined the level of the sea bed and the material below it. Fifteen boreholes were dug in the harbour to a depth of up to 80m below sea level to sample sediment and rock conditions, and another 30 probes were carried out to classify the sediment.
“This work resulted in a new map of the bottom of the Sydney Harbour,” says Kruti Joshi of Sydney Metro. “It revealed that the Hawkesbury Sandstone under the harbour is deeper than first thought. The rock level at the bottom of this section of the harbour, west of the Sydney Harbour Bridge, is about 16m deeper than previously mapped.”
The survey also added to knowledge of the ancient prehistory of Sydney: “We were able to learn the full extent of the ancient palaeovalley under Sydney Harbour. It was revealed that Sydney Harbour was a valley 20,000 years ago with a small river running through it, which was about the size of the current Lane Cove River or Woronora River.
Choosing The TBMS
“The boreholes found shells in the sediment, 38m below the surface, and charcoal layers and timber, about 52m below sea level, that are the products of bush fires more than 20,000 years ago, reinforcing the view that Sydney Harbour was once a vegetated valley.”
More pertinently to the metro project, the survey influenced the choice of TBM. As a result of the findings, a mixed-shield machine, rather than a double shield, was used for tunnelling under the harbour to deal with the identified ground conditions.
“The geology varied for each TBM tunnel,” says Sleiman. “The two double-shield machines on the south side of the harbour tunnelled through 75% Hawkesbury Sandstone and 25% Ashfield Shale. To the north of the harbour, the other double shields tunnelled through 92% Hawkesbury Sandstone and 8% Ashfield Shale.
“The 800m under-harbour drive was through sandstone on either side of a paleo valley of marine sediments of sand and clay. The transition zones from the sandstone to the sediments were through the buried former cliff lines.”
“No ground preparation was needed,” says Sleiman. “The material below Sydney Harbour is soft by the standards of rock excavation that tunnellers are normally used to in Sydney. But it is still a stable medium and experiences very little compaction as the weight of the tunnel rings settle into the soils overlying the bedrock.
“The greatest groundwater pressures were under the harbour of course, but there were no dewatering issues even there.” Maximum pressure encountered was 4bar. “At the Barangaroo Station site, a permanent anchor solution had to be adopted to resist the upward ground water pressures on the underside of the station box due to the depth below ground.”
Challenges
One minor sub-harbour problem was dealt with effectively: a piece of the cutterhead came loose while excavating below the harbour. Hyperbaric intervention was called for: “The team overcame the challenge of having to inspect the cutterhead in a compressed air environment; but generally, the cutterhead performed well during the excavation on both drives under the harbour.
“After the slurry TBM completed the first drive, the shields and cutterheads were retrieved from a shaft on the north side of the harbour and barged back to the start point for the second drive. The gantries were pulled back through the completed tunnel to the launch location.”
It took three months to excavate the first under-harbour tunnel and two months to excavate the second. The increased speed came from lessons learned modifying the cutter head, and from changing tunnelling processes to better deal with the clay material at the bottom of the harbour.
The other major challenge was the city centre. Here, says Sleiman, careful planning in advance was the key. “The tunnelling route and cavern locations required a detailed interrogation of anthropogenic change, including of buildings and basements. Sydney Park is built on former brick pits; there is a historic tunnel at Sydenham, and of course many transport and utility assets were operating above and below ground.
“Careful planning was required to operate tunnelling sites in highly urbanised environments without disrupting the functioning or amenity, or the safety of the city. There was high pedestrian traffic all around some sites. Disposal of spoil, working hours, dust, noise and traffic flow had to be carefully considered. Those drove innovation in safety, staging, excavation and lining methodology, spoil transport, heavy vehicle management and impact mitigation.
Investigating Buildings
“Heritage-listed buildings, high existing building loads, eccentrically loaded pad foundations and concentrated lift-core loads in the CBD required an intensive investigation of their respective load impacts on the tunnel lining and excavation.” The search involved documents, people and structures: “Often this meant approaching building managers, the original consulting engineers’ archives, or undertaking physical survey of building basements and below ground structures.
“The exercise involved a specialist group of structural engineers and façade specialists, who often had to interpret and develop drawings for updates and alterations to existing building plans. The buildings varied in the quality of their construction and in their degree of documentation; some had no representative documentation at all.”
JHCPBG also undertook a detailed ‘predicted effects’ assessment of high-rise buildings in locations where tunnelling was 6m or less from the basements. “That was a first for Sydney” says Sleiman. “Due to the lack of reliable documentation – and in cases due to poor construction – detailed analysis was required to determine maximum allowable settlements and to assess a building’s reaction to the risk during construction.
Several dozen detailed investigations and complex ground and building structure analyses and assessments were undertaken to inform a strategy to manage the excavation of the tunnels and station excavation sites.”
A particularly difficult element was excavating around the 70-year-old Eastern Suburb Rail tunnels. “That was one of the biggest challenges of the Martin Place Station tunnelling works.
It required construction methodology to be carefully considered and regularly assessed to minimise impact on this structure and operations of the rail lines.” Martin Place is in one of the busiest areas of the Sydney CBD, with large volumes of pedestrian traffic among heritage buildings and existing rail lines. A temporary twin-deck pedestrian bridge was constructed to divert pedestrians around the shaft constructed for the southern entrance to the station.
“At Pitt Street Station the site is bounded beneath by the CrossCity Tunnel, with only 4.6m between the two structures,” says Sleiman. The permanent support bolts of the Cross City Tunnel are 3.5m long, leaving only a 1.1m beam of sandstone between the two structures. This had to be carefully excavated out and replaced with a reinforced concrete arch bridge structure that would transfer the enormous loads of the TBM passing over the top of the Cross City Tunnel as it traversed the Pitt Street Station cavern.”
The new underground stations on the line are designed to be as close to the surface as possible to allow customers to get in and out easily. At Crows Nest, Barangaroo, Central and Waterloo the stations will be excavated from the surface using cut-and-cover. At Victoria Cross, Martin Place and Pitt Street the station caverns will be mined out underground.
The stations are designed as either single-span or binocular caverns. A single-span mined cavern was proposed at Victoria Cross station; single-span cut-and-cover stations are proposed at Crow’s Nest, Bangaroo, Central and Waterloo. Binocular mined cavern stations are planned for Martin Place and Pitt Street. The decisions on the type of station cavern were based mainly on constraints to the tunnel alignment, such as basement buildings or other underground infrastructure.
Linings were segmental for the bored tunnels and cast in situ for adits and station caverns. Six segments per ring were used for the hard-rock tunnels from Marrickville to Barangaroo and from Chatswood to Blues Point, and seven segments per ring were used in the under-water tunnels. Over 100,000 segments were produced in all. Environmental considerations played their part: an innovative concrete mix reduced the carbon footprint without affecting durability. Both steel and polymer fibres were used; all the linings also included polypropylene fibres, not for reinforcement but as a means of fire protection.
Spoil Handling
Spoil and muck were extracted from the tunnels by conveyor; there was over 30km of conveyor system in total, with capacities of up to 800t/hr. Barges were used to transport the spoil from the excavation of the crossover cavern at Barangaroo, saving significant traffic impacts, but most of the rest of the excavated material had to be moved by truck.
Over 200,000 truck movements had to be organised, avoiding queuing and putting pedestrian safety first. “Successfully managing a large fleet of trucks in a congested city and suburban environment was a major logistical challenge” says Sleiman.
“Detailed traffic management plans had to be devised, particularly during peak hours. Vehicles were tracked using onboard GPS monitoring and site-traffic access plans.” A total of 662 spoil drivers and a similar number of trucks collectively travelled over 21 million kilometres.
“A slurry treatment plant was used for the under-harbour tunnelling. Approval was obtained from the Environment Protection Agency to reuse the marine sediments. All of the clean spoil across the project has been reused on over 100 approved road, housing and airport projects across Sydney.”
The combined average rate of progress of the TBMs was 120m/week, which was as expected. The Barangaroo Station box is where the two southern TBMs completed tunnelling with their final breakthrough. The two northern TBMs from Chatswood broke through to finish tunnelling at a retrieval shaft at Blues Point. The shaft measured 10m x 21.4m for the first 21m in depth and 14m x 21.4 for the last 9m. The Under-Harbour TBM broke through twice into the Blues Point shaft. The last TBM broke through to complete its drive in March 2020 Once each of the TBMs finished tunnelling it was retrieved by crane.
All were returned to the supplier; major parts from two of the TBMs are being reused on the Cross River Rail project in Brisbane.
The tunnelling part of the project is now complete. Major portions of the project have now been handed over to the client Sydney Metro for the next stages of work to build the stations and integrated developments and install the rail systems. It is hoped to begin operations in 2024.
