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Metro Tunnel

Town Hall Station piling works

Before bulk excavation can commence, we need to install a series of piles within each site to support some structures.

This will enable workers to safely carry out excavation underground. Piling can generate medium to high levels of noise.

What we are doing

To construct the new Town Hall Station, we will be undertaking excavation at three sites in the south of the CBD (City Square, Flinders Quarter, and Federation Square).

Before excavation can commence, we will be installing over 500 piles to a maximum depth of approximately 38 metres across our City Square, Flinders Quarter (located next to Young and Jackson) and Federation Square sites.

These new foundations will enable excavation and station construction works to be safely carried out underground.

We will excavate to a maximum depth of approximately 40 metres using a combination of ‘top down’ and ‘bottom up’ construction methods.

The ‘bottom up’ method involves excavating the box down to its maximum depth before the construction and fit out of the station.

The ‘top-down’ method involves building each level during excavation before the construction and fit out of the station.

Surfaces will then be reinstated above the station.

Stages of piling

Setting up the rig

Deliver the rig to site and move it into position

Drilling

Drilling holes into the ground using a core barrel, and excavating material

Steel reinforcement

Removing the material and inserting a prefabricated steel reinforcement cage into the hole

Concrete pouring with tremie pipe

Pouring concrete into the hole to form the pile. Once the concrete sets, the pile is complete

Capping the piles

Once the piles are constructed, the top layer of concrete is removed and topped by a capping beam to prevent lateral displacement.

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Station piling processClick to enlarge

Pile top removal

As the piles are constructed, there will be some instances where the top of the pile may need to be removed.

Where possible to avoid hammering, we use a more innovative method to cut the piles. The method involves the use of an expanding mortar to create pressure and generate a horizontal cut, so the top layer of concrete can be easily removed by a crane.

This helps minimise the need to use jack hammers and excavators in the process, reducing noise, dust and vibration impacts on the surrounding community.

Following the removal of the top layer of concrete, a temporary concrete capping beam is cast on top of completed piles to prevent any displacement during subsequent excavation works.

Process

  1. Polyethylene sleeves are placed over the reinforcement cage down to the desired cut off level
  2. An arrangement of purpose designed flasks are fixed to the reinforcement cage to the cut off level inside the concrete
  3. The concrete is then poured into the hole
  4. During concrete curing, excess water reacts with the flasks, creating expansive stress to crack the concrete
  5. In this process, the pressure from the expanding agent forms a perfectly horizontal crack at cut-off level
  6. The top layer of concrete can then be removed using a crane and the breaking back of the pile is complete.

Pile top removalClick to enlarge

Timing

Piling works are underway at City Square, with Flinders Quarter and Federation Square piling to start in 2019.

At the completion of piling, shaft excavation across the three sites will commence and continue until 2020.

Piling works will be completed during normal construction hours.

Managing noise and vibration

The use of heavy machinery such as piling rigs, cranes and excavators can generate medium to high levels of noise and some vibration at various stages of construction.

While every effort is made to minimise impacts from construction, a project of the size and scale of the Metro Tunnel can have an impact on the comfort of people living, working or studying near construction sites. We recognise there are some sensitive buildings and stakeholders located close to the Town Hall Station worksite.

Prior to works commencing, qualified consultants have and will continue to undertaken extensive modelling of the predicted noise and vibration impacts. This will allow us to determine appropriate mitigation strategies, and ongoing discussions with key stakeholders allow us to identify any additional mitigation measures that may be required.

Mitigation measures may include (but are not limited to) offering periods of respite, selecting less noisy equipment, shielding noise sources, and installation of acoustic timber hoarding around the site.

Noise and vibration monitoring will continue throughout the project to inform the construction process, validate the modelling, and demonstrate compliance with the Metro Tunnel Environmental Performance Requirements (EPRs).