Customer: City Rail Link Limited
Location: Auckland CDB
Contract type: ECI
- 350m trench in Albert St, 18m deep and 13m wide
- 4 shafts for the pipe-jacking works
- 362 reinforced concrete piles
- 250 services
- 2 tunnel boring machines (SP2 works)
- Live traffic maintained around and over the top of the trench throughout.
McConnell Dowell is half of Connectus, a JV responsible for delivering the City Rail Link (CRL) Contract 2 (C2). Procured under an Early Contractor Involvement (ECI) model, C2 is part of the early works package for the overall NZ$4.4 billion CRL project.
C2 physical works began December 2015 and comprised two separable portions:
- SP1: 350m of cut and cover tunnels under Albert Street between Customs Street and Wyndham Street, and re-instating the streetscape on completion.
- SP2: Pipejacking 516m of new stormwater main along the eastern side of Albert Street, between Swanson Street and Wellesley Street, and strengthening a 40m length of the Orakei Main Sewer beneath Victoria Street.
A key part of the enabling works has been coordinating a total of approximately 250 services that need to be relocated, protected or abandoned.
This includes 700m of communications, 200m of gas, 550m of electricity, 300m of water, 150m of wastewater and 1,000m of stormwater. This work has required dynamic traffic management across various work fronts in the CBD, coordinated with construction staging, and complying with consent requirements.
Located in the Auckland CBD, this project presented a number of challenges, including:
- Ensuring public and pedestrians safety
- High-risk working in a constrained site (including plant and people interaction, lifting and working around high=risk services)
- Temporary support of high-risk services
- Maintaining traffic flow at all times along the work site
- Rigorous consent conditions
- Extensive stakeholder management
- Deep excavation (18m) in the middle of Auckland’s CBD
Temporary support of high-risk services
The extent of unknown utility services was a key project challenge. We have dealt with this challenge by employing a dedicated utilities manager, draftsman, site engineer and supervisor. A significant effort was made by the team to investigate and model all the utilities within the site boundaries. We did not rely on the standard ‘dial before you dig’ or utility providers’ records, as these were found to be largely inaccurate and only roughly indicative of where services existed.
To ensure minimal disruption to utility providers, we invested significant engineering and drafting resources in reviewing and interpreting the various sources of data, as well as performing targeted additional site investigation work.
The team then closely monitored and authorised all works that had potential to impact services, and amended and updated the model accordingly. For example, the design of the piling works had to be continuously updated as we discovered, tested and located unknown services. Approximately 50 of these were encountered.
To build the underground rail tunnels up Albert Street from Customs Street to Wyndham Street, a trench 13m wide by 18m deep at the deepest point (about the same height as a five-storey building) is being excavated. A ‘cut and cover’ method is used because the depth along Albert Street is too shallow for a Tunnel Boring Machine (TBM) and a number of underground utilities run down its length.
With the team having reached the deepest point of the trench, the twin tunnels are being constructed using custom-designed mobile formwork. These will be re-used for each 12m pour, before backfilling the trench and then completing the final streetscaping reinstatement works.
The road area on Albert Street is only 20m wide from kerb to kerb, and the consent conditions require an active traffic lane to be maintained on either side. The trench to be constructed is 14.25m wide externally (12.75m wide internally) and due to the required rail alignment, it is not in a consistent position relative to the kerbs. This meant that the team had no available working space outside of the trench position.
Other complications involved numerous side streets, which could be closed only temporarily and not at the same time, and numerous property driveways that had to be kept open. To reduce disruption, the team designed a suspended working space above the trench in the form of deck structures. To build this suspended working space, we adopted a complicated methodology that divided the work into dozens of different traffic configurations with hundreds of micro-activities to achieve everything that was possible within the tight geometric constraints.
The project is being delivered under a suite of resource consent conditions and designation conditions which cover social impact and business disruption, water and air quality, noise and vibration, and traffic access. Achieving the project target noise limit is particularly challenging with the limit being 75dB during daylight hours and 60dB at night. There are also requirements to keep a lane of traffic open in both directions and the footpaths on each side open at all times. To comply with these conditions, we had to carefully select the tools and equipment used and plan our works around the available spaces.
There are multiple sensitive businesses, residential towers and hotels on the project which were resistant to the works and frequently objected to works occurring close by. To develop and maintain trusted relationships with these local stakeholders, the team frequently goes above and beyond contractual obligations. Examples of this include stopping works for the lunchtime period in front of restaurants and other food outlets, or scheduling our works to avoid turning away business from licensed bars during important sporting events.
The team also adjust traffic control barriers, fences and hoardings to improve sightlines to businesses and their advertising, clean windows and frequently meet with various stakeholders to understand their concerns. They also made abundant use of the cafes, lunch bars and used restaurants and bars for frequent social engagements.
Innovations and solutions
Extensive forward planning has been key to construction progress, including regular coordination meetings with stakeholders, the consent holder (City Rail Link Limited) and regulatory authorities. These meetings provide an effective forum to update third parties on upcoming works, ensure that there are no surprises and manage expectations. Other innovations include:
- Piling around utilities with auger piling
- Customs Street intersection
- Suspended working deck and suspended pedestrian crossings over the trench. These have proven very popular with the public as viewing platforms.
Piling around utilities
The project required the installation of 362 piles in Albert Street, within ground highly congested with underground utilities. The piles needed to be installed extremely close to (within 100mm of) existing services. Maintaining the large clearance distances that would typically be applied on a greenfield or brownfield project (~1m) was not an option. The team had to approach utilities in a different manner and develop robust controls to manage this significant risk. We built a strong utilities team with in-house expertise and implemented a ‘permit to pile’ system, whereby a permit was issued for each individual pile location (as opposed to the standard ‘permit to excavate’ used on other projects that covers a much larger geographic area).
The team also performed a comprehensive review of the existing utilities data. A risk assessment looked at the different potential utilities that could be encountered at each pile location and determined an appropriate level of site investigations to be performed at that location.
The project also made use of a Cased, Continuous Flight Auger (CCFA) piling rig, imported from Italy. It provided both the size and the accuracy required to deliver immense piles to tight tolerances. The CCFA process ensured each pile hole was never left unsupported, which was essential to minimise settlement and improve verticality of piles.
Customs Street Intersection
The intersection of Customs Street and Albert Street is one of the busiest in New Zealand and a major thoroughfare for traffic coming into and out of the Auckland CBD. It is used by a quarter of central Auckland’s public transport passengers and thousands of workers, residents, tourists and shoppers every day. Minimising the duration of disruption caused here was extremely important to the client, City Rail Link Limited. We had to find a way to keep it functioning while carrying out the critical enabling works.
The project needed to construct a 12m-wide trench across the intersection and build the tunnel across it.
The original concept that City Rail Link Limited had envisaged, was to install reinforced concrete piles and headstocks on either side of the intersection and then build a temporary steel deck on top of this. The deck would be demolished at the end of the project, 2.5 years later.
We designed and implemented a better solution, whereby a permanent deck was built at the same level as the headstocks. By removing the need to demolish it at the end, we were able to reduce the duration of disruption at the intersection from 2.5 years to 11 months, and traffic kept flowing throughout.
Suspended working deck over the trench
Because Albert Street was to remain open to public traffic throughout the project, there was no space on either side of the trench for the vehicles and plant required to remove spoil and deliver concrete, steel and backfill materials. The team came up with an innovative solution to use the top level of steel holding the trench open to also support a large working deck (6.3m) suspended above the trench. The deck provides space for a portion of the public traffic lane and most of the construction space to ‘fly’ over the trench excavation and tunnel construction works.
Key to Success
The Infrastructure Sustainability Council of Australia (ISCA) awarded CRL C2 a ‘Leading’ design rating - the highest possible achievement in the scheme. The submission to ISCA consisted of design documentation produced through the ECI phase, including well prepared management plans, good policies and procedures, opportunities for energy reduction and strong stakeholder and Mana Whenua engagement.