The construction of the City Rail Link (CRL) tunnels underneath Auckland’s city centre is a major challenge requiring a robust and innovative approach to construction monitoring.
NEXT TO QUEEN STREET, Albert and Victoria Streets in Auckland have some of the most valuable real estate in the country. As the owner of a multimillion-dollar building on Albert Street, knowing that a 20 metre deep trench was going to be dug close to your foundations, you’d want reassurance that systems were in place to protect your interests.
Soldata, global experts in building monitoring in urban tunnel environments, has been engaged to provide assurance for Albert Street’s property owners, insurers, Auckland Transport and the Connectus project team, a joint venture partnership combining Hawkins Infrastructure and McConnell Dowell.
Soldata’s remotely and robotically controlled laser system installed for CRL has five units and a network of 270 prisms (monitoring prisms, reference prisms and reflectorless measurement points) covering the length of Albert Street, including Swanson, Victoria and Wellesley Streets with sub-millimetre precision at up to 180 metres from their location. It also monitors potential settlement of Albert Street using “reflectorless” technology, which does not involve any installation of prisms or other equipment. Evolving and adjusting to ongoing works, the network will expand to include seven units and over 500 measurement points once ‘cut and cover’ trenching works begin on lower Albert Street.
The units measure the prisms’ exact location using an invisible laser beam emitted from the unit and reflected by the prism. From this beam the exact displacement between the unit and the prism is known. Successive measurements track the displacement of the prisms over time and consequently give advance warning of potential ground and building changes adjacent to construction, as well as performance of construction techniques as movement trends become visible. The reflectorless technology also allows for ground movement to be monitored in roadways and footpaths with active traffic. Monitoring data is available 24/7. If the defined threshold of movement is exceeded, an alarm email is issued immediately.
There are other monitors operating as well including inclinometers (measures lateral change over the full depth of the installed borehole) and piezometers (measures variation in ground water pressure within a defined aquifer). Each piling shaft has these monitoring the movement of the retaining walls during excavation. Once excavation of the cut and cover begins, additional piezometers and inclinometers will be installed around the perimeter and strain gauges are to be installed on struts within the excavation.
To gain permission and access to buildings for the installation of the monitoring instruments Connectus stakeholder manager Alan Howard-Smith has been communicating and coordinating site visits with Albert Street’s building owners.
“In general, building owners and tenants have been very cooperative,” says Alan. “We’ve been busy installing units on their buildings’ roof tops and fixing prisms to their canopies from Sky City to the harbour’s edge. It’s given the owners reassurance that their safety and needs are being considered and that Connectus is tracking the impact of this construction every moment of the day.”
The urban environment is always presenting construction challenges, and Connectus and Auckland Transport are at the forefront of industry best practice, employing Soldata’s stringent monitoring technology to provide significant risk management innovations for the CRL project and its stakeholders.
Project Background
- CRL will use twin 3.4 kilometre long tunnels up to 42 metres below the city streets to create an underground rail line linking Britomart and the city centre with the existing western line near Mt Eden.
- It is estimated to take five and a half years to build at a cost of $2.5 billion.
- It will feature two new underground stations at Aotea Square (11 metres deep) and Karangahape Road (33 metres deep) and a redeveloped Mt Eden Station.
- Most of the twin 3.4 kilometre long tunnels will be built with a tunnel boring machine. The 7.5 metre diameter tunnel boring machine will be about half the size of the one used at Waterview.
- Connectus is delivering Contract 2 which is one of CRL’s first two contracts and involves the construction of 350 metres of twin cut and cover tunnels along Albert Street, between Custom Street and Wyndham Street. It also involves diverting an existing stormwater tunnel that runs under Albert Street. The diversion will be constructed by pipejacking a two-metre diameter pipeline along the eastern side of Albert Street between Swanson Street and Wellesley Street.
Parting words from Jeremy Sole- a final column