Construction of the city rail link (CRL) tunnels under Auckland’s city centre is a major challenge requiring a robust & innovative approach to construction monitoring. Who does it?
Connectus, a joint venture between McConnell Ltd subsidiary Hawkins Infrastructure Ltd & McConnell Dowell Constructors Ltd, won the tunnel contract in April last year and Soldata Group Oceania has the job of monitoring the work to ensure surrounding structures aren’t damaged.
Hawkins Group chief executive Geoff Hunt said it was the first time in recent history that the 2 McConnell entities had worked together.
Malcolm “Buck” McConnell & Jim Dowell formed McConnell Dowell Ltd in 1961. The company went public in the 1980s and is now owned by Johannesburg company Aveng Ltd. Mr McConnell’s family got back into business buying Hawkins Construction Ltd and forming a group parent, McConnell Ltd. Group companies Hawkins Infrastructure & Harker Underground Construction Ltd are in the tunnel team.
Under rail link enabling contract 2, Connectus will construct the cut-&-cover rail tunnels under & along key transport corridors in the cbd. Harker will deliver the stormwater tunnel package, using specialist micro-tunnelling technology.
The monitoring expertise is held by Soldata, formed as Sol Data Ltd after a collapse above the Jubilee Line extension being built in London.
Connectus has engaged Soldata to provide assurance for Albert St’s property owners, insurers, Auckland Transport & the Connectus project team. Soldata’s remotely & robotically controlled laser system installed for the city rail link has 5 units & a network of 270 prisms (monitoring prisms, reference prisms & reflectorless measurement points) covering the length of Albert St, including Swanson, Victoria & Wellesley Sts, with sub-millimetre precision at up to 180m from their location.
It also monitors potential settlement of Albert St using “reflectorless” technology, which doesn’t involve any installation of prisms or other equipment. Evolving & adjusting to ongoing works, the network will expand to include 7 units & over 500 measurement points once cut-&-cover trenching works begin on lower Albert St.
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 & the prism is known. Successive measurements track the displacement of the prisms over time, and consequently give advance warning of potential ground & 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 & footpaths with active traffic. Monitoring data is available 24/7. If the defined threshold of movement is exceeded, an alarm email is issued immediately.
Other monitors also operating include inclinometers (which measure lateral change over the full depth of the installed borehole) & piezometers (which measure variation in groundwater pressure within a defined aquifer). Every piling shaft has these monitoring the movement of the retaining walls during excavation. Once excavation of the cut & cover begins, additional piezometers & inclinometers will be installed around the perimeter and strain gauges are to be installed on struts within the excavation.
To gain permission & access to buildings for the installation of the monitoring instruments, Connectus stakeholder manager Alan Howard-Smith has been communicating & co-ordinating site visits with Albert St’s building owners. He said: “In general, building owners & tenants have been very co-operative. We’ve been busy installing units on their buildings’ rooftops and fixing prisms to their canopies from SkyCity to the harbour’s edge. It’s given the owners reassurance that their safety & needs are being considered and that Connectus is tracking the impact of this construction every moment of the day.”
- The city rail link will use twin 3.4km-long tunnels up to 42m below the city streets to create an underground rail line linking Britomart and the city centre with the existing western line near Mt Eden.
- The build estimate is 5½ years at a cost of $2.5 billion
- It will have 2 underground stations at Aotea (11m deep) & Karangahape Rd (33m) & a redeveloped Mt Eden Station
- Most of the twin 3.4km-long tunnels will be built with a tunnel boring machine. The 7.5m-diameter tunnel boring machine will be about half the size of the one used at Waterview
- Connectus is delivering contract 2, one of the rail link’s first 2 contracts, involving the construction of 350m of twin cut-&-cover tunnels along Albert St, between Customs & Wyndham Sts. It also involves diverting an existing stormwater tunnel that runs under Albert St. The diversion will be constructed by pipejacking a 2m-diameter pipeline along the eastern side of Albert St between Swanson & Wellesley Sts.
Image above shows sample ground deformation monitoring at the Albert-Victoria Sts intersection.
Attribution: Company release.