What often gets lost in the gloss of large
construction projects are the small, precise elements that lead to the high-profile finish – the nitty gritty points and nuts and bolts that surveyors meticulously measure, set out and measure again to ensure buildings are straight, floors are level, tunnels are the right shape and railway lines are correctly aligned.
But surveyors are used to succeeding in the shadows – sometimes literally.
Nearly 40m below Auckland, the survey teams with contractor Link Alliance have been guiding the successful build of New Zealand’s first underground rail network for the past two years.
The largest transport infrastructure project ever to be undertaken in New Zealand, the NZ$4.4 billion (US$3.2 billion) City Rail Link (CRL) will build a 3.4kmlong twin tunnel underground rail link that runs up to 42m below the city center.
With the tunnels varying in depth along the alignment, two different construction methods are required: cut and cover where the tunnels are dug from the surface, and mined tunnels using excavators or advancing by tunnel boring machine (TBM).
Crews will also redevelop two existing stations (Mt Eden and Britomart) and construct two new underground stations (Aotea and Karangahape). When complete, CRL will carry up to 54,000 people an hour, moving the equivalent capacity of three Auckland Harbour Bridges or 16 extra traffic lanes into and through the city at peak times.
“As surveyors, we’re like the hunters and gatherers; we collect a lot of data and provide it to many teams,” says Sam Williams, surveyor manager with Link Alliance, a group of New Zealand and international companies. “And it needs to be good data because the success of this entire project rides on the exactness of our survey points and measurements. We have to be right all the time.”
To Williams and his survey crew’s credit, their survey marks have been right 100 percent of the time since they first arrived on site, in September 2019.
“With the accuracy, automation and user-friendliness of our gear, and the skills of our team, we’ve been able to work smoothly and quickly to keep everyone on track without sacrificing tight control, process or precision,” he adds.
UNDER CONTROL
The first critical element they needed to get right was the primary survey control network, which was no easy feat given the project extends through three suburbs and principal construction would run through the heart of Auckland’s built-up central business district. Unlike with previous projects where tunnel survey teams have set up their own control networks, Williams wanted to establish a high-quality primary control network that could also be used for the entire project.
Setting up on the summit of Mt Eden, 4km south of the business district, Williams used Trimble R10 GNSS receivers to verify the Land Information New Zealand (LINZ) continuously operating reference station (CORS) at the port of Auckland and established that as his base station. He then carried out static GNSS observations on three existing LINZ datums and verified those accuracies.
Satisfied with that foundation, he performed a series of four- to six-hour observations with R10 receivers to extend the GNSS campaign to marks more relevant to the project. They established two new marks at each of the three project stations visible from the summit of Mt Eden – two of which were at heights of 10 stories and six stories.
After the GNSS survey, Williams performed terrestrial observations with the Trimble S9 total station on the same intervisible marks to further improve the horizontal accuracy. He then input those measurements into Trimble Business Center (TBC) network adjustment software, to more accurately compute each station’s coordinates, ultimately creating a primary network baseline precise to 1-2mm.
For vertical control, Williams performed a 10-km level run with the Trimble DiNi 03 level, measuring from the base of Mt Eden, down through each station and back again with an impressive misclosure of 4mm.
“The S9, DiNi and TBC network adjustment were game changers for this,” says Williams. “The long-range fine lock allowed us to measure and establish a 3-km baseline, in a built-up city environment, with a single prism at 1-2mm accuracy. Pairing that with our vertical control results gave us the confidence that when we took that network down to street level and created secondary construction control for the guys on site, our tunnels and our stations were going to tie-in perfectly.”
In the tunnels, the teams have been setting out targets at set intervals – every 80m inside the TBM tunnels and at 50m spacing inside the mined tunnels – and then build a secondary network off those to fill the gaps between and provide control on both sides of the tunnel.
With those primary and secondary control networks, Williams has maintained site control at the surface and in the tunnels at 3mm or less.
TRACKING THE TUNNELS
Whether setting out, confirming earthworks, guiding machines, marking bolt locations or capturing as-builts, life in the tunnels has been ruled by cramped quarters and “Hurry up.”
“Nothing advances without our survey support or confirmation, but mine tunneling is very production focused so the tunnelers want you in and out as fast as you can,” says Darrell Wymer, Link Alliance’s tunnel survey manager.
“Once an excavation or shotcreting is deemed complete, we need to scan the cut and check that against the design model stored on our Trimble TSC7 controller to identify any underbreaks or overbreaks,” he adds. “From that scan we need to produce a point cloud to support the geologist’s geological mapping and create reports that show conformance to design for the construction, engineering and client teams – all in less than an hour. So, we’re under significant pressure to perform our work accurately without causing great delay to the tunnelers.”
They began to relieve some of that pressure in February 2021 when they acquired the Trimble SX12, a total station and scanner combination that can scan a full dome at 100m with a point density of 0.1m in 11 minutes, offers a 1.6-second total station measuring time in prism mode, and can pinpoint a 3-mm diameter spot at 50m with its green, eye-safe laser pointer.
“With tunnels you need both total station and scanning functionality,” says Williams. “With the SX12, we have both in one unit and that alone saves us hours in the field because we can do our set outs, volume calculations and as-builts from one set up. And because the scans are automatically geo-referenced, it saves us significant time in the office as well.”
The ability to perform multiple functions from the same set up has boosted their efficiency, precision and speed, particularly at Mt Eden and Karangahape, two of their busiest work sites.
For the Karangahape station tunnels, teams sunk a 25m diameter shaft 25m deep and then tunnelled 20m both east and west to provide launch points for excavating the station’s 278m-long rail tunnels, which will include 203m-long platforms. As the two excavators and road header advance, survey teams use the SX12 to guide the excavation, set out rock bolts, and confirm shotcrete layers.
“With the SX12, we can monitor the earthworks in real time and confirm excavation and shotcrete accuracy on the fly,” says Williams. “Once teams get to a certain advance point we set up and guide the last pieces using the instrument’s quick scanning and calculating functions in the TSC7’s Trimble Access software. When that’s done, we scan the finished excavation, compare it to the design model on our controller, and then scan the first layer of shotcrete. Then, with Access software, we can use the previous excavation scan to produce a heat map that’ll indicate areas that don’t have proper thickness and they can top it up with shotcrete. It removes any need for post-processing in the office and keeps us moving.”
At Mt Eden, speed and agility have been particularly critical. As the first launch point for the 130m-long TBM, teams first needed to create a 50m-long tunnel to set the TBM on the right course for its 860m-long journey to Karangahape station. In parallel, they began building three tunnels – each in different directions and at different heights – and constructing twin rail lines that will tie-in to an existing line. The functionality of the SX12 has enabled a two-person crew to handle the aggressive workload.
“The ability to seamlessly switch between surveying and scanning has cut the time we need to be at the face checking and collecting data by 60 percent,” says Wymer. “We can collect a polygon scan of a tunnel face in about 2-3 minutes, and that’s including photos. That speed allows us to be so much more responsive.”
The confidence in knowing they’ve captured all the points they need at each set up has also been a boon.
“Usually we’re about 10m-15m away from whatever we’re surveying,” says Williams “With traditional terrestrial total stations, we produce reports every 500mm in chainage and every 300mm of profile around the tunnel. Measuring each of those points manually takes a long time. With scanning, you pick up every surface in its entirety at 95 percent accuracy and you capture it all in minutes.”
ALL BY DESIGN
Equal to the versatility and efficiency of their instruments has been their software functionality, particularly the TBC tunnel module where all the design and as-built data comes together. Wymer has been using the specialized software to create three design profiles: for excavation, for the shotcrete lining and for the final concrete lining, respectively – a process repeated for each tunnel section. All of those models are exported to the controllers for field crews to support the excavators and roadheader guidance systems and to guide, monitor and verify all the construction work, like installing the rock reinforcement layers.
Sometimes, because of geology and the dynamic nature of tunnel work, Wymer has had to be ready to respond to urgent requests for design model changes.
“There have been times when they’ve hit bad rock and the geologists have called for a design change,” says Wymer. “That means we have to generate a new design by the time the next cut is complete. On a good day we may have six hours to make changes to the profile and adjust bolt positions but if the call has been made overnight, we’ll often only have an hour to turn it around. With the scan detail and the automation and bulk data import features in TBC, we can meet these tight timelines so crews can keep working at speed.”
The crews received their first major validation that the CRL is on track on 16 October when the TBM broke through at Karangahape station – right on target. It was a significant milestone and nod to Williams and his survey crews, giving them the confidence that their process, precision and tight control will serve the team well in their drive to complete the project by 2024.
“I’ve never been on a project of this scale, for this length of time and not had some sort of survey incident,” says Williams. “That’s a great testament to our team and our technology.”
It’s also a testament to survey crews who sweat the small stuff to achieve big rewards for tunnel projects.
