Delivery of New Zealand’s record breaking City Rail Link project through complex ground conditions is being eased through use of digital ground model solutions.
Auckland’s City Rail Link (CRL) project is the largest infrastructure project ever to be undertaken in New Zealand. It is a 3.45km twin-tunnel underground rail link up to 42m below the city centre, transforming the downtown Britomart Transport Centre into a two-way through station that better connects the Auckland rail network.
The total cost of the project is estimated to be around NZ$3.4bn (£2.2bn) and is jointly funded by the government and Auckland Council. Aurecon was awarded the role of principal technical advisor (PTA) and is working alongside a consortium of engineers and architects on its main responsibility is to deliver the reference design.
Ground risk is a critical factor on any major infrastructure project and has a direct impact on time and costs – and CRL is no different. Aurecon needed to do a detailed technical analysis of the ground conditions to identify the risks more clearly and then be able to communicate and mitigate these risks. Aurecon has used Seequent’s 3D geological modelling solution – Leapfrog Works to enable the team to achieve this.
Construction of the rail link is now underway and the completion date is set for early 2024.
This is a vital project for Auckland as population growth is averaging 2.9% every year and the city’s transport infrastructure needs to accommodate this growth. During the past decade, Auckland’s rail commuters have increased from 2.2M trips a year to 19M.
As well as the construction of the tunnel, the CRL adds two new underground stations – Mount Eden and Karangahape Road. When finished, it will allow trains to pass through most Auckland stations every 5 to 10 minutes at peak times.
Put simply, the new CRL will double the number of trains on the network, double the capacity of people on trains to 30,000 an hour and double the number of people living within 30 minutes of the city centre.
The scale and location of this project poses challenges.
The geology in the Mount Eden area of the project is particularly complex. There are three volcanoes with quite different characteristics to the north and south of the rail alignment. Over time, the volcanos have produced ash and lava deposits, which are mixed in with soft sediments from historical swamps and ponds, hard basalt rock, and sandstone.
Aurecon’s team undertook an extensive drilling programme to provide information on these underlying ground conditions.
“Understanding the ground conditions and the insights gained through the drilling programme are crucial in terms of progressing and refining the design on the CRL in the next 18 months,” says Aurecon geotechnical team leader Philip Kirk. “We have already unearthed volcanic rock, soft sediment and reclamation material along different parts of the CRL route.”
Over 200 boreholes have been drilled in various stages of this project, recovering almost 5km of core. Historical records of over 1,000 boreholes with over 21km of core from around the wider project area have also been used to understand and model the ground conditions.
Engineers and construction teams need to see how these ground conditions interact with structures so they can make informed decisions on structural design, services relocations and construction methods.
“Leapfrog Works has been an instrumental tool to support our ground engineering team, who are the interface between science and engineering,” says Kirk. “Geologists need to be able to communicate the uncertainty in the ground conditions in a way that the engineers understand, relevant to the design.”
Aurecon decided to use Leapfrog to assist in the detailed assessment of ground conditions. According to the consultant, as it transforms into a design-led organisation, rather than a traditional cost-out engineering company, it is technology like Leapfrog Works that supports its mission. Built specifically for the civil engineering industry, Leapfrog Works provides a complete solution for fast and dynamic modelling of ground conditions, analysis of the interactions with the engineering design and intuitive 3D visualisation.
Aurecon prepared 3D geological models for parts of the CRL alignment and they used Leapfrog Works to deliver these geological assessments.
crl complex geology
“A lot needs to happen in order to construct the tunnels in the area with the most complicated geology on the project,” says Aurecon engineering geologist Anita Nagy. “This includes relocating stormwater and bridges and constructing several new structures within a tight corridor.
“Leapfrog Works was really able to support the ground engineering team by facilitating better communication and interaction with our engineering design colleagues. The team were immediately surprised by how effective it was as a communications tool. This richer output of information is a vital tool for communication with project stakeholders. But even more importantly it’s about getting the designs right, from the start”.
Leapfrog Works has helped Aurecon identify key opportunities in the reference design, that will ultimately save time and money over the course of the CRL project.
For example, shaft design details and ground improvement works at the Water Street shaft were revised and optimised to mitigate the ground risks in the area. This was an essential refinement to avoid project delays associated with inappropriate construction methods.
Aurecon project director for the PTA Tom Ireland says: “I have found the 3D geological model an invaluable tool used to communicate ground risks to both stakeholders and contractors during the procurement phases. Better understanding of the ground conditions results in much better time and cost certainty.”
A great benefit of Leapfrog Works is the dynamic nature of the application. The Aurecon team were able change the ground model immediately, based on the live data coming from the onsite shaft borehole drilling. These changes to the model were then communicated quickly to the designer, who changed the types of pile to be used, thus saving the project team days and making sure the structure was as accurate as it could be.
Leapfrog product manager Pat McLarin adds: “We are really pleased that Leapfrog Works has been embraced by Aurecon on this major infrastructure project. The visualisation of the models is key and what that means in terms of risk and decisions to be made on the design. This has been really important to Aurecon’s ground engineers so that they can communicate more effectively to such a diverse group of stakeholders.”
As work continues on the CRL project, Aurecon continues to use Leapfrog Works to deepen its understanding of the ground conditions and communicate changes and risks to the rest of the project team.