Back in the 1980s French engineers Gilles Piquereau, Remi Moreau and Maurice Debray were working on the TGV Atlantique rail link building two major 4.8km tunnels as part of the link between Palaiseau and Villebon-sur-Yvette.
Construction of the first tunnel was complex but the two TBMs boring either side of the tunnel struggled to deliver the capabilities required and needed a lot of manual adjustment. This made progress slow and the engineers decided that a new approach was needed for the second tunnel. The team replaced the two machines with just one 9.2m hydroshield TBM, one with more automation and the power to be able to be operated continuously. A new steering system was tested, relying on real-time analysis of data from the sensors on the machine’s thrust cylinders. This system meant the TBM could be controlled more precisely and could operate autonomously 24 hours per day, six days per week.
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By 1988 the system had proved itself so successful that it led to the creation of a new department called Conception d’Automate de Pilotage (CAP), which today continues to specialise in this type of excavation. “Most other systems use the position of the machine to make the steering while our system works with the difference of elongation of the push rams,” explains Lionel Milesy, one of two project engineers that run the CAP department at Vinci.
The TBM position calculated using the elongation measurements is then used to calculate the catching curve for the trajectory, determine the ring sequence, create the ring laying plan, and set steering orders for the ring excavation.
These steering orders are based on the horizontal and vertical difference between the thrust cylinder elongations.
“The ring programme software allows operators to pre-calculate in advance the next 20 rings according to the trajectory and generate a laying plan with the characteristics of the adopted catching curve, ring positions and associated steering orders for the pilot,” says Milesy.
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By GlobalDataAt the same time the continuous monitoring of the push ram elongation data means that the position and hence the catch-up curve can be tracked according to the TBM’s position ensuring it remains as close as possible to the excavation axis.
CAP also hosts pilot assistance software to assist in maintaining position. This constantly indicates on screen to the pilot a point representing the drift between the target position and the current position of the TBM. “It is a tool for the pilot to ensure good pressure distribution. The red cross should be in the middle. If he does it means he follows the steering instruction,” says Milesy.
At the same time, there is also the option of using an automated steering function where CAP, rather than the operator, ensures precise and accurate distribution of thrust in the different groups of cylinders in line with the steering orders.
Earlier versions of the CAP system did not have their own navigation modules but in 2008 CAP added their own. A motorized total station fixed on the tunnel lining determines the coordinates of two prisms on the TBM, using a reference prism whose coordinates are known. The coordinates of these two prisms are then used to calculate the TBM’s position. A bi-axis inclinometer determines the roll and pitch of the TBM. These data are used to determine the front position of the TBM and to check the accuracy of measurements from the theodolite on the two navigation prisms. The data are transmitted to the system’s computer to display the front and rear position of the machine on a screen in the control cabin.
A recent update to the system has been the use of the ring programme software for automatic calculation of the catching curve. Previously this had to be carried out by a surveyor who would set the curve according to the measured position, but with the increased data available and growing automation this can now be undertaken by operators. “We make automatic calculations so that everybody can use the software. We introduced this solution in 2014 in Doha,” says Milesy.
New Ground
Breaking through in October 2017, Glasgow’s 5km Shieldhall Tunnel waste water tunnel is the latest example of the successful implementation of the system. The GBP 100M (USD 140M) project for Scottish Water, carried out by the Costain/Vinci Construction Grands Projets joint venture (CVJV), saw a 4.7m diameter, 1,000t Herrenknecht machine tunnel at depths of up to 32m in mixed conditions.
“Ground conditions in Shieldhall had bedrock of mudstone or sandstone, a mix of both. The UCS can be variable from 10 up to 80 or 90MPa,” says Rafik Karaouzene, tunnel manager for Vinci Construction Grands Projets on Shieldhall. “This makes steering more difficult.”
The CAP system is good as with the ring laying plan it will give you position at each millimetre during excavation regarding the steering axis which is good,” he says noting that earlier systems simply use the navigation system to show the TBM position according to the project axis, meaning that the actual position of rings could only be measured once placed.
By monitoring the position of the thrust cylinders using sensors the pilot can also monitor the annular gap. “When central the gap is 35mm on each position giving a 70mm clearance. Regarding this value the ring builder is comparing this to the ring laying plan, because the ring laying plan is also using this value for each ring in the sequence,” explains Karaouzene.
In homogenous ground, this is less of a concern but for variable conditions progress at the face will vary, which is reported by the CAP system in real time ensuring that the rings are not built at too large a variance from the centre line.
Dig Data
As CAP has evolved the transformation to digital systems has revolutionised monitoring and reporting with all parameters such as torque, thrust, face pressure, pressure on the ground, temperature, pulling forces, shield articulation, all reported in real time and sent back to the above ground data centre. “I can look at the parameters at the same time as the operator sees down on the excavation. When he is in difficult ground an engineer can help monitor the parameters. Having two people can be very helpful,” says Karaouzene.
Other advantages include the ability to review all aspects of project performance retrospectively to readjust parameters on future schemes; and maintaining performance between shifts.
The ability to retain and review so much information means that each new project benefits from the experiences of those that went before. “We started with a small tool. It got bigger and bigger as we moved through each project. Every time pass a line you find another way to use the information after the line,” says Karaouzene, giving the examples of how torque, cutter head rotation and advance speed interact according to face pressure or how to improve performance of slurry treatment plants.
Another tool within the CAP system is the ability to integrate surface settlement data allowing the analysis of settlement as the TBM passes, reporting to the construction team in real time enabling operators to respond to any issues.
Recording manufacturing information about the segments is also possible as data from radio frequency identification chips, barcodes or QR codes can be connected into the database.
“When segments arrive on site an operator scans the segments, and then they are scanned when they arrive on a TBM and assigned to a ring number so you know where they are,” says Milesy.
There is also a tablet based tool for checking, analysing and creating a diagrammatic representation of any cracking.
Looking ahead CAP continues to evolve as the data is gathered from more and more sources and its engineers look for more opportunities to automate processes and optimise tunnelling.
“The best tool is not to have to intervene as a human with the ring laying plan but we still need to report clearance.
Anything which can avoid operative having to do something will be helpful,” says Karaouzene. “We are putting some trigger level on the data station so when you reach levels it sends you an SMS or alarm, for example if the face pressure is too high.”
Currently the system is used on all Vinci projects, and growth is likely thanks to projects such as the Grand Paris Metro Expansion, which will see 85 per cent of the new 200km metro underground.
Around 24 TBMs are expected to be in simultaneous operation by 2020, and Vinci is currently bidding for contracts, having been awarded the EUR 926M (USD 1.14bn) contract for 9.2km tunnel on Line 15 Sud.
“For projects like a metro where we need to follow a precise curve CAP is a useful tool,” says Milesy.
