Within the last five years, tunnelling equipment has steadily grown to become more effective in tougher ground conditions, mixed geology, and fractured rock.
International safety standards and equipment have become more sophisticated to maintain safety in these adverse conditions. Today’s tunnelling projects are longer, deeper, and more involved, and with new tunnels such as the proposed undersea link between Russia and Alaska, they are likely to keep pushing the limits.
Innovations in TBM design
Larger machines
Bigger machines are becoming a trend on more and more projects. Currently the largest machine is a 15.43 m diameter Herrenknecht EPB machine for an under-river tunnel in Shanghai, but this record will most certainly be exceeded.
In hard rock, the record was recently surpassed by the 14.4 m diameter Robbins TBM boring the Niagara Tunnel Project in Ontario, Canada. Larger machine sizes carry a new set of design challenges, including cutterhead vibration as the machine bores. Vibration must be kept to a minimum, and has not been a problem at Niagara so far. In addition, larger machines necessitate more complex systems to keep all the operations from the machine to the back-up to muck removal coordinated and running smoothly.
Larger TBMs are also being initially assembled onsite, rather than in a manufacturing facility, to reduce overall assembly times and costs. The Niagara TBM was assembled onsite in less than 12 months from the date of order, and began boring the project a full month early. More onsite assemblies are planned, including two 10m Double Shield machines that will bore a water tunnel for the AMR Project in India, and a 12.4 m diameter Main Beam TBM for a hydroelectric project in China.
Retractable TBMs
Multiple headings on a given project using one machine require that TBMs be designed for swift removal and re-launching. Several recent projects, including a 6.7m diameter TBM being assembled for the East Side Access Project in New York, USA, have made use of design changes such as bolt-only cutterhead segments to reduce the number of welds for rapid disassembly. Other components mounted to the perimeter of the TBM, such as the front, side, and roof supports, can also be removed or made hydraulically retractable so the TBM can be pulled from the tunnel and past any ring beams or other ground support structures. Specially designed transport dollies on rails are used to lift the TBM structure and slowly retract it to its new head.
Disc cutters
Record production rates in extremely hard rock up to 400 MPa UCS are a real possibility using larger diameter disc cutters. Disc cutters excavate material by penetrating rock and creating a “÷crush zone’ through which fractures propagate until the material between adjacent crush zones is chipped from the rock face.
Beginning in 2004, three Robbins TBMs for the KÌÁrahnj̼kar Hydroelectric Project in Iceland were designed with larger diameter, back-loading 19-inch cutters. The larger diameter cutters have higher roller bearing capacity as well as greater wear volume in the cutter ring. 19-inch disc cutters are capable of withstanding loads up to 311 kN, compared to the industry standard 17-inch cutters capable of a 267kN load on each cutter. A higher capacity roller bearing results in better penetration and higher excavation rates. The tip of a typical 17-inch cutter can be buried into a rock face up to 15 mm, while a 19-inch cutter has up to 20 mm of penetration, meaning that each time a 19-inch cutter rotates it has up to 33 per cent more penetration than the industry standard. The larger ring size has also substantially reduced the cutter change frequency by a factor of two or better.
The three Iceland machines bored a total of 39.5 km in varying geology including extremely hard but non-abrasive ground (up to 300 MPa UCS) consisting of Basalt, Moberg, and Pillow Lava. The average number of cubic meters excavated per cutter change, for all three machines, was very high. In total, the machines excavated 1,320 m3 per ring on average, using just 1,282 rings to bore the entire tunnel. More recently, one of the machines boring an additional tunnel on the project has turned in three world records in its size class of 7 to 8 m, including a record for boring 115.7m in 24 hours.
Several projects are beginning to use even larger 20-inch cutters. The Main Beam TBM at Niagara has been boring with 20-inch cutters since August 2006, and the two Double Shield TBMs for the AMR Project in India will use the same cutter type. The larger diameter cutters offer reduced costs for contractors since they have an even longer cutter life. More wear material results in fewer cutter changes and less down time during those changes while boring on a project, resulting in reduced labour costs. Even larger diameter disc cutters are possible, but they are necessarily limited by factors including the size of the cutterhead. Optimal cutter spacing to obtain efficient rock cutting is critical to high performance. If disc cutters become too large, optimal cutter spacing would not be possible on cutterheads below a certain diameter.
Unique cutterheads
It is becoming easier to customise cutterheads for optimal performance in specific ground conditions. Fractured or blocky rock is one potential problem, which can substantially reduce cutter life and detrimentally affect cutterhead wear. Large blocks coming from the tunnel face and crown can potentially impact the cutters with great forces that result in premature cutter disc failure. Smaller blocks can also wedge between the rotating cutter parts and the static cutterhead structure, resulting in cutter blockage.
The three TBMs on the KÌÁrahnj̼kar Project were designed to prevent against these situations. Specially shaped structures, known as rock deflectors, were designed and installed to keep blocks away from sensitive areas such as the leading edge of the disc cutter, with superior results.
Abrasive ground is another problem, and an even larger factor in cutter wear than rock hardness. The 6.7 m diameter Main Beam TBM for the East Side Access Project has been designed with abrasion-resistant wear plates on the cutterhead. These plates will protect the cutterhead from abrasive wear as it bores in schist and gneiss geology. Durable carbide buttons can also be used on TBMs in the gauge area, where abrasion is highly pronounced.
Monitoring progress from the surface
High-tech data acquisition systems are now becoming standard on many projects. These systems use real-time monitors to sample and log data on a number of machine parameters such as cutterhead power and speed, TBM advance rates, hydraulic cylinder positions and pressures, etc. For Main Beam machines, the system will record data on each TBM stroke. In Double Shield machines, such as the two TBMs for the AMR Project in India, more parameters will be monitored and data will be recorded per each ring built.
The data collected is relayed to computers on the surface to allow managers, owners, and consultants above ground to monitor machine performance underground. The data is also viewable in graph form to analyse trends over time. Optionally, access to the information can be granted by customers to allow selected parties to view machine operations via the internet anywhere in the world.
Testing the limits of geology
Probe drills
Boring in more extreme geology is driving the development of better geological investigation systems to check for conditions such as squeezing ground and pockets of water ahead of the TBM. TBMs can now be equipped with 360o probe and grouting systems to allow full face ground conditioning in advance of the boring. For example, a 5.6 m diameter Robbins TBM in Papua New Guinea will utilise probe drills that can bore ahead anywhere from 30 to 100 m to check for underground aquifers far ahead of the machine. The advance notice will allow engineers to determine if depressurisation of the aquifers is needed before the TBM reaches them.
Hybrid machines
Hybrid technology has been a long-standing goal in the tunnelling industry. Double Shield TBMs are an earlier example of hybrid capabilities. Invented by Robbins, the machines are designed to excavate and simultaneously line tunnel at high speeds through fractured rock, mixed face and hard rock conditions. The machines make use of principals from both open type, hard rock TBMs and shielded TBMs with segment erection capabilities.
More recently, mixed ground machines are taking TBM excavation to the next step. A recent 3.1 m diameter machine boring the Kavacik-Beykoz wastewater tunnels in Istanbul, Turkey, has the ability to be converted from an earth pressure balance machine (EPBM) with screw conveyor to a hard rock, single shield machine with conventional TBM conveyor. The design allows the machine to excavate low permeability soils with high silt or clay content under the water table, as well as hard rock. It has been very successful thus far.
Slurry Shields are another type of convertible machine used to excavate coarse sands and gravels, containing little or no clay or silt, under the water table. Slurry Shields can become open mode, hard rock single shield machines by fitting disc cutters on the cutterhead, adding a belt conveyor, and altering the cutterhead. The ability to convert to open mode machines is advantageous as slurry separation plant operations can be expensive and some slurry additives can be environmentally difficult to work with.
Safety gets priority
Rescue chambers
Rescue chambers are becoming more common on TBM projects and are required by some customers. These chambers, located on the back-up, are provided in case of a tunnel fire as a safe refuge for workers. Various models include up to 24 hours of oxygen, air conditioning and refrigerant systems, food, water, and bathroom facilities.
Back-loading cutterheads
Now required by law in Europe, back-loading cutterheads not only guarantee faster cutter changes, but also safer ones. The back-loading design allows workers to change cutters from within the machine, rather than in front of the face where dangerous rock falls often occur.
What’s next?
Universal machines
Universal TBMs, which can change modes from Double Shield to EPB to slurry, are the dream. Many projects require tunnelling below the water table in both soft ground and hard rock, necessitating machines that can be converted in the tunnel. However, there are many hurdles yet to overcome. The number of joints in Double Shield TBM designs makes them a challenge for use underwater, while soft ground would not support the gripper pressure required for Double Shield TBM advancement. The slurry/open mode rock shield mentioned above was developed for use in these situations but the method has proved costly and time consuming during conversions between modes. In addition, next-generation machines should have a method to allow for efficient cutter changes in underwater situations when the geology will not allow water to be purged from the cutting chamber.
Stronger steel
Better cutter ring steel is needed to support even higher cutter loads than the current maximum of 311 kN for 19 and 20 inch cutters. Higher TBM performance rates are limited by the maximum load that can be placed on the cutters, which is in turn limited by contact stress in the ring itself. The materials and processes used to manufacture large diameter hard rock cutter rings are pushing the very limits of modern metallurgy.
A more efficient total system
Efficiency in the back-up system, TBM, and conveyor design will ultimately allow projects to become more cost-competitive. Stations on the back-up system can be re-configured and many tasks automated, allowing for multiple functions to be performed by one person rather than several. For example, in recent years the job of the TBM operator has expanded to include management of muck train movement using cameras in the operator’s cabin and a system of stoplights. These types of significant savings can arise from well-thought-out changes in logistical design using existing technology.
While advancements in mechanical excavation have been dramatic in the past 25 years, the next years promise to hold even larger changes. Projects such as the massive AlpTransit scheme in Switzerland and the proposed Gibraltar undersea tunnel between Spain and Morocco will continue to pose challenges that must be answered with innovative tunnelling solutions.