Difficult ground doesn’t begin to describe the challenges overcome at a recent tunnel in central Turkey. The breakthrough of a 5.5 m diameter Robbins Crossover XRE TBM at the Gerede Water Transmission Tunnel was a feat of modern construction. The 9 km leg was the final section of the 31.6 km long water supply line bored through what is widely considered to be Turkey’s most challenging geology: from 48 fault zones to water pressures up to 26 bar, the ground put the machine and the crew to the test.
Listen in on the conversation with Robbins Vice President Doug Harding as we find out how the unique TBM design and experienced crew overcame a gamut of challenges.
Excavation in mixed ground conditions is always a challenge, but under a densely urban environment the stakes become even higher. At India’s Mumbai Metro, two 6.65 m hybrid-type rock/soft ground Single Shield TBMs are successfully boring parallel 2.8 km tunnels in basalt rock with transition zones of shale, tuff, and breccia below the city. They have made intermediate breakthroughs at the 1.2 km mark and overcome rock strengths up to 125 MPa UCS with significant water ingress, all just one year after factory acceptance, shipping, site assembly, and launch. The hybrid machines are optimized for abrasive rock geology using a robust cutterhead mounted with disc cutters and a reinforced screw conveyor at the centerline. The machines can also operate in closed or semi-closed mode using features designed to advance in soft ground with water inflows: dual ratio gearboxes to adjust cutterhead speed and torque to the geology, screw conveyors with bulkhead gates and discharge gates, ground conditioning with foam and polymers, and probe drills for pre-excavation grouting.
TBM Excavation in Himalayan Geology: Over 1,200 Meters per Month at the Bheri Babai Diversion Multipurpose Project
A Double Shield TBM achieved in 17 months what was projected to have taken 12 years with Drill & Blast: The 12.2 km long Bheri Babai Diversion Multipurpose Project (BBDMP). Bored in Himalayan geology including sandstone, mudstone, and conglomerate, the excavation was able to achieve over 1,200 m advance per month on multiple occasions. Crews achieved this while traversing a fault zone and getting through one section that required a bypass tunnel constructed in just five days. The success of this tunnel is not only in breaking through a historically difficult mountain range, but also in changing the notion, to the people of Nepal, that drill and blast is the way to excavate mountainous rock tunnels.
Tunneling through 48 Fault Zones and High Water Pressures on Turkey’s Gerede Water Transmission Tunnel
The December 2018 breakthrough of a 5.5 m diameter hybrid-type Single Shield/EPB TBM at the Gerede Water Transmission Tunnel in Central Turkey was a feat of modern construction. The 9 km leg was the final section of the 31.6 km long water supply line bored through what is widely considered to be Turkey’s most challenging geology. The project was originally started with the contractor selecting three Double Shield machines, which were procured and supplied without Robbins involvement. When two of the machines became stuck and were unable to continue, the solution of the hybrid-type TBM was developed to complete the rest of the tunnel. The TBM was assembled and launched more than 7 km from the tunnel portal and successfully navigated 48 fault zones as well as hydrostatic pressures up to 26 bar.
In April 2019, a 3.5 m diameter open-type, Main Beam TBM and its crew broke through at the Galerie des Janots Tunnel in La Ciotat, France after encountering two large, uncharted caverns. The 2.8 km long tunnel, excavated in limestone known to have groundwater, karstic features, and voids, took two years to complete due to the challenges encountered. Limestone and powdery clays made for slow going early on in tunneling, until a cavern measuring 8,000 cubic meters in size was encountered on the TBMs left side at the 1,035 m mark. The crew had to erect a 4 m high wall of concrete so the TBM would have something to grip against—a process that took about two weeks. The first cavern, while the largest, was not the most difficult void encountered. At the 2,157 m mark, crews encountered a 4,500 cubic meter cavity extending directly below the bore path. This cavern required stabilization, filling, six bypass galleries, and four months of work to get through.
On July 24, 2020, a jubilant ceremony marked a milestone for southern Turkey’s arduous Bahçe-Nurdaği High-Speed Railway Tunnel. The first TBM-driven portion of tunneling using an 8.0 m (26.2 ft) diameter Robbins Single Shield machine is now complete. The 8.9 km (5.5 mi) long TBM tunnel was no easy bore, as it was excavated through some of the hardest and most abrasive rock ever encountered in the country.
“We are proud of the TBM crew who acted rapidly and were well organized to overcome the challenging ground conditions with a unique Single Shield TBM for the completion of the first tube of the Bahçe-Nurdağı Railway Project,” said Deniz Sahin, TBM Chief for contractor Intekar Yapi A.Ş.
Ground conditions during tunneling ranged from abrasive, interbedded sandstone and mudstone with quartzite veins to highly weathered shale and dolomitic limestone. The TBM encountered rock measuring between 136 and 327 MPa (19,700 to 47,400 psi) UCS. Water ingress with fines was expected in fault lines and shear zones affected by the East Anatolian Fault. “The TBM became stuck in three different fault zones, which we got through by building bypass tunnels. In smaller fault zones, we encountered excessive material flow and the TBM had to be stopped, while ground had to be stabilized with chemical injections while we cleaned the cutterhead,” said Sahin. Water inflows of 10 liters per second on average were removed using a dewatering system.
The majority of tunneling, said Sahin, was in metasandstone with quartz, with an average of 220 MPa (31,900 psi) UCS and a Cerchar abrasion value of 3.87. In such regions, the TBM’s 19-inch back-loading disc cutters had to be changed frequently and there was high vibration. Despite the challenges, Sahin was impressed by the machine’s overall capacity: “The Robbins Single Shield TBM’s motor power, hydraulic power and cutterhead torque were quite strong. The secondary ventilation and air suction systems inside the TBM were powerful. The connections between the gantries, scaffolding systems, walkways and working areas were good.”
The TBM ultimately achieved up to 456 m (1,500 ft) per month, a result achieved with the help of a Robbins continuous conveyor system for muck removal. “The electric motor and gearbox capacity of the conveyor system was quite enough for a 10 km (6.2 mi) tunnel and we had no failure on them. The conveyor performed well even under excess material and the whole system was quite robust,” said Sahin.
The owner, Turkish State Railways Directorate (TCDD), is aiming to overhaul the railway connection in southeastern Turkey by providing a shorter, faster route in one of the country’s busiest railway corridors. The new rail line between the towns of Bahçe and Nurdağı includes two parallel 9.8 km (6.1 mi) tunnels being excavated by both NATM (850 m / 0.5 mi) and TBM (8.9 km / 5.5 mi).
Nepal’s first TBM-driven tunnel was a success by any standard: The Robbins Double Shield machine bored up to 1,200 meters a month and finished the Bheri Babai Diversion Multipurpose Project nearly a year early. Listen in on our conversation with Brad Grothen P.E., Robbins Technical Director, and Missy Isaman P.E., Robbins Project Engineer, as we discuss the challenges, lessons learned, and recommendations for future tunnels in mountainous geology.
Come visit Robbins at the BTS Conference & Exhibition in London, stand B26 and hear relevant talks on industry hot topics while getting expert advice and information from our team of experienced staff. Check back here for updates as the conference date approaches.
Nepal’s first TBM-driven tunnel was a success by any standard: The Robbins Double Shield machine bored up to 1,200 meters a month and finished the Bheri Babai Diversion Multipurpose Project nearly a year early. But how were crews able to bore so quickly? And what made the contractor and owner ultimately decide to use a TBM for the first time?
Watch our complimentary 30-minute webinar with Brad Grothen P.E., Robbins Technical Director, and Missy Isaman P.E., Robbins Project Engineer, as we discuss the challenges, lessons learned, and recommendations for future tunnels in mountainous geology.
To Grout or Not to Grout? In Rock Tunnels encountering High Water Pressure, Grouting can offer Great Benefits over Slurry
When you’re faced with a hard rock tunnel where there are expected significant sections under high water pressure, which tunneling method do you choose?
While Slurry Shield tunneling has a long history of addressing this problem, this method has not always been problem free. I would argue Slurry tunneling in rock is not, in most cases, the lowest risk or the most cost-effective method.
At recent projects around the world, we have seen that two non-slurry methods can be highly effective: use of a shielded, Non-Continuous Pressurized (NCP)-TBM in rock with a comprehensive grouting program, or sequential advance in EPB mode. Both types of tunneling operations have proven themselves safe, and have saved a significant amount of money for both contractors and owners.
Grout vs. Slurry
There are certain inherent traits to a Slurry tunneling operation that appear to give a lower level of risk: the entire operation is sealed; the slurry itself is conveyed to the surface through a system of pipes. But is this low risk truly the case?
Hyperbaric interventions are high-risk operations, particularly as water pressures go up. In water pressures over 6.5 bar, divers are often not permitted to enter the cutterhead, so grout must be used or there must be an alternate plan to bring down the high pressure. Higher pressure hyperbaric interventions up to approximately 12 bars have been successfully performed, but at what risk? Pressures in some tunnels have far exceeded 12 bars and would make hyperbaric interventions even more costly, risky and time consuming or impossible.
In ground with fines, slurry separation can be costly and difficult. Slurry tunneling is also not immune to problems such as blowouts/loss of face pressure when a fault zone or low cover zone is encountered, as is well-known in our industry from projects such as Hallandsås in Sweden and the SMART Tunnel in Malaysia.
Probing and Grouting
In an NCP-TBM operation, crew members may be more exposed to the tunneling environment but risks are not increased. With a good geotechnical baseline report and ground investigation tools, contractors can determine the zones requiring grouting ahead of the machine. It is now common to drill probe holes accurately of plus 100 meters with Down-the-hole (DTH) drills.
While grouting does take time and cost money, this cost has to be balanced against the cost and time to do hyperbaric intervention during slurry tunneling. Even 100% grouting in a rock tunnel could require less time than high-pressure hyperbaric interventions. The practice of pre-grouting has been done for years in drill & blast rock tunnels in Scandinavia and worldwide.
This video shows the basic process of probe drilling and grouting in a shielded rock TBM.
Grouting can also be done from a Slurry TBM of course, and is normally done to set up safe zones. However, it is worth noting that based on having a pressurized face filled with slurry, drilling through the head is very difficult. Sealed pipes/ports need to be installed in advance, eating up space and compromising the working conditions during hyperbaric interventions.
There has been recent development to enact cutter changes by accessing the cutters through the cutterhead under atmospheric pressure. However, this system requires a large diameter machine as well as a deep cutterhead structure. The deep structure severely affects muck flow and substantially increases the need for more frequent inspection and cutterhead repairs. These atmospherically accessed cutterheads do not address the problems of cutterhead repair, changing center cutters, or replacing scrapers, all of which are high wear items in rock tunneling at large diameters.
Are there times when a Slurry TBM has an advantage over an NCP-TBM in rock? Yes. Rock properties can drive the decision: Some rock formations are very difficult or nearly impossible to grout, and therefore the success of pre-excavation grouting will not be a given. If significant water inflows are predicted and the rock will not readily take common grouting material, or chemical grouting is not an approved option, a slurry TBM is the logical TBM selection.
Lining requirements are another potential reason not to go with Slurry: The operation of a slurry TBM goes hand-in-hand with the use of an (often expensive) segmental lining. Pre-excavation grouting using an NCP-TBM offers tremendous cost savings when done in a non-lined tunnel or when the liner can be installed independently after excavation.
In cases where a final liner has to be installed with tunnel boring, and often in cases where excessive water inflows are predicted, a slurry TBM may make more sense. Under excessive water inflows a grouting operation may still experience leakage after the initial tunnel construction, making installation of a final liner afterwards potentially costly and time consuming.
In Depth: Slurry Tunneling vs. NCP-TBM Shielded Tunneling in Rock
Cutterhead inspections in rock must be viewed with a different mindset than in soft ground tunneling. When tunneling in abrasive rock with any type of machine, inspections should be performed regularly; once per shift can be a requirement. This is in contrast to tunneling in soft ground, where cutterhead inspections are often planned and based on a set number of meters, for example every 100 m.
The Contractor Experience
Contractors who are used to tunneling in soft ground may not realize that when using a Slurry TBM in rock, inspections must be frequent due to increased cutter consumption. We have seen this borne out on recent projects such as the Hiroshima Expressway Line 5 in Japan. On that project, a 13.7 m diameter Robbins Slurry TBM is boring in granitic rock. The contractor opted for a Slurry machine because that was their historic experience, and they were expecting up to 13 bar water pressure. This high pressure water zone was only in a small section of the overall tunnel length, about 5 percent.
The contractor in Hiroshima had grouted off from the surface a planned safe zone in which to inspect the cutterhead without requiring a hyperbaric intervention, but this strategy did not go according to plan. The abrasive rock damaged the cutters and cutterhead before they could reach the safe zone, resulting in unplanned delays.
By far the biggest benefit of using a shielded NCP-TBM in rock, rather than Slurry, is the ease of cutter and cutterhead inspections. In areas with no pressure and with frequent or continuous grouting, the cutterhead can be inspected regularly and without the requirement of expensive, time consuming, and often risky pressurized interventions or complicated procedures to remove slurry from the cutterhead. Frequent inspections mean that cutter and cutterhead damage can be caught early before they cause significant downtime.
To go along with the above point, abrasive wear in any type of TBM is obviously always higher in rock than in soft ground, particularly when the rock has a high quartz or other abrasive mineral content. However, in Slurry machines, which crush the rock and send the rock chips through a system of pipes, abrasive wear is of even greater concern. Even with using durable slurry piping, transfer points and pipe elbows will require higher rates of replacement, causing more delays associated with muck removal than a typical NCP-TBM operation using a conveyor belt.
Dealing with Water Inrushes
If sudden water inrushes at high water pressure are a known risk, NCP-TBMs can effectively be designed to statically hold the pressure using sealable muck chutes in the bulkhead. This type of design can be used as a pressure-relieving gate in semi-EPB mode, opening by pressure and allowing muck to be metered out onto the belt. Or in extreme cases, the sealed gates can be activated and probe/grout drills can be used to forward drill and grout for ground consolidation and to seal off the water. Extra seals around the main bearing can be filled with pressurized grease and other vulnerable points can be sealed off in the same manner.
A Crossover TBM can also be designed to keep boring under pressure by implementing a center-mounted screw conveyor. A long screw conveyor can be used to draw down high water pressures and abrasion resistant hard facing can be added to the screw conveyor flights for abrasive wear. Under such conditions, a machine could operate continuously with, say, 3 bar pressure and sequentially in high pressure of 15-20 bar. An example of this is our ongoing project Mumbai Metro, where two Robbins Crossover TBMs are excavating in mixed ground. In these machines, the center screw conveyor is able to seal itself off/hold pressure so the TBM can continuously bore or operate using the screw conveyor in a sequential fashion. Boring is done when there are not enough fines to form a plug. Take a look at the below figures for a summary of sequential boring operation.
Dealing with Gasses and Contaminated Ground
In Slurry tunneling, dealing with gasses in the tunnel is relatively easy because the gasses are contained in the slurry pipes. Gasses can also effectively be contained and safely dispersed on non-pressurized TBMs using scrubbers and high volumes of air. On a recent Robbins TBM in Australia a machine was capable of operating in open mode with gasses using a bulkhead fitted with suction ports to draw any gas from the top of the cutterhead chamber and directly into a sealed ventilation system.
Contaminants such as asbestos may be better contained in slurry pipes, but many other types of contaminants may not be easily separated from the slurry and therefore easier to deal with using NCP- TBMs. In Slurry operation the quality of Bentonite itself can vary widely, with some lower cost material containing heavy metals, which has the potential to be detrimental to the environment. The slurry solution itself also tends to bind well with heavy metals, contaminating the slurry and making separation difficult.
The conclusions to draw from this discussion are straightforward. Is Slurry tunneling still a valid option in rock with potential of high water pressure? The simple answer is yes. Is it the most cost-effective option? Is it safer than any other option? In many circumstances the answer is no.
Slurry TBMs need a level of expertise in operation that NCP-TBMs simply don’t require. The operation of most NCP-TBMs is both simple and straightforward, which in turn saves on personnel costs.
My hope is that consultants and owners realize that Slurry TBMs are not the only option when high water pressure is expected. Slurry TBMs are not in most cases the lowest cost, and other methods can be just as safe while being simpler to operate. While grouting takes time, so does slurry tunneling with its typically lower advance rates and possible need for expensive, high risk hyperbaric interventions. When Slurry machines operate in rock, the need for frequent cutterhead inspections ultimately makes their use questionable. In most cases NCP-TBMs are the better option.
For Further Reading: Recent Industry Examples
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- The Next Push: Robbins TBMs on the Move Around the Globe
- To Grout or Not to Grout? In Rock Tunnels encountering High Water Pressure, Grouting can offer Great Benefits over Slurry