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.
Event Name: Bauma CONEXPO India
Dates: April 20-23, 2021
Location: New Delhi, India
Come visit Robbins at Bauma CONEXPO India in New Delhi to get expert advice and information from our team of experienced staff. Stay tuned for more information on what you can expect to see from Robbins at the show.