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Long-distance tunneling; Challenges and trends of large projects in Europe

FIG. 1
BBT – Overview of the TBM tunneling lots.

FIG. 1-BBT - Overview of the TBM tunneling lots.

Mechanized tunnel construction has enormous potential to drive innovative developments and technologies in underground mining in the future. One factor is the steadily increasing need for mobility around the world, and the trend toward urbanization that makes it necessary to move more infrastructure underground and to connect the surrounding regions with central points and cities, and to create corridors of connectivity. On the basis of highly specialized solutions, know-how and valuecreating interaction between humankind and technology, technological progress in the planning, construction and implementation of modern infrastructure in varied and difficult terrain is constantly striving for new records. New tunneling length records are set with mega infrastructures such as the railway projects Gotthard Base Tunnel, Brenner Base Tunnel and the High Speed Two railway systems currently under construction in Europe. These are corridors of connectivity and transnational supply chains that reshape our social and physical worlds toward a borderless world. Numerous ambitious, long-distance tunnel drives are in the planning and implementation phases. This article focuses on challenges, current trends and innovations in mechanized tunneling, with exemplary projects that represent a multitude of specific geological and hydrogeological challenges at the highest level, and with regard to safety of operation.

Long-distance tunneling projects

Brenner Base Tunnel. The Brenner Base Tunnel (BBT) with a total length of 64 km (40 miles) will be another “project of a century” and the longest railway tunnel in the world since the 57-km (35-mile)-long twin tube Gotthard Base Tunnel (rail connection between Switzerland and Italy) that was commissioned in 2017.

The BBT will be the centerpiece of the Scandinavian- Mediterranean TEN-T Corridor from Helsinki (Finland) to Valetta (Malta) and will contribute to sustainable transnational mobility. It is a twin-tube single-track rail tunnel (internal diameter, ID = 8.1 m or 26 ft) with a service and drainage gallery (ID = 5 m or 16 ft) in between that is about 12 m (40 ft) below the main tunnel tubes over its entire length. The service gallery enables ground improvement measures to be carried out for the construction of the main tunnels and reduces the extent of preexploration measures from the main tunnel tubes. The rail link, connecting the countries of Austria and Italy, follows the highest safety standards for tunnels with the main tubes linked every 333 m (1,100 ft). In emergencies, these cross passages will be used as escape routes.

Construction of the transnational century project started in 2007. More than 75 km (46 miles) are being built using a total of four tunnel boring machines (TBMs) designed and manufactured by Herrenknecht in Germany.

One gripper TBM with a diameter of 7.91 m (26 ft) has been excavating the exploratory tunnel from Austrian Ahrental toward the Italian border in predicted loose to friable rock mass with squeezing rock zones. The gripper TBM started tunneling on Sept. 26, 2015 and excavated about 16.7 km (10.3 miles) southward toward Italy, completing its work on July 6, 2019. In solid rock, the gripper TBM performed with daily top performances of 61 m (200 ft) and monthly performances of 825 m (2,706 ft), while regular weekly performances of around 200 m (650 ft) were standard over many months. Extremely difficult rock formations with highly squeezing and swelling rock conditions with extensive invert heave along with encountering fault zones, sometimes with large-volume rock collapses, were some of the challenges along the exploratory tunnel section. The TBM advance rates were determined in these conditions largely by the high degree of rock support required in the L1 and L2 area of the gripper TBM. In the L1 area directly behind the gripper TBM anchors and steel mesh were standard except in extremely difficult rock formations that required shotcrete and 360° TH arches as initial rock support that were replaced with U-profiles in the course of further advances. The shotcrete equipment in the L2 area was used regularly and required greater availability. The applied gripper TBM technology for that exploratory tunnel section was proven technology and was already successfully used in numerous other complex projects in the Alps such as the Lötschberg rail tunnel with a bored tunneling length of 20 km (12 miles) and Gotthard Base tunnel with a total of 85 km (52 miles) of bored tunnel length. New in the evolutionary stage of this technology were systems such as the disc cutter rotation monitoring (DCRM) that helped to optimize tool maintenance intervals and thus improvements in tunneling in terms of performance, safety and quality.

FIG. 2
Exploratory tunnel Tulfes-Pfons, gripper TBM, and rock support due to prevailing convergence.

FIG. 2-Exploratory tunnel Tulfes-Pfons, gripper TBM, and rock support due to prevailing convergence.

For the excavation of the 48-km (30-mile) main tunnel tubes of the BBT, two large-diameter double-shield TBMs (Ø10.65 m or 35 ft ) were employed. One smaller diameter double-shield TBM (Ø6.82 m or 22.3 ft) excavated an additional section of the exploratory (and later drainage and maintenance) tunnel over a length of 14 km (8.7 miles). The three double shields were applied to excavate and line part of the Italian construction lot “Mauls 2-3.” TBM supply for the mechanized tunneling lot of Mauls 2-3 is supported by train and mucking out by tunnel belt. In March 2021, a new record was set by one of the doubleshield TBMs (Ø10.65 m) that tunneled a section of 14 km (8.7 miles) in geology that is mainly characterized by Bünder schist with locally unstable tunnel face conditions. The TBM is operated by the contracting joint venture BTC S.c.a.r.l. (Astaldi S.p.A., Ghella S.p.A, P.A.C. S.p.A. and Cogeis S.p.A). The team achieved performances of up to 860 m (2,821 ft) in one month with an average daily advance rate of 27.7 m (90.1 ft) and peak performances of up to 36.75 m/d (120.5 ftpd).

FIG. 3
High Speed Two railway line. Overview of alignments, phase one.

FIG. 3-High Speed Two railway line. Overview of alignments, phase one.

High Speed Two (HS2). High Speed Two (HS2) is a new national high-speed rail network north of London currently under construction in the United Kingdom. The new railway line will address the rising demand for intercity journeys, commuting and freight rail transport. Phase one of HS2 is currently under construction and will run between London and Birmingham. Phase one comprises more than 100 km (62 miles) of tunneling. It is built to ease congestion on the West Coast Main Line. Phase two of the new high-speed rail line will focus on the connections further north, from Birmingham to Manchester and Leeds.

Phase one comprises six twin running tunnels with inner diameters of 8.8 m (29 ft) spaced approximately 20 m (65 ft) apart (center line to center line). Tunnel sections were introduced to improve the environmental impact and were thus defined beneath built-up areas where disruption at the surface would be severe. The tunnels connect to portal structures at either end and are connected along their lengths by cross-passages and ventilation shafts. The route alignment of Phase one comprises long tunneling sections such as the twin-bore Chiltern Tunnel that extends over a length of approximately 16 km (10 miles) and is thus one of the longer tunneling sections along the route. Two variable-density TBMs (Ø10.24 m or 33.6 ft) are being used to excavate the twin tunnels that are mainly driven through firm to hard chalk. Locally very high permeability in chalk and the likelihood of karst formation were predicted as well as the presence of minor layers of marl and nodular flint.

FIG. 4
Chiltern Tunnel, variable density TBMs (Ø10.24 m) with hydraulic mucking.

FIG. 4-Chiltern Tunnel, variable density TBMs (Ø10.24 m) with hydraulic mucking.

The design and construction work for the Chiltern Tunnel was contracted to the Align Joint Venture (JV) of Bouygues Travaux Publics, S.A.S, Sir Robert McAlpine Ltd. and Volkerfitzpatrick Ltd. The JV decided to use variable-density TBMs with hydraulic mucking instead of a mixshield or earth pressure balance (EPB) TBM to deal with the variable and locally highly permeable chalks along the 16-km (10-mile)-long tunnel sections and with hydrostatic pressures of up to 5 bar at TBM axis. It is considered that the chosen variable density technology has less impact on the nearby potable water stratum as less slurry flow to the face is required than with a standard slurry or mixshield TBM. This will reduce the risk of losing slurry in the highly fractured chalk that could possibly pollute drinking water wells. The 10.24-m (33.6- ft) diameter variable-density TBMs used for the Chiltern Tunnel are designed with only one muck transportation system in the tunnel, with a slurry circuit that functions in the corresponding closed operating mode. The muck is extracted from the pressurized excavation chamber by a 20-m (65.6-ft) long screw conveyor. At the discharge end of the screw conveyor, the excavated soil is transported into a slurryfier box where the muck is liquefied. The slurryfier box is fitted with a rotary sizer that can deal with the predicted flint nodules and processes the material to a size suitable for hydraulic mucking through the slurry circuit.

An additional two variable-density TBMs are used by the Balfour Beatty Vinci Joint Venture for the northern tunnel sections closer to Birmingham, the N1 Long Itchington Wood Tunnel (TBM-Ø 9.92 m or 32.5 ft) and the Bromford Tunnels (TBM-Ø 8.56 m or 28 ft). The N1 Long Itchington Wood Tunnel is a twin bore (ID = 8.8 m or 28.8 ft) of 1.58 km (1 mile) in length and the Bromford Tunnels (ID = 7.55 m or 24.7 ft) comprise twin bores of 5.7 km (3.5 miles) in length. Both tunnel sections are being built in Mercia mudstone. In its unweathered state, the Mercia mudstone may be described as an intact, jointed, weak rock, whereas in its fully weathered state, it is a reddish-brown, very soft to hard silty clay, but frequently containing less weathered mud-rock clasts.

The specific added value of variable density TBM application for these tunnel sections had been based on the predicted geological conditions, environmental sensitivity around the tunnel and muck-out management. The variable density TBM technology shows the advantages regarding the excavation through soils of different behavior in respect to face-pressure control, especially in areas where ground cover is limited, or in areas where the TBM passes beneath or close to sensitive assets and residential properties. The technology also shows advantages regarding achievable performances in ground conditions with high fines content, and shows an added value with regard to muck management. Hydraulic mucking out of the excavated material and related slurry treatment plant enables, through the filter press process, cakes with a reduced moisture content. Muck treatment with a high percentage of lime addition in urban environments can thus be avoided and reduces transportation due to limited weight by about 10 percent with better opportunities to reuse excavated muck for landscape fill.

FIG. 5
Assisted segment ring building.

FIG. 5-Assisted segment ring building.

The tunnel sections for Lot S2 Northolt Tunnels comprise twin tunnels (ID = 8.8 m or 28.8 ft) of about 8 km (5 miles). This contract was awarded to the Skanska Costain Strabag JV that will use two EPB TBMs with a shield diameter of 9.82 m (32.3 ft). The subsurface conditions along the tunnel sections comprise clay of the Lambeth Group and mixed face conditions with chalk containing flints. Hydrostatic pressures of up to 5.5 bar at tunnel invert will be faced. Another EPB shield (Ø7.08 m or 23.2 ft) is applied to the 850-m (2,770-ft)-long section of lot S1 Atlas RLT.

As most of the defined tunneling sections within phase one of HS2 are characterized by long tunneling drives, innovative systems such as partially or fully automated subsystems in the TBM supply chain were designed that support the TBM supply logistics. The Chiltern TBMs, for example, are designed to operate in a semicontinuous advance mode. The tunnel lining for Chiltern comprises 2-m (20-ft)-long segment rings, and the TBM is equipped with 14 thrust cylinders pairs. All pairs have a stroke of 3,200 mm (125 in.) and are equipped with stroke measurement systems. In semicontinuous advance mode, the excavation cycle starts with excavation in standard mode for 1,200 mm (47 in.), allowing a 400 mm (15.7 in.) clearance to insert the two first segments while the machine is stopped. Then the excavation continues in semicontinuous mode with the operator setting the advance speed. The thrust force is thereby regulated by the programmable logic controller.

The Northolt TBMs are designed with innovative packages such as fully automatic segment transfer and assisted segment ring building. The fully automatic segment transfer, for example, starts with the pickup of the first segment stack by the segment stack transfer wagon from the quick unloading system and ends at the position where the erector picks up the first segment from the segment feeder. The automatic segment transfer system includes a segment scan system that can evaluate, with the help of artificial intelligence, possible defects to the segments such as spallings on segment edges or missing dowels or defects to the segment gaskets. This information is visualized in the control cabin.

The benefits of such a fully automatic process are improvements in safety and quality and a more economical operation with savings in personnel costs and time. In countries with high labor costs, such technological innovations gain more in importance. The assisted ring building system is another major advantage with respect to safety. This assistance system consists of segment picking and segment positioning that support the erector driver placing the segments in their correct positions. Live camera pictures help to assist the erector operator during picking up the segment, and sensors support the operator while placing the segment in its correct position.

Santa Lucia Tunnel. The Santa Lucia project is a 7.5-km (4.6-mile)-long twin-tube highway tunnel north of Florence in Italy. Apart from long tunneling drives, the project is also unique due to the TBM size applied for this highway section between Bologna and Florence crossing the Apennine mountains. The twin tunnels each carry three traffic lanes with a width of 3.75 m (12 ft) and two 0.7-m (2.3-ft) wide side-strips. With a diameter of 15.87 m (52 ft) the EPB TBM is Europe’s largest TBM, and in combination with long tunnel sections, the project was an unprecedented logistical and technical challenge that was successfully mastered within less than three years.

Extraordinary challenges in addition to the tunneling length and machine diameter also include project specifically defined ground conditioning limitations, alternating geology with sedimentary rock mass and slope deposits, and gassy soil conditions (radon, carbon dioxide and methane). This demanded a specific TBM solution affecting, for example, the design of the shield structure, main bearing and its seal system, thrust forces, mucking and equipment for interventions. The applied EPB TBM was designed for 6 bars. The tunnel has an inner diameter of 14.3 m (47 ft) and consists of a segmental lining with an outer diameter of 15.4 m (50.5 ft). The precast reinforced concrete rings of 2.2 m (7.2 ft) in length are 550 mm (20 in.) thick and are fitted with gaskets to guarantee watertightness.

The EPB TBM had to cope with sections characterized by instable tunnel face conditions, sections with potential gas presence in the rock mass and fault, fractured and/or transition zones with locally low cover. The overburden along the tunnel alignment was characterized by mainly a cover of more than 48 m (158 ft) with up to 270 m (886 ft) as a maximum. In these conditions, the EPB operated in closed mode with the excavation chamber completely filled with excavated and conditioned material to actively support the tunnel face and to avoid possible explosion hazards in the gassy ground conditions. The 130-m (426-ft)-long TBM was designed with a nominal thrust force of 314,218 kN. A total of 25 electric motors generated a maximum torque of 123,000 kNm. The cutting wheel rotated at speeds of up to two revolutions per minute. It was fitted with 78 face and gauge 500 mm (19-in.) single disc cutters, six double discs in the center, 156 soft ground tools and 16 buckets.

A special TBM design feature to counteract the potential risk of gases and thus explosions was a special double-walled enclosure of the 70-m (230-ft)-long conveyor belt that was permanently ventilated. This double-walled structure included the section from the screw conveyor discharge gate to a transverse conveyor belt on the TBM backup. Possible gas emission was avoided by keeping this double-shell structure in overpressure. This was supplemented by the continuous supply of large quantities of fresh air that would dilute any gas presence within this encapsulated system before reaching the transfer belt conveyor and the loading chute to the tunnel belt. All the equipment in that area is fully explosion proof, and the air quality and the tightness of the systems are permanently monitored. The system comprised an elaborate gas detection, alert and shutdown scheme that was developed by Herrenknecht to meet specific regulations and to ensure the safety of personnel on the TBM and behind it at any time. Depending on the sensor position, the equipment reacted with different sensitivity. For example, the detection of a low methane concentration of 10 percent of the lower explosive limit (LEL), would not cause an advance stop if detected in the belt conveyor enclosure. On the other hand, if detected outside this channel, an immediate shutdown would have been triggered. The TBM also had a duplicated ventilation system installed that circulated fresh and clean air supplied by two air ducts installed from the portal. They worked independently of each other and guaranteed the supply of fresh air to the TBM. Key information from the ventilation and gas monitoring systems was transmitted to the control cabin along with the other current tunneling parameters so that the TBM operator could monitor the data on the display screens and could take corrective action if needed.

FIG. 5
Encapsulated conveyor belt to avoid gas emission.

FIG. 5-Encapsulated conveyor belt to avoid gas emission.

Rubber-tired multiservice vehicles are used for the tunnel supply logistics that carry one complete tunnel ring. The 2.2 m (7.2 ft) long precast reinforced concrete rings (9+0 elements) have a weight of 123 t (135.7 st). The EPB TBM for the Galleria Santa Lucia highway tunnel project was designed and manufactured by Herrenknecht in Germany. After a one-year predesign and manufacturing period, the TBM was accepted in August, 2016 at the manufacturer’s factory in Germany. The company Pavimental S.p.A started excavation in early May 2017. It took less than three years to construct the 7.5-km (4.6-mile)-long twin tubes. The tunneling team achieved an overall average performance of five rings per day, respectively 11 m/d (36 ftpd). A best daily performance of 10 rings or 22 m (72 ft) was achieved with a best weekly performance of 55 rings (121 m or 397 ft). The overall project duration was impacted by numerous factors. This comprised regular planned maintenance stops for cutting wheel and tailskin refurbishments due to abrasive geology and long tunnel drives, logistics impacts, probe drilling for geological exploration, and jobsite closures. During the COVID-19 pandemic, the project region was heavily hit between March 2020 until the TBM breakthrough on June 8, 2020 that made it also necessary to close down the jobsite for a period of time. Nevertheless, the project was successfully completed safely and in time with the EPB TBM that mastered challenges such as a very large TBM diameter, long-distance tunneling, permanently variable geological conditions, cover, abrasiveness and gas potential.


The mega infrastructure projects highlighted in this article are all comprised of long tunneling sections. The projects are remarkable corridors of connectivity and differ in terms of their complexity, risk sensitivity and requirements. This, in combination with the tunneling lengths, sets an impulse in the technological development to construct corridors, respectively foundations of social mobility and economic resilience with the help of mechanized tunneling technology.

Long tunneling drives demand resilient equipment to construct the infrastructure. All mentioned projects involve TBM technology that has already proven its reliability and dependability in a multitude of complex projects worldwide. When supporting projects of this kind with particularly long tunneling drive challenges such as uncertainties in ground conditions, rock burst and squeezing phenomena, gassy soil or rock conditions are common and demand robustness and durability of applied technology and the logistics of the supply chain. This, in combination with an experienced tunneling team, is the key to project success.

Part of the evolutionary stage of mechanized technology and with the focus on long tunneling drives is the trend to have systems on board of the machines that help to optimize tool maintenance intervals. The DCRM and DCLM systems provide information to help with assessments about the nature of the tunnel face. This in combination with camera systems in future realtime preliminary exploration will be possible. Innovative packages such as semicontinuous operation, automatic segment transfer and assisted segment ring building complement innovative developments and trends and support to increase safety of workers and work processes, but they also positively impact performance rates, quality and on more economical operations, assist with savings in personnel costs and time.


Bäppler, Flora, Brenner Base Tunnel – challenges of gripper TBM application for the 15km long exploratory tunnel Ahrental in challenging rock mass, Tunnels and Underground Cities, Engineering and Innovation meet Archaeology, Architecture and Art – Peila, Viggiani Celestino (Eds), pp. 3525-3530

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