Robbins double shield TBMs for Austrian headrace 16 Dec 2015

The new GKI scheme is one of the largest hydro installations in the Austrian Alps, with the capacity to generate 40GWh/year and is being developed by a private enterprise group under three main construction contracts. Hochtief of Germany is contracted to excavate the 22km long TBM headrace and to have it ready for the scheme’s scheduled commissioning date in 2018.

The geology along the 6.53m o.d. headrace comprises mainly hard schist of an average 70 MPa with the possibility of faulted and squeezing conditions as the overburden rises to a maximum 1,200m above the alignment. Headrace excavation is divided into two TBM drives from a central adit with TBM 1 progressing south to the dam location and TBM 2 heading north to the 160m deep power shaft, which is part of the powerhouse contract. The working site of the headrace tunnel is squeezed into a tight and limited area between the River Inn on one side and the rise of the mountains on the other. The headrace runs largely parallel with the course of the river with a perpendicular drill+blast adit intersecting the alignment at about 1,000m inside the mountain.

Hochtief won the headrace tunnel contract in July 2014 and offered to complete the 22km long tunnel with one TBM, completing one drive first and returning to the central working site to complete the second. The project owner however, led 80% by the Austrian hydro company TIWAG, considered this too great a risk to the project programme, given the unpredictability of the geology, the potential for squeezing and faulted rock, and the relatively tight construction schedule. Hochtief therefore ordered two identical double shield TBMs from Robbins to complete the headrace drive and its specified precast segmental lining.

Designed for conditions

According to Robbins, the new 6.53m diameter machines are designed specifically for the task at hand.

“The machines include many standard Robbins elements and there are several new features that will help them cope with whatever conditions may arise,” said Andy Birch, Project Site Manager of the Robbins onsite Field Service team for the assembly, commissioning and launch of both TBMs. “As standard, Robbins build TBMs with the largest bearing to TBM diameter ratio in the industry. The bearings in these two 6.53m o.d. machines are 3.2m in diameter and are powered with seven 350kW VFD [variable frequency drive] electric motors that provide a cutterhead power of 2,310kW. Together with a maximum cutterhead thrust of 40,712 tonne at 450kN bar, the maximum cutterhead torque is 6,000kNm at 0-5.4 rev/min. Seven motors connected to the 3.2m diameter bearing ensure no overload of the bull gear.”

Among the specific features adopted for these TBMs are gear reducers on the motors. By reprogramming the gearboxes on the VDM motors, extreme breakout torque can be increased to 8-9,000Nm at up to a maximum 4.5 rev/min. “The gear reducer allows a greater torque at lower cutterhead rotations,” explained Birch, “with higher cutterhead speeds at lower torque levels of up to 8.7 rev/min.”

The GKI TBMs can generate a forward thrust of 57,000kNm with the two 1.2m x 1.2m gripper pads making contact with the rock at the 10 o’clock and 2 o’clock positions through the gripper shield. This is different to the more usual horizontal grippers engaging on the lateral axis of main beam TBMs and provides for a three-point gripping action – two grippers onto the crown and bracing the gripper shield onto the tunnel invert. It also provides for a clear working space within the shielded TBM ahead of the segmental lining build area.

Overcutter capacity

The hard rock backloading cutterheads of each TBM are dressed with 41 x 17in Robbins wedgelock disc cutters and have a 15mm wear capacity on the gauge cutters. To be better prepared to cope with fractured and squeezing conditions, the cutterheads are also equipped with an overcutter facility. Once engaged, the overcutters can increase the bore dimension by 25mm and 50mm on the radius, corresponding to a tunnel increase of 50mm or 100mm on the diameter. The 50mm overcutter setting on the radius is engaged in extremely squeezing ground to prevent the shielded TBM becoming trapped and still provide time for the segmental lining to be installed some 12m behind the face.

An overcutting facility can only be through the crown and must be non-centric to the normal cut diameter to keep the invert of the extended cutterhead diameter on the same line. To achieve this, the cutterhead and the bearing unit is lifted onto a new axis either 25mm or 50mm higher. “It takes four or five days to fit the extending overcutter facility,” explained Birch, “but the effort is well worth it if potentially squeezing conditions are predicted.”

Probing and pre-excavation grouting

Another special feature on the TBMs is the system for probing and pre-excavation drilling should it be needed. Probing to 30-60m ahead of the face is a systematic requirement. Probing can provide significant information, to indicate either very hard rock or squeezing conditions, as well as forewarning of groundwater inflows, their volume, pressure and temperature with the drill flushings providing more information. A new development is the use of sensors on the drill-bits or drill-strings to provide more accurate and more wide ranging information about conditions ahead.

The drill ports also allow for forepoling in an array over the crown and, in potentially squeezing conditions, lubrication of the shield can be injected through the same shield ports. Control of water ingress from the face is possible on the machines by closing off the muck hopper in the excavation chamber with a horizontal guillotine door.

On the two Robbins machines, the drills are mounted on their own ring beam rather than on the segment erector. “This provides for a permanent drill installation immediately behind the segment erector,” explained Martino Scialpi, the Robbins Service Project Manager for the project. “From this location, the drill can install 12 drill holes on a 6.2^o outlook through the drill ports around the forward shield. It can drill an additional six horizontal holes through the cutterhead should that be needed, as well as 10 injection holes through ports in the gripper shield. A second drill can be fitted to the ring beam mount should that be needed for the time consuming drilling for pre-excavation grouting rounds.”

Blind hole end for 11km long drives

At end of each 11km drive there is no possibility of breakthrough across the interface into the neighbouring contracts – neither at the upstream site of the new dam with its 100m of tunnel, nor at the downstream junction with the 650m deep x 5.76m i.d. high-pressure shaft into the underground powerhouse. Both TBMs therefore, will have to be dismantled within the finished tunnel and the components, including the sections of the cutterheads, retrieved back through the completed and segmentally lined tunnels. The machines have been designed and manufactured specifically to achieve this final operation with efficiency.

Two-stroke logistics

In operation, the trailing backups of the two identical machines are designed to comply with the logistics chosen by Hochtief for the project. These include:

• mucking out by railbound systems, rather than continuous conveyor application;
• mucking into stationary muckcars using a transfer conveyor shuttle, rather than by shunting the muck train;
• a routine based on two 1.66m long strokes per train-set, rather than one; and
• a mechanical segment erector, rather than a more usual vacuum erector.

Basing logistics on a two-stroke cycle will help optimize the programme on the long 11km+ headings for each TBM but has made for long rolling stock service trains and a long trailing backup of 18 gantries and a total length of 196m for the two machines.

Each service train comprises:

• ten 21m³ Mühlhaüser side tipping muck cars;
• four segment cars to carry the four segments per each 1.66m wide ring of lining;
• one flatcar for service extensions;
• two pea gravel cars for annular backfilling;
• a grout car and
• a man-rider.

Pea gravel is transported in auger hoppers that are lifted into position on the backup and the empty ones taken out for recharging. On their way in, each service train weighs about 240 tonne and on the way out, about 480 tonne, with the ten muck cars full. Both drives are on a slight uphill gradient towards the dam in one direction and to the high pressure shaft in the other.

Once at the TBM, the conveyor shuttle and muck shoot running on rails across the top of the backup gantries fills each of the ten stationary muck cars in turn. A curtain of plastic strips prevents anyone holding on to the shuttle’s roller tracks and potentially losing their fingers. The service-train loco operator also operates the muck filling shuttle through the two strokes of each round.

Segment design and production

The segmental lining is designed by Vigl Consult of Austria. It is a conservative design with rebar reinforcement cages, rather than fibre reinforcement, and the four segments per ring are not bolted or gasketed, but rather have dowel connections on the longitudinal joints and dowel locators on the circle joints. The invert segments weigh 7 tonne and are cast with a central drainage gutter and footings for the service train rails either side. The trailing gantries of the TBMs run on rails set outside and train tracks. These are moved forward as the TBM advances.

Segment production is under a subcontract with Hilti & Jehle, an aggregate supply and concrete casting company in Pfunds which provided land for part of the tunnel project working site. Hilti & Jehle is also taking all the tunnel muck and recycling it into the segments. “This all made perfect sense,” said Wilhelm Kohl, Tunnel Manager for Hochtief. “The quality tunnel muck is going into the segments and any unsuitable material is being used to fill spent gravel pits along the river bank. The segment casting yard is also on site, which does make available space very tight. We will stockpile segments in three-ring stacks in due course, to maximize space.”

The casting yard is equipped with a steam-curing chamber and a carousel of 48 Formwork moulds for a 12 ring-set casting cycle. The rebar cages are also fabricated on site in the casting factory’s and on top of the steam-curing chamber. Production of the 270mm thick x 1.66m wide segments is on a programme of 40 segments/day with each segment spending 24hr in the 4-track steam-curing chamber.

The 7-tonne invert segments are cast with pads on the extrados to compensate for the open invert in the ring build shield of the TBMs. The pads allow the invert segment to be installed directly onto the invert of the tunnel bore with the other three segments built within the protection of the ring-build shield of the double shield machine. Under the conservative approach to the project, Hochtief also selected a mechanical erector for build of the segmental lining in the ring-build area of the machine.

Access adit and TBM launch

When TunnelTalk was on site, Tunnel Manager Kohl introduced the project and explained progress to date including drill+blast excavation of the 1,000m long access tunnel and of the TBM launch and working chamber. The access adit is a permanent installation as it is through this adit that the hydro system will be dewatered. Gravity flow will empty the entire headrace to the River Inn on one side of the working site.

Originally, the adit, that runs perpendicular rather than at an angle to the headrace, was to advance to about 750m by drill+blast before TBM1 would be walked in to take over the drive through the curve onto the headrace alignment. Drill+blast was to continue on an opposite curve to the alignment of the second TBM with drill+blast completing the main tunnel section between the two adit curves. This was eventually changed to drill+blast for the full adit and to a 200m long drill+blast cavern on the main headrace alignment. Both TBMs will travel the adit to the chamber for launch into their opposite starter tunnels.

Work on the adit started in October 2014 on a 1m rounds topheading and bench cycle using a Leibherr excavator and systematic spiling across the crown to support the 80m² heading through the soft weathered rock of the portal zone into the mountain. The first blast in hard rock occurred in November after the first 170m of excavation. The face was reduced to 60m² and progressed on a fullface cycle of up to 3m/rounds. This continued for excavation of the on-line TBM working cavern.

Drill+blast progressed on three shifts/day using a three-boom Sandvik jumbo, two wheel loaders and three 30m³ dump trucks. Fullface excavation of the permanent headrace-dewatering adit was supported with steel arches, two layers of wire mesh and synthetic fibre shotcrete applied using a Meyco Potenza robot. The last 14cm of the invert was excavated with a road miner and finished with a steel-fibre reinforcement concrete invert slab. The working shifts of 5-6 crew members progressed at 6m/day to 14 June when it junctioned with the 150m² on-line TBM working chamber.

Once the main components of the TBMs are assembled on the site, each is transported into the working chamber for final assembly and commissioning. Transportation is a tricky operation made easier with the shield travelling on a cradle or dolly that is pulled on the twin set of rail tracks by six 35 tonne locos with a combined power of 273kW or 620 tonne. The set of six locos were synchronized so that the operator in the lead loco operating all six simultaneously. Despite the 50m radius curve at the adit portal, the walk of the first TBM along the 1km long adit took about 1 hour. The backup gantries followed, two at a time. This too was a tricky operation with the load passing through the 50m radius curve of the service rail tracks on the articulated cradle.

The cradle system was first used by Hochtief on its Thames River crossing contract in London for the Crossrail project to transport the twin tube TBMs across the contract’s open station boxes.

The first TBM was pulled into the adit and working chamber cutterhead first. Second was pushed backwards through the adit into the cavern and moved forward into its opposite heading in the common working chamber. TBM1, heading towards the reservoir was launched in mid-October and was more than 311m into the drive by 16 December 2015, while the second TBM will be transported through the adit and into the assembly chamber for its scheduled launch northwards towards the powerhouse in January 2016.

Service trains to TBM1 will run directly in and out of the heading and the adit. Service trains to TBM2 will reverse into and out of the heading in the working chamber. At the portal, the service trains navigate the sharp 50m radius curve to the muck-skip discharge bunker and its conveyor transfer to the aggregate stockpiles at the Hilti & Jehle factory.

Safety considerations

Safety of the crews working on the TBMs on their long headings is a high priority on the project and Hochtief has procured three rail-mounted refuge chambers on the contract. These are 12m long and are able to accommodate 20 people. There is one stationed on the middle of the California switches at back of each TBM and the third is stationed at the adit portal and available to emergency services for going into the distressed heading and to help manage the emergency situation. Particular to the units procured for the project is that they are self-powered and can be operated from inside the safe environment.

Another sign of heightened safety and ergonomic comfort for the crews is design of the new Mühlhaüser man-riders. These are insulated against the raw sheet-metal walls, have proper seats, rather than metal benches, have glazed windows that are sealed against noise, dust, water and wind as they travel, and are fitted with first aid equipment and rescue stretchers. The comfort was appreciated by the visitors on the Robbins-hosted trip to the project in October, even though we were riding just the 1km adit into the TBM assembly and launch cavern, rather than journeying to a 6.5m diameter TBM nearing the end of its 11km long drive.

The two new TBMs were fabricated by Robbins at a Robbins facility in Rome, Italy. The factory is capable of manufacturing four TBMs at once in four TBM bays, and can fabricate machines up to 10-14m in diameter. In recent times, as well as the two double shield machines for the GKI project in Austria, Robbins has produced five other TBMs in the facility – three machines for Turkey, one for Malta, and another for a project in Albania. Once fully assembled and factory tested, as per the purchase order from Hochtief, each of the GKI TBMs was disassembled for the 800km road trip to site in a convoy of 70 trucks for each machine and via the highways over the Brenner pass to Innsbruck and on to Pfunds.

TBM procurement, design and benefits

While on the Robbins hosted site visit, it was explained that here is often discussion with customers about the benefits of OFTA or the Onsite First Time Assembly option that is offered by Robbins to its customers. “There are major time savings to be had along with other cost and transportation benefits,” explained Martin Rauer, Project Manager and Sales Manager for Robbins Europe GmbH. “The two TBMs here in Pfunds were fully manufactured and tested in the factory before being disassembled for transportation to site. OFTA would have taken about 25,000 man-hours each. As it was, they required 25,000 man-hours in the factory plus another 15,000 man-hours, or two months, on site to assemble each again and prepare them for launch. It is however a personal preference of the client and there is also the logistics of the manufacturer to consider. OFTA would work against the need to keep a large manufacturing factory fully employed and optimized.”

Another discussion with many clients and TBM customers revolves around the best method of excavation for each rock tunnelling project. “This spans from TBM versus drill+blast, and from a main beam TBM to a single or double shielded machine,” said Detlef Jordan, Sales Director for Robbins Europe GmbH. “There is always the discussion of the flexibility offered by drill+blast against the potential for higher rates of excavation by rock TBMs and then also the discussion of the ability of a double shield TBM to mine and line at the same time. Of course there are many variables that influence the decision but there are some historical and case study data to add to the discussion. On one project where a TBM was advancing from one end and drill+blast was progressing from the opposite end, the TBM achieved a best advance of 723m/month and up to 40m/day, while the drill+blast heading recorded rates of 281m/month and 12m/day.”

“There are also the safety issues to consider,” said Jordan. “A TBM cutterhead provides support of the face and a shielded machine provides overhead protection of the work area, as opposed to the open face and environment of a drill+blast heading until primary support is installed or forepoling is used. A TBM adds the ability to build a precast segmental lining and the double-shield introduces the additional feature of advancing the stroke, off the grippers, and erecting the segmental lining at the same time. While many variables need to be inline to achieve optimum performance of a mine and line double shield operation, for me, the double shield concept, with the added features of these TBMs, is the best combination for mechanized excavation of this project and for many others like it,” said Jordan.

The two double shield Robbins TBMs at GKI are programmed to progress at about 900m/month and to complete the two 11km long drives of the 22km long headrace in about 8-9 months achieving 20-30m/day. TunnelTalk, with the agreement of project contractor Hochtief and TBM manufacturer Robbins, will follow progress through the coming months and report on the ups and downs as the machines progress through the mountain to their blind-hole end points and out again.

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