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Under Pressure | Eurasia mega-project sets bar for complex subsea tunnelling

Eurasia Tunnel

The Istanbul Strait Road Tube Crossing Project has had to navigate awkward marine deposits, volcanic rock, seismic threats and extreme high water pressure working conditions.

“From a tunnelling point of view, this project is critical; it’s not the largest but it’s a large diameter; it’s not the deepest, but one of the deepest, and the geology is not the worst geology but it’s one of the most complicated,” states Başar Arıoğlu, chairman of Avrasya Tüneli, which is overseeing project delivery and has as its equal shareholders the two contracting firms Turkey’s Yapı Merkezi (of which Arıoğlu is also board member) and South Korea’s SK&EC delivering the scheme in JV.

“The extra depth, the pressure [up to 10.3 bar], the large diameter and the erratic geology – these altogether make a very technically challenging project,” he adds.

So it was understandable that during New Civil Engineer’s visit in January there remained genuine glee that almost five months before, the Herrenknecht Mixshield tunnel boring machine (TBM) broke through into the project’s European worksite – having completed a 3.4km journey from the starting point on the city’s Asian side. The tunnel reaches depths of up to 106m below sea level and is never shallower than 26m beneath the seabed.

Location of the new Eurasia Tunnel

Location of the new Eurasia Tunnel

The event’s significance is not limited to marking a major milestone in the project’s progress, but is seen at the highest levels as genuinely breaking boundaries for future tunnel design.

The full $1.25bn (£867M) Eurasia project stretches 14.6km from Kazlıçeşme on the Europe side to Göztepe in Asia and includes a 5.4km highway extension on the Europe side, a 5.4km twin deck tunnel, including the major TBM element, and a 3.8km upgrade where the tunnel meets the D100 on the Asian side.

The extra depth, the pressure the large diameter and the erratic geology – these altogether make a very technically challenging project – the Bosphorus is not just a challenge for vehicles to cross. 

Famously dividing the city onto two continents, its geology is intricate. Starting at the Asian side, the tunnel encounters the Trakya formation (mostly mudstone and sandstone with volcanic diabase), then soft marine deposits, comprising predominantly clayey silty sand, sandy clay and gravels, before advancing through the Trakya formation again at the European side.

Hence the need for the all-rounder Mixshield, whose cutter head is equipped with large cutters for scraping the softer subsoil away. For the harder-going rock, 35 twin disc cutters take the strain. It is a vast machine – 3,300t in weight, 120m long and with an excavation diameter of 13.7m. Its cutter head power amounts to 33.3kW/m2.

So, the team had the right machine for a complicated geology. Except it is often not always that straightforward when it comes to geotechnics.

“Our geological survey was pretty good,” says Ulaş Erboylu, contracting JV TBM works and planning & reporting engineer. It helped having consultant Parsons Brinckerhoff (now WSP Parsons Brinckerhoff) as scheme designer. It has international experience of dealing with seismic challenges and has local experience of having recently designed the nearby Marmaray rail link.

Eurasia Tunnel geological profile and tunnel boring machine design

Eurasia Tunnel geological profile and tunnel boring machine design

Eurasia Tunnel geological profile and tunnel boring machine design

But as every geotechnical engineer and engineering geologist knows, no amount of experience or heavy engineering of your machinery can give 100% certainty about what challenges lie ahead. Geological surveys carried out with Fugro and 23 borehole investigations along the tunnel alignment helped verify the plan ahead of construction.

Getting that alignment right was tricky. “When you get shallower you are more in the marine sediments,” explains JV project manager Naim İşli. “If you could go deeper with a longer tunnel it could be easier.

“It is not a short tunnel – 3.4km – and with this diameter and this pressure, that is a big challenge. So if you make it longer you never know what will happen with your machine, whether your machine will live for that long or not. There is always an optimum length of tunnel for a machine that you design. Otherwise you need to design a very heavy machine and the challenges increase.”

Still changes did need to be made. “In the early days of the project the alignment of the main TBM tunnel was changed – it got deeper and longer,” says İşli. “You cannot set the two entry and exit locations in the early days so, depending on the land availability, you need to change your entry and exit structures. If you start moving them, the alignment will change.” Which it did. Then the team had to face the reality of construction. “The geology created a very big challenge for us to pass the boring machine under the sea,” says JV deputy project manager Jin Moo Lee.

“There are some alluvium formations, such as the sand and the clay and the gravel. This is not a rock. It was very difficult to test this formation.”

“The geology was a little bit worse and a little bit more complicated than we expected,” Arıoğlu explains. “We underestimated the effects of this geology on the machine. The rocks were more abrasive than expected [particularly on the latter European drive] and we had to change the cutters more than expected.

The rocks were more abrasive than expected [particularly on the latter European drive] and we had to change the cutters more than expected.

“It is a little bit artistic, and a little bit engineering. As the machine is digging, it’s like trying to look at somebody’s poop to understand what he was eating. It’s the same idea. We can look at what’s coming from behind but we cannot see what is in front.”

The TBM embarked on its 16-month journey in April 2014 and advanced on average 8m to 10m a day.

Its 18 operators and supporting engineers held the project’s success in their hands. “It’s a very important part of tunnelling that if you have people on the job who know what they are doing then if they can take care of it in the first 10 or 20 minutes then it doesn’t become a huge problem,” says Arıoğlu.

What that meant, he continues, is that with the greater compressive strength of the tricky volcanic rock, operators needed to increase the rotation speed of the head but reduce its penetration. 

Eurasia Tunnel construction

Eurasia Tunnel construction

“So you want to push the machine softer, let’s say on each turn you want to push it half a centimetre,” he explains. “If you have softer ground then you want to slow down the rotations and push it let’s say three centimetres each time.”

“If you don’t do this properly and you push the machine too hard onto the hard rock then you destroy all of the cutters or you shorten the life expectancy. And we had many breaks on these cutters. Because you can’t see and because the only way you can find out is that maybe there is a little bit more noise coming from the front and the torque is increasing. So the operator has to be very cognisant of this and should be expecting these changes.”

As the tunnel advanced, a geotechnical engineer was on hand to aid the operators’ judgment of whether the rock was worsening. It slowed work down a margin but was vital in avoiding larger stoppages caused by misjudging what rock the TBM was dealing with.

It’s a very important part of tunnelling that if you have people on the job who know what they are doing then if they can take care of it in the first 10 or 20 minutes then it doesn’t become a huge problem.

The exceptional ground conditions and extreme water pressures meant avoiding the need to replace the vital cutter head tools wherever possible. In reality, around 430 changes of tools were required. Most significant, around halfway through the journey, the machine ground to a halt – huge flow rates of excavated material, high water pressure and large rocks being handled inside the excavation chamber had taken their toll.

Which meant calling on the uniquely designed TBM maintenance protocols. And this is where this project really pushed the boundaries of what can now be achieved in tunnelling.

Although tested on two notable Herrenknecht projects in Hamburg, Germany and Shanghai, China previously, this machine went a step further and incorporated a bespoke maintenance chamber that for the first time would be able to be maintained in pressures of up to 12 bar.

The machine was built to enable access to the damaged parts – the intake screen and jaw crusher – despite them being in the excavation chamber, under high pressure.

Professionally trained saturation divers capable of steel work spent three weeks working in conditions not far off what faces those working on the International Space Station.

See how the team fixed the tunnel boring machine under extreme water pressure here

Successfully repairing the cutter heads meant the TBM could continue its journey and carry out the placement of the around 15,084, 14.6t, tunnel lining segments. Eight – plus one keystone – form each of 1,672 rings, each 2m long. Some 30,765 bolts connected the segments.

Eurasia Tunnel TBM

Eurasia Tunnel TBM

A 30mm cementitious fire resistant lining is applied and the tunnel will have a final inner diameter of 12m.

Those segment dimensions were typical in almost all cases – all except two. 

It is not news to anyone familiar with Istanbul that the city lives with the constant threat of earthquakes – thanks to its proximity to the Northern Anatolian Fault.

The location of the tunnel in the southern part of the city makes this tunnel the closest to the fault – just 16km away – of all of Istanbul’s structural crossings.

“Tunnels are generally safe in earthquakes,” states Arıoğlu. But that didn’t preclude a specific design feature being needed. The tricky challenge for the seismic design of the tunnel once again comes back to the change of ground conditions midway along the route. The contrasting properties of the Trakya formations and the marine deposits at the centre mean in the event of a quake the two would behave quite differently.

“We have the soft ground and the hard ground and when an earthquake happens and the ground starts shaking the motion and acceleration is communicated by the rock,” says Arıoğlu. “The rock part is moving in a certain way but the soft rock is not attached like the hard rock so it’s moving with a different frequency.”

So, where the two key soil types interface, there are two seismic joints installed, 524m apart. These allow for tunnel sections to move 50mm transversely and 75mm longitudinally in the event of an earthquake. In this case the tunnel would resist the largest predicted earthquake for 2,500 years.

Parsons Brinckerhoff specified the tolerances needed for what Arıoğlu says is a “one of a kind joint”. Japanese expertise came in at this stage. Consultant NCC generated the design for a rubber and steel flexible section. Seibu manufactured the rubber joint that can yield and allow independent movement of the tunnel elements in the event of a quake.

However, the TBM could not simply pause while site workers installed the joints manually. So the designer ensured that they could be formed in a way that meant the TBM could automatically handle them as it did regular lining segments.

“We needed to have some extra steel pieces to keep the seismic joints very rigid to withstand all the forces [during tunnelling] on the lining,” explains Arıoğlu. “And after the machine is far away you remove these restraints and then the flexible joint becomes flexible.”

While the high risk work is complete, work continues on building the final tunnel structure. Advancing at 12m a day, site workers installed the steel reinforced, 360mm thick C40 concrete upper deck slab, cast insitu on 600mm deep concrete corbels either side of the tunnel. Work was set to finish the week after our visit and by the end of January the 350mm lower deck slab construction was set to get underway with a completion date three months beyond that.

Works were also running concurrently on the combination of U-section, cut and cover tunnels adjacent to both ends of the tunnel and a twin tube, 900m long New Austrian Tunnelling Method section on the Asian side, as well as shafts and the not insubstantial highway upgrade works, which include underpasses, overpasses and junction improvement works. A new administrative building is being added on one side and toll plazas and ventilation shafts will be installed at both ends of the tunnel.

Who’s the Eurasia tunnel for?

Eurasia tunnel graphic

Eurasia tunnel graphic

Istanbul’s flourishing economy and population has made this new link essential – in recent years the Marmaray rail tunnel crossing was added to the two existing road bridges spanning the Bosphorus Strait, which divides the Turkish city into two continents.

Despite numerous car and pedestrian ferries, the local sport for commuters is driving with one eye on their favourite smart phone live traffic apps in the vain hope of finding a traffic-free route. Huge queues for the two key bridges or ferry routes are the only option. Even ignoring the traffic problems, there is a current journey time between the soon-to-be connected areas of an hour and 40 minutes. So by later this year, this new, tolled tunnel will open and offer those prepared to pay $4 (£2.77) a lifeline and a comparative travel time of just 15 minutes.

The plan is to complete the project to ensure it opens to traffic by the end of this year.

If that target is met, the JV will have shaved almost a year off the expected construction time.

And then the focus for Avrasya will be on welcoming the flood of traffic in the form of cars, small vans and minibuses. Traffic volumes are expected to hit 120,000 vehicles a day in the first year. Egis has won the role of operator for the over 22 year concession.

Beyond this year, the project may have spawned an imitator in the form of a newly proposed, three deck rail and road tunnel, again under the Bosphorus.

Quite an indication of how much this scheme has already achieved.

Skin in the game Delivering the project as a PPP

The burden of delivering a PPP in Turkey threw a curve ball at the project in its early days and it came down to the engineers to eliminate issues as they arose. 

This is the first build, operate, transfer form of PPP of its kind in Turkey.

For the engineering partners – the contracting joint venture of Turkey’s Yapı Merkezi and South Korea’s SK&EC – there was good reason to be creative. Together they contributed $285M (almost £200M) investment into the scheme.

The ride was not smooth. 

Discussions with the consortium of nine banks both local from Turkey and others from wider Europe and Asia got underway.

“There was definitely a very strong appetite for the project,” says Arup lenders technical adviser – for the banking consortium – Tom Corke. “But the commercial realities became much more acute and the banks were much more risk resistant at the time.

“We were dealing with parties that weren’t familiar with the processes and requirements. It made things challenging and it took us more than two years to reach financial close on the project,” says Corke.

Corke adds that negotiations were “tough” but everyone wanted the project to go ahead.

None moreso than the contractor investors.

Fearing that delays might send the £867M costs upward, the team made the bold move of moving on regardless.

So, no financial close, but that didn’t stop the team, explains Yapı Merkezi board member Başar Arıoğlu: “During this time we didn’t stop working.” 

Eurasia Tunnel graphic

Eurasia Tunnel graphic

Among the activities that went ahead pre-close were appointing Parsons Brinckerhoff designer, advancing the design to just short of the basic design, geotechnical studies, 3D geology modelling and offshore borings.

“That gave us a lot of confidence and we took most of the decisions during that time regarding what the final structure was going to look like, adds Arıoğlu.

In one of the boldest moves, the team went ahead and began ordering parts of the mammoth Herrenknecht machine that would bore the main tunnel, which typically would take 12 months to procure.

“We took a staged approach and worked with the machine producer … each time, we gave them one more stage and [committed] say $1M, $2M or $5M,” explains Arıoğlu.

“So we didn’t take the risk of ordering a full machine but we ordered it piecemeal.”

The added bonus of getting the project well underway before financial close was an increased confidence about deliverability.

This allowed the contractors to present the bank with a proposal to cut the construction period from 55 months to 48. 

“Finally we told them, okay we have the machine under production, the design is advanced, so we can agree to reduce the construction time,” explains Arıoğlu.

Arıoğlu even hopes to cut another three to four months off the March 2017 deadline.




Under Pressure | Eurasia mega-project sets bar for complex subsea tunnelling

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