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Tunnelling: Spanish lessons

Buried deep under a mountain in north-west Spain, innovative methods are helping extend the country’s high speed rail network.

The €209.08M (£188M) 6km twin bore Bolanos Tunnel is located 175km south east of Santiago de Compostela. It is on the 450km line that links Madrid with the region of Galicia, via the cities of Olmedo, Zamora and Ourense.

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The Bolanos Tunnel is the biggest on the 450km line that links Madrid with the region of Galicia

The client is Adif Alta Velocidad, rail infrastructure administrator for Spain. Adif bid each bore separately but combined both contracts after their award, with the contractors working together with the same methods, equipment and sharing knowledge.

But taking the lead in the work is FCC (who is in a joint venture with Acciona Infrastructuras, taking 47.5% of work, and Collosa, 5%). FCC has a long history with high speed rail in the country, and is increasingly looking overseas for work, including on the UK’s High Speed 2 project.

The Madrid-Gallicia line is full of aqueducts and tunnels over mountainous and remote terrain, with the Bolanos Tunnel the biggest and most expensive of them all. The majority of the track stretches in the province of Orense are in tunnels, and these works account for 88% of the total budget.

A Herrenknecht S-511 tunnel boring machine (TBM) is creating the 9.9m diameter tunnels, says Juan Margareto, tunnel deputy project director. Owned by FCC, it has worked on sections of the 14km-long Pajares Tunnel and the 6km-long Sotiello Tunnel. Concrete lining segments are 370mm thick, resulting in an internal tunnel diameter of 8.76m. Except for the outer 15m of the eastern end and 70m of the western end (both cut and cover), most of tunnel’s length will be constructed using the TBM.

So is it worth the price tag, for 12km of work? “Three kilometres [length of tunnel] is the point at which we can start to consider using a TBM. But there are other factors: other jobs nearby; how many TBMs you want to use. Each situation is different,” says Magareto.

On site, at the west portal, is a concrete ring segment factory, water treatment plant and ring storage area.

Road bridges and extensive earthworks were also needed to get heavy equipment into the hilly landscape.

The contract started in September 2013, and soon ran into some problems. To get the equipment into action, power was needed. But in the remote mountainous area, there is not enough electrical power supply, and diesel generators were thirsty and polluting. Instead, the team built 35km of power lines (12km underground) costing £3.6M.

Another hurdle was sourcing the large volumes of water required for the TBM that requires 100m3/h. Futhermore, equipment outside the tunnel needed water (15m3/h) and so did the nearby concrete ring segment factory (127m3/h).

The solution was an on-site water treatment plant, with dirty water from the TBM filtered, directed through 1,028m3 of storage tanks, and pumped back out for re-use. The water goes into the plant with a solids concentration of 10,000mg/l, and comes out looking pretty clear at about 60mg/l. In the end, with the recycling plant creating 95m3/h, the team only had to take an additional 32m3/h from a nearby river.

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A Herrenknecht S-511 tunnel boring machine is creating the 9.9m diameter tunnels

Margareto says the power lines and water treatment plant will remain on site after the build, potentially for use by the local community. FCC has also heavily repaired many of the winding mountain access roads.

The geology of the area that will be excavated is roughly 50% of shales of different types and 50% quartzite, phyllite quartzite and sandstones.

Traces of carbonate ampelita of sandy volcanic levels are also found in the area. The highest outcrop over tunnel´s main length is 210 m.

While the below ground profile is mainly slate, (slate 50%, laminated slate 10%, sandstones 25%, quartzite 5% cannelcoal 10%) the cannelcoal deposits need to be dealt with to meet tight Spanish regulations on heavy metals.

The excavation started on the Campobecerros (eastern) side, with a rising slope of 0.5% over about

5,500m, then descending with a slope of 0.3%.

There was an area (around 3.5 km from the portal), that the TBM crossed along an anticline with laminated slates. This combination stopped the TBM several times because the cutter head was blocked by material. To solve this problem the team had to inject a foam made of two components on to the face of the tunnel. When component A is mixed with component B the result is an expansive foam that allows the TBM drill to re-start.

Close to the portal, a major fault could have caused a problem for the TBM. So, FCC’s technical department designed a mixed drill system. The upper part of the tunnel was drilled with a conventional method, while the TBM drill was used on the bottom part. The TBM is worth protecting, not only because it needs to be used twice, for the left (southern) tunnel, then the right (northern).

According to studies carried out during the design stage, the expected water pressure at certain points of the excavation is 14bar, with water flow peaks up to 100 l/s.

To improve the tunnel’s resistance to water ingress, FCC is using a two component mortar system for the first time in Spain (or Europe for that matter) in an open shield TBM.

First used in Japan in the early 2000s, it has been used solely in earth pressure balance TBMs. Two types of liquid are mixed together, which according to FCC, then take 10 seconds to form an impermeable gel-like barrier.

Simulations were carried out to test its effectiveness. One component is cement, bentonite, water and a retardant; and the second component is an accelerant, liquid sodium silicate.

The amount of accelerant can be adjusted, depending on where the TBM is progressing and how much of the tunnel needs to be done.

Another innovation claimed by FCC is a double joint system between ring sections. To prevent angular deformation and a broken seal between concrete tunnel sections, the double joint system distributes the load more evenly, also making the structure less prone to leaks.

The use of trapezoidal segments instead of rectangular ones has also been implemented, which improves longitudinal joint compression.

Production of rings needed to begin two to three months before the TBM started, and they have been churned out consistently ever since, with a stockpile of about 1,000 waiting outside in the hot Spanish sun at any one time.

All up, the tunnel’s concrete ring segments amounts to 159,380m3 of concrete and 15,906t of reinforcement.

Margareto says with about 300 people working on the massive site at any one time, safety is paramount, with an ambulance and nurse service available around the clock. “It’s not mandatory to have a medical site, but we do it,” he says.

And to make sure communication lines remain open in the tunnel, there’s separate wi-fi, mobile and digital walkie-talkie systems.

Margareto says the project is on course to finish in November as planned, providing another link in the chain for Gallicia-Madrid line.

He says it could have all been done faster, using two TBMs concurrently, but the cost would have been higher, and Adif is managing the rollout of infrastructure alongside other tunnels and viaducts on the line.

On 29 June this year 925,000m3 had been removed from a total excavation of 1,140,000m3. As of 14

October 81.4% of the two tunnels was complete; with each tunnel 6.7km long, the first has been completed and the second is on 4,025m.

Parts of the Madrid-Gallicia line – with speeds up to 350km/h – are already open, with the full line expected to be open in 2018.

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