Tunnelling: Spanish lessons

Buried deep under a mountain in north west Spain, innovative methods are helping extend Spain’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.

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


The western (Zamora) portal to the tunnel, with plant and concrete segments on display.

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-Galicia 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 (clearance area of 52m2 ) says tunnel deputy project director Juan Margareto. Owned by FCC, it has worked on sections of the Pajares Tunnel (14km) and the Sotiello Tunnel (6 km). Concrete lining segments are 370mm thick, resulting in an internal tunnel diameter of 8.76m. Except for the outer 15 m of the eastern end and 70 m of the western end (both cut and cover), most of the tunnel´s length will be constructed using the TBM.So is it worth the pricetag, for 12km of work? “3km [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,” Margareto says.


Bolanos Tunnel cross section

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 use of diesel generators would have used almost 600l/h. And created 17kg/h to 20kg/h of carbon.

Instead, the team built 35km of power lines (12km underground) costing £3.6M. “It was a big investment, but we think it was necessary,” Margareto says.

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).


Bolanos Tunnel Location

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 relatively 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.

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 scores of the winding mountain access roads.

The geology of the area that will be excavated is made up roughly of 50% of shales, of different types and approximately 50% of quartzite, phyllite quartzite and sandstones. The highest outcrop over tunnel´s main length is 210 m.


Bolanos Tunnel Route

While the below ground profile is mainly slate, (slate 50%, laminated slate 10%, sandstones 25%, quartzite 5% cannel coal 10%) the cannel coal deposits need to be dealt with in consideration of 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 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.

Fault line

The upper part of the tunnel was drilled with a conventional method, whilst the TBM drill was used on the bottom part.

At the start of the tunnel, close to the portal, there was a big fault (with only 17 meters of depth). To prevent the fault causing the TBM a problem, FCC’s technical department designed a mixed drill system. The upper part of the tunnel was drilled with a conventional method, whilst 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).

“A big TBM”, Margareto calls it. It is single-shield, open-mode, with a total length of 300m, diameter of 9.9m and weighing 3,000t.

According to studies carried out during the drafting process of construction project, the expected water pressure at certain points of the excavation is 140 m of water leakage column, with water flow peaks up to 100 l/s. To improve the tunnel’s resistance to water ingress, it uses a two component mortar system, used for the first time in Spain (or Europe for that matter) in an open mode 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, takes 10 seconds to form an impermeable gel-like barrier. Simulations of the tunnel situation were done to test its effectiveness. One component, 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 – 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.


Work goes on inside the tunnel on the TBM.

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 about 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 24/7. “It’s not mandatory to have a medical site, but we do it.”

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


Concrete segments are stacked outside the Ourense portal of the tunnel.

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

“From 2002 to 2010, all investment was Santiago to Ourense. Now, the investment is more Ourense to Zamora,” he says.

He says it could have all been done faster, using two TBMs concurrently, but the cost would have been higher, ande 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 1,140,000 m3. As of 14 October 81.4 % of the two tunnels is complete; with each tunnel 6.7km, 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.

Source: newcivilengineer.com


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