Spain’s High Speed Rail Network, known locally as the Alta Velocidad Española (AVE), is the envy of countries around the world. Since it opened with a link between Madrid and the southern city of Seville in 1992 it has continued to grow into Europe’s largest high speed network with more than 3,100km of connections linking centrally-located Madrid with the rest of the country, taking travellers from Malaga in the south all the way to Barcelona in the north and on to France.

Expanding the network remains a high level priority with new links planned or in many cases already underway throughout the network. One such link is a new high speed connection to the Galicia region in the north west of the country where construction of the new Bolaños tunnel is a critical part of the works. Undertaken by Spain’s FCC Construccion and Acciona Infrastructure (in a 50/50 joint venture) for state-owned client ADIF in a contract worth EUR 162M (USD 175.34M), the work comprises the delivery of two parallel, single-track tunnels with a length of more than 6km each, excavated by a single shield rock TBM manufactured by Germany’s Herrenknecht.

“The main work in the scope of the contract is the excavation of the Bolaños Tunnel on the left track,” explains Oscar Alvarez, project manager for FCC. “This is a single track tunnel running for 6.785km including cut-and-cover tunnels of 18 and 57m each at both ends. The bored tunnel is 6.079km long,” he says.

After considering the boring options available the team opted to use a hard rock TBM. “Due to the geological conditions of the mountain and the length of the tunnel, the most suitable option was to drill it with a TBM,” says project engineer Florencio Milla. “Any alternative method, such as NATM, would have increased the tunnel delivery time and therefore the remainder of the construction works.”

Construction of the tunnels began in August 2012 under a strict 38 month time limit, meaning that all project works will be completed by November. Tunnel boring was completed in December 2016 meaning that the scheme remains on track to meet its deadline.

Complex Geology That is not to say that the boring was without its challenges. “The main challenge throughout the delivery of both tunnels was the geology,” says Alvarez.

“Excavating a terrain that had laminated slates with a very high degree of alteration was not possible with an open-headed TBM.”

The geology of the excavated area was made up of around 50 per cent shales of different types, and approximately 50 per cent of quartzite, phyllite quartzite and sandstones. Traces of carbonate ampelita of sandy volcanic levels were also found in the area. The highest outcrop over the tunnel´s main length was 210m. The result was high geological instability of the area, which led the team to implement an innovative stabilisation system for the backfill ring of the TBM during the tunnelling process.

“Although the lithology of the tunnel was as expected according to the GI the geotechnical characteristics of the rock were worse, causing serious excavation problems at different times,” says project engineer Florencio Milla. “For this reason we needed to use consolidation foams along some areas of the tunnel. The foam was injected on to the face of the tunnel.

The mixing of the components created expansive foam that allowed the TBM drill to re-start.”

The two component foam of a solvent free polyurethane and silicate dealt with the unstable geology. “In one part of the tunnel we found material with very little stability and cohesion that caused a blockage to the TBM’s cutter head, which stopped the tunnel excavation. To solve this problem we had to reinforce the tunnel from the cutter-head of the TBM with the specific dual-component foams,” says Milla.

Given the challenging nature of the ground the contractors chose a rock TBM that had been used in a project with similar geotechnical conditions. The selection was a Herrenknecht S-511, which became a HK S-805 for the project. It has a drilling diameter of 9.9m, and a head of 10m, with a total length (including the back-up) of 180m.

“Following the investigation into the geology we anticipated water inflow in the tunnel. As a result we decided to design and use a waterproofing system based on a two-component mortar,” says Milla.

Considering the challenging ground progress has been good. The team reports that in 85 per cent of the tunnel performance was better than the best possible forecasts.

For the first tunnel where the geological issues were encountered average progress was 19.4m/d meaning that the entire bore took 11 months. Progress was faster in the second tunnel at 24.3me per day and the 6km bore took only nine and a half months.

On Site Fabrication

In terms of the precast tunnel lining segments ring manufacture took place on site by FCC. The ring’s design was the universal type, which the team points out is the best design for the different tunnel layout changes. The concrete changed from HA-40 to HA-60 according to the geological zones of the tunnel, and the reinforcement was also adapted in the same way. Bridge Technologies had involvement with the lining design. Across the tunnels a total of 4,185 ring segments were placed, each with a lining thickness of 370mm and a ring length of 1.6m. “The high quality standard to which the tunnel is finished is a major achievement for everyone involved in the project. Seeing the results all over the tunnel has been very rewarding for all of us,” says Alvarez.

Alongside the TBM bore the project used conventional NATM as well as taking a more unusual approach to deal with a geological fault. “For the tunnel portals, a mixed drilling system designed by FCC’s technical services team was used to avoid a geological fault with 17m of depth at the beginning of the tunnels,” says Milla. The TBM assembled the corresponding rings and left the tunnel finished. At the end of the tunnel, the last 14m were excavated by NATM, for the reception of the TBM.

Muck from the works was transported to an authorized waste disposal area near the site. At the end of the tunnel excavation FCC-Acciona restored the landscape. “We took care to protect the environment in all its project works, a clean point segregation of hazardous waste and transfer points were installed in sensitive areas of the work for nonhazardous waste and valuable waste, which is destined for recycling,” says Milla. The project employed 250 workers on site during peak construction with an average of 125 still on site. Current activities involve dismantling the TBM and the facilities as well as completing the connecting tunnels located every 25-30m along the bore.

“The Bolanos Tunnel has been a technically complex project that has required the full breadth of our tunnelling expertise,” Alvarez reflects. “Developing and implementing innovative solutions to address the geological instability we encountered was only possible with experience, spanning 550km of tunnelling works, and dedicated teamwork. We have now built more than 35 per cent of all the high speed lines in Spain, and with this project we are proud to have delivered another important part of the network”.