The US$3.2bn metro Line 9 is one of Barcelona’s most significant infrastructure projects in recent years. When complete, the new driverless subway line will be the longest in Europe, with an overall length of more than 44km. The new line runs across the city connecting Badalona and Santa Coloma de Gramanet, in the north-east, with Zona Franca and the airport, to the south-west (figure 1). Construction of the line is divided into four sections (or Lots) based on the typology of the ground and, in total, involves the use of four TBMs – one 9.4m diameter and three 12m diameter machines.
Due to the lack of surface space in the city, a single large tunnel with a double-deck rail track configuration has been selected for the line. This allows station platforms to be located within the running tunnel itself, allowing surface disruption to be limited to access shafts in highly congested areas rather than large station box structures.
A deep tunnel alignment, at an average depth of 35m below ground, was selected for a combination of reasons, allowing the alignment to pass below existing underground infrastructure, through better ground conditions, but primarily to reduce the risk of surface settlement as much as practically possible.
The tunnel lining consists of pre-cast concrete segments. Each ring has an outer diameter of 11.7m and is formed of six segments plus a key. The width of the ring is 1.8m and it has a thickness of 400mm. This Universal ring allows horizontal and vertical curves greater or equal to a radius of 300m.
Concrete mix design
Manufacturing of the segments is undertaken in a factory located in the “Torre de Corb” industrial estate, in Balaguer (Lleida). The manufacturing process is automated with rolling moulds and fixed workstations, which facilitates high productivity and an excellent level of quality.
According to the Spanish Code EHE, the concrete used for segment manufacture is not considered conventional. Two characteristics make it “high performance” concrete. Firstly, is the concrete’s ability to exceed 50N/mm2 strength at 28 days. The second is the use of a combination of steel bar cages and steel fibers for reinforcement.
Prior to the comencement of segment manufacturing for Line 9, the Structural Technology Laboratory of Barcelona´s Civil Engineering School carried out a study to optimise the concrete mix design. The test-based design process adopted by the Structural Technology Laboratory involved three main stages: Definition for the cement paste composition; the granular aggregate mix optimisation; and the cement paste verification in the concrete. Once defined, a toughness study was undertaken for the concrete mix reinforced with steel fibers.
Ultimately, the pre-cast segments for this section of the Line 9 project, consisted of HA-45/P/20/IIb-H concrete. In addition, 25kg/m3 of steel hook fibers (0.75mm diameter and 50mm long) is added to the concrete mix, to give an equivalent flexural strength of 2.9N/mm2 after 28 days.
TBM sliding operation
The UTE Gorg Joint Venture selected a 12m diameter Herrenknecht EPBM, at the time the largest machine of its kind, to excavate through the soils from Gorg Station to the open cut box of Sagrera Station (Lot IV) and from Zona Franca on the eastern branch of Lot II to Zona Universitaria Station.
On October 2007, ALE Heavylift Iberica, in cooperation with the Tunnel Department of Acciona Infraestructuras SA jointly started to look at solutions for sliding the TBM across Sagrera Station following breakthrough. The main challenge was the need to negotiate two opposing curves, of 285 and 300m respectively, within the station box (figure 2) and avoid disassembly and relocation of the back up equipment using heavy cranes.
Typically for this kind of slide, a concrete cradle and thrust frame would be employed, however due to the curved path this method was deemed unsuitable. In addition, the casting, assembly and demolition of additional reinforced concrete structures would be required in order to lift and relocate the back up equipment.
After several meetings a solution was reached based on an existing skid system that could be modified to suit the site conditions and restrictions.
In December 2007, the 1500t TBM shield and its back-up (equating to an additional 1000t), was skidded a length of 80m, successfully negotiating the two curves.
Equipment
The slide was carried out by means of standard ALE SKS5000 skidding equipment modified to suit the site conditions. Hydraulic skid shoes moved over PTFE (Teflon) blocks that were incorporated into steel skid tracks. The system included a 500t skid shoe incorporating a 500t capacity cylinder with a working stroke of 620mm. On top of this cylinder a pivot afforded longitudinal and transverse movement, which allowed adjustment to the curved path.
The force required for skidding the structure was generated by hydraulic push-pull cylinders, which were an integral part of the skidway. Each horizontal jack had a total pushing capacity of 64.3t and a pulling capacity of 33.9t. The units were coupled to the skid shoes by means of a pin-construction. Centralised diesel-driven power packs generated the hydraulic power required for operation of the skid shoes’ and push-pull units’ hydraulic cylinders.
Skid shoe stability was designed with a side-force of 20t – up to a maximum of 10% of the vertical load on the skid shoe involved. Four skid shoes were employed for the operation. These were arranged under special brackets welded to the TBM shield at positions previously approved by the TBM manufacturer.
The skid tracks were placed at a distance of 5580mm from the axis of the TBM, with 11700mm between the two tracks. The tracks were positioned on concrete slabs and fixed using specially designed anchors.
Once the TBM arrived at the station, preparations began with the welding of brackets onto the shield and installation of the final portion of the skid tracks. A 960mm high x 640mm wide and 6m long beam was installed over each pair of skid shoes and positioned under the brackets that had been welded to the TBM.
In this position the beams were welded to the brackets and the weight of the TBM was transferred to the skid shoes by means of the two rear pushing jacks and two front pulling jacks. The shield and backup were then slid simultaneously through the station box. To assist this task a special device that allows the back-up to be rolled across the rails was designed by Acciona Infraestructuras SA and Herrenknecht’s technical department.
Once the TBM reached its final position, the shield was again transferred onto concrete supports using the jacks and the skidding system was removed.
The entire sliding operation took eight days and was considered a complete success.
FURTHER READING
1. N Della Valle, 2005. “The Barcelona TBMs’ Learning Curves†T&TI, February 2005, p26.
2. N Della Valle, 2003. “The new Line 9 of Barcelona Metro†Proc. RETC 2003, New Orleans, SME.
Stacked configuration of the new metro Line 9 Inside view of the concrete segment plant, with rolling moulds and water steam curing chamber Segment storage for Barcelona Line 9’s TBMs The Herrenknecht shield is moved across the station The Herrenknecht TBM is slid through Sagrera shaft Assembly of one of the rolling supports for the back-up The TBM sliding process underway in Sagrera shaft Fig 1: Route map showing the alignment of Barcelona’s new metro Line 9 Fig 2: Sliding alignment for the TBM at Sagrera shaft
