Barcelona, capital of the north east region of Spain, is a bustling city that has been continuously renovating since hosting the Summer Olympic Games in 1992. Recently two large civil and electromechanical contracts were awarded by GISA (the Barcelona region infrastructure agency) for the construction of the bored tunnel section of the all new Line 9 of Barcelona’s Metro. The line, when completed, will connect the new airport terminal (under construction) located south of the city to the Badalona & Santa Coloma municipalities in the north (see map). Works on site are expected to start in May 2002.
This long-awaited underground line will become the backbone of Barcelona’s public transport system, crossing and connecting to all of the city’s five metro lines and six railway lines, as well as linking with the high speed line now under construction which will connect the French border to Madrid. All these interconnections and other peculiarities of the route have led to the adoption of innovative technical and construction solutions.
The project
Barcelona is home to around 2.7 million people, who rely heavily on public transport for their daily commuting. The new line, with an investment of US$1.6bn, a total length of 41km (34km of which will be in tunnel), 43 stations and several strategic connections (airport, high speed railway, metro) will greatly increase the travelling possibilities across the city, making public transport even faster and more attractive for residents and visitors alike.
For more than two thirds of its length the line will pass below densely populated zones where surface areas are scarce and roads severely congested with traffic at peak hours.
Because of these constraints, which make it necessary to minimise surface and road access disruptions and the requirement for the new line to pass below existing metro and railway lines, an innovative and ambitious scheme has been developed. It involves constructing a deep line with stations integrated inside the tunnel section and large-diameter shafts equipped with high capacity elevators providing passenger access to the system.
This solution will allow an average of two diameters of overburden to be maintained on top of the tunnel crown to minimise surface influence, while allowing safe under crossing of all the other shallower metro systems, which were mainly constructed by cut and cover. The reduced access shaft area (24m i.d.) when compared with a cut & cover station will help to minimise surface impact, reduce property/land acquisitions and lessen the need for diverting traffic and utilities.
In addition, fast passenger transfer from one metro line to the other will be enabled by directly connecting the corresponding platforms by high-speed elevators. To implement this integrated station solution it has been decided to construct a bored tunnel with an internal diameter of 10.9m. This will facilitate the siting of two platforms and other station facilities directly inside the tunnel.
Works alignment
Because of the constraints posed by the intersections, connecting points and locations of the shafts, the alignment of the Line 9 has been mostly geometrically driven rather than ground driven; only small adaptations to the tunnel trace could be done to avoid bad ground areas such as faults or other sensitive situations. So far, although the design & build contract for the whole of the bored tunnel has already been awarded, the detailed geometry of the stretch between the airport and Sagrera Station is still in the design stage, while the geometry of the stretches from Sagrera to Sta. Coloma (Can Zam station) and Badalona (Gorg Station) are completely defined. Geometrical data includes a minimum bending radius of 300m and a maximum bored tunnel slope of 4%.
Variable geology
The Barcelona area is composed of a Palaeozoic crystalline basement of sedimentary origin including slates, limestone and micro-conglomerates which underwent a light regional metamorphism and cornubianites which were affected by a batholitic intrusion showing a certain degree of thermal metamorphism. These rocks, especially the granodiorite forming the batholith, show a high degree of weathering when close to the surface. Lying on this Palaeozoic basement is a Cainozoic series of Pliocenic argillites and Miocenic conglomerates (breccia type) with varying clay content. Further on top we find a Quaternary series of alluvial and delta origin composed of clay, silts, sands and gravel, which are related to the fluvial bed of Llobregat y Besós rivers.
These Palaeozoic and Cainozoic sequences have been highly disrupted by the batholitic intrusion, past orogenesis and neogene deposits resulting in a series of bending and faults causing wide displacement of all the indicated terms.
Going from south to north, the tunnel is bored through the alluvial formation of the Llobregat river mainly composed of sandy and clayey silts, sandy gravel and silty sands. Successively, close to Zona Universitaria station, it enters the Palaeozoic basement crossing all the geological formations which compose it, from the cornubianites to the granodiorite and encountering various tectonic contacts with relative displacement. At Sagrera Meridiana station the tunnel enters into the alluvial plain of Besos river of similar characteristics to that of Llobregat. At this point the line bifurcates; the north west portion enters into the granodioritic batholith and the south east section passes through the Miocenic conglomerate and more recent alluvial deposits.
A detailed geological investigation which required additional boreholes and test on samples was recently completed to increase the knowledge of the subsurface conditions and to detect critical areas which might require preventive ground treatment to decrease the risk to the passage of the TBM in such bad stretches.
The contracts
Following the indicated need for an integrated station inside the tunnel section, it was decided to construct a bored tunnel of approximately 12.1m od, which takes into account the 400mm lining thickness and the necessary shield and tail brushes allowance. The expected geology indicates that different machines are needed to bore through the mainly rock or soil formations. Accordingly, the bored tunnelling has been divided in two parts (mainly rock, and mainly soft ground) which have been tendered separately.
Two design & build contracts worth US$351.8M and US$265.4M respectively were awarded last September by GISA to two Spanish JVs involving all of Spain’s major general contractors. They are the Linea 9 JV led by FCC, and the Gorg JV led by Dragados Obras y Proyectos. Engineering supervision of the entire stretch of bored tunnel has been awarded to Paymacotas, a leading Spanish engineering firm.
Funding for the two contracts will be provided initially by the JVs through a construction loan from a group of Spanish and European banks. On completion and final acceptance of the works, the client will pay the total cost in one or more installments, plus the financing cost already agreed in the contract and based on Euribor index plus a fixed fee. Monthly certification issued by the Engineer will give the contractors access to relevant funds.
The first part of the line north of Sagrera station is scheduled to be operational by 2005, while the second part from the Airport to Sagrera is planned to come into operation in 2007. The design & build contracts also include all the electromechanical installation needed to operate the line excluding the supply of the rolling stock.
Record TBMs
Although the final construction design is yet to be finalised a general outline on the works execution can be given. According to the varying ground conditions along the alignment, both JVs opted for new machines designed to tackle different grounds; a Herrenknecht EPB shield has been preferred for the soil section (fluvial alluvium and Miocenic conglomerate), with the possibility of installing disks and operating in full open mode to cross a 180m-thick porphyritic dyke, which is inevitably present along the designed route. This EPB shield, with an overall diameter of 12.06m, is claimed to be the largest EPB contracted so far, with impressive nominal thrust of 110MN and torque of 38MNm capacity, adding up to a total installed power of 5.32MW. The cutting wheel will be of closed type, with material flow openings accounting for 35% of the total wheel area.
For the mostly hard rock sections (granodiorite, slates and cornubianites) a shielded Wirth–NFM Hard Rock TBM has been preferred. This machine, equipped with 76 disk cutters and capable of a total thrust of 110MN, will be adaptable to work in closed mode should it be required for unstable ground or fault zones. This will be done by replacing the first belt conveyor with a screw conveyor, and other ancillary modifications. A total installed power of 7.15MW is indicated for such a machine.
In both TBMs, special care is being taken in the design of the articulated shields and back-up trains, which will be capable of negotiating tight 300m radius curves.
The bored tunnels will be lined with a tapered single pass segmental ring 1.8m wide, 400mm thick. The rings will be either a 6+1 or 8+1 segment configuration. Waterproofing will be guaranteed by segment EPDM gaskets and 150mm thick annular gap grouting. Outside water pressure at the invert of excavation is expected to be well in excess of 3 bar at the deepest points.
Soil conditioning systems will be required on both machines for different reasons. The EPB shield will have to bore through low flowable and highly pervious sandy/gravelly formations well under the water level, where it is expected that bentonite and foam injection will play a fundamental role in a smooth progression. On the other hand, the hard rock machine will need substantial conditioning when working in closed mode, to lower material friction and improve ground flow through the cutting wheel openings.
Boring activities are scheduled to start in early spring 2003 for both contracts. In the meantime, the contractors are busy working on the detailed construction design and setting up starting shafts and access shafts to the stations. Both JVs will operate a three-shift/day, seven days a week, to achieve a planned average advance of 230m to 270m a month.
TBM boring routes
As previously indicated, the two contracts will run through completely different ground conditions along the planned route. Therefore, tunnelling has been divided into four stretches to allocate the correct machine to the surrounding rock type.
According to the schedule, the TBMs will start in the north section of the project and progress towards the southern section. Final decisions on the boring direction of the south section has still to be taken, but is likely to be as follows.
The Wirth Hard Rock TBM being used by the Linea 9 JV, will start from Can Zam Station and bore approximately 4km to just past Can Peixauet Station. It will then be dismantled and transported to Zona Universitaria Station. Once reassembled, the machine will then bore the 9km rock section up to Segrera Station.
The Gorg JV’s Herrenknecht EPBM will start boring close to Gorg Station in the north and will construct the 4km of tunnel south to Segrara Station. After disassembly at Segrera, it will be transported and reassembled at Zona Franca Station to bore the remaining soil section to Zona Universitaria.
Stacked tracks
The standard 10.9m tunnel id on both the hard rock and soil drives, which takes into account 100mm of construction tolerances, is necessary to house the split track system that runs one track above the other in opposite directions.
A concrete cast slab located close to the spring line separates the upper and lower sections, with each half carrying one track in one direction. For safety provision, the tracks will be sealed off from each other by the concrete slab. In the case of a fire, two-way fireproof escape doors will allow access between the two sections.
Shafts and stations
A particular challenge on this huge project will be constructing the connections between the shafts and the tunnels in soft ground. While all the details have not yet been worked out, an indicative procedure, with a similar one for the rock section of the project, has been identified.
Before the passage of the shield, the diaphragm walls will be constructed down to the design level. The area corresponding to the opening, which will be cut by the TBM, will be un-reinforced or fibreglass reinforced. The inner portion of shaft will be only partially excavated to just above the tunnel crown level to allow for necessary counter-pressure as the shield advances. As the shield hits the diaphragm wall it will bore through it, generating an elliptical intersection figure which will be supported by the segment rings. Proper tail-shield grouting will seal the segmental lining against the diaphragm walls allowing for a safe shaft excavation. Successive steps will require segment removal to gain access to the tunnel, structural connection between shaft and tunnel, and integrated station construction.
Ground monitoring
Special consideration is being given to the ground and structural monitoring of both tunnels and shafts, not only to control the structural performance of such elements, but mostly in consideration of the strict requirement to prevent and avoid any surface disturbance which may compromise old buildings or other sensitive structures.
Emphasis will be on the continuous recording of any movement ahead, during and after the TBMs’ passage. For this task GISA will award an independent contract to a specialised firm. The scope of works of this contract will include the fully automated monitoring of piezometers, stand pipes, settlement benchmarks and multi-point extensometers.
A geotechnical model will be constantly updated with the field data to verify the design assumptions and give an immediate feedback to the Contractors regarding their machines’ operating parameters. An automatic real-time alarm system will alert all involved parties when set trigger levels are approached.
Related Files
Figure 1: The full Barcelona transport system
Figure 2: The wide diameter tunnel
