In the construction of the new high-speed Córdoba-Málaga line, which crosses the Betic System, it was necessary to dig various tunnels whose total length adds up to 19 kilometers. Notable among them for its length (over 7 km) and for the complexity of its terrain is the Abdalajis tunnel which, starting from near the Antequera Santa Ana station to the west of the Guadalhorce reservoir, crosses the mountains of the same name.
Its more than 7 kilometers traverse an area of major hydrogeological complexity in which the subsoil is a continuous chain of aquifers.
- At over 7 kilometers long, it is the largest ever dug in Andalusia
- The excavation work was begun in November, 2003, and continued for 26 months
- It is composed of two tubes connected by a total of 19 evacuation and safety corridors, with a total length of 500 meters
On the east tunnel, the first to be completely bored, the work was completed in 26 months. 10 more months of additional conditioning work were needed to meet railway use requirements, permitting the installation of the tracks and electrification and signalling superstructure.
THE CHAIN OF AQUIFERS: AN ENGINEERING CHALLENGE
The project constitutes a major engineering challenge due to the structural complexity and the lithologic diversity of the Abdalajis Valley mountain range.
The area is a continous chain of aquifers.
Informative and environmental impact studies
The studies carried out in the project phase advised following the course undertaken, as it would affect the aquifers as little as possible. The alternative, through the area of El Torcal de Antequera, would have affected larger aquifers and would have required digging longer tunnels.
The hydrogeological study on the aquifers was undertaken as part of the construction project, as was the development of the obligatory Environmental Surveillance Plan.
The impact on the aquifer
In March 2005 the first encroachment on the aquifer occurred, in about the middle of the excavation.
The Tunnel Boring Machines continued to advance in order to waterproof the lining rings. As this work moved forward, the flow of draining water was reduced.
The settling and purification systems for drained water guaranteed the absence of pollutants.
Water coming from solid rock was channeled toward the settling basins built for that purpose at the tunnel mouths. These basins purified the water in three successive phases.
Once it was purified, the water drained from the tunnels into the Arroyo del Quinto could be used for irrigation and to replenish the Cuenca del Guadalhorce aquifer, which suffered greatly during the last few years' drought.
The permanent monitoring and quality analysis of the waters removed was performed by independent laboratories.
Supplying the population
From the moment of the encroachment, Adif serviced and fulfilled the water supply needs of the town of Valle de Abdalajis by means of tanker trucks (about 450,000 liters per day) and constructed two new wells that will ensure the supply for the population and other uses.
Final waterproofing of the affected area
Once the tunnel boring was completed and the first waterproofing of the affected zones was accomplished, the next step was to reinforce them by building interior rings of reinforced concrete which would give the affected areas greater stability and greater resistance to the hydrostatic pressure.
In this way continuous drainage of the aquifers, which would have been the solution with the greatest environmental price, is ruled out; recovery of underground waters is made possible and tunnel safety during railway use is ensured.
For the construction of the tunnel, the project was divided into two stretches (stretches 12 and 13 of the 'Gobantes - Abdalajis tunnel' project) which included the building of the two parallel tunnels and the conditioning of the tunnel mouths. With a length of 8,970 meters, they traverse the municipal jurisdictions of Antequera and Alora (Málaga). The Railway Infrastructure Management (GIF) tendered for these stretches on November 28, 2001.
The project was designed using a bi-tubal typology, consisting of two identical one-way circular section tunnels with an available interior diameter of 8.80 meters.
The principal dimensions of the 'Gobantes - East and West Abdalajis Tunnel' stretches are:
- Length: 8,970 m.
- East tube length: 7,043 m., plus 227 m. of false tunnel; total 7,270 m.
- West tube length: 7,067 m., plus 230 m. of false tunnel; total 7,297 m.
- Minimum schematic radius: 6,900 m.
- Maximum tunnel gradient: 16.02 mm/m (descending toward Málaga)
Both tunnels are united by a total of 19 cross-galleries, almost 350 meters, which allows the evacuation of one tunnel to the other in the event of an emergency or incident in either of them.
One out of three of these galleries was built during the construction phase for safety reasons, to facilitate tunnel evacuation in the event of an incident. Thus, there was always a gallery less than 1000 meters from the front of the excavation.
TUNNEL BORING MACHINES (TBMs)
The Abdalajis tubes were bored with double-shield TBMs.
- The tube TBM was baptized "La Alcazaba", while the western tunnel TBM was named "La Mezquita"
- Each of these machines has a diameter of 10 meters and is 11.89 meters long
- Their total length, including the back-up where the control room and other installations are, is 110 meters
One of the principal auxiliary installations is the electric substation for the tunnel's power supply. The station has 22 MW of power which is supplied by means of a new 66 kW (kilowatt) line that is 1,400 meters long.
Due to the structural complexity and the lithologic variety of the Abdalajis Valley mountains, the project was divided into five differently lined sections, depending on the conditions created by the terrain traversed throughout the route. Four different types of concrete and steel voissoirs were used; types A, B, C and H, from lesser to greater resistence.
Complete waterproofing is also an essential requirement to guarantee that the tunnels are fit for the use of the high-speed Córdoba-Málaga railway line.
One of Adif's greatest priorities in the pursuance of its railway infrastructure construction activity is its commitment to the environment, which forms a part of its Social Responsibility. For this reason 12.8 million euro has been earmarked to guarantee the environmental integration of the project, by means of the adoption of measures to protect the fauna, the flora, and the hydrologic system, as well as more for the recovery of the natural surroundings and integration into the landscape
Protection of the hydrological system
- The project's auxiliary installations were located outside of the most permeable areas and the area of influence of streams and drainage channels.
- A transverse drainage project was planned to guarantee that the water would drain correctly.
- Two settling basins were designed for each area, to collect the water coming out of the tunnel as a consequence of the boring. They were designed so that they could also be used to facilitate cleaning the mud from the bottom.
- In the environmental surveillance programme a quality analysis follow-up is included for the water contained in the settling basins.
- The treatment and management of all kinds of contaminant residues (oils, lubricants, etc.) generated during construction is accounted for.
- In the environmental surveillance programme the surveillance and checks necessary to perform a follow-up on the behavior of the aquifer were included, with regards to both the quantity and quality of the resource.
Protection of the flora and fauna
- A study of the fauna in which the principal mating, feeding and breeding grounds were identified, as well as the corridors along which the fauna habitually travels.
- Elaboration of a calendar setting seasonal limits on blasting, cleaning and clearing of the area to avoid impacting the animals during periods of breeding and nesting, corresponding to the period from january to august.
- Measures aimed at preserving protected animal species and areas of faunal interest, nesting, sheltering or hunting grounds, from the disturbances or impacts inherent in the construction phase.
Recovery of the natural surroundings and integration into the landscape and environment
The construction of the Abdalajis tunnels is an environmentally friendly project. The work has followed the directives of the Córdoba-Málaga High-speed Line Informative Study and the subsequent Environmental Impact Statement.
- The landscape integration measures are focused on the treatment of the most visually fragile areas, taking into consideration the resulting views from different strategic points outside the train, and the impact of different construction elements (false tunnels, dumps, etc.)
- For replanting on slopes the criteria set in the Environmental Impact Statement were followed, and the slopes were classified into different replanting typologies depending on their gradient, type of soil, existing microclimates, etc. In this way, each type was replanted using the optimum species and methods.
- All of the dirt left over from the excavation was defined and evaluated environmentally in such a way that its use in the project itself was planned, thus reducing the volume of materials to be dumped as much as possible.