There is not small project when it comes to take part in an ambitious challenge. The N1 tunnel project in Porto, Portugal, or Ceuta tunnel as it is also known, is a segment of large-scale civil engineering to improve traffic flow between Porto's downtown district, the Santo Antonio hospital district and the highway access. The tunnel is 650 m long, 400 m of which blasted. The tunnel was excavated into the massive Porto granite, at a depth of 4 to 20 metres below a dense urban environment.
The project was originally started in 2000 and was stopped due to the inability to mechanically excavate the granite rock. The City of Porto then turned to traditional drilling and blasting techniques. The use of pyrotechnical delay detonators did not easily comply with the specified environmental and safety requirements concerning blast charging, firing and vibration control which resulted in the process taking far too much time. The introduction of digital blasting was proposed to overcome these concerns. Excavation of the Ceuta tunnel was entirely performed in an environment where concerns were expressed regarding the implementation of the drilling and blasting technique. A combination of both geological and economical requirements led the contractor to propose Daveytronic®, a digital blasting solution.
Drill/blast tunnelling resumed in January 2004 with blasts at 4.5 m below the foundations of Porto's Forensic Medicine Institute. Blast holes, 38 to 42 mm in diameter, had to be drilled at 1.5 m length maximum. This method would result in over 1,000 blasts for the excavation of the upper section. This implied a low excavating rate of 1.5 m per day maximum in the best rockmass and worksite conditions. The final blasting pattern was defined after about 10 blasts with the following characteristics: 160 boreholes and drilling length of one metre.
The excavated cross section is 100 square metres. The top heading is circular-shaped with an external radius of 5.8 m. The average top heading is 55 sq m and the average bottom section is 45 sq m. Excavation was performed in two steps, first the top heading, or upper half section, then the benches and invert, or lower half section. The overburden above the tunnel ranges from 4.5 m at the western portal to 23 m.
The safety features of the Daveytronic® solution were taken into account throughout the designing process. The system relies on rigorous security principles: separation of the testing and firing circuits within the detonator, with the use of two independent capacitors; blast data storage (detonator identification number and delay) in a non-volatile memory unit; unique firing capacitor energy level test (firing order can only be sent once all detonators have reported a fully charged firing capacitor).
The use of the Daveytronic® digital blasting system improved the tunnel advance rate by +190% in sound granite and reduced tunnelling time by 30% compared to the initial method. An overall reduction of 60 operating days in the duration of the excavation of the upper section of the project was reached, while maintaining all specified environmental standards. The saved time allowed the contractor to complete the tunnel construction in the period of one year, as specified by the City of Porto. In spite of depths ranging from 20 m to 4.5 m, tunnelling of the top heading was performed with no damage to the many structures in the area. Spie Batignolles was assigned with the construction of the tunnel.
Daveytronic® offers efficiency and performance. In tunnels, mines, quarries and construction, the Daveytronic® solution has proven its performances in terms of vibration control and reduction, wall control and stability, improved fragmentation, lower excavation and crushing costs, dilution control, and inventory management.
After 10 years of prevarication, controversy, bankruptcies and litigation, Porto's Ceuta tunnel fully opened in October 2006. The tunnel, which starts at Praça Felipa de Lencastre and feeds into two exits - one at the Santo Antonio Hospital, the other at the Soares dos Reis Museum - was a story of incompetence and a catalogue of disasters from start to finish. It was EUR5 million over budget, took six years longer to complete than was necessary and involved the mayor of Porto, Rui Rio, in a court case.
Work on the EUR27 million tunnel, designed to ease traffic congestion in the city, began in 1996. The initial proposal was that it should exit in front of the Forensic Medicine Institute on the right side of the Carregal Gardens and the Soares dos Reis Museum. This plan was opposed by the Portuguese Architectural Heritage Institute (IPPAR) because it was too close to the protected Santo Antonio Gardens, the museum and the Santo Antonio Hospital's accident and emergency department.
In 1999, the then mayor of Porto, Nuno Cardoso, ignored the IPPAR and ordered the excavation of the tunnel's exit, right in front of the hospital's accident and emergency department, while at the same time digging the entrance in front of the Hotel Infante de Sagres.
In 2000, the project ground to a halt because the construction estimate of EUR22 million was too low and the project ran out of cash. The City of Porto was faced with a difficult and embarrassing decision - either sack the construction company and pay legal compensation, or find the money and continue with the project. The city decided to continue with the work, by extending the mouth of the tunnel away from the hospital and gardens and asked the EU for more financial help. However, the IPPAR turned down the project, proposing that the only solution was to place the tunnel's exit further along by the Palacio de Cristal.
For years, the Ministry of culture ordered work to be stopped and the case spent months in court until the stalemate was resolved by altering some details of the initial project. The tunnel's exit is now 78.3 metres from the museum's entrance and the speed limit was reduced to 30 km/h. Visit
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