Metro Format Comprises 70km of light railway, of which roughly 6.1km is in bored tunnel. 12 stations are located underground and were excavated for using the NATM tunnelling system. Original Budget was 750 million Euros (55million euros allocated to tunnelling) Tunnelling started in June 2000 and finished 42 months later.
Tunnel – standard underground line Surface –urban areas with crossroads and traffic lights Surface – segregated channel with level crossings
Major constraint was in the CBD Porto is built on a relatively high gradient river valley.
Porto’s Geology Porto is built on a highly variable granite Batholith. Medium grained 2 mica (igneous rock) granite, which has undergone deep weathering due to the help of tectonic movements, such that decomposed granite are felt next to fresh granite at various depths and locations. Tunnels pass through ~ 6 grades of weathering (W1-W6) and alteration ranging from fresh granite to cohisionless granular soil. High water levels, man made ‘minas’ and deep wells increased the challenge ahead.
A typical distribution of weathered granite taken found at the face of the EPB driven Tunnel. The different degrees of weathering of the granite taken from a borehole. The weathered granite on the left side is at a depth of 24m under the sound granite of the right.
Tunneling 3 tunnels were to be excavated by a TBM EPBM boring at up to three bar pressure. They were lined with pre- cast concrete segments of 7-9 m internal diameter The machine had a nominal face opening of 20% to allow excavated material to flow through the machine face. A continuous conveyor belt was used to remove the soil before trucks took it away.
COLLAPSE OF TUNNEL No sooner had the boring started work on the tunnel, it was stopped as two houses collapsed resulting in the death of one. This was the third and largest collapse till the surface and it left a 250m³of hole. This happened due to the over excavation and face collapse in the extremely heterogeneous geology.
First Action A commission of inquiries was set up by the government. 16 recommendations were concluded as a specification guide. The Engineer, Construction manager and the contractor were forced re-think their roles.
First Action (cont.d) It concluded that the accident could have been avoided. Reinforcing team with trained geotechnical engineers. Advance knowledge of area to be drilled before the boring is used. Investigation of the ground structure resulted in an additional machine, in preparation for advance tunnelling.
Solutions Panel of experts after reviewing came up with two proposals about the EPBMs - The use of an automatic bentonite injection system. - An additional double piston pump.
CONTRACTOR’S MODIFICATION 10 independent foam generators. New rotary fluid joint. New twin belt weighing system. An early alert system in place when an operator reaches any critical level.
CONCLUSION Despite the safety precautions in place the use of TBM was recommended. Daily meetings held with management team to share information and any problem that may arise. Could have tried other tunneling methods (i.e.Cut-and-cover method, Drill and blast, Immersed tube method, Lunardi Method, Slurry-shield machine, Wall-cover construction method )