The upgrading of trunk road SS 47 in Valsugana valley in Trento province aims at ensuring smooth and safe traffic from east to Martignano and the other way round. The bypass road mainly consists of a twin bored tunnel with two lanes per tunnel.
The project spans approx. 4,295 metres. There are actually three bored tunnels, two main tunnels and one slip road tunnel. The Padova-Trento uphill tunnel is 2,964.53 metres long while the Trento-Padova downhill tunnel is 2,929.04 metres long. The slip road tunnel is approx. 293 metres long.
The main tunnels were excavated by means of a 12.11 m-diameter Herrenknecht TBM whereas traditional means were preferred for the slip road tunnel (drilling and blasting and hydraulic hammer in the first 200 metres, then pipe umbrella support system, steel arches and shotcreting). A 2 m-diameter tunnel, in which a water supply pipe is installed to supply Trento city’s aqueduct with water, was also reinforced. Reinforcement work was mainly performed on a 160 m-long segment where the twin tunnels pass beneath the Cantanghel aqueduct. It consisted in overexcavating 40 centimetres around the entire diameter of the existing cross-section, then a reinforced concrete shell has been poured. This was necessary when the TBM had to bore underneath the water tunnel, leaving just an approx. 1.5 m-thick rock nucleus in between.
Herrenknecht’s 140 m-long, 2,000-tonne machine S-251 was equipped with a cutterhead dressed with 68 tungstene cutters, delivering a maximum thrust of 7,061 tonnes, a rotation speed of up to 4.5 rpm and a cutterhead power of 3,600 kW. The machine’s price tag is EUR16 million. The TBM commenced digging the second tunnel on 3rd June, 2005 and completed the job on 12th December, 2005. Read E-News Weekly 2/2006. Breakthrough of the first tunnel took place on 29th January, 2005. Read E-News Weekly 7/2005, 45/2004 & 3/2004. Visit www.herrenknecht.com
The main tunnels are lined with precast reinforced concrete segments that were produced in a segment plant outside the construction site. The rings are made of six segments plus the keystone, each 40 cm thick, 1.5 m long and 148 to 152 cm wide. The trapezoidal rings with universal segments enabled crews to assemble the rings in 19 different positions, thus allowing to achieve curves both horizontally and vertically. A total of 24,570 segments were installed in both tunnels, which represents 3,510 lining rings. The segments were produced from CIFA moulds. Visit www.cifa.com
The two main tunnel are connected by 11 cross passages, at intervals of 250 metres each. Two emergency lay-bys, each 60 metres long, have been constructed in each bore. Both the cross passages and lay-bys were constructed by drill and blast and hammer and subsequently supported with steel arches, bolts and shotcrete. A PVC membrane covered with a 80 cm-thick reinforced concrete layer was installed for waterproofing insulation.
The geology encountered during tunnelling is calcareous, in particular red scale, multicoloured scale and grey limestone. In the last 150 metres, loose ground was encountered with consolidated marly material. TBM excavation in the main tunnels was carried out from a single drive, one tunnel after the other whist the slip road tunnel was dug from two attacks. The rock was hauled away from the tunnel face by a conveyor belt up to a dump area outside before being transported away.
A service tunnel was dug under the roadway to house the technological and safety equipment and drainage system.
Design analysis undertaken by SWS Engineering required the implementation of analytical models and subsequently the working out of 2D FEM models in most critical stretches. The challenge of boring under the water tunnel strategically vital for the city of Trento and the east slip road tunnel required the working out of BEM 3D models. Visit www.swsengineering.it/progetti/geotecnica/martignano.html 33/07.
