North Bore Breaks Through at Piedicastello Tunnel in Trento, North ItalyThe objetive of the environmental and urban redevelopment in the Piedicastello area, the re-routing of the Trento ring road and the associated works is to divert traffic away from the ongoing infrastructure. The project involves the construction of a new road bypass to break the isolation of Piedicastello caused by the construction of the ring road. The carriageway width is identical to the existing motorway. The most significant piece within the project is the Piedicastello twin bored tunnel, provided with a pedestrian emergency escape situated halfway through and pedestrian cross connections between the two bores every 100-150 metres. These crosscuts will ensure, in case of incident or fire in one of the twin tunnels, the safe evacuation to the parallel tunnel of motorists possibly caught inside. Excavation of the southbound tunnel has been completed on 19th September, 2006. On 8th February, a hydraulic breaker holed through the north tunnel. The twin bored tunnel crosses the Doss Trento headland and the Sardegna foothill on the slopes of Mount Bondone, on the right bank of Adige River, at the western end of the city of Trento. The northbound tunnel stretches 864.53 m, with south and north cut-and-cover portions of 41.72 m and 11.58 m respectively. The total length is therefore 917.83 m. The southbound tunnel spans 908.16 m, with cut-and-cover segments of 18.73 m at the south end and 9.16 m at the north. The tunnel totals 936.05 m. The typical cross section is characterized by an inner radius of 5.15 m with and without invert. The roadway width is 8 m, with service sidewalks. Click here to see the cross section type A. The pedestrian emergency exit about halfway through the tunnel is a shaft with staircase.About 87% of the tunnel path crosses a calcareous formation named Malcesine limestone whereas 13% approximately is alluvial deposits consisting of sand gravels. Piedicastello tunnel, north portal Piedicastello tunnel, south portal From north to south, the tunnel alignment runs through Malcesine limestone under the Doss Trento on its first 350 metres, then meets alluvial deposits (116 metres) in Piazzale Divisioni Alpine, and again Malcesine limestone on its last 420 metres on the slopes of Mount Bondone. The calcareous bedrock named Malcesine limestone comprises of marly limestone alternated with marly interbeds of the Lower Eocene. In the bedrock, tufaceous calcarenite with thickness ranging from 4 to 5 metres is also present.To complete the south tunnel, crews have still to build 307 metres of invert and concrete bed and 444 metres of final lining at the vault. In the south tunnel, 124.5 metres remain to be excavated, 479.5 metres of invert and concrete bed and 553.5 metres of final lining at the vault have also to be finalised. At the moment, tunnelling is being done from two attacks, one from south and another one from north. Breakthrough took place under the Doss Trento at chainage 354 from the north portal. Through rock, the tunnel has been excavated by drilling and blasting and, nearby buildings, by means of hydraulic breakers. The last blast was performed on 16th October, 2006. Tunnelling continued exclusively with the usage of hydraulic breakers. The sections in soft ground have been dug by conventional excavator with bucket. The following machinery has been employed for tunnelling: Atlas Copco Boomer H173, second category explosive, detonating fuse and detonators supplied by Pravisani, two Fiat Hitachi EX 285 excavators equipped with breakers and a Cat 320 BSVA excavator with bucket. Visit www.atlascopco.it, www.pravisani.com, www.hitachi-c-m.com and www.cat.comThe rock is reinforced with primary supports, i.e. steel arches, Swellex bolts, welded fabric and shotcrete. The choice for supports depends on the geomechanical features of the bedrock. Visit www.swellex.comGround support in soft soil combines forepoles of steel pipes, jet-grouting columns, steel arches, and shotcrete sprayed on welded mesh. This was also conditioned to the ground conditions at the face.For support tasks, the following means have been mobilized: Atlas Copco Boomer H173 (for bolting), Swellex bolts supplied by Emam, CIFA shotcreting pump, steel arches from Sidertam, steel reinforcements from AltoLago and jet-grouting and forepoling performed by ILESP. Visit www.emam-italia.it, www.cifa.com and www.altolago.com.The rock and earth have been hauled by loaders and trucks: Fiat Hitachi V170 FHE loader, Fiat Hitachi V270 Evolution loader, Iveco 410E37H truck and Astra HD6 84.38Z truck. Visit www.iveco.com and www.astraspa.comThe main technical problems that teams had to overcome throughout excavation are related to tunnelling near the A22 motorway tunnels located at about 20 metres only, which could not tolerate any traffic interruption. Crews adopted sequential excavation and supporting techniques, using both the drill/blast method and hydraulic hammers. Also, the gravel stretch was bored under a thin cover of only three metres, with the presence of buildings at only five metres from the tunnels and road traffic at the surface of the ground above the tunnels. In that case, the ground was reinforced with jet-grouting columns. Lastly, another obstacle was a precariously balanced rock mass of 10,000 cubic metres on the walls of the Doss Trento, lying above a few houses and located at only 30 metres from the tunnel. The vibrations generated by each blast are continuously monitored by crack meters installed into the walls and the data is transmitted in real time.The contract was awarded on 26th January, 2004 to Lauro and construction is programmed to be completed on 26th May, 2008. Tunnelling commenced on 27th January, 2005. The difficulties encountered during tunnelling have pushed the project one month behind schedule but the time lost will be regained in the second, less difficult phase of works, which could be completed at year end.Altogether, an approx. 90,000 cu m rock volume will be excavated. Read E-News Weekly 36/2003. Excavation was performed at two drives from south and is underway at two drives from north since February 2006. The full length of the tunnel will be fitted out with lighting and all the necessary safety equipment such as ventilation, fire fighting devices, SOS signage, traffic control and variable message signs. The tunnel cost, not including the equipment, amounts to around EUR20 million. Visit www.provincia.tn.it 09/07.