Tunnellers on the latest section of Naples' Metro Line One had to cope with old mineworkings and fractured pyroclastic material while keeping a close eye on historic buildings above.Andrew Mylius reports.
By 2011, after 36 years of construction, a new metro line will form a 25.5km loop around Naples'city centre.The latest 4.5km stretch of Line One, which will eventually link suburban and inter-city rail lines and the airport with the centre, opens this month.
Work has already begun on the next 5km section.
Construction of the first 9km of Line One, from Piscinola to Vanvitelli, began in 1976. The district it runs through lies on a 400-450m high shoulder of volcanic tuff west of Naples' low-lying city centre. From the outset it was intended the line should be extended to connect with the centre, but only a funicular could have coped with the steep hillside. The section of Line One now in the final stages of commissioning has therefore been tunnelled in a spiral, softening the gradient to a manageable but still fairly severe 5.5%.Minimum radius is 170m.
Tunnelling in tuff is relatively straightforward, says Carlo Viggiani, head of geotechnical engineering at Naples University and adviser on construction of the metro. The rock is homogeneous and has good bearing properties.
Three separate consortia working under concessionaire Metropolitana di Napoli Construction carried out the latest ItL900bn (£285M) section. The first package, Cilea and Salvator Rosa stations and the interconnecting tunnel from Vanvitelli, went to Metrosud, a joint venture between ICLA Costruzioni Generali, Vianini and Costruire.
The next section of line, including Materdei and Museo stations, was built by the Comena joint venture, headed by De Lieto Costruzioni Generali with Astaldi and Giustino Costruzioni.
The line through to Dante and construction of Dante station was carried out by Cesif joint venture, led by Impresa and including Della Morte, Carola di A Carola, Grassetto Costruzioni, Torno International and Impregilo.
Metropolitana Milanese was responsible for design and project management.
Before tunnelling began, station shafts were built and used for access and mucking out.The route skims close to the top of the tuff strata soon after Museo station through to Dante, breaking through into overlying layers of fractured and in places cohesionless pyroclastic material. Here excavation of the twin 6.75m diameter tunnel was by road header and back-hoe operating within a shield.
Antonio de Risi, project engineer for client Commune di Napoli, says permeation and jet grouting was used in the crown and walls to stabilise the relatively loose pyroclastic material.Resin-anchored rock bolts stabilised fractured rock and steel arches were erected for temporary support.Glassfibre anchors were used to stabilise the face ahead of excavation.
An earth pressure balance tunnel boring machine made the drive from Materdei to Vanvitelli and back.
Advancing through stable tuff, there was little call for ground support, de Risi adds.
However, cavities in the tuff were encountered all along the 4.5km section, created by the quarrying of stone for construction many centuries ago.These had to be backfilled with vast quantities of grout.
De Risi says where the line crossed through or directly above a cavity, contractors were forced to advance a pilot tunnel and carry out micropiling to ensure loads would be transferred to competent underlying rock before pushing ahead with full-bore excavation.
Linings were a combination of cast insitu reinforced concrete, particularly in the looser, lower-lying ground, and bolted segemental precast elements.
De Risi has been watching closely for settlement and damage to buildings during the work. They have been monitored using extensometers and reflective targets while stress gauges have been installed in the tunnel linings.
Track bed is designed to minimise the transmission of vibration to surrounding ground and ultimately to delicate historic buildings in Naples' city centre.
The system was developed after Metropolitana di Napoli was sued by owners of buildings damaged when the first section of Line One was built.
Site preparation and archaeological investigation is now going ahead on the next 5km section of the line, through to Naples' rail terminus at Piazza Garibaldi. Five new stations will be built and it is intended they will be up and running by the end of 2005.
De Risi will need to be extra vigilant along this section, which passes through the heart of historic Naples.
Buildings with shallow foundations will be highly susceptible to movement in the predominantly pyroclastic material and made ground in which contractors will be working.The tunnel will be 10m above bedrock.
Historic structures will also be at risk if the high water table along the alignment falls.Construction so far has been well above the water table, but the Dante-Garibaldi section will be up to 25m below it. Original designs for cut and cover construction were abandoned in favour of full-face excavation to minimise above-ground disruption.
As on the previous sections, construction will begin with station shafts. Reinforced concrete diaphragm wall boxes will be installed. Ground anchors and temporary steel trusses may be needed to support the walls as the spoil is dug out, until the cast insitu reinforced concrete floor slabs are in place.