Swiss spotlight refurbishment and upgrades 15 Jun 2017

Warm sunshine greeted delegates to the Swiss Tunnels Congress in Lucerne in late May to discuss approaches to rail and road tunnel refurbishment and upgrades.

Stefan Maurhofer, President of the Swiss Tunnel Society, welcomed 375 attendees and stressed the importance of maintenance and modernisation of underground infrastructure and the demands of performing such vital works while maintaining existing rail and road services.

Presentations explored a number of aspects of the standard construction system that has been under development, and implemented by, rail operator Rhatische Bahn AG (RhB, or Rhaetian Railway) to enlarge and improve many of its older tunnels. The delegates were also briefed about the Swiss Federal Roads Office (ASTRA) work, currently underway, to enlarge the Stalvedro Tunnel on the A2 motorway, near Gotthard road tunnel.

Materials were also discussed, including approaches to understanding and using sprayed concrete linings by Karl Gunnar Holter, technical manager for underground construction at BASF and Assistant Professor at NTNU, Norway.

He referenced a number of recent projects employing the system in Norway (Gevingås; Holestrand) and Switzerland (Lausanne Metro L2-Tunneld Viret; Lungern; Mormont; Gobet), and mentioned also a recent trend for cast-in-place concrete lining in hard rock tunnels, such as Ulvin rail tunnel in Norway. He added that benefits to owners, with respect to investment cost, can include using only thin linings in tunnel rehabilitation projects with space constraints, and generally being able to reduce total lining thickness and avoid excessive amounts of concrete.

A standard upgrade approach to rail tunnels was discussed. In the Swiss Alps, RhB owns a total of 115 rail tunnels with a combined length of 58.7km, standardised single-track structures, usually with masonry linings, and all of which are more than a century old.

Except for the Engadine Line, the rail tunnels are 4.7m high with horseshoe profiles topped with circular vaults (2.15m radius), and have slightly inward leaning straight side walls for a base width of 4.04m, all together giving a net profile area of 17.9m2. The profile was designed for the era of steam trains.

An asset condition assessment in 2012 revealed that about two-thirds of the tunnels were classed as either poor (13 structures) or damaged (64) and the respective time-frames for improvements are 5-10 years and 25-35 years, respectively.

The problems relate to the similar construction methods used, and arise from water and moisture ingress and frost, such as damage to masonry joints or breakdown, especially at the crown and sides, and/or excessive horizontal loading at the straight and sloping side walls – which are not as structurally robust as curved vaulted walls; and, ineffective dewatering with consequent loading effects. But bearing capacity is not an issue, leaving the challenge as a serviceability problem, said RhB head of construction, Urs Tanner. Increasing the tunnel area to hold the kinetic envelope of the moving train with more margin is also important.

Early initiatives to develop a standardised approach for repairs and improvements were tried over 2010-13 on three tunnels – Argenteri, Charnadura and Klosters – using thin (50mm) shotcrete layers and local damp patch fixing. Only limited, medium-term benefits were gained and the approach was re-thought to obtain long-term economic improvements from methods capable of being rolled out over the whole of the RhB network, he said.

A 2012 concept study explored having a standard new profile constructed of precast, high side walls topped off by a sprayed concrete crown below the existing tunnel top, and lowering the track bed. Critiques over aspects, including discontinuities between materials, led the concept towards a fully precast profile with lowered track bed.

There was also design development, with the help of Herrenknecht, to employ a telescopic, steel protective formwork system to help staged, ongoing construction activities at night and during the day while ensuring operational system as rail services continue.

Trials of different precast systems were run at Hagerbach Test Facility.

The prototype standardised system was tried out at RhB Glatscheras Tunnel, the 334m long structure on the Albula Rail Line chosen for its benefits of access and relatively simple geology. Glatscheras was originally built in only 144 days.

At Glatscheras, the new larger profile comprises five elements of 300mm thick precast concrete in 1.5m long rings. The segments were cast with moulds in factory conditions, where they were trial erected in early 2015. Specially designed sealing profiles were also manufactured for the axial and circumferential joints of the segments. Limited space on site called for regular deliveries of the segments, and tight logistical control.

In the tunnel, the telescopic formwork was used for staged breakout of the masonry lining and rock, and shotcrete support applied. Excavation shifted from transverse cutter to drill and blast as the rock quality was found to be even better. The floor sides were excavated to establish foundations for the precast rings. RhB rail wagons were adapted to be segment delivery trains, and also carried a specially-designed manipulator to place them. Dewatering channels were installed at the sides of the profile. The tunnel floor was lowered by 1.2m, slab track installed in stages. Pea gravel backfill was placed in the 150mm annulus behind the new lining.

Details of the segment train, telescopic formwork and staged bridging system of progressively lowering the track bed while ensuring normal train service operations every day following were discussed in detail by Christian Schreiber of Rhomberg Sera rail Group. He added that an Atlas Copco Hagloader was used for mucking out as the tight space prevented having a machine with swivel action.

The pilot standardised construction works at Glatscheras ended in late 2016, and equipment installation is nearing completion, said Peter Kirchhofer of consultant Gruner. He adds that the project has increased the net cross-sectional area of the tunnel by more than a third, which is equivalent to obtaining a new build tunnel.

RhB has around 75 tunnels to improve using the system over the next 25-35 years, equating to approximately 26km at a rate of 500m per year while working safely in tight spaces and keeping rail services running. Much of the approach can be prepared for and standardised, said Kirchhofer, but every tunnel is different in terms of particular challenges of geology, hydrogeology, access, routing, connections and more, and adaptions will be needed to local situations.

Reflecting on the development of the standardised approach by RhB for its rail tunnels, STS president and session chair, Stefan Maurhofer observed the importance of contractor involvement in the planning and project development process. “Early integration of the contractor is important – it is a central thing to do,” he said. “It will be interesting how this is further developed.”

Adding a third lane to the Stalvedro road tunnel was discussed, with the challenges of tunnel upgrade on road infrastructure on works which are nearing completion, with road surfacing the latest stage of activity.

Built in only the 1970s, the two-tube Stalvedro Tunnel needed a hard shoulder, or third lane, added to one of the tunnels for traffic safety and congestion reasons. The other tube is being renovated. Contractor on the works is a JV of CSC, Impresa Costrzione and Edilstrada. The client is ASTRA.

Boundary conditions were among the prime challenges of the project. These included: the tunnel being enlarged as it was near its sister tunnel; the tunnels passing below a federal rail tunnel with a minimum clearance of 8.6m, and leading to the enlarged profile being a relatively flat vault; the poor ground at the north end of the works; traffic routing; and, seasonal restrictions of concreting works that required the road to stay open in summer and left the activities at portals to winter months.

Excavation in good ground was performed by drill and blast with support employing shotcrete, bolts and mesh. Poor ground required an excavator or hydraulic breaker and heavier support, including a spile canopy. Heavier support was also used where the tunnel passes below the rail tunnel, and monitoring performed. Weathered rock at the portals called for pipe roofing umbrellas

ASTRA’s roads manager, Mario Fioroni, highlighted for delegates key lessons from the project that include the importance of analysing both the geology and construction history of an existing asset.