The Steg-Raron contract forms the southern section of the 35km Lötschberg tunnel, part of the AlpTransit project, in Switzerland. In total the Lötschberg project consists of 81.5km of tunnels, to form the twin bores that will carry the railway. The US$410M Steg-Raron civil works contract (excluding trackbed) is notable for being the only portion of the main Lötschberg tunnels to be constructed using TBMs (Figure 1). At the northern end of the project, there is one other TBM section – the 9.6km Kandertal exploration/service tunnel.

The four main partners of the MaTrans joint venture for the Steg-Raron contract are Marti Tunnelbau AG of Switzerland (the lead partner), Balfour Beatty of the UK, Porr Tunnelbau GmbH of Austria and the Walter Group of Germany. The Steg and Raron sections were initially awarded as separate contracts but, after MaTrans won both, it was decided to combine them into one contract. The first of these was awarded in November 1999 and runs for 6 years.

While the bulk of the Steg-Raron section is driven using two TBMs – with drive lengths of 10km and 8.9km – there is also a 4.6km length of running tunnel, 8.4m i.d., constructed using drill and blast. In addition, there are also 30 cross passages, located at intervals of 333m, a turn-out tunnel and two operations chambers.

Geology

Along this section of the alignment overburden reaches up to 1700m (Figure 2). The geology primarily consists of homogeneous, hard rock formations. The first section consists of the Lias-Dogger-Malm limestone sequence, which is in some cases very schistose and dips flat in the direction of the drive. The Aar Massif follows this, consisting of granodiorite, very hard massive granitic gneisses, hard and compact granites (coarse and fine-grained), very hard gneisses and gneisses that are schistose. In this latter rock there was an added and unexpected complication in the form of a thin stratum (approximately 50mm thick) of naturally occurring asbestos (see T&TI, Jan 2003). The exception to hard rock formations was the 500m long soft Trias zone.

Overall, the ground encountered differed considerably from that anticipated at tender stage, being much harder and more abrasive. Crushing strengths of up to 250N/mm² and Cerchar values of up to 6 were measured. This was reflected in significant changes in the proportions of each support type used.

TBM drives

The hard and highly abrasive rock posed a significant challenge to the two Herrenknecht open face TBMs (T&TI, Jan 2003), with penetration rates reducing to as little as 1mm per revolution in the granite. Temperatures in the cutterhead reached 80ºC, and individual cutter discs up to 200ºC, in these sections.

Despite deflection wedges, incorporated to protect the discs, heavy wear required up to 26 of the 66 discs to be replaced per day, with each disc taking up to 60 minutes to be changed.

Overstress in the rock mass also led to block failure in the face and resultant large blocks created problems in the cutterhead and on the conveyor belts used for spoil removal. Overall there were wide fluctuations in daily production rates due to variations in the rock and breakdowns. Table 1 summarises the advance rates for the TBM drives, both of which have now been completed.

Two factors were identified as being essential to achieving faster advance rates. Firstly, the need to have a well-trained and motivated team and, secondly, the importance of maximising the availability of the TBM and the conveyor belt muck-out system. To achieve this a maintenance and servicing regime was applied to both the TBM and conveyors. Also reserve equipment and spare parts were made readily available to minimise downtime.

Typically rock support consisted of 80mm of sprayed concrete, mesh and 3m long Swellex rock bolts. Invert segments were placed 30m back from the face. To reduce the environmental impact of the tunnel, spoil from the drives was recycled as aggregate for the invert segments.

Drill and blast drives

From the Raron portal, the northbound running tunnel was constructed using drill and blast. Each round consisted of 110 blast holes, which were charged with emulsion explosives. Typically the pull length was 4m for the tunnel with its 67m² cross-section. Rock support consisted of steel fibre reinforced sprayed concrete. The drill and blast tunnel achieved advance rates of 6m/day (average) and 17m/day (peak) in contrast to the TBMs, which reached an average of 12.4m/day and a peak of 50m/day. The actual advance rates for the drill and blast section rose as the tunnel progressed.

In perhaps the first ever application in tunnelling, logistical support for the drill and blast tunnel was carried on a raised platform, which was supported by rails suspended from the tunnel crown. This novel design by ROWA left the tunnel invert clear for construction traffic and casting of the invert slab. The suspended back-up proved to be very successful, despite initial scepticism amongst some of the tunnelling crews.

As on the TBM drives spoil was removed via conveyors. To facilitate this there was a crusher unit to reduce maximum block size to approximately 200mm.

Drill and blast was also used in the East Tunnel to excavate the soft Trias zone ahead of the TBM, the machine was then pulled through to recommence boring on the other side of the zone. In the West Tunnel, the Trias zone fell within the drill and blast section anyway.

Permanent works

The design for the permanent lining embodies a drained solution, with invert drains provided to remove any infiltration. In wetter areas of the tunnel, primarily near the portals, a waterproofing sheet membrane is being installed to direct the water to the drains. In total, approximately 125,000m² of sheet membrane has been installed.

Currently, casting of the permanent lining and formation of the walkways is underway. The planned progress rate for the secondary lining is 210m/week, using 12.5m long shutters for the main arch. Each pour for the secondary lining in the TBM drives requires 90m³ of concrete, while 155m³ is required per pour in the drill and blast section due to overbreak. Casting of the permanent lining is executed in two stages, first casting a kicker and then the arch. Walkways are then added.

This work represents a considerable logistical challenge and requires more labour than the excavation phase. There are walkways on both sides of the single track in each tunnel and cables and services are laid within the walkway concrete.

Safety

The project’s Accident Frequency Rate has exceeded 15.0, although now it has been reduced down close to 8.0. This contrasts with Balfour Beatty’s operations target of under 0.3 in the UK. Local factors play a large role in inflating the figures to high levels, for example the degree of absence of more than three days is far greater than in the UK, there being a different social system applying. However, it is important to note that there has not been a single serious accident during the entire excavation. A more accurate reflection of the safety record on the project was provided by the Swiss Accident Insurance Institute assessment, which stated that the MaTrans contract was the lowest risk site, for its size, in the whole of Switzerland.

Contract and staffing

The contract is a re-measurable Bill of Quantities with a defined payment plan. This payment plan is adjusted according to actual progress. The contract was written in accordance with the Swiss standards, SIA 118 for General Conditions for Construction Work and SIA 198 for Underground Construction Work, and includes a three-year defect liability period. Penalty clauses exist for deficiencies in safety, quality and compliance with the contract and there is a Disputes Review Board to mediate in areas of disagreement. As noted earlier, the ground encountered differed from that anticipated and this remains the subject of contractual discussions.

In line with European practice, staffing levels were low and there was a lean management team on site, consisting of 32 staff for a multinational workforce of around 300. Despite being a re-measurable contract there were no quantity surveyors on site, as the section engineers handled all aspects of the work. Inside the tunnels the crew for the 9.4m external diameter TBMs consisted of 14 men, supported by an additional five who drove trains or worked on the surface. In the drill and blast section, each shift comprised nine men working on excavation and mucking, seven casting the invert and a further seven installing rock support or performing other work. The shift pattern was 7 days on/1 day off/7 days on/6 days off, with 3 shifts per day to enable 24hr working. Two 9hr shifts were for production, with a 6hr maintenance shift each day during mining. During the lining of the tunnel no maintenance shift is required. The invert segment factory was run by a team of 15 men, who produced 25 segments per day, working a 5 day week.

Conclusions

Having been awarded the contract in November 1999, construction has progressed well. To date, almost 93% of the tunnelling has been completed. Most of the remaining excavation relates to the two operations chambers. Casting of the permanent lining is now underway.

The project is due to finish on programme in 2005. The Steg-Raron project has demonstrated the cost effectiveness of constructing long large diameter tunnels in hard rock using TBMs.

Related Files
Fig 2 – Longitudinal section of the Lötschberg Tunnel
Fig 1 – Plan map of the Lötschberg Tunnel
Fig 3 – Cross sections of the drill and blast and TBM driven tunnel profiles