Swiss road upgrade sees TBM finish at Belchen 20 Jul 2017

Upgrade and rehabilitation of the Belchen road tunnel in Switzerland has reached a milestone with completion of TBM excavation of a third tube to expand capacity on the heavily trafficked section of the A2 motorway between Basel and Egerkingen.

Breakthrough celebration at Belchen

Contractor Marti completed the 3.18km long new bore with a 13.97m diameter Herrenknecht TBM in late June, three months ahead of schedule. The single shield TBM was launched in February 2016, and, while limited to one-shift operation on weekdays only, the shield achieved progress of up to 90m in five days.

ASTRA, the Swiss Federal Roads Office (FEDRO) delivery agent and client for the Belchen Tunnel upgrade, was supported on the complex planning and excavation operation by consultants Emch+Berger, Aegerter & Bosshardt and ILF Beratende Ingenieure.

The Belchen area of the Jura Mountains presents geological challenges for tunnelling, as they did for construction of the existing twin two-lane tubes on the motorway. The geology consists of tightly compacted folds and thrust faults, and includes a large amount of swelling rock comprising gypsum Keuper marl and opalinus clay.

Fig 1. Belchen road tunnel route with the new bore on west left side

Increasing rock pressure over time now requires repair in the existing tunnels. Those works will progress once the new tube is commissioned in 2022. During the works, traffic will use two of the three tubes until all completed. The current west tunnel will then be reassigned as a central escape and rescue tunnel, linked by new passenger and traffic cross passages. ASTRA’s budget for the entire upgrade project is approximately CHF500 million or about US$525 million.

Marti Site Manager Sergio Massignani said that the construction experience of the existing tubes provided comprehensive information for anticipating conditions on the new tunnel and for design of the TBM to cope with the swelling rocks. The new tunnel runs at about 40m to the west of the existing tubes, increasing to 116m apart in one area to ensure sufficient cover, and narrowing to 19m towards the north portal (Fig 1).

At the recent 2017 Swiss Tunnel Congress, attended by TunnelTalk, the client’s consultants explained that during design of the third tube, both TBM and drill+blast methods were under consideration for excavation of the complex geology, which includes 15 expected transition and fault zones.

Risk analyses had shown that additional measures would be required for both methods. The TBM option included facilities to drill boreholes, with or without core recovery, to investigate ahead of the face and installation of GFRP rock bolts through openings in the shield skin to create a spile canopy or pre-excavation pipe roof umbrella. Special measures were also specified for dealing with any interruption to TBM tunnelling at the transition zones.

Herrenknecht third bored TBM set for launch

Design of the lining behind a TBM or drill+blast excavation had to counter the development of swelling pressure over time. Congress delegates heard that, during design, there were no recognised and verified approaches for assessing the development of swelling pressures. The design approach therefore was to adopt a force-locking ring closure system, as based on empirical values from earlier works at Belchen and on the experience of other projects in Switzerland including the Adler, Mont-Terri and Chienberg tunnels.

The result was a double lining concept, which added an secondary steel-rebar reinforced inner lining to either a TBM or drill+blast operation. For drill+blast excavation, the challenge would be to ensure a circular primary lining ring closure within a month of excavation and in weak zones, before the face had advanced 50m ahead of ring closure. For either excavation method, installation of the reinforcing steel work for the inner lining would have to be completed within four months after excavation and primary lining, to obtain secondary ring closure within the specified time limit.

Segmental lining build behind the TBM

Consequently, planning of the primary and secondary lining activities needed to be decided well in advance. To support and ensure that the advance planning work was done, ASTRA implemented an additional payment within the scope of the tender.

Of a total of nine bids received for the project and with contractor variants were also welcome:

• two were for both TBM and/or drill+blast solutions;
• three for the TBM solution only;
• one for the drill+blast solution only; and
• three offering contractor variants.

Marti won the contract in mid 2014 for a TBM solution and the Herrenknecht machine was assembled on site in 2015. It is the largest single shield TBM built by Herrenknecht and has a drive power of 3,500kW.

Casting the invert section of the secondary reinforced inner lining

TBM excavation of geological transition zones had proven difficult on previous projects and as experienced at the Adler Tunnel. The benefit of the earlier tunnelling works at Belchen indicated where the most serious conditions would be encountered. This information enabled a successful TBM bore and lining work.

Project representatives reported to the congress that, having addressed the auxiliary construction measures well in advance, all transition zones were identified and that the TBM was not stopped by the fault zones, with only minor difficulties experienced. They added that inner lining measures also proved their worth and that the existing tunnels were found to have a more positive effect than expected on controlling groundwater, although it was noted that the weather had a significant and direct influence on the ground water behaviour.

The secondary cast concrete inner lining progressed behind the excavation and within the specified time limits, with about 11,000 tonne of reinforcement steel supplied to the project by Armaspez Schweiz AG.

Completed inner lining of the new tube

By the end of 2016 the TBM had bored about 60% of the new tunnel with the phases of the follow-up inner lining at the required stages behind the machine. The TBM completed the drive on 21 June 2017, after 16 months of passing through the complex geology.

Following the end of excavation, further construction work on the new tunnel is planned to progress through to 2019. Fit-out of the operational and safety equipment in the new tube is planned during 2019-2021 with commissioning of the third bore scheduled for 2022.

A third two-lane tube at Belchen was vital for upgrade of the A2 motorway. The route is on the Germany-Italy axis through Switzerland and currently carries about 55,000 vehicles/day with HGV trucks accounting for about one in 10 of the traffic flow.