Freeze assist on Swiss Albula rail link 13 Dec 2018

Main excavation has been completed on the Albula 2 project in Switzerland where client Rhaetian Railway (RhB) is adding a new 5.8km long rail tunnel that called for major ground freezing to overcome a reach of complex geology.

Tunnelling underway in good rock

Albula 2 runs 30m parallel to the existing tunnel, which opened in 1903, and will be linked by a dozen cross passages at intervals of about 450m. The experience of building the original tunnel and its use, have been key factors in the ground freezing success on the second tunnel. The existing 5.8km long x 6.65m high x 6.1m wide single-track horseshoe profile tunnel has a vaulted masonry lining of 650mm to 950mm thick. It is the longest tunnel on the spectacular 62km long mountainous rail line that climbs the Albula Valley. The line was granted UNESCO World Heritage status 10 years ago and includes another 41 tunnels and 144 bridges.

Following a condition assessment in 2006, RhB considered options to upgrade the Albula tunnel, which had suffered long-term weather, water and frost damage. The option chosen was to add a new parallel single-track tunnel, to open by about 2022, and convert the refurbished old tunnel into a safety facility. Management of the project is provided for RhB is by Amberg Engineering.

While the first tunnel awaits progressive rehabilitation, using a standardised technique that TunnelTalk has reported as being developed by RhB, it is the harsh lessons of a tragic collapse suffered during its original construction that has been vital in developing the engineering and tunnelling plans for the new Albula 2 Tunnel. During construction of the original tunnel in 1901, a zone of cellular dolomite known as the Raibler-Rauwacke zone, was encountered unexpectedly in the northern section near the Preda portal. Tunnellers had been dealing with better schist rock before they met the surprising weak and wet conditions under a 130m head of groundwater. The result was a collapse and flooding inflow of mud and water that claimed lives of the tunnel workers.

In light of the experience, extensive early planning has been crucial to reducing risk when excavating the new tunnel through the same weak and faulted dolomite zone.

Frozen zone open as drill+blast progress from the portal

Second tunnel ground investigation

In addition to desk studies of the geological conditions encountered by tunnellers more than a century before and their construction experience, the planning for the new tunnel called for probing into the weak zone. Working from a small cavern at the end of a planned new cross passage, two 160m long horizontal, core drilling probes fitted with inflow-preventers were extended into the weak zone.

The porous structure of the dolomite was caused by leaching of water-soluble gypsum and anhydrite from dolomite and limestone breccia and karstic cavities along the division surfaces in the rock. The probing data established the weak zone to be a varied formation lasting about 110m along the tunnel axis.

The worst section in the varied zone is a 20m long faulted complex at the south side which sits along side mylonite and granite. In a presentation to the Swiss Tunnel Congress in 2017, attended by TunnelTalk, ground freezing specialist Dr.-Ing Orth of Swiss consultant Rothpletz, Lienhard + Cie, leaders of the overall project design team, described the worst part of the weakest zone of the Raibler-Rauwacke as a fault zone full of fine-grained broken material.

First breakthrough into the freeze zone heading

Today, due to long-term drainage following construction of the first tunnel, the head of water is about 50m, and no further drawdown of groundwater table is allowable in building the second tunnel. This objective is achieved by constructing the Albula 2 Tunnel with a full seal and ring closure as it passes through the weak zone.

Probing into the Raibler-Rauwacke also found some difficulties in penetrating parts of the zone due to the high silt content which would be hard also to consolidate by grout injection. The only remaining option would be to use ground freezing.

Ground freezing

The new tunnel is 35-40m² in cross-section with a single lining for most of its length. To pass through the weak zone a larger oval-shaped cross section of 10.06m high x 8.27m wide was selected to accommodate a fully sealed double shell lining of 650mm and 600mm thick.

Construction management of the ground freezing operation was provided by a team of consultants from Lombardi and geological specialist for the project Sieber Cassina + Handke.

It was decided the ground freezing zone needed to extend through about 60m or slightly more than half of the entire weak zone, through the worst of the varied sections and through solid to weathered mylonite, silty fine sand and also the soft cellular dolomite.

To establish the freeze, contractor Zublin Spezialtiefbau worked from inside the same cavern used for probing to install an array of highly accurate horizontal drill holes, all the while working against a 50m head of water. To avoid the risk of mud inflows Zublin used a releasable lost drill bit. The challenging task eventually called for a few extra holes to ensure full closure of the freeze body. In total, 50 holes were drilled for the freeze pipe array, reported Rothpletz, Lienhard + Cie. Other members of the design team included Gahler und Partner and Straub.

Nearing the end of the majority drill+blast excavation

The majority of the new Albula 2 tunnel is excavated in three sections advancing drill+blast headings from the two portals, lined with shotcrete, and excavation through the freeze zone. Contractor joint venture of Porr Suisse, Walo Bertschinger and Societa Italiana per Condotte d’acqua advanced all three excavations concurrently. A limitation on construction activity is weather in the high Alpine location which limits works over the winter period from December to early March each year.

At the north portal near Preda, the drive began in August 2015. The plan was to advance the drive to reach and open up the frozen zone. However, as progress through variable quality schist using short rounds or excavator or blasting excavation, was slower than anticipated, the contractor began the frozen zone from the probe drilling and freeze installation chamber at the end of a cross passage off the old tunnel. In the ice plug, a jack-hammer tool on an excavator opened up the tunnel to a cross-section of about 65m². The freeze extends about 2.5m beyond the limits of the tunnel. Excavation support comprises shotcrete and wire mesh.

As it approached the excavated freeze zone, the Preda heading used grout injection to stabilize the northern end of the weak zone. Excavation advanced in 1m rounds and support was double layer wire mesh and shotcrete and rock bolts in the crown. Breakthrough into the frozen cavern was achieved in October 2017.

Final breakthrough celebrations

By late 2017, excavation had advanced southward from the cavern into granite with good progress being achieved using 150mm steel fibre reinforced shotcrete with friction anchor bolts. The same lining system as being used on the heading from the south portal near Spinas, which had started in September 2015.

By early 2018 it was expected the Albula 2 Tunnel would see final breakthrough by late August. TunnelTalk, having followed the development of the challenging project, joined delegates at the Swiss Tunnelling Congress in June 2018 to hear the site management consultants Poyry Schweiz and AF Toscano brief on progress and the final stages of excavaation.

Final breakthrough was achieved and celebrated in early October. Despite the variety of major tunnelling and logistical challenges, tunnelling for the Albula 2 project was still completed in the same year as planned at the outset of the main underground works three years ago.