Paula Wallis, TunnelTalk
Glacial lake level in May 2008
- Last Thursday, November 5, relieved authorities in Bern, announced that a glacial lake no longer posed a danger to communities living in the shadow of the Lower Grindelwald Glacier following breakthough of a 2km drainage tunnel. For the past 10 months construction crews have been in a race against time to blast a tunnel though the flank of Mättenberg Mountain in the Alps to drain the lake of melt-water.
- For decades the Grindelwald Glacier has been one of the region's major tourist attractions, but in recent years rising temperatures in the Alps have turned the massive ice flow into a menacing threat.
Fig 1. Alignment of drainage tunnel
- Without the supporting pressure of the glacier, large landslides and rockfalls, including a 2006 slide that released 2 million m3, have covered the lower end of the glacier protecting the ice below from melting in the sun. Higher up, the glacier continues to recede creating a natural reservoir with no means of over ground drainage.
- The lake has increased in size each year since its first appearance in spring 2005. As the threat increased a sophisticated early warning system of cameras and alarms have kept an ever-watchful eye on the receding glacier and the welling melt-water for any sign of a sudden breakout that would require the immediate evacuation of the communities below.
- In May of 2008, with a lake volume of 800,000m3, a breakout did occur releasing 110m3/s. While minor flooding and damage was reported, the threat of more severe events demanded action and in the summer of 2008 the decision was made to build a 3.2m wide x 4.64m (13.30m2) drainage tunnel.
For designer B+S AG of Bern, in corporation with Geotest AG of Zollikofen, time was a major consideration. "To be successful the tunnel had to be constructed within one season," said Walter Steiner, Tunnel Consultant for B+S AG, which also holds the Project Management contract. "At the upstream end of the tunnel, lake level work had to be completed before winter, when snow and the threat of avalanches shut down construction. To keep to the very tight timeline the team had to function on all levels from owner, specialists, designers and constructors."
- The inlet is above the current waterline allowing the tunnel to skim the raising water table. Melt-water will travel some 700m in the tunnel before cascading in a waterfall of about 110m into the Glacier Gorge from Adit 2. The lower 1.3km of the tunnel serves purely as an access tunnel for heavy machinery to reach the glacier to maintain and modify the lake inlet as the reservoir deepens over time, said Steiner (Fig 1). "As the glacier melts and the natural dam deepens, new tunnel entrances will have to be constructed at lower elevations so that the seasonal lake does not exceed a safe size."
- Construction of the SFr15 million project ($US14.7 million) began in January 2009 from the portal near the Glacial Gorge Restaurant (Fig 1). Employing an Atlas Copco Boomer 282 drill rig, local contractor Gasser Felstechnik AG, used more than 90 tonnes of emulsion explosives to excavate the drill+blast tunnel. A second idential drill rig was kept in reserve onsite in case the first was down for maintenance or repairs.
- From January to March one shift of 5 crew, working 9-10hours/day, six days/week averaged 32m/week. From April to September three shifts working around the clock, seven days/week averaged 77m/week.
- The geology along the alignment is mainly massive limestone with a small section of Grindelwald marble at the northern portal. The last 30m were though frozen or partly frozen moraine and slope debris of silty, sandy soil and ice. When an initial attempt to tunnel though this ground proved too risky the decision was made to move into more competent ground with a steeper 100m by-pass tunnel. According Steiner ground water posed little problem along the alignment with only one zone of slight water inflows.
- In the upper part of the tunnel a concrete invert was installed to prevent erosion and cavitations by the inflow of rushing water. Shotcrete was applied with a Paus shotcrete robot and a SIKA PM 500 for added support primarily in the crown of the tunnel.
- With excavation complete, the contractor is currently adjusting the alignment of the tunnel near Adit 2 to ensure the water will flow through the adit into the gorge, and not into the lower part of the tunnel.
- Full completion of the project is anticipated by March 2010.
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