Lausanne metro tunnel collapse

On 22 February a tunnel collapse on Lot 1200 of Switzerland's M2 Project in Lausanne (T&TI, November 2004, p7) displaced 50m3 of material into the tunnel and caused extensive damage as it cratered towards the surface in the busy St. Laurent's commercial district.

The incident took place at around 6pm, at a depth of approximately 12m below the surface. Thankfully no-one was injured, although two buildings, a supermarket and a takeaway food outlet, were evacuated when their cellars collapsed. Reports said that no work was being carried out in the tunnel at the time of the incident, which is believed to be due to the sudden inrush of groundwater from a pocket in the glacial moraine the tunnel was being driven through.

Contractor for Lot 1200, the Ouchy – Croisette JV, had been using an Eickhoff ET 380-L roadheader to cut the tunnels within its scope, but at the time of the incident, investigation works were said to be underway following an earlier inrush.

Following an overnight assessment of the void, owner Metro Lausanne-Ouchy SA, moved equipment to the location on surface and began breaking the overlying flagstones in order to access the cavity for further investigation and remedial works. Within 44 hours of the original collapse, Metro Lausanne-Ouchy SA said the critical phase had passed and allowed some building works to recommence on the subway, although retrieval of the collapsed area was ongoing.

A curtain of 11 piles was drilled and concreted ahead of the collapsed face to consolidate the ground and limit the flow of further material into the tunnel, in conjunction with grouting. By 3 March, backfilling of the void with sand from crushed glass got underway. T&TI learnt roughly 800m3 of glass-sand is estimated to be required for backfill to the height of the bases of the damaged buildings. The material was selected as it is cheap and has good compaction properties. It is obtained by crushing the glass collected by the town for recycling.

An independent expert is due to be appointed to head an investigation into the circumstances surrounding the incident. In addition, a compensation fund was set up with Zurich Insurance for any injury or property damage claims. A source told T&TI that Metro Lausanne–Ouchy SA could not comment further on the incident, as it did not wish to prejudice the inquiry.

Two senior officials, one representing the canton and the other the town, released a statement showing continued confidence in the project teams and said: "The questions which currently arise on the knowledge of the ground are legitimate. On the other hand, it is premature to imply that the experts elected for the Project M2 did not hold account of essential information in their possession."

Costing US$472M, Lausanne's 6km long metro system is expected to take four years to complete. Underground works only commenced late last year, with tunnel profiles varying from 9.94m wide x 6.74m high to 11.7m wide x 7.61m high.