Collapse of the Rastatt tunnel in Germany during construction last summer was unforeseeable by the contractor, according to rail operator DB Netz.
DB Netz said that the “accident was unforeseeable” for the contractor as part of its due diligence in the latest extension of the contract announced in the Official Journal of the European Union.
The collapse on 12 August last year occurred behind the tunnel boring machine and caused subsidence of the at surface Karlsruhe to Basel railway line that was closed for six weeks while a new piled slab deck was constructed to take the line over the collapse. The tunnel boring machine and bore were backfilled with concrete to stabilise the area.
Initial investigations focused on failure of the precast segmental lining behind the tunnelling shield and the use of ground freezing techniques with just 5m of cover between the tunnel and live rail lines above.
No details of the investigation into the collapse have yet been released, but DB Netz has issued a number of contract extensions for the contacting joint venture which involves both Hochtief Solution and Züblin.
The subsequent contract extensions have focused on recovery and repair of the tunnel and the latest extension for the 4.2km long tunnel takes the value of the work to over €333M (£299M). The last estimate put the cost at €312M (£280M)