News

On the above-mentioned day at 11:35 p.m., a loss of the overburden stability occurred in the central top heading section of the northern tunnel tube. A crater with approximate dimensions of 24 x 15 meters formed above the tunnel. The crater was backfilled with concrete before the end of the last week and the surroundings has thus been fully stabilized. This was the third overburden collapse on this tunnel project, however, the type of the collapse is absolutely different.

In Stromovka park, the tunnels were driven in weathered Dobrotiv shales covered with a thick, water-saturated sand - gravel horizon. To the contrary, in the case of Prašný Most, the tunnel was driven in weathered shales covered with a layer of loess where no significant water horizon was encountered.

The investigation into the collapse at Prašný Most is proceeding under enormous pressures for the fast completion, however, final conclusions cannot be drawn in such a short time due to both technical and legislative reasons. The case can be closed after the collapsed material is removed, retrieved structures are documented and assessed, and a number of tests and calculations are performed. The period of 2-3 months indicated by the Regional Mining Authority is a commonplace and cannot be considerably reduced.

Metrostav a.s. cooperates with representatives of the Municipal Authority of the City of Prague, Regional Mining Authority, and the Police, who are independently investigating the incident. In addition, its technical departments are analyzing other possible links.

On July 12, the company management decided to release preliminary results of internal discussions, despite the fact that these were only theories not substantiated by any evidence and that these results did not provide a complex explanation of this extraordinary event.

Based on the evaluation of all known data, the experts of Metrostav a.s. assume three possible causes of the collapse:

1) A hydrogeological anomaly in the collapse location that changed the load-bearing capacity of the overburden. The existing overburden – soils, loess, and claystone are highly sensitive to saturation with water, which can cause a sudden change in their properties. Water could have been introduced into the tunnel overburden for example by improperly constructed parking area lacking a proper drainage, Baroque fortification works mentioned by Professor Jiří Barták, or unrecorded historic excavations.

2) One of the triggering mechanisms of the incident could have included a human failure caused by the contractor’s personnel – for example a failure to observe the specified procedure for demolishing the temporary vertical support .

3) It is not possible to exclude a fault in the design. It is necessary to re-assess whether the selected variant of the excavation sequencing as it was accepted by the parties involved in the construction was optimal at given place and whether it took into consideration all known factors.

The collapse could have also happened due to a combination of the above-mentioned factors because the occurrence of only one factor would have probably caused only a local deformation of the primary lining. Final conclusions cannot be made and the contribution of individual factors to the failure cannot be stipulated yet because the required data and evidence are not available at the moment.

Elimination of consequences of the collapse should not affect the construction completion date. Metrostav a.s. has mobilized all its capacities and is able to eliminate the delay which has developed at the given point. This promise is realistic because the Myslbekova – Prašný Most section is ahead of the schedule by approximately 2.5 months. There are only 130 meters to excavate in the northern tunnel tube and 180 meters in the southern tunnel tube. The works can be accelerated by additional geological exploration through exploratory galleries driven from the Prašný Most portal.

Tunnels in the second mined section between Letná and Troja have been completed and the only remaining driving works are carried out in ventilation tunnels. Works on other parts of the project continue as planned. Both individual and public transport will return to Prašný Most – Špejchar – Letná area on September 1 as planned. Pedestrian ways will be also completed in accordance with agreed schedules.

Blanka Tunnel Complex is presently the largest European urban tunnel project. It will relieve residential areas of the city from unnecessary automobile traffic and will significantly contribute to reducing the number of accidents. Its contribution to environmental protection is indisputable.


Prague, July 12, 2010

Ivan Hrdina

Production and Technical Manager
Metrostav a.s.


For more information contact:
Frantisek POLAK
spokesman Metrostav a.s.
Tel.: +420-266 709 294
E-mail: polakf@metrostav.cz