Günter Letzel

Road collapses during construction of underground station

The loss described in this article happened during the construction of an underground line. It involved damage to a road. In this case, the tunnel was being built using the open-cut method.

The construction phase was insured under a CAR policy and incorporated the construction of an underground station with a ventilation shaft. In order not to disrupt the traffic above ground, the eight-lane road ran over the construction site on steel plates fixed to a steel supporting structure.

The collapse occurred one Saturday morning at 6:10, at which time road traffic had not yet built up. A bus that was waiting for the lights to change suddenly plunged 20 m into a gaping hole 30 m long and 40 m wide. All three passengers in the bus were killed, the driver was seriously injured.

In the search for the cause of loss it was found that a 26 m high supporting wall in the excavation area had given way because it was not sufficiently anchored in the ground at the side. This led to the ground caving in and finally to the complete collapse of the steel supporting structure.

Soil investigations: no indications of difficult soil conditions

Before construction work began, soil investigations had been carried out at 50 m intervals along the intended route. There were no indications of difficult soil conditions. The investigations carried out after the accident showed that there were inhomogeneous soil layers of sedimentary rubble and sandstone with embedded layers of clay behind the supporting wall.

It was assumed that the change in the water table caused by the construction work had made the soil begin to slip on the partially inclined clay layers. As a result of the collapse of the supporting wall, adjacent buildings were damaged by subsidence, with losses exceeding the US$ 1m limit of liability under the CAR policy. Also numerous gas, water, electricity, and telephone lines were cut in the area.

After the loss event the road had to be made passable for traffic as soon as possible. To stabilize the construction site, the soil in the surrounding area was strengthened by squeeze cementing.

The excavated area was filled in again and strengthened using the same method. Those sections of the supporting walls that were still in place were reinforced. The steel structure was assembled again. Once the steel plates had been laid, the road was opened to traffic again.