C8/​1996R Clay slid­ing onto track from cut slope at Paimio, on July 7, 1996

06.07.1996

On Saturday, 6 July 1996, about 200 – 300 cubic metres of heavy clay slid onto the track between Paimio and Salo, blocking the entire track. The engineer of an express passenger train proceeding from Turku to Helsinki saw the clay sliding from the cut slope and succeeded in stopping the train before the blocked section of the track. The incident did not result in personal injury or loss to equipment.

The soil along the costal track is known to be sensitive to disturbances. During the previous two weeks, it had rained considerably in the dangerous zone, and at the time of the incident it was raining heavily.

At the site of the mud slide, lime pillars had been imbedded eight years earlier in order to stabilize the soil bed. The pillars did not serve the intended purpose, since the bedrock beneath the pillars is sloping and smooth. The pillars did not extend down to the bedrock, and the surface of the cut went between the pillars and the bedrock. In the calculations, the surface of the cut had been assumed to go through the pillars. Mud slides had occurred previously on the cut slope next to the mud slide now in question. At that time, the lime pillars had been embedded at this latter point. Also at that time, such an approach to stabilizing the soil bed was new, and there had not been much experience with it.

There is reason to review the basic assumptions and values of stability calculations in areas known to be prone to disturbances.