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Landslide at Sköttorp on the Lidan River

Swedish title: Landslide at Sköttorp on the Lidan River
Author Sten Odenstad
Series: SGI proceedings
Number: 4
Year: 1951
Pages: 44
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Summary In 1946 a large flake of the clay plain at Sköttorp suddenly moved towards the Lidan River and filled some 800 m of its channel. In a few days the upstream water rose 12 m and then began to run off över the clay mass. During the following two years the river cut down through this mass, the upstream water sinking successively nearly to its old level. The slide caused a temporary inundation of the valley upstream of Sköttorp. Some secondary slides occurred and others were feared. A floodwave running downstream of Sköttorp was also apprehended. Remedial and preventive measures were taken, mainly consisting in building dams and digging channels to conduct the water through the site. Extensive soil investigations were made on the site and in the laboratory; most of the results are given in this report. It was found that the river bank had been severely stressed before the slide. Several factors can have acted a.s "trigger agents" e. g. the erosion in the river bed, the increasing weight of the grove on the bank, the weakening of the clay due to leaching of salt, or, most probably, a temporary increase of the ground water pressure. In order to explain how the slide could spread horizontally 200 m in the direction away from the river two assumptions must be made. One is that the clay soil had a horizontal surface or layer that was particularly weak, owing to its constitution or to a high water pressure in an adjacent sand layer. The other is that the strength of this surface or layer was almost completely destroyed, when rupture was produced in it by stress concentration. Probably a front of such rupture travelled rapidly landward, thus forming the slide bottom. At the same time and rate, rupture in the soil above this bottom spread landward in the following way. Slip surfaces, parallel to the river and inclined 45 degrees to the horizontal, appeared, forming a zigzag line in cross section. Those soil wedges thus formed which had their base on the slide bottom slipped apart without any appreciable deformation; their edges constitute the clay ridges visible in the slide cavity. The interjacent soil wedges subsided and were deformed so as to fit between the former.