Kompleksowe badania osuwisk w dolinie Brdy w Koronowie k. Bydgoszczy

Abstract

Complex investigations of landslides in the Brda river valley in Koronowo near Bydgoszcz.A b s t r a c t. In the recent years intensified landsliding was observed in the surroundings and in the town of Koronowo near Bydgoszcz. (Krajeńskie Lake District). The gently undulated moraine upland, a wide valley of the Brda river, its relatively steep valleysides and incised valleys of tributaries are morphologic features related to modeling by North Polish Glaciation. Quaternary tills alternated with fluvioglacial sands and gravels are underalined by Miocene clay and mud with brown coal intercalations. Due to such morphologic and geologic setting the study area is prone to slope instability. Failure apt soil massifs were assigned to 4 groups. Out of 32 identified landslides, 9 were selected for detailed examination. To register a nature and rate of failures a complex monitoring system was installed on slopes where the selected landslides resulted in severe damages to municipal infrastructure. The system was facilitated with inclinometers and piezometers, supported by a network of GPS-RTK geodetic benchmarks as well as with a recording weather station. The performed examination revealed that the most unfavourable arrangement of the layers is, when under a non-cohesive soil (e.g. sand, gravel) lies a cohesive impermeable layer (e.g. clay, loam). The landslides are rather shallow ones (except 2 incidents) with slow rate of displacement of an order of few mm/year. The landslide triggering factor is water originating from precipitation and snow melting. Influence of water was especially significant in early Spring 2011, due to the combined effect of snow melting and infiltration of thawing water originating from the former, exceptionally high rainfalls. The influence of hydrologic conditions on slope deformations is complex. There is a significant time-lag between a movement initiation and unfavourable hydrometeorological conditions. That is exemplified with the landslide that was initiated in February-March 2011 in consequence of atmospheric conditions of November- December 2010. The obtained results formed the background for inventing engineering treatment measures aiming at current remedial stabilization of slopes and mitigation of possible landsliding in the future.