PROBLEM

In covered karst regions the causes of structural damages are often in coherence with the effect of seepage-water movement in the underground. In such regions, where a karsted and/or fissured hard rock is overlain by cohesionless soil, hydraulic induced erosion and suffosion processes occur within the overburden. Erosion is the hydraulic induced mobilisation and transportation of all diameters of grains in soil. It results in total destruction of the grain skeleton of the soil. Suffosion is also a hydraulic induced process, but only the small grains of a loose soil get mobilized and transported in the pores of the skeleton of the coarse grains. As a consequence of these processes, an abrupt or creeping material displacement from the underground to the cavities in the deeper underground is possible. The situation is characterized by känozoic, especially quarternary loose soil, overlying a karsted sulphate of carbonate rock (MOLEK ET AL., 1997). The hydraulic conditions in the underground can be generally characterized by - in the most cases - vertical seepage water movement to a deeper aquifer (Fig. 1). This water may derive – for example - either from natural rainfall infiltration or from leaky sewer pipes. At surface these processes result in - more or less widespread - depressions or in form of collapse craters with endangering of the stability in built-up areas. The developement of such depression- and collapse forms is discribed by REUTER & MOLEK (1980). Because of these described effects, water has generally two different possibilities for direct or indirect influence to buildings by endangering either their stability or at least by restricting their use (FISCHER ET AL., 1991; VOGEL & KAST, 1994). The same effects of erosion and suffosion as described before, with comparable consequences at surface, can also run within the layers of stratified river deposits.

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