Hangstabilitäten im Mainzer Becken

In the Tertiary sediments of the Mainz Basin west of the Rhine slope stabilities have been studied and mapped. Mineralogical investigations of the slip horizons have been made.The immediate cause of the landslides in 1982 were extreme weather conditions. The development of the mass movements is analysed in-situ and by soil mechanical, hydrogeological and air-photogeological investigations. 90 percent of the recent landslides are reactivated old slides.Diverse measurements and fieldwork have been made to delineate the kinematic patterns of the landslides and the forms of their slip planes.For the first time in the Federal Republic of Germany a slope stability map at a scale of 1: 50 000 has been prepared. There is potential danger of mass movements in 8 percent of the studied area. The occurrence of 197 landslides in 1982 demonstrates the necessity of this mapping.A proposal is given for the “semi-quantitative” appraisal of labile slopes.Grain size distribution, chemical and mineralogical composition from samples of slip horizons of recent landslides have been examined quantitatively. The material is very poorly sorted and the samples are characterized by a homogeneous distribution of grain size fraction in the ränge of 0,2 — 20 μm. The clay — silt — ratio is about 1: 1, coarser grain sizes are rare.Considering the clay minerals their chemical composition of the fraction < 2 μm has been analysed and a normative mineral composition was calculated. Hüte in concentrations of 40 – 50 wt. -percent is the dominant clay component. Montmorillonite was found in all samples with 20 – 30 wt.-percent whereas the values of chlorite and kaolinite were smaller than 7 wt.-percent. The calculated mineral content agrees with the data from X-ray determination. Compared with the total sample there are only 33 wt.-percent of clay minerals while the rest consists of calcite/dolomite ≧ quartz ≫ goethite > feldspar. The exceptional good crystallinity of the clay minerals, their grain size distribution and mineral composition indicate that the slip horizons correspond to sedimentary and diagenetical structures.For the development of slip planes and slip horizons the grain size distribution and the mineralogical composition of the clay fraction are important: the silt fraction facilitates rapid water resumption whereas the clay fraction tends to retain the water over longer periods. When sufficient water is supplied this interaction leads to a greater fixation of water and to swelling of the sediment, which greatly reduces its shear strength.