ACTION OF RAINFALL AND MOISTURE INCREASE TO DEVELOP LANDSLIDES

Correlation of landslides and precipitation has long been observed. A lot of landslides ensue in springs following long, wet winters. Cyclic yariations of weathering and yearly rainfall are reflected by the yearly frequency of landslides. Shear strength decreases exponentially with increasing water content. Shear strength values of clays are seen to haye decreased after drying wetting cyles. From the aspect of clay hehaviour in shear, the most sensitive and most important parameter is the water content: namely, thc presence and motion of water affect the development of most subsurface soil motions and landslides. Correlation of landslides and precipitation has long been observed [7]; among landslide causes rainfall is mostly- directly or indirectly- comprised [5]. This is especially true for stratum slides [8]. Most slope slides occur directly after a long or intensive rainfall period or important sno,,,,-mclt. It will be endeavoured to support introductory state-ments on hand of examples, case studies at home and abroad, to point out the correlation between ·water content and shear strength, shear parematers, to warn slope designers of water content increase. Examples, ohservations in this country Precipitation is decisive for the development of seepage flo-w. A lot of landslides ensue in springs following long, wet "inters (in particular, if pro-tracted snowmelts forward water seepage into the soil, in winter, also evapora-tion is reduced). Slope slides after development of a cut for the Godisa-Abaliget railway line vs. monthly precipitations over 90 mm have been plotted in Fig. 1. The correlation is unambiguous. The effect of rainfall on the development of landslides is manifest from another example illustruted in Fig. 2 where precipitation in each ten days has been plotted, together with 60-year averages. The year 1976 is seen to b