Reliability of Geotechnical Parameters in the Analysis of Ancient Landslides

More than 850 large scale landslides in the two main active seismic zones (Alborz and Zagros) in Iran have been studied over the past nearly four decades. Of these, remedial designs made by the author for major engineering works in the 180 or so landslides encountered in the projects have, to the present, been successful. This work has led me to the conclusion that, regardless of their geotechnical parameters measured on intact samples in the laboratory, the main cause of forming of these landslides has been occurrence of high magnitude recent and historical earthquakes, combined with micro/mega discontinuities existing within the soil/rock masses. In this study, according to the results of geotechnical studies, the stability analysis of ancient landslides in the area of gas transmission lines (IGAT) in Iran, is discussed. Three samples, taken from slip surfaces of ancient landslides of the two main active zones are discussed. The samples were taken from apparently weak soils, but, unexpectedly, they showed good stability strength. The major occurrences of landslides were in the two mentioned active zones (Zagros and Alborz). In the sampling process of the three landslides, samples representative of apparently geotechnically weak rock or poor soil type were the only ones selected for testing and were taken from slip surface/zones with well-defined natural deformation, remolding and slicken siding. Geotechnical and soil shear parameters measured on samples taken from the slip surface showed apparent good results indicating that a landslide should not occur. However this assumption proved to be misleading, for despite these measurements, land sliding did occur. This was especially so in the case of rockslides.