Evaluation of the role of Neogene clay sediments in landslide occurrences, south of Ardebil Province, northwest of Iran

The southern areas of Ardebil Province have clay sedimentary rocks and many damages have been made because of new landslide occurrence or reactivation of old landslides every year. The clay sedimentary layers are related to the Upper Red Formations to the Neogene period in the study region. The clay minerals could be the main reason of the landslide occurrence because of swelling and shrinkage characteristics of soils. This research was carried out to test the hypothesis of the effect of clay expansive soils in slope instability. This area extends to 164500 ha, and 11.91% of it found to be affected by landslides. 32.8% of the whole area of the clay sedimentary rocks had experienced landslides. Minimum percentages of sand, silt and clay particles in the soil samples are 0.0, 23.4 and 8, and the maximum values ​​are 39.9, 72 and 54.5, respectively. Minimum liquid limit of samples is 36.7% and its maximum is 67.66. The plastic limit of the samples also varies between 19 and 33.13%. Soils have a pH above 8. The cation exchange capacity above 30.41 milligrams per 100 grams of soil, which reaches up to 76.52, is one of the main characteristics of the soils in the region. The swelling potential rates of the soils of landslide area have been classified to four respective zones indicating the low, medium, high and very high swelling potential using physical and chemical properties of soils and based on different methods. At least 80% of the soil samples in the area have a swelling potential with medium to high intensity and 87.2% of them are located in zones of high and very high landslide hazard. The expansive clay soils are one of the main causes of landslide occurrence in the region.