Geotechnical characteristics of residual soils in rainfall-triggered landslides in Rize, Turkey

Abstract

Every year, hundreds of landslides are triggered by rainfall in the city of Rize, northern Turkey, resulting in casualties and devastating social and economic consequences. Although these landslides have been observed every year, for a long time in Rize, there is limited data on characteristics of these soils in the literature. Characterizing these soils is of paramount importance for numerical modeling of the landslide mechanisms, for stabilization works, for landslide susceptibility mapping and for establishing rainfall intensity-duration thresholds. This study seeks to investigate the properties of residual soils decomposed from volcanic rocks that are involved in rainfall-triggered, shallow (< 5m thick) and rapid landslides of the flow type. Disturbed and undisturbed soil samples are taken from a total of 12 landslide sites to evaluate index properties, physical and mechanical properties and the mineralogy. Also, in-situ unit weights are determined on undisturbed samples, and portable hand vane tests are conducted to evaluate the in-situ undrained shear strength. Investigations reveal that soils at the shear surfaces, at the main and side scarps, and within the landslide mass are similar materials, i.e. there is no material difference above and below the shear surface. Results indicate that these materials are relatively loose, medium-stiff, mostly fine-grained soils (low and high plasticity silts and high plasticity organic soils), with liquid limits in the range of 35–87%, with some organic content (1.8–12.1% by dry mass) and have relatively low pH (3.5–5.3). As for the mechanical properties, a total of 72 direct shear tests are conducted on undisturbed samples (which sometimes included a plant root or a stone) in the normal stress range of 10–65kPa, in unsaturated and in saturated condition. The average internal friction angle is 36.3° (range 31.1–38.0°) in the saturated condition. The undrained shear strengths are 10 to 52kPa and 42 to 103kPa from unconfined compression tests and hand vane tests, respectively. The results of this study can be used for future research on rainfall triggered landslides.