LANDSLIDE SUSCEPTIBILITY MAPPING OF THE SLOPES IN THE RESIDUAL SOILS OF THE MENGEN REGION (TURKEY) BY DETERMINISTIC STABILITY ANALYSES AND IMAGE-PROCESSING TECHNIQUES

The aim of the present study is to prepare a landslide susceptibility map of a region of about 120 km(2), between Gokcesu and Pazarkoy (arou nd Mengen, NW Turkey) at approximately 10 km north of the North Anatol ian Fault Zone, where frequent landslides occur. For this purpose, mec hanisms of the landslides were studied by two-dimensional stability an alyses together with held observations, and the parameters controlling the development of such slides were identified. Field observations in dicated that the failures generally developed within the unconsolidate d and/or semi-consolidated soil units in forms of rotational, successi ve shallow landslides within the weathered zone in Mengen, Cukurca and Sazlar formations. Although consisting of residual soils, Capak and G okdag formations do not exhibit landslides as the natural slopes forme d on these, do not exceed the critical slope angles. Statistical evalu ations and distribution of the landslides on the topographical map sho wed that such parameters as cohesion, angle of internal friction, slop e, relative height, orientation of slopes, proximity to drainage patte rn, vegetation cover and proximity to major faults were the common fea tures on the landslides. Digital images were obtained to represent all these parameters on gray scale on the SPOT image and on the digital e levation model (DEM) of the area using image processing techniques. So il mechanics tests were carried out on 36 representative samples colle cted from different units, and parameters were determined for two-dime nsional stability analyses basing on ''sensitivity approach'' and for the preparation of digital shear strength map. In order to determine t he critical slope angle values for the residual soils, a series of sen sitivity analyses were realized by using two-dimensional deterministic slope stability analyses techniques for varying values of cohesion, a ngle of internal friction and slope height along with varying saturati on conditions. According to the results of the sensitivity analyses, t he Mengen formation was found to be most susceptible unit to landslide s, covering about 33.5% of the region studied in terms of surface area . The distribution of the critical slopes were determined by superimpo sing the critical slope values from sensitivity analyses on slope map of the study area. On the other hand, iso-cohesion and iso-friction ma ps were produced by locating the values of cohesion and internal frict ion angles in a geographic coordinate system such that they coincide w ith sample locations on the DEM and by further interpolation of the va lues concerned. The pixel values were evaluated in gray scale from 0 t o 255, 0 representing the lowest pixel value and 255 representing the highest. Sensitivity analyses on cohesion and angle of internal fricti on to investigate the effects of these parameters only on stability, r evealed that cohesion was effective at a rate of 70% by itself, while angle of internal friction alone controlled the stability by a rate of 30%. The iso-cohesion and iso-friction maps previously obtained were digitally combined in these rates and a ''shear strength map'' was pre pared. The geographic setting of the study area is such that northern slopes usually receive dense precipitation. In relation to this fact, about 42% of the landslides are due north. Thus, a slope orientation m ap was prepared using the DEM, and slopes facing north were evaluated as being more susceptible to sliding. Proximity to the drainage patter n was another important factor in the evaluation, as streams could adv ersely affect the stability by either eroding the toe or saturating th e slope, or both. When considered together, in conjunction with the fi eld observations, faults and landslides showed a close association. In the area, about 88% of the landslides were detected within an area cl oser than 250 m to major faults, therefore, a main discontinuity map w as produced using the SPOT image of the region, and ''proximity to maj or faults'' was evaluated as a parameter as most of the landslides dev eloped in areas where the vegetation was rather sparse. A vegetation c over map was therefore obtained from the SPOT image, and the areas wit h denser vegetation were considered to be less susceptible to sliding with respect to the areas with less or no vegetation. Having prepared the maps accounting for the distribution of critical slopes, shear str ength properties, relative height, slope angle, orientation of the slo pes, vegetation cover, proximity to the drainage pattern, geographic c orrections were carried on each of these, and a potential failure map was obtained for the residual soils by superimposing all these maps. N ext, a classification was performed on the final map and five relative zones of susceptibility were defined. When compared with this map, al l of the landslides identified in the held were found to be located in the most susceptible zone. The performance of the method used in proc essing the images appears to be quite high, the zones determined on th e map being the zones of relative susceptibility.