Applied Clay Science 52 (2011) 11–19Contents lists available at ScienceDirect Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y Research Paper A geotechnical study on the landslides in the Trabzon Province, NE, Turkey Ali Yalcin⁎ Dept. of Geological Engineering, Aksaray University, 68100, Aksaray, Turkey a r t i c l e i n f o a b s t r a c t In the Trabzon province at the Eastern Black Sea region in Turkey, there are small and large landslides, especially following heavy rainfall periods in every year. These landslides regularly result in the loss of lives and properties. In this study, an assessment on the geotechnical characteristics of the areas where the landslides occur and the immediate vicinity were compared with the characteristics of non-landslides areas. A total of 50 landslides occurring in different locations were studied. As a result of the geotechnical investigations, the average of liquid limit values were between 49% and 69%, and the average plasticity index of the units ranged from 9% to 19% in the overlying materials in the landslide areas. Heavy rainfall exceeding boundary saturation of soils plays a critical role in causing landslides. According to the Unified Soil Classification System, the soils were “silts of high plasticity and silts, silty or clayey fine sands of low plasticity”. 80% of the total landslides occurred within very high and completely weathered units of basalt, andesite, or pyroclastics, and intercalations of sandstone, clayey limestone and siltstone. This study shows that the shear strength parameters decreased with increasing moisture content and that the number of landslides increased in relation to the particle size distribution of the clay. © 2011 Elsevier B.V. All rights reserved. Article history: Received 1 July 2010 Received in revised form 12 January 2011 Accepted 12 January 2011 Available online 21 January 2011 Keywords: Landslide Lithology Slope Vegetation Geotechnical properties 1. Introduction Landslides are major natural hazards in many areas of the world. As in most countries, the economic losses and casualties due to landslides in Turkey are greater than generally recognized, and they cause a yearly loss of property larger than from any other natural disaster. In recent years, some devastating landslides in Turkey were reported (Gokceoglu et al., 2005; Ocakoglu et al., 2002). Generally, landslides are caused by intense rainfall, seismicity, water level change, storm waves or rapid stream erosion. These effects increase the shear stress or a decrease in the shear strength of materials in the slope. Other causes that make an area vulnerable to landslides are residential and commercial development expanding into unstable slope areas under the pressures of population growth and urbanization and human activities such as deforestation or excavation of slopes for road cuts and building sites. They became significant causes of landslides. In the area, heavy rainfalls generate a rapid increase in pore pressure in the unsaturated zone and groundwater flow in the saturated area which can give rise to landslides. The properties of the lithological units are of crucial importance in terms of the propensity for landslides to occur. The weathered units act as a soil from a mechanical perspective, thus determining the geotechnical properties of the unit is very important. In a previous study of the Black Sea region, the depth of the completely weathered zones in rocks were determined at the end of the geophysical studies performed via land observation and Ground Penetrating Radar (GPR) surveys. Calculation was carried out at 9 different points using the RAMAC/GPR ground radar system and evaluating the obtained profiles. The main scarp of the landslide and road cutting indicated that the weathering depths varied between 15 and 20 m. These measurements supported the values obtained by GPR surveys (Yalcin and Bulut, 2007). Water content, consistency limits, grain size distributions, the type and characteristics of fine grained lithological units must be determined in relation to landslide behavior. The detailed investigations of the geotechnical properties of the units are a major aspect of the prediction of landslides. The characterization of landslides could provide very useful clues in terms of the nature of the slope failure and subsequent debris movement, thus indicating where landslides could potentially occur. This study can also provide the information to locate safe construction areas in regions that are prone to landslides thus safeguarding lives and property. In this study, fifty landslides were investigated in the Trabzon province. 2. The study area The study area extended from longitude 39°15′E to 40°15′E and from latitude 40°30′N to 41°8′N in the middle of the East Black Sea region (Fig. 1). The climate is characteristically Black Sea, with temperate climate summers and a rainy season generally lasting from September to April. The rainfall regime is irregular, with some periods when precipitation is rare and at other times there are long-lasting heavy rains. Very intense precipitation episodes also occur, causing ⁎ Tel.: + 90 382 2882328. E-mail address:
[email protected]. 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.01.015 The slope angle is directly related to the occurrence of landslide. an extensive geotechnical study was carried out on 50 different areas where landslides occurred. The main commercial agricultural products are hazelnuts and green tea. the geotechnical properties of the areas of landslides were compared with non-landslides areas.2 analysis was performed to discover in which slope group . the geotechnical properties related to the overlaying lithological units are the most decisive for landslides. However. The variations in the slope of hills. slope. The overlying units widely affected by weathering in the study area. Yalcin / Applied Clay Science 52 (2011) 11–19 Fig. Methodology In this study. and the rivers extend from north to south direction give this region a rough morphology. 3. mountains. Lithology. It is frequently used as the key measure in landslide studies. ArcGIS 9. 2007a). aspect. stream. In respect of the geological conditions. Location map of the study area. Thus. 2). apart from the agricultural area the other main land cover types are forest and pasture areas. disastrous flash floods in river basins and many landslides on the slopes (Yalcin. 3).1. the study area mainly comprises alkaline volcanic units outcropping parallel to the Black Sea coast. 1. Morphology The topography in the study area rises abruptly from coast.12 A. and roads were examined by comparing each of these parameter maps with a landslide inventory map (Fig. land cover. Heavy rainfall and dense vegetation have increased the speed of weathering which then considerably reduced the resistance of the overlying units to the risk of landslides. 3. The slope map of the study area was divided into five slope categories (Fig. Slope map. Landslide inventory map. . Yalcin / Applied Clay Science 52 (2011) 11–19 13 Fig. 3. 2. Fig.A. 2 °C.1 0.4 5.3 1. 2006).6 0.. Several researchers (Bathurst et al. Rainfall is one of the most significant factors causing for landslides. 2009. the landslide inventory map was compared with the land cover maps (Table 1).1 0. a single date image from Landsat ETM+ (Path 173.9 11. To show the relationship between the land cover and landslides. such as cohesion. The total precipitation in Trabzon city is 838 mm.1 18. and. The main ones corresponded to deciduous. 2010.1 1. 6). 2008) have revealed a clear relationship between vegetation cover and slope stability.1 % of landslide area (b) 2. 3. short heavy rainfalls further increase the pore water which consequently can give rise to landslides. 5). Landslides were largely seen in deciduous and hazelnut areas. the landslides occur in the areas with moderate slope (Fig. vegetations tend to increase the infiltration rate of soils.7 0.2 13. continental units.7 23.1 26. volcanic facies. the number of landslides has increased in these areas. This table indicates that most of the landslides occurred at a slope angle N50%. 2009).3. pasture. as a consequence. Yalcin. and also draw water from beneath the soil surface and transpire it through the leaves back to the atmosphere. 2000. The landslide rate in the moderate slope (0–30%) has about 20%.5 1. and agriculture areas. thicket and small wood. In this study. As a result. 2007b. 2000 was used to generate land cover types (Reis and Yomralioglu. Such a relation revealed by many earlier studies of the Black Sea region (Akgun and Bulut. Vegetation can both Table 1 Distribution of landslides. These types obstruct the surface flow of rain fall water. 3.2 80. Therefore. thus maintaining the geological material close to saturation. Parameter Slope (%) Classes 0–10 10–20 20–30 30–50 N50 Tea Hazelnut Deciduous (Forest) Rocky Pasture Agriculture Settlement Alv Pl Ev Kru Jkr Jlh γ2 Kk % of total area (a) 13. There is very rainy season from September to April. 4.00 Fig. the hottest month is August with an average temperature of 23.3 1. The number of landslides was relatively high on bare land. 2008). Gokceoglu and Aksoy.5 1. changes in land cover resulting from human activities and climate interact with slope stability.5 0.00 0. A landslide event is a sophisticated process that can be powerfully affected by the vegetation. andesite. Yalcin / Applied Clay Science 52 (2011) 11–19 landslides happened.2 49.1 15.8 30. 2006. Monthly average precipitation vs. 5.5 2. enhance effective soil cohesion due to root matrix reinforcement and soil suction or negative water pressure through evapotranspiration and interception (Karsli et al. nine different types of land cover described. internal friction angle.9 2. and precipitation dispersal pattern is symmetrical (Fig. as well as raising the soil unit weight and reducing the anti-shear strength of rock and soil (Ardizzone et al. Jkr—Jurassic–Cretaceous units. Row: 32) on October 19.7 1.7 6. 2004. A plant cover and litter layer of dead vegetation protects the soil surface from compaction by heavy raindrops. Plant stems help slow down water that flows over the soil surface.2. or main water table. From data accumulated over a period of 22 years. and intercalations of sandstone clayey limestone and siltstone. 2007. Yalcin. This increases the pore water pressure of the soil. The frequency of landslides was found by determining the relationship between the slope and the landslide (Table 1). 2007). thereby increasing the infiltration capacity (Dingman.. are influenced by the vegetation. especially for shallow landslides as in the Trabzon province. As a general rule. Geotechnical parameters.6 54. the effect of rainfall infiltration on slope can change the suction of the soil and the positive pore pressure.4 1. Plant roots help create openings in the soil.4 36. A typical shallow failure in the study area. temperature graphic. γ2—Kaçkar granites.. Land cover Land cover acts as a protection and reduces the susceptibility of soil erosion and landslides.7 11. In general. Climate The climate in the Black Sea region is dominantly a typical temperate climate with little snow during the winter months. 4). Some water usually evaporates from plant surfaces before it can fall to the soil surface.2 3. it rains all the year. .14 A. In this province. and low on forest canopy (Fig. Kru— Basalt. pyroclastics.0 8. Begueria.8 49. Karsli et al. Jlh—Lias units. 1996. Decayed plant matter keep fine soil particles (such as clay particles) from sticking together..0 16. and also slows the delivery of water to the soil surface. This is a higher for the tea areas because tea plant is very common the surface and rain water infiltrate the soil.4 45. and the rate of occurrence was recorded. Land cover Geologya a Alv—Alluvium.7 °C. The deciduous areas include different tree types of tree growth such as brake. 4). 1994). In particular. For this reason. As results. Ev—Eocene. weight of the soil and pore-water pressure. Fig.0 0. the coldest month in Trabzon Province is is February with an average temperature of 6. Jakob.6 17.00 3. Pl—Pliocene. Kk— Carboniferous units. Tangestani.8 41. transient pore pressure in response to short intense rainfall plays an important role in the occurrence of shallow landslides which are the main type in the study area (Fig. Land cover map. 6. Yalcin / Applied Clay Science 52 (2011) 11–19 15 Fig. Fig. Lithology map. .A. 7. plasticity index (PI).5 65.4 34.0 Standard deviation 15. Specific gravity 3. andesite.3 58.. value 13.0 2.7 kN/m3 and 18. natural unit weight (γn). The degree of weathering of rocks was determined following the method suggested by ISRM (1981).9 kN/m3 to a low of 11.5 Arithmetic mean 35. porosity (n). The average of the grain size distributions belonging the completely weathered section of each lithological unit is presented in Table 3 whereas Fig. Specific gravity was influenced by the presence of dolomite. Yalcin / Applied Clay Science 52 (2011) 11–19 Table 2 The distribution of number of samples in the lithological units.0 4. As the moisture content increases.3 0. The geotechnical tests.6 1.5. 3. If the moisture content is further increased. Then. Particle size distribution The influence of particle size distribution on the occurrence and speed of landslides was significant (Yalcin. Table 3 Grain size distribution of number of samples in the lithological units.2 0. calcite.0 27.0 7. Eight lithological units distinguished in the Trabzon province (Fig. .6 1.3 1.3 4. and quartz.7 3. The average specific gravity of all soil particles was about 26–27 kN/m3.3 29.8 6. 2007b). Index characteristics Fifty samples were analyzed to measure the saturated unit weight (γs).7. X-Ray Diffraction revealed illite and kaolinite as the main clay minerals in the weathered materials (Fig. and moisture content (ω).8 55. 7). and intercalations of sandstone clayey limestone and siltstones (Table 1). In the study area.3 5. 8). Wu and Qiao. The results of the soil analysis are given in Table 4. void ratio (e). because lithological variations often led to different the strength and permeability values of rocks and soils (Dai et al.4 31.6. Yalcin.8 30.4 kN/m3. plastic limit (PL). Luzi and Pergalani. or the moisture content.4 47.3 14.6 34.3 3.7 5.5 4.4. 2001. 2009. Sieve and hydrometer analyses were undertaken to designate the grain size of soil in the landslide area.3 66. 2008. a clayey or silty soil will become softer and stickier until it cannot retain its shape. The samples for laboratory tests were collected from the landslide area according to the lithological units (Table 2).16 A. Günther and Thiel. depending on the nature of the mineral constituents. in accordance with ASTM (American Society for Testing and Materials) standards (Bowles. The average values of the dry unit weights ranged from a high of 13.7 50.8 5. liquid limit (LL). pyroclastics. The average values were separately calculated for each lithology in the laboratory tests.. 1999. it is considered to be in a liquid state. shear strength) and the landslide were found in highly weathered parts of lithological unit.9 Min. 9 shows the grain-sized distributions of the samples.4 48. the majority of the landslides (80%) were found to be located within basalt.2 3.3 63. and a dispersion is formed (Barnes.5 5.3 1. 3. 2007b). The geological units in the landslides areas highly weathered.9 kN/m3.0 62.1 30.4 33. dry unit weight (γd). plastic index. fine sieve.5 Silt (%) Clay (%) Fig. 1992).0 38. XRD pattern of the clay in the weathered rocks. This characteristic depends on the amount and mineralogical composition of the fines and the amount of water present. 2001).. Geology Alv Pl Ev Kru Jkr Jlh γ2 Kk Number of specimen 3 3 10 30 0 0 4 0 3.4 64.1 63.4 60.8 9. The average values of the saturated unit weights were between 16. The grain size distributions of the soil in the landslides areas and their neighborhood were determined by separating the particles using a conventional sieve. Lithology The nature and rate of geomorphological processes including landslides depend on the lithology and weathering characteristics of the underlying materials (Dai et al.4 32.0 5.9 58. 8. Soil containing fine particles showed the plasticity and cohesiveness so that a lump of soil can have its shape changed or remolded without the soil changing in volume or breaking up.8 61. GarcíaRodríguez et al. 2000).5 39.3 4. Many authors agree that lithology has a significant impact on the occurrence of landslides. value 59. Grain size Sand (%) Kru Ev γ2 Pl Alv Kru Ev γ2 Pl Alv Kru Ev γ2 Pl Alv Max. As a result of the aerial distributions analysis performed according to the lithological units. and hydrometer methods.3 67. 2009. the rock outcrops were formed by physical and chemical factors.7 34.6 3. specific gravity (Gs).8 35.9 1. Geotechnical properties Fifty disturbed–undisturbed samples from the landslide and stable areas were analyzed (grain size distribution. were performed on disturbed and undisturbed samples collected from the slopes during dry and rainy seasons.3 5. feldspar.1 8. there are less interactions between the soil particles and the slurry.3 2. 2 7.4 35. The larger the plasticity index the greater will be the volume change.3 68.8 1.9% to 18.0 47.5 0.6 5. Soils of the same geological origin are usually found on the plasticity chart as straight lines parallel to line A.6 25. in which an empirical boundary known as line “A” separates clays from silty.1 118.5 to 0. Particle size distribution.0 3.4 26.9 26.2 151.0 44. 80% of samples are in the MH group (silts of high plasticity) soils.5 7.6 31.9 1. with low plasticity) soils (Fig. The plasticity index values are N20 above the line C.1 44.0 149.1 0.7 0.5 17.5 11.9 11.2 23.5 3. The shear strength was higher in the dry season samples. 10).2 8.0 0.1 26.5 56. and the average moisture content changes from 40% to 62% (Table 4).9 42. Most failures involve a shear-type failure of soil due to a soil composed of individual soil particle that slide when the soil is loaded.6 51.0 0.6 28.6 8.5 0.0 27.4 154. may be used to determine other important properties such as void ratio. the soils had very low shear strength parameters. Therefore.5 0. were determined by the widely used triaxial tests suitable for all types of soil.5 46.8 17.7 27.0 54.9 Fig. Table 4 Geotechnical properties of the soil samples.4 42. As a convenience for comparing different soils.5 14.3 43.2 26.0 13.A.1 0.9%.8 29.8 41.9%.5 22.7 20.2 0.17 kN/m2 to 149.7 0. the average void ratios ranged from a high of 1.1 1.6 12.2 10.5 Arithmetic Standard mean deviation 17.0 8.4 0.3 142.4 62.3 1.5 0.7 56.6 11.0 38.8 12.3 40.6 27.9 14.4 13.0 57. “Fat” or plastic clays are found above the line.5 27.8 16. 20% of samples are ML group (silts. Line D separates clays of high plasticity from clays of very high plasticity.6 0.9 4.6 1.6 9.2 19.6 1. Yalcin / Applied Clay Science 52 (2011) 11–19 17 the specimens collected in the rainy season (Table 4). The average plasticity index of units extended from 8. Shear strength In soils failure occurs as a result of exceeding the maximum shear stress that the soil can maintain.3 to a low of 0.1 52. 2000).9 66.7 10.6% and 68.1 28.1 26.8% to 49. 9.3 46.0 0.0 0.5 0.0 52.0 12.9 12.6 0.9 28.3 4.5 36.3 0.7 12. 3.5 10.4 27.6 3. 10). The test consists of the application of shear stress within a cylindrical soil sample by changing the principal stresses σ1 and σ3.6 17.0 18.6 0.4 1.9 1.5 0. Line C is the boundary of plasticity index for 20 values.4 48. whereas the averages plastic limit ranged from 37.0 0.2 22.7 0.4 4.1 31.3 16.2 47.3 18.2 31.2 85.4 0.2 12.8 51.2 44.6 6.2 27. In this study.2 1.6.9 26.0 16.6 3.0 6. The average values of the effective cohesions varied from 118.9 9.2 31.9. with the average of liquid limit values between 48.8.3 40.7 9.0 133.8 21.4 39.9 28. The shear strength of soil depends on the water content.9 9.9 0. The shear strength parameters of a soil are cohesion (c) and angle of friction (ϕ). porosity.1 5.0 27.5 32.5 0.4 33.5 0. The samples tested were of intermediate and high plasticity.0 124. The higher values of cohesion (c) and the angle of friction (ϕ) are higher than the shear strength of the soil (Barnes.4 32.3 1. Silts and clays containing a large portion of “rock flour” (finely ground non-clay minerals) are below line A four separate charts help to make clear the use plasticity charts (Endurazyme.1 9.2 0.3 26.4 12.3 10.1 0.1 0.8 23.5 14.3 63.4 3.0 17. The average values of the effective cohesions ranged from 37 kN/m2 to 17 kN/m2.0 8. value 13.5 42.5 62.8 8.4 0.5 7.3 1.6 1.9 6.8 27.5 0. Soil with high shear strength will be able to support a slope without failing.3 0.0 0.4 17.7%.3 3.2 1.7 119.4 8.4 20.7 43.7 11.3 57.2 68.8 1. and degree of saturation.9 0. the shear strength is the main factor for understanding the behavior of a soil mass.2 16.5 63.3 13.5 0.9 17. According to the plasticity chart.3 110.0 126. Thus.5 0.07 kN/m2 and the average values of effective angle of frictions ranged 28° to 43° (Table 4). and the average values of effective angle of friction ranged from 11° to 26° for .2 24.2 0.2 5.2 41.1 1.9 26.3 13.7 51.2 16.3 0.8 2. The most common procedure is to maintain a steady triaxial cell pressure σ3 and increase the axial or vertical stress σ1 until failure is achieved.5 38.8 18.3 4.0 74.5 0.5 29.4 22.7 4.9 13.4 25.3 11.6 144.2 111. silty or clayey fine sands.0 0.3 5.4 8.5 49.9 16.4 5. 6 5.6 7.0 51.5 30.6 7.3 37. Geotechnical properties Saturated unit weight γs (kN/m3) γs-Kru γs-Ev γs-γ2 γs-Pl γs-Alv 3 Dry unit weight γd (kN/m ) γd-Kru γd-Ev γd-γ2 γd-Pl γd-Alv Specific gravity Gs (kN/m3) Gs-Kru Gs-Ev Gs-γ2 Gs-Pl Gs-Alv Liquid limit LL (%) LL-Kru LL-Ev LL-γ2 LL-Pl LL-Alv Plastic limit PL (%) PL-Kru PL-Ev PL-γ2 PL-Pl PL-Alv Plasticity index PI (%) PI-Kru PI-Ev PI-γ2 PI-PI PI-Alv Porosity (n) n-Kru n-Ev n-γ2 n-Pl n-Alv Void ratio (e) e-Kru e-Ev e-γ2 e-Pl e—Alv Moisture content ω (%) ω—Kru ω—Ev ω—γ2 ω—Pl ω—Alv Cohesion c (kN/m2) c—Kru c—Ev c—γ2 c—Pl c—Alv Internal friction angle ϕ (°) ϕ—Kru ϕ—Ev ϕ—γ2 ϕ—Pl ϕ—Alv 2 Cohesion c (kN/m ) c—Kru (for dry season) c—Ev c—γ2 c—Pl c—Alv Internal friction angle ϕ (°) ϕ—Kru (for dry season) ϕ—Ev ϕ—γ2 ϕ—Pl ϕ—Alv Max.8 6.8 13. Casagrande (1936) devised a plasticity chart (Fig.9 1. The average porosity varied from 0.0 1.2 49.3 13.3 36.0 39.8 27. 7 26. 2010).2 1.1 0.0 2.1 16. To obtain the residual shear strength parameters drained triaxial tests were performed of 20 undisturbed samples collected from various depths of sample pits in the sliding mass (3–5 m). value 19.6 40.9 48.2 Min.3 127.4 22.3 26.7 18.8 120.4 11.9 11.1 18.2 1.6 0. the shear the strength parameters (c and ϕ).4 23. 637–650. silty or clayey fine sands. In other words. C. Eng. Although these factors vary from region to region. New Jersey. 44..62 and the void ratio was 1. 172–191. Ecol. and high pore pressure is maintained during movement.L. Nat. M.. A. 1992.. Soil Mechanics and Foundation Engineering. Field studies of 50 landslides in highly weathered units and the laboratory analysis indicated that there is an approximate correlation between the geotechnical properties of the units and landslides. The water infiltration rate also decreases with the soil density. In such situations where the moisture content of the soil may be larger than the liquid limit. landslides prone areas can be more correctly identified. Sci. Eng. the lithology parameter is determined with a very general approach. 1936. J. Barnes. CL: Clays. known and do not show a great variation with the exception of geology. S. 43 (3). The shear strength can be major factor landslides. Based on these test results. The shear strength of materials varies with the water content. 381–391. Imperial College of Science and Technology. 2007. OH: Clays of very high plasticity). In conclusion. Cardinali.C. 1981.. current landslide susceptibility maps should be produced in such a way that they have information about the geotechnical parameters of the landslide prone areas. For this reason. these studies classify the units according to names in geologic terms and relate these units to the occurrence of landslides.. Changes in land cover and shallow landslide activity: a case study in the Spanish Pyrenees.html. Palgrave. Turkey) and landslide susceptibility map of its near vicinity. the soils in the study area slowly became fully saturated with water and after a period of strong rain falls in the rainy season a landslide occurred.. Xu. 7. F.C. and the stability of the masses is disturbed. One of the greatest flaws in landslide studies applied in regional scale is that. M. 10. Therefore. The soils in the study area had very low shear strength values when saturated with water. Bulut. thus the risk of a landslide decreases.A. 687–698.. 2006. Because of the increase in the amount of fine grain. Thus. H. 9. vegetation cover. aspect..com. particularly in the rainy seasons. and vegetation cover do not distinctly influence the risk of landslides. This is a source of different effect to landslides.mite... NJ. Rock characterization testing and monitoring (ISRM suggested methods). The maximum values of the effective cohesion and angle of friction were 37 kN/m2 and 26° for the rainy season samples. 60–64. 2000. a mixture of different particle sizes and the distribution of these particles gives very useful information about the engineering behavior of the soil.E. Eng. MH: Silts of high plasticity. 2008. an increase in clay content also increased the potential for a landslide. Hazards Earth Syst. OL: Silts and silty clays of intermediate plasticity. Nat. London. 1994.. 147–161. strength. North Turkey) region. Bovolo. 81 (1). C.. Li..W. http://www. However. discontinuity. García-Rodríguez. the denser the soil and less the pores or the water passage in the soil. Endurazyme. The grain size distribution plays an important role in the movement of a landslide mass and the pore-pressure increase after failure. G. climate. this is a rather rough approach as these variations in these units have different effects on landslides. A soil with high shear strength is stabilizes slopes without failing. Can. F. to some degree. 1377–1387. Z. New Jersey.. a reduction in strength due to wet condition is mainly responsible for failure in the study area. Assessment of landslide susceptibility on the natural terrain of Lantau Island. Environ. C. Duman. Jakob. Samples with finer grain sizes or larger fine-particle contents fail more easily. decreasing resistance to shear. The experimental data and field inspections show that landslides mostly occur in the clayey soils. Gokceoglu. ISRM. while the moisture content changed from 40% to 62%. landslides can occur after heavy rainfalls or at the end of the rainy season since the water content of the soil may be more than the liquid limit of the deposit. USA. Royal School of Mines.. M. their effect is.F. 1st Int. Benito. the geological units that reached an average clay content of 14% were found in approximately 80% of the landslides in this study... Sci. H. and etc. Hong Kong. T. F.E. F. An unstable slope can have short or long term side effects depending on the shear strength. pore water pressure increases. Dingman. the maximum effective cohesion and effective angle of friction values reached 149 kN/m2 and 43°. Bathurst. Nefeslioglu. The March 17. In the dry season samples. CH: Clays of high plasticity. Geol. Landslide susceptibility mapping of the slopes in the residual soils of the Mengen region (Turkey) by deterministic stability analyses and image processing techniques. drainage is reduced and thus..Y. The effective stress is reduced by increased water pressure. C. Geol. A. Günther. Gokceoglu. 3.61 in the study area. Soil Mechanics Principles and Practice. Proc. The lithological units could not continue the primary properties of discontinuity.. Therefore.A. A. Catena 38. 317–327.18 A. Results and conclusion There are a wide variety of factors that contribute to the occurrence of landslides. Geol. pp. The higher the density. GIS-based landslide susceptibility for Arsin-Yomra (Trabzon. J. In the study. 2001. The clay content in the soil is very important in relation to the potential occurrence of landslides. Sönmez. The determination of the preconsolidation load and its practical significance. B. To determine the saturation level. Most natural soils are composite. Casagrande. Ardizzone.. C. Susceptibility assessment of earthquake-triggered landslides in El Salvador using logistic regression. 36 (3). M. New York. Hazards Earth Syst. Factors such as morphology.. silty clays. Editor.. Díaz. Lee. Geomorphology 74 (1–4).J.I. slope. Combined rock slope stability and shallow landslide susceptibility assessment of the Jasmund cliff area (Rügen Island. Physical Hydrology. 279–300. Environ. T. Thiel. The average of liquid limit values was between 49% and 69%. 1996.. They are generally related to lithology. with slight plasticity. P. Upper Saddle River. grain size and fine-particle contents can have a significant impact on the mobility of rainfall-induced landslides. 65–83.. 2005 Kuzulu landslide (Sivas.. M. Identification and mapping of recent rainfall-induced landslides using elevation data collected by airborne Lidar. Aksoy. the moisture content of the slope was compared with the liquid limit values in the layers most prone to failure.. 196–206. F. the units show different degrees of weathering.au/ manual/aline.. J. cited 30 November 2010.. Guzzetti. Prentice Hall.. Yalcin / Applied Clay Science 52 (2011) 11–19 Fig. Germany). Slope stability depends principally on the friction angle (ϕ) and at very limited cohesion. Modelling the effect of forest cover on shallow landslides at the river basin scale. As a consequence of infiltration and the deep leakage of spring waters. the location of rivers and roads. The porosity of soils was found to reach 0. Malpica. Geomorphology 95. sandy clays of low plasticity. Mass.. Cisneros. Geol. the movement is usually fast. 2000. sudden failure can occur. . These values presented the optimal conditions for a landslide in the Trabzon study area. 2007. References Akgun. elevation. 2010. H. Soil stabilization of clay based soils. 2009.. S. Conf. J. Reichenbach. Dai.. 2005. McGrawHill. elastic. Galli. The natural moisture content (ω) was very close to the liquid limit value because the study area is humid and rainy. The high soil porosity and void ratios permit water to flow easily in the soil and favor the infiltration process. High-pore water pressures contribute to most of the slope failures and most landslides in the Trabzon province during rainstorms. Engineering Properties of Soils and their Measurements. Distributions of the samples on the plasticity chart (ML: Silts. The impacts of logging on landslide activity at Clayoquot Sound British Columbia. Begueria. As a final conclusion. 4.. Bowles. Cambridge. 2010. 51 (8). S. F. F.. Nat. Hazards 41. A.. 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