Applied Clay Science 52 (2011) 11–19

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Applied Clay Science

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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

⁎ Tel.: +90 382 2882328.E-mail address: ayalcin@aksaray.edu.tr.

0169-1317/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.clay.2011.01.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 July 2010Received in revised form 12 January 2011Accepted 12 January 2011Available online 21 January 2011

Keywords:LandslideLithologySlopeVegetationGeotechnical properties

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 livesand properties. In this study, an assessment on the geotechnical characteristics of the areas where thelandslides occur and the immediate vicinity were compared with the characteristics of non-landslides areas. Atotal of 50 landslides occurring in different locations were studied. As a result of the geotechnicalinvestigations, the average of liquid limit values were between 49% and 69%, and the average plasticity indexof the units ranged from 9% to 19% in the overlying materials in the landslide areas. Heavy rainfall exceedingboundary saturation of soils plays a critical role in causing landslides. According to the Unified SoilClassification System, the soils were “silts of high plasticity and silts, silty or clayey fine sands of lowplasticity”. 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 showsthat the shear strength parameters decreased with increasing moisture content and that the number oflandslides increased in relation to the particle size distribution of the clay.

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1. Introduction

Landslides are major natural hazards in many areas of the world.As in most countries, the economic losses and casualties due tolandslides in Turkey are greater than generally recognized, and theycause a yearly loss of property larger than from any other naturaldisaster. In recent years, some devastating landslides in Turkey werereported (Gokceoglu et al., 2005; Ocakoglu et al., 2002). Generally,landslides are caused by intense rainfall, seismicity, water levelchange, storm waves or rapid stream erosion. These effects increasethe shear stress or a decrease in the shear strength of materials in theslope. Other causes that make an area vulnerable to landslides areresidential and commercial development expanding into unstableslope areas under the pressures of population growth and urbaniza-tion and human activities such as deforestation or excavation ofslopes for road cuts and building sites. They became significant causesof landslides. In the area, heavy rainfalls generate a rapid increase inpore pressure in the unsaturated zone and groundwater flow in thesaturated area which can give rise to landslides.

The properties of the lithological units are of crucial importance interms of the propensity for landslides to occur. The weathered unitsact as a soil from a mechanical perspective, thus determining thegeotechnical properties of the unit is very important. In a previousstudy of the Black Sea region, the depth of the completely weathered

zones in rocks were determined at the end of the geophysical studiesperformed via land observation and Ground Penetrating Radar (GPR)surveys. Calculation was carried out at 9 different points using theRAMAC/GPR ground radar system and evaluating the obtainedprofiles. The main scarp of the landslide and road cutting indicatedthat the weathering depths varied between 15 and 20 m. Thesemeasurements supported the values obtained by GPR surveys (Yalcinand Bulut, 2007). Water content, consistency limits, grain sizedistributions, the type and characteristics of fine grained lithologicalunits must be determined in relation to landslide behavior. Thedetailed investigations of the geotechnical properties of the units are amajor aspect of the prediction of landslides. The characterization oflandslides could provide very useful clues in terms of the nature of theslope failure and subsequent debris movement, thus indicating wherelandslides could potentially occur. This study can also provide theinformation to locate safe construction areas in regions that are proneto landslides thus safeguarding lives and property. In this study, fiftylandslides were investigated in the Trabzon province.

2. The study area

The study area extended from longitude 39°15′E to 40°15′E andfrom latitude 40°30′N to 41°8′N in the middle of the East Black Searegion (Fig. 1). The climate is characteristically Black Sea, withtemperate climate summers and a rainy season generally lasting fromSeptember to April. The rainfall regime is irregular, with some periodswhen precipitation is rare and at other times there are long-lastingheavy rains. Very intense precipitation episodes also occur, causing

Fig. 1. Location map of the study area.

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disastrous flash floods in river basins and many landslides on theslopes (Yalcin, 2007a).

The main commercial agricultural products are hazelnuts andgreen tea, apart from the agricultural area the other main land covertypes are forest and pasture areas. In respect of the geologicalconditions, the study area mainly comprises alkaline volcanic unitsoutcropping parallel to the Black Sea coast. The overlying units widelyaffected by weathering in the study area. Heavy rainfall and densevegetation have increased the speed of weathering which thenconsiderably reduced the resistance of the overlying units to the riskof landslides.

3. Methodology

In this study, an extensive geotechnical study was carried out on50 different areas where landslides occurred. Lithology, slope, aspect,

land cover, stream, and roads were examined by comparing each ofthese parameter maps with a landslide inventory map (Fig. 2).However, the geotechnical properties related to the overlayinglithological units are the most decisive for landslides. Thus, thegeotechnical properties of the areas of landslides were compared withnon-landslides areas.

3.1. Morphology

The topography in the study area rises abruptly from coast. Thevariations in the slope of hills, mountains, and the rivers extend fromnorth to south direction give this region a rough morphology. Theslope angle is directly related to the occurrence of landslide. It isfrequently used as the key measure in landslide studies. The slopemap of the study area was divided into five slope categories (Fig. 3).ArcGIS 9.2 analysis was performed to discover in which slope group

Fig. 2. Landslide inventory map.

Fig. 3. Slope map.

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Fig. 4. A typical shallow failure in the study area.

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landslides happened, and the rate of occurrence was recorded. Thefrequency of landslides was found by determining the relationshipbetween the slope and the landslide (Table 1). This table indicatesthat most of the landslides occurred at a slope angle N50%. Thelandslide rate in the moderate slope (0–30%) has about 20%. This is ahigher for the tea areas because tea plant is very common the surfaceand rain water infiltrate the soil. For this reason, the landslides occurin the areas with moderate slope (Fig. 4).

3.2. Climate

The climate in the Black Sea region is dominantly a typicaltemperate climate with little snow during the winter months. There isvery rainy season from September to April. From data accumulatedover a period of 22 years, the coldest month in Trabzon Province is isFebruary with an average temperature of 6.7 °C, the hottest month isAugust with an average temperature of 23.2 °C. The total precipitationin Trabzon city is 838 mm, and precipitation dispersal pattern issymmetrical (Fig. 5). As a general rule, the effect of rainfall infiltrationon slope can change the suction of the soil and the positive porepressure, or main water table, as well as raising the soil unit weightand reducing the anti-shear strength of rock and soil (Ardizzone et al.,2007). Rainfall is one of the most significant factors causing forlandslides. In particular, transient pore pressure in response to shortintense rainfall plays an important role in the occurrence of shallowlandslides which are the main type in the study area (Fig. 4). In thisprovince, it rains all the year, thus maintaining the geological materialclose to saturation. Therefore, short heavy rainfalls further increasethe pore water which consequently can give rise to landslides.

3.3. Land cover

Land cover acts as a protection and reduces the susceptibility ofsoil erosion and landslides. Several researchers (Bathurst et al., 2010;Begueria, 2006; Gokceoglu and Aksoy, 1996; Jakob, 2000; Tangestani,2004; Yalcin, 2008) have revealed a clear relationship betweenvegetation cover and slope stability, especially for shallow landslidesas in the Trabzon province. Geotechnical parameters, such ascohesion, internal friction angle, weight of the soil and pore-waterpressure, are influenced by the vegetation. Vegetation can both

Table 1Distribution of landslides.

Parameter Classes % of total area (a) % of landslide area (b)

Slope (%) 0–10 13.7 2.410–20 6.9 5.720–30 11.1 11.830–50 26.8 30.8N50 41.5 49.3

Land cover Tea 1.1 1.7Hazelnut 15.4 23.2Deciduous (Forest) 45.5 49.3Rocky 1.1 1.0Pasture 18.6 8.0Agriculture 17.1 16.5Settlement 1.2 0.4

Geologya Alv 3.9 1.5Pl 2.4 2.2Ev 36.6 13.2Kru 54.7 80.0Jkr 0.1 0.00Jlh 0.7 0.00γ2 1.6 3.1Kk 0.1 0.00

a Alv—Alluvium, Pl—Pliocene, continental units, Ev—Eocene, volcanic facies, Kru—Basalt, andesite, pyroclastics, and intercalations of sandstone clayey limestone andsiltstone, Jkr—Jurassic–Cretaceous units, Jlh—Lias units, γ2—Kaçkar granites, Kk—Carboniferous units.

enhance effective soil cohesion due to root matrix reinforcement andsoil suction or negative water pressure through evapotranspirationand interception (Karsli et al., 2009). In general, vegetations tend toincrease the infiltration rate of soils. Some water usually evaporatesfrom plant surfaces before it can fall to the soil surface. A plant coverand litter layer of dead vegetation protects the soil surface fromcompaction by heavy raindrops, and also slows the delivery of waterto the soil surface. Plant stems help slow down water that flows overthe soil surface. Plant roots help create openings in the soil, and alsodraw water from beneath the soil surface and transpire it through theleaves back to the atmosphere. Decayed plant matter keep fine soilparticles (such as clay particles) from sticking together, therebyincreasing the infiltration capacity (Dingman, 1994).

A landslide event is a sophisticated process that can be powerfullyaffected by the vegetation. As a result, changes in land cover resultingfrom human activities and climate interact with slope stability. In thisstudy, a single date image from Landsat ETM+ (Path 173; Row: 32) onOctober 19, 2000 was used to generate land cover types (Reis andYomralioglu, 2006). The number of landslides was relatively high onbare land, and low on forest canopy (Fig. 6). Such a relation revealedby many earlier studies of the Black Sea region (Akgun and Bulut,2007; Karsli et al., 2009; Yalcin, 2007b, 2008). As results, ninedifferent types of land cover described. The main ones correspondedto deciduous, pasture, and agriculture areas. To show the relationshipbetween the land cover and landslides, the landslide inventory mapwas compared with the land cover maps (Table 1). Landslides werelargely seen in deciduous and hazelnut areas. The deciduous areasinclude different tree types of tree growth such as brake, thicket andsmall wood. These types obstruct the surface flow of rain fall water.This increases the pore water pressure of the soil, and, as aconsequence, the number of landslides has increased in these areas.

Fig. 5. Monthly average precipitation vs. temperature graphic.

Fig. 6. Land cover map.

Fig. 7. Lithology map.

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Table 2The distribution of number of samples in the lithological units.

Geology Number of specimen

Alv 3Pl 3Ev 10Kru 30Jkr 0Jlh 0γ2 4Kk 0

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3.4. Lithology

The nature and rate of geomorphological processes includinglandslides depend on the lithology and weathering characteristics ofthe underlying materials (Dai et al., 2001). Many authors agree thatlithology has a significant impact on the occurrence of landslides,because lithological variations often led to different the strength andpermeability values of rocks and soils (Dai et al., 2001; García-Rodríguez et al., 2008; Günther and Thiel, 2009; Luzi and Pergalani,1999; Wu and Qiao, 2009; Yalcin, 2007b). Eight lithological unitsdistinguished in the Trabzon province (Fig. 7). As a result of the aerialdistributions analysis performed according to the lithological units,the majority of the landslides (80%) were found to be located withinbasalt, andesite, pyroclastics, and intercalations of sandstone clayeylimestone and siltstones (Table 1). The geological units in thelandslides areas highly weathered. In the study area, the rock outcropswere formed by physical and chemical factors. The degree ofweathering of rocks was determined following the method suggestedby ISRM (1981).

X-Ray Diffraction revealed illite and kaolinite as the main clayminerals in the weathered materials (Fig. 8).

3.5. Geotechnical properties

Fifty disturbed–undisturbed samples from the landslide and stableareas were analyzed (grain size distribution, plastic index, shearstrength) and the landslide were found in highly weathered parts oflithological unit. The samples for laboratory tests were collected fromthe landslide area according to the lithological units (Table 2). Thegeotechnical tests, in accordance with ASTM (American Society forTesting and Materials) standards (Bowles, 1992), were performed ondisturbed and undisturbed samples collected from the slopes duringdry and rainy seasons. The average values were separately calculatedfor each lithology in the laboratory tests.

Fig. 8. XRD pattern of the clay in the weathered rocks.

3.6. Particle size distribution

The influence of particle size distribution on the occurrence andspeed of landslides was significant (Yalcin, 2007b). Sieve andhydrometer analyses were undertaken to designate the grain size ofsoil in the landslide area. The grain size distributions of the soil in thelandslides areas and their neighborhood were determined byseparating the particles using a conventional sieve, fine sieve, andhydrometer methods. The average of the grain size distributionsbelonging the completely weathered section of each lithological unitis presented in Table 3 whereas Fig. 9 shows the grain-sizeddistributions of the samples.

3.7. Index characteristics

Fifty samples were analyzed to measure the saturated unit weight(γs), natural unit weight (γn), dry unit weight (γd), specific gravity(Gs), liquid limit (LL), plastic limit (PL), plasticity index (PI), porosity(n), void ratio (e), and moisture content (ω).

Soil containing fine particles showed the plasticity and cohesive-ness so that a lump of soil can have its shape changed or remoldedwithout the soil changing in volume or breaking up. This character-istic depends on the amount and mineralogical composition of thefines and the amount of water present, or themoisture content. As themoisture content increases, a clayey or silty soil will become softerand stickier until it cannot retain its shape. Then, it is considered to bein a liquid state. If the moisture content is further increased, there areless interactions between the soil particles and the slurry, and adispersion is formed (Barnes, 2000).

The results of the soil analysis are given in Table 4. The averagevalues of the saturated unit weights were between 16.7 kN/m3 and18.4 kN/m3. The average values of the dry unit weights ranged from ahigh of 13.9 kN/m3 to a low of 11.9 kN/m3. The average specificgravity of all soil particles was about 26–27 kN/m3, depending on thenature of the mineral constituents. Specific gravity was influenced bythe presence of dolomite, feldspar, calcite, and quartz. Specific gravity

Table 3Grain size distribution of number of samples in the lithological units.

Grain size Max. value Min. value Arithmetic mean Standard deviation

Sand (%) Kru 59.4 13.4 35.9 15.3Ev 64.5 47.3 58.4 5.3γ2 39.8 29.0 34.1 4.5Pl 35.7 27.4 30.4 4.6Alv 34.8 32.6 33.7 1.1

Silt (%) Kru 61.4 34.4 50.0 8.3Ev 48.3 31.8 38.0 4.8γ2 67.3 55.3 62.1 5.0Pl 66.5 58.4 63.3 4.3Alv 65.8 60.3 63.3 2.8

Clay (%) Kru 30.7 0.3 14.2 9.3Ev 5.3 1.7 3.6 1.2γ2 5.0 3.0 3.8 0.9Pl 7.5 5.6 6.3 1.0Alv 5.9 1.5 3.0 2.5

Fig. 9. Particle size distribution.

Table 4Geotechnical properties of the soil samples.

Geotechnical properties Max.value

Min.value

Arithmeticmean

Standarddeviation

Saturated unit weight γs (kN/m3) γs-Kru 19.0 13.9 17.6 0.6γs-Ev 17.8 16.3 17.0 0.5γs-γ2 18.5 18.2 18.4 0.1γs-Pl 17.3 16.8 17.0 0.3γs-Alv 16.9 16.5 16.7 0.2

Dry unit weight γd (kN/m3) γd-Kru 14.4 10.3 12.6 1.2γd-Ev 13.5 10.9 12.3 0.7γd-γ2 14.0 13.7 13.9 0.1γd-Pl 13.0 11.9 12.6 0.7γd-Alv 12.1 11. 7 11.9 0.2

Specific gravity Gs (kN/m3) Gs-Kru 28.0 26.1 26.9 0.4Gs-Ev 27.6 26.2 26.8 0.4Gs-γ2 27.5 26.4 27.0 0.5Gs-Pl 27.4 26.9 27.1 0.3Gs-Alv 27.3 26.3 26.9 0.5

Liquid limit LL (%) LL-Kru 63.2 36.0 48.6 7.4LL-Ev 68.5 39.0 51.1 8.0LL-γ2 62.0 44.5 52.3 8.4LL-Pl 74.0 63.0 68.7 5.5LL-Alv 54.0 47.5 51.0 3.3

Plastic limit PL (%) PL-Kru 51.5 29.5 38.3 5.4PL-Ev 46.5 32.4 37.8 3.9PL-γ2 49.5 35.3 41.2 6.1PL-Pl 56.7 46.3 49.9 5.9PL-Alv 43.7 40.6 42.2 1. 6

Plasticity index PI (%) PI-Kru 27.5 3.6 10.3 5.6PI-Ev 22.4 6.6 13.3 5.8PI-γ2 20.4 7.1 11.1 6.3PI-PI 22.7 16.5 18.8 3.4PI-Alv 10.3 7.0 8.9 1.7

Porosity (n) n-Kru 0.6 0.5 0.5 0.1n-Ev 0.6 0.5 0.5 0.0n-γ2 0.5 0.5 0.5 0.0n-Pl 0.6 0.5 0.5 0.0n-Alv 0.6 0.5 0.6 0.0

Void ratio (e) e-Kru 1.6 0.8 1.2 0.2e-Ev 1.4 1.0 1.2 1.1e-γ2 0.1 0.9 0.9 0.0e-Pl 1.3 1.1 1.2 0.1e—Alv 1.3 1.2 1.3 0.1

Moisture content ω (%) ω—Kru 57.0 23.5 40.1 9.4ω—Ev 52.0 38.4 44.4 4.6ω—γ2 57.9 39.7 48.4 8.2ω—Pl 66.8 56.3 62.2 5.3ω—Alv 51.2 43.4 47.8 4.0

Cohesion c (kN/m2) c—Kru 31.4 8.8 21.4 8.2c—Ev 33.3 13.7 22.8 8.3c—γ2 26.5 20.6 23.5 4.2c—Pl 42.2 31.4 36.8 7.6c—Alv 22.6 11.8 17.2 7.6

Internal friction angle ϕ (°) ϕ—Kru 25.7 12.9 19.3 3.9ϕ—Ev 18.3 9.2 13.4 4.0ϕ—γ2 26.4 24.6 25.5 1.3ϕ—Pl 12.5 9.9 11.2 1.8ϕ—Alv 14.3 11.2 12.7 2.2

Cohesion c (kN/m2)(for dry season)

c—Kru 142.2 85.3 119.0 16.9c—Ev 151.0 110.8 126.3 17.7c—γ2 133.4 120.6 127.0 9.0c—Pl 154.0 144.2 149.1 6.9c—Alv 124.6 111.8 118.2 9.0

Internal friction angle ϕ (°)(for dry season)

ϕ—Kru 40.4 27.4 31.9 3.7ϕ—Ev 32.2 23.2 28.4 4.0ϕ—γ2 44.5 41.2 42.9 2.3ϕ—Pl 30.1 27.6 28.8 1.8ϕ—Alv 31.2 28.5 29.9 1.9

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may be used to determine other important properties such as voidratio, porosity, and degree of saturation.

The samples tested were of intermediate and high plasticity, withthe average of liquid limit values between 48.6% and 68.7%. Theaverage plasticity index of units extended from 8.9% to 18.9%, whereasthe averages plastic limit ranged from 37.8% to 49.9%. As aconvenience for comparing different soils, Casagrande (1936) deviseda plasticity chart (Fig. 10), in which an empirical boundary known asline “A” separates clays from silty. Soils of the same geological originare usually found on the plasticity chart as straight lines parallel toline A. The larger the plasticity index the greater will be the volumechange. “Fat” or plastic clays are found above the line. Silts and clayscontaining a large portion of “rock flour” (finely ground non-clayminerals) are below line A four separate charts help to make clear theuse plasticity charts (Endurazyme, 2010). Line C is the boundary ofplasticity index for 20 values. The plasticity index values are N20above the line C. Line D separates clays of high plasticity from clays ofvery high plasticity. According to the plasticity chart, 80% of samplesare in the MH group (silts of high plasticity) soils, 20% of samples areML group (silts, silty or clayey fine sands, with low plasticity) soils(Fig. 10).

The average porosity varied from 0.5 to 0.6, the average void ratiosranged from a high of 1.3 to a low of 0.9, and the average moisturecontent changes from 40% to 62% (Table 4).

3.8. Shear strength

In soils failure occurs as a result of exceeding the maximum shearstress that the soil can maintain. Therefore, the shear strength is themain factor for understanding the behavior of a soil mass. Soil withhigh shear strength will be able to support a slope without failing. Theshear strength of soil depends on the water content. Most failuresinvolve a shear-type failure of soil due to a soil composed of individualsoil particle that slide when the soil is loaded. The shear strengthparameters of a soil are cohesion (c) and angle of friction (ϕ). Thehigher values of cohesion (c) and the angle of friction (ϕ) are higherthan the shear strength of the soil (Barnes, 2000). In this study, theshear the strength parameters (c and ϕ), were determined by thewidely used triaxial tests suitable for all types of soil. The test consistsof the application of shear stress within a cylindrical soil sample bychanging the principal stresses σ1 and σ3. The most commonprocedure is to maintain a steady triaxial cell pressure σ3 and increasethe axial or vertical stress σ1 until failure is achieved. To obtain theresidual shear strength parameters drained triaxial tests wereperformed of 20 undisturbed samples collected from various depthsof sample pits in the sliding mass (3–5 m). The average values of theeffective cohesions ranged from 37 kN/m2 to 17 kN/m2, and theaverage values of effective angle of friction ranged from 11° to 26° for

the specimens collected in the rainy season (Table 4). Thus, the soilshad very low shear strength parameters. The shear strength washigher in the dry season samples. The average values of the effectivecohesions varied from 118.17 kN/m2 to 149.07 kN/m2 and the averagevalues of effective angle of frictions ranged 28° to 43° (Table 4).

Fig. 10. Distributions of the samples on the plasticity chart (ML: Silts, silty or clayey finesands, with slight plasticity, CL: Clays, silty clays, sandy clays of low plasticity, OL: Siltsand silty clays of intermediate plasticity, MH: Silts of high plasticity, CH: Clays of highplasticity, OH: Clays of very high plasticity).

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4. Results and conclusion

There are a wide variety of factors that contribute to theoccurrence of landslides. They are generally related to lithology,slope, aspect, elevation, vegetation cover, discontinuity, the locationof rivers and roads. Although these factors vary from region to region,their effect is, to some degree, known and do not show a greatvariation with the exception of geology. One of the greatest flaws inlandslide studies applied in regional scale is that, the lithologyparameter is determined with a very general approach. In otherwords, these studies classify the units according to names in geologicterms and relate these units to the occurrence of landslides. However,this is a rather rough approach as these variations in these units havedifferent effects on landslides. The lithological units could notcontinue the primary properties of discontinuity, elastic, strength,and etc. For this reason, the units show different degrees ofweathering. This is a source of different effect to landslides.

Most natural soils are composite, a mixture of different particlesizes and the distribution of these particles gives very usefulinformation about the engineering behavior of the soil. The grainsize distribution plays an important role in the movement of alandslide mass and the pore-pressure increase after failure. Sampleswith finer grain sizes or larger fine-particle contents fail more easily,and high pore pressure is maintained during movement. Based onthese test results, grain size and fine-particle contents can have asignificant impact on the mobility of rainfall-induced landslides. Theexperimental data and field inspections show that landslides mostlyoccur in the clayey soils. Thus, an increase in clay content alsoincreased the potential for a landslide. In the study, the geologicalunits that reached an average clay content of 14% were found inapproximately 80% of the landslides in this study. The clay content inthe soil is very important in relation to the potential occurrence oflandslides. Because of the increase in the amount of fine grain,drainage is reduced and thus, pore water pressure increases, and thestability of the masses is disturbed.

Slope stability depends principally on the friction angle (ϕ) and atvery limited cohesion, sudden failure can occur. In such situationswhere the moisture content of the soil may be larger than the liquidlimit, the movement is usually fast. To determine the saturation level,the moisture content of the slope was compared with the liquid limitvalues in the layers most prone to failure. The natural moisturecontent (ω) was very close to the liquid limit value because the studyarea is humid and rainy. The average of liquid limit values wasbetween 49% and 69%, while the moisture content changed from 40%to 62%. Therefore, landslides can occur after heavy rainfalls or at theend of the rainy season since the water content of the soil may bemore than the liquid limit of the deposit. As a consequence of

infiltration and the deep leakage of spring waters, the soils in thestudy area slowly became fully saturated with water and after aperiod of strong rain falls in the rainy season a landslide occurred.

The high soil porosity and void ratios permit water to flow easily inthe soil and favor the infiltration process. The water infiltration ratealso decreases with the soil density. The higher the density, the denserthe soil and less the pores or thewater passage in the soil, thus the riskof a landslide decreases. The porosity of soils was found to reach 0.62and the void ratio was 1.61 in the study area. These values presentedthe optimal conditions for a landslide in the Trabzon study area.

The shear strength can be major factor landslides. A soil with highshear strength is stabilizes slopes without failing. An unstable slopecan have short or long term side effects depending on the shearstrength. The shear strength of materials varies with the watercontent. High-pore water pressures contribute to most of the slopefailures and most landslides in the Trabzon province during rain-storms. The effective stress is reduced by increased water pressure,decreasing resistance to shear. The soils in the study area had very lowshear strength values when saturated with water. The maximumvalues of the effective cohesion and angle of friction were 37 kN/m2

and 26° for the rainy season samples. In the dry season samples, themaximum effective cohesion and effective angle of friction valuesreached 149 kN/m2 and 43°. In conclusion, a reduction in strength dueto wet condition is mainly responsible for failure in the study area,particularly in the rainy seasons.

Factors such as morphology, climate, and vegetation cover do notdistinctly influence the risk of landslides. Field studies of 50 landslidesin highly weathered units and the laboratory analysis indicated thatthere is an approximate correlation between the geotechnicalproperties of the units and landslides. Therefore, current landslidesusceptibility maps should be produced in such a way that they haveinformation about the geotechnical parameters of the landslide proneareas. As a final conclusion, landslides prone areas can be morecorrectly identified.

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