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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
2. STUDY AREA 
The study area is located 419?09'54" ve 41912'04" east 
meridians and 40°57'17" ve 41°06'39" north parallels, and take 
part in Ardesen district, Rize (Figure 1). 
  
  
  
  
  
Figure 1. Location of study area 
According to observations result 27 yearly in Ardesen region, 
the coldest month is January that its temperature average is 6.7 
°C, the hottest month is July that its temperature average is 22.2 
°C . Rize is the rainiest city in Turkey, its total precipitation is 
more than 2300 mm., and precipitation disperse every month 
symmetrical. Heavy rainfall is the triggering variables which 
shift the slope from a marginally stable to an unstable state and 
thereby initiating failure in an area of given susceptibility. So, 
landslides in the study area have been widely affected by heavy 
rainfall. At the same time, heavy rainfall has been increased to 
speed of weathering materials in the region. Thus, weathering 
has been considerably reducing materials! resistance for 
landslides susceptibility in the area. Observations result 27 
yearly in Ardesen region has been evaluated and is shown the 
results in Figure 2 and 3. 
Tea gardens have been great majority constituted land cover 
and tea farming has been widely made in the study area. But, 
the roots of tea tree are tightly set and have been obstructed 
surface flow. So, superficial water has been gotten into soil 
materials and its saturation increased. The soil materials that 
saturated have been showed higher stress- lower resistance. 
Thus, movements have been become easy. Also, distribute 
acidic manure over a tea garden have been increased 
weathering speed of soil materials and this event invest the 
situation with additional horrors for landslide. 
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Figure 3. Average temperature according to months 
3. METHODOLOGY 
3.1 Geotechnical properties of landslide materials 
The properties of soil materials have been investigated for the 
effects on landslides. For this aim, geotechnical properties have 
been outlined in the study area. Geotechnical investigations 
carried out after Ortaalan landslide (Figure 1) revealed that 
much of the topsoil in the study area, consisting of soil. 
Weathering in the region has presumably altered and broken 
down the upper part of the bedrock by chemical decomposition 
and physical disintegration. In decomposed rocks, inherent 
bonding in the parent rock has gradually deteriorated in the 
course of decomposition because cohesion due to bonding 
generally decreases as the material decomposes. Due to limited 
cohesion, local soils, if unprotected (i.e., subjected to 
saturation), are susceptible to erosion during heavy rainfall. The 
shear — strength characteristics of such soils are dependent on 
the material type, density, stress level, permeability, etc. For the 
effects of soil on the Ortaalan landslide, representative 
disturbed and undisturbed soil samples were collected for 
conducting laboratory investigations including consistency of 
clays, specific gravity, unit weight of soil and direct shear test.