LIQUEFACTION ANALYSIS of YOUNG SEDIMENTS in WESTERN PART of İZMİT BASIN

The İzmit Basin has developed under the control of North Anatolian Fault Zone (NAFZ). The basin is morphologically located in a plain between the Kocaeli Peneplane in the north and the Samanlıdağ Highlands in the south. NAFZ covering the study area in east west direction affects the region. Layers of sand and silty sand deposits are widespread in the alluvial basin. The groundwater level is considerably shallow. Soil liquefaction and settlements due to seismic shaking were assessed by this study. Necessary borings, field and laboratory tests are densely existent in the western part of the basin. Thus, liquefaction studies were mainly performed in the western part of the basin at Saraybahçe, Kullar and Kosekoy areas. Two evaluation methods were carried out for soil liquefaction analyses based on soil and seismic properties. A computer program assessed soil liquefaction potential of coarse grained soils based on Seed and Idriss approach originated in 1971. SPT N30 values of coarse grained soils were used in the liquefaction analyses and settlements due to liquefaction and seismic shaking were also determined. This method which is called Standard Liquefaction Analysis indicated that 36 out of 61 borings had liquefaction potential in different sand and gravelly sand levels. Calculated total average settlements varied from 7.83 to 13.27 cm. The lowest average settlement was observed in Saraybahçe area. The results obtained from standard liquefaction analyses were imported to Geographical Information System (GIS) to map the liquefaction potential and seismic settlements. The second method, Modified Chinese Criteria (MCC), was performed for liquefaction susceptibility of fine grained soil layers. This method which takes into account consistency and index properties of soils identified two boring areas that are susceptible to liquefaction. LL versus wn graphs displayed the results of this method.