17 Ağustos 1999 tarihinde meydana gelen Mw=7.4 manyitüdlü depremin sonucunda Kuzey Anadolu Fay zonu üzerinde bulunan Adapazarı’nda çok sayıda sıvılaşma ve taşıma gücü kaybı sonucu yapılarda yana yatma, oturma ve yanal deplasmanlar gözlenmiş ve çok sayıda insanın ölümü ile büyük ekonomik kayıplar oluşmuştur. Bu nedenle Adapazarı’nda yer alan zeminlerin deprem yükleri altındaki davranışlarının bilinmesi önem kazanmıştır. Bu amaçla Adapazarı'nın sekiz farklı bölgesinde hasar görmüş yapıların etrafında yapılan arazi çalışmaları kapsamında açılmış on sondaj kuyusundan alınmış olan örselenmemiş zemin numuneleri üzerinde dinamik üç eksenli basınç deneyleri yapılmıştır. Deneylerde efektif jeolojik yükün 100 kPa’a yakın olduğu numuneler kullanılmış ve bütün numunelere σ c’=100 kPa konsolidasyon basıncı izotropik olarak uygulanmıştır. Konsolide olan zemin numuneleri üzerinde ilk önce maksimum elastisite modülleri belirlenmiş ve daha sonra göçme kriteri ε=±% 2.5 deformasyon seviyesi meydana gelinceye kadar farklı gerilme oranlarında ( σ d/2 σ c=0.205-0.515) gerilme kontrollü drenajsız deneyler yapılmıştır. Boşluk suyu basıncı oluşumunu kontrol edebilmek için yükleme frekansı f=0.1Hz seçilmiştir. Bu çalışma sonucunda farklı zemin ve plastisite grupları için gerilme-şekil değiştirme-boşluk suyu basınç davranış biçimleri incelenmiş ve ayrıca dinamik mukavemet eğrisi elde edilmiştir. Zeminlerin dinamik davranışı incelendiğinde 17 Ağustos 1999 Kocaeli depreminin Mw=7.4 manyitüdüne karşılık N=20 çevrim sayısında değişik plastisitede olan zeminlerin yumuşayarak büyük deformasyonlara uğradığı gözlenmiştir.

The August 17, 1999 Kocaeli Earthquake had a moment magnitude of Mw = 7.4 with a focal depth of 17 km. The earthquake has affected the city of Adapazari which is located on the North Anatolian Fault Line. Several liquefaction and bearing capacity loss cases involving tilting of buildings, excessive settlements and lateral spreading were encountered in the city. The earthquake caused several thousands of casualties and an enormous loss of assets. Consequently, it becomes important to determine the cyclic behavior of Adapazari soils under earthquake loads. Accordingly, undisturbed soil samples were taken from 10 bore holes drilled next to the damaged structures in Adapazari City within the context of geotechnical studies. The soil samples taken from the borings were determined to be as silty clay (CL), clay (CH), clayey silt (ML), silt (MH), silty sand (SM) and sand (SP). For the silty sands and very sandy silts, the fines content varies between FC=39-56%. The high plasticity silts have natural moisture content values changing between wn=41-40% while fines content varies between FC=92-99% and plasticity index values change between Ip=16-38%. The natural moisture content values change between wn=41-40% and fines content varies between FC=87-99% for nonplastic silts. The low-plasticity and high-plasticity clays have natural moisture content values varying between wn=32-45% and fines content values changing between FC=95-100% while plasticity index values change between Ip=15-44% and consistencies vary between wn/wL=0.55-0.90. Slightly overconsolidated soils with OCR=2.75-3.85 are lying on the surface of Adapazari. Silst and clays below 4.0 m depth are normally consolidated soils. Cyclic triaxial tests were conducted on undisturbed normally consolidated soil samples having different fines content and plasticity index values. The soil samples with an effective geological overburden stress near to 100 kPa were used for the experimental studies and all samples were consolidated under a consolidation stress of σ c=100 kPa. All tests were performed by cyclic triaxial testing apparatus located at Soil Dynamics Laboratory of Civil Engineering Faculty of ITU. Undrained cyclic tests at different cyclic stress ratio values were conducted on undisturbed soil samples and cyclic loadings had been applied regarding the failure criterion of the axial strains reaching ε= ±2.5%. In order to control the pore pressure build-up, the cyclic loading frequency was chosen to be f=0.1 Hz. According to the strength curves obtained by using the results of the cyclic triaxial tests performed within this study obeying the failure criterion of reaching an axial strain of ε=±2.5%, the cyclic stress ratio values obtained for a number of cycles of N=20 which represents an earthquake with magnitude Mw =7.4, are determined to be as CSR=0.49 for inorganic silts (MH) having plasticity index values change between Ip=27-31%, CSR=0.360 for inorganic silts (MH) having plasticity index values change between Ip=15- 22%, CSR=0.430 for inorganic clays of high plasticity (CH), CSR=0.325 for inorganic clays of low to medium plasticity (CL), CSR=0.240 for inorganic silts and very fine sands (ML) and CSR=0.220 for silty sands ( SM). It has been determined that sands having a high proportion of silts show the lowest resistance against liquefaction. For the whole range of silts with both low and high plasticity, non-plastic silts have the lowest resistance while the resistance increases as the plasticity increases. Clays are more resistant to liquefaction when compared with silty sands and non-plastic silts and they reach the failure (ε=±2.5%) at higher number of cycles. Moreover, it has been observed that even high-plasticity soils present a softening behavior accompanied by a high amount of deformation for the number of cycle N=20. On the other hand, while the pore pressure build-up is lower for the non-plastic soils than the high-plasticity soils during initial loading stages, the rate of pore pressure build-up gets higher in nonplastic soils as the soil gets near to the failure point defined as ε=±2.5% or ru=1.0. In addition cyclic test, initial elasticity modulus has been measured by using gap sensors connected the upper plate of the triaxial chamber. The elasticity modulus of fine grained soils change from 55 MPa to 375 MPa.