Centrifuge modeling studies of site response in soft clay over wide strain range Abstract: Centrifuge models of soft clay deposits were shaken with suites of earthquake ground motions to study site response over a wide strain range. The models were constructed in an innovative hinged-plate container to effectively reproduce one dimensional ground response boundary conditions. Dense sensor arrays facilitate back-calculation of modulus reduction and damping values that show modest misfits from empirical models. Low amplitude base motions produced nearly elastic response in which ground motions were amplified through the soil column and the fundamental site period was approximately 1.0s. High intensity base motions produced shear strains higher than 10%, mobilizing shear failure in clay at stresses larger than the undrained monotonic shear strength. We attribute these high mobilized stresses to rate effects, which should be considered in strength parameter selection for nonlinear analysis. This nonlinear response de-amplified short period spectral accelerations and lengthened the site period to 3.0s. The nonlinearity in spectral amplification is parameterized in a form used for site terms in ground motion prediction equations to provide empirical constraint unavailable from ground motion databases.

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This paper presents one-dimensional ground response simulations of centrifuge models involving soft clay deposits subjected to ground motions of varying intensity. Total stress ground response simulations were performed using equivalent-linear (EL) and nonlinear (NL) methods. Shear strains higher than 10% were mobilized during large ground motions, therefore undrained shear strength of the clay is an important parameter for the simulations. Testing shows that the Bay Mud materials used in centrifuge modeling have monotonic shear strengths that increase by 13% per log cycle of shear strain rate. Comparison of simulation results to observations reveals the importance of incorporating shear strength into the development of stress-strain backbone curves, with appropriate consideration of rate-adjustments to shear strength and stiffness. NL ground response simulations provide a good match to observed pseudo-spectral accelerations only when rate-adjusted shear strengths are properly accounted for, otherwise the NL simulations have significant under-prediction bias at oscillator periods less

Under repeated loads, stress-strain and strength properties of soils play an important role in solving the engineering problems. The resistance of a material is defined by the slope of the initial part of stress-strain curve. However, this value changes due to cyclic loading and decreases cycle by cycle. As a result, deformations occur on the ground and resulting structural damage. Engineering properties of soils (such as modulus and damping properties of soil, shear wave velocity, unit weight etc..) must be known to minimize the effects of earthquakes. Another important parameter is the frequency content of earthquakes which directly affects the wave propagation through the soil. In this study, the effects of frequencies, obtained from different earthquakes, on the dynamic characteristics of the soil have been investigated. A dynamic triaxial test set has been used to examine the dynamic properties of soils. The variation of dynamic properties of sand samples under the experimental conditions has been determined for regular and irregular loading types.

Zemin tabakaları, sismik yükleme sırasında değişken zemin koşulları nedeniyle farklı davranış gösterebilmektedir. Zemin özelliklerindeki bu değişkenlik, deprem sırasında oluşan tekrarlı gerilmelerin özelliklerini değiştirebilmekte ve buna ek olarak yapılara etkiyen yer ivmesi genlikleri, frekans içerikleri ve buna bağlı olarak oluşan tepki kuvvetlerini ve dolayısıyla yapı davranışını da büyük ölçüde etkilemektedir. Bu çalışmada 2019’dan itibaren güncel olarak kullanılan Türkiye Bina Deprem Yönetmeliğinde özellikle sahaya özel zemin davranış analizinin zorunlu kılındığı ZF zemin grubunda yer alan zeminler ve diğer gruptaki zeminler için bir boyutlu zemin davranış analizleri yapılmış ve farklılaşan kuvvetli yer hareketleri ile zemin özelliklerinin değişimine bağlı etkisi ortaya konmuştur. Çalışmada gerçek deprem kaydı olarak 11 kuvvetli yer hareketi ile Eskişehir il merkezinde üç mahallede yer alan 6 sondaj üzerinde bir boyutlu eşdeğer doğrusal ve doğrusal olmayan analizler yapılmıştır. Analiz sonuçlarına göre yerel zemin koşullarının yüzey davranışını etkilediği, eşdeğer doğrusal analiz yaklaşımıyla genel itibariyle daha büyük tasarım parametrelerinin ortaya çıktığı, sahaya özel analizlerin tasarım parametrelerini kontrol ettiğini ve daha geniş kapsamda kullanılması gerektiği belirlenmiştir.