Dolgu barajların deprem analizleri
Seismic analysis of earth dams
Yazar: CEM SELÇUK
Danışman: PROF. DR. HÜSEYİN YILDIRIM
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Barajlar su kaynaklarının kontrolünün sağlanması ve ihtiyaca yönelik kullanılması için toplumların gelişmesinde rol sahibi olan ve aynı zamanda büyük yatırımlar gerektiren yapılardır. Barajlar farklı nedenlerden dolayı hasar görebilir ve bu hasarlar çok büyük zararlara, hatta felaketlere yol açabilir. Bu nedenle baraj yapılarının güvenli tasarımı son derece önem teşkil etmektedir. Depremler, barajlar üzerinde meydana gelen hasarların kaynaklarından biridir. Deprem etkisi ile baraj rezervuarındaki suyun kontrolsüz boşalmasının sebep olacağı yıkıcı etkiler, bir depremde oluşabilecek en kötü sonuçlardan biridir. 
Dolgu barajlar, istatistiksel olarak ülkemizde ve dünyada kullanımda olan barajların büyük çoğunluğunu oluşturmaktadır.  Mevcut ve inşa edilmekte olan dolgu barajların birçoğu sismik aktivitelerin olduğu deprem bölgelerinde bulunmaktadır. Ülkemizde de bilindiği üzere birçok bölge fay etkisinde ve deprem bölgesi olarak tanımlanmaktadır. Bu sebeple bu bölgelerde yapılması tasarlanan ya da kullanımda olan dolgu barajların deprem etkisi altında davranışları irdelenmesi gereken önemli bir konudur. Bu bağlamda dolgu barajların deprem analizleri tezin konusu olarak seçilmiştir.
Dolgu barajlar, depreme dayanıklı tasarlanan ilk yapılardandır ve bu alanda yapılan çalışmalar geoteknik deprem mühendisliği açısından oldukça değerlidir. Dolgu barajların deprem etkileri gözetilerek tasarlanması 1930'lu yıllara dayanmaktadır ve günümüze kadar ulaşan bir gelişim söz konusudur. Bu tarihten sonra yapılan çalışmalardan elde edilen bilgi birikimi ve gelişen teknoloji, günümüzde malzemenin ve depremin karmaşık yapısının gerçeğe yakın modellenebilmesine olanak tanımıştır. Bu tez çalışması kapsamında dolgu barajların deprem analizleri üzerine yapılan çalışmalar ve geçmişten günümüze kullanılan analiz yöntemleri araştırılmış ve sunulmuştur.  
Çalışma kapsamında dolgu barajlar ve beton barajlar hakkında genel bilgiler verilmiştir ve dolgu barajların beton barajlarla mukayese edildiğinde söz konusu olan bazı üstünlüklerine değinilmiştir. Ülkemizde var olan dolgu ve beton barajlardan örnekler sunulmuştur. Depremlerin yapısı, deprem dalgaları, barajların deprem analizleri için depremlerin seçilmesi ile ilgili deterministik ve istatistiksel sismik tehlike analizi yöntemleri ve dolgu barajların deprem analizlerinde kullanılacak depremlerin seçilmesi konusunda yapılan literatür taramasına yer verilmiştir. Depremlerin dolgu barajlar üzerinde meydana getirdiği hasarlar, yer değiştirmeler ve sıvılaşma kaynaklı hasarlar başlıkları altında irdelenmiştir. Kretlerde meydana gelen oturmaların hesaplanmasında kullanılan yarı-ampirik formüller ve zeminlerin sıvılaşma potansiyelinin belirlenebilmesi için olasılıksal yaklaşımlarla yapılan çalışmalarda önerilen formüller sunulmuştur.
Dolgu barajların deprem analizler yöntemleri ile ilgili geçmişten günümüze literatürde bulunan çalışmalar ve analiz yöntemleri taranmış, yarı-statik yöntem, Newmark kayan blok analizi, kayma kirişi yöntemi ve dinamik sonlu elemanlar yöntemi başlıkları altında ele alınmıştır. Bahsedilen yöntemlerin dolgu barajlar üzerinde kullanıldığı çalışmalara yer verilmiş, eksik ve üstün yönleri sunulmuştur. Dinamik sonlu elemanlar yöntemi ile yapılacak doğrusal olmayan analizlerde kullanılan malzeme modelleri, doğrusal olmayan elastik, elasto-plastik ve kritik durum yaklaşımı başlıkları altında incelenmiştir. Deprem etkisi altında hasar gören dolgu barajlar vaka analizleri başlığı altında çalışılmıştır. Bu kapsamda tarihte meydana gelen önemli depremler sonucu birçok araştırmaya konu olan Aşağı San Fernando, La Villeta, El Infiernillo, Sürgü ve Los Angeles dolgu barajlarının yapısal performansları değerlendirilmiştir.

Abstract:
Dams are structures which play a significant role in the development of societies and countries by enabling them to control their water sources and to use it according to their needs. Building dams require large investments and a high level of engineering knowledge. Dams can be damaged in many different ways such as settlements, cracks, accidents and earthquakes which can result in catastrophic disasters. Therefore, safe design of dam structures is extremely important. Earthquakes are one of the main reasons of damages on dams. Due to the forces of an earthquake, the destructive effects caused by the uncontrolled discharge of water from the dam reservoir is one of the worst case scenario in an earthquake.
 Earth dams statistically constitute a big majority of the existing dams in Turkey and in the world. Most of the existing and under construction earth dams are located in seismic zones. As it is a well known fact, many regions in Turkey are located near by fault and seismic zones which means dams located in these areas have a great potential to experience an earthquake. For this reason, it is necessary to design and analyse earth dams, which are located on seismic zones, with respect to their earthquake loads. In this context, seismic analyses of earth dams has been studied in this thesis.
Earth dams are one of the first structures designed to be earthquake resistant and the studies in this field are considered as milestones in terms of geotechnical earthquake engineering. The first studies on designing earth dams with concern to the effects of an earthquake first started in 1930s. Since then, various studies have been made on seismic analyses of earth dams and new methods have been proposed by different researchers. The knowledge gained from these studies and the developing technology has enabled the modeling of the complex structure of the materials and earthquakes more realistically. In this thesis, the studies on earthquake analyses of earth dams and the methods used from past to present are investigated.
In this study, general information about earth dams and concrete dams and examples of each dam types are given in an order. Some advantages of earth dams over concrete dams are mentioned. Earthquake mechanism, fault types and earthquake waves are explained briefly. Deterministic and probabilistic seismic hazard analyses are investigated including the advantages/disadvantages of both approaches. The earthquakes, which represent the ground motion that is expected to occur at the dam site, are defined by different safety aspects. The terminology for selecting an earthquake is mentioned. 
Earthquake damages on earth dams are investigated under two topics; displacements and liquefaction. Displacement caused by earthquake loads can cause the instability of slopes, cracks and settlements. Shear strength of soils and Mohr-Coulomb failure criterion are often used for evaluating the slope stability of earth dams. The dam cracks caused by earthquakes are identified by longitudinal and transverse cracks. Longitudinal cracks are expected to occur around the dam crest due to the tensile and shear stresses that occur during earthquakes. Transverse cracks are less common types of cracks to occur during an earthquake as they might also occur due to different settlements. Crest settlements during earthquakes are a serious problem which may cause loss of freeboard and serious damages in case of uncontrolled water release from earth dams. Many studies have been made to determine crest settlements of earth dams during different ground motion levels. Swaisgood and Jansen semi-empirical formulations for determining crest settlements are presented in this study. The liquefaction phenomena in soils is described by the means of excessive pore pressure build-up and a decrease in effective stress. The most common type of liquefaction for earth dams is flow liquefaction which may cause serious damages as Sheffield dam failure is given as an example. The likelihood functions of liquefaction which is obtained by probabilistic approach are explored by means of two different works.
The methods for seismic analyses of earth dams are grouped under four approaches in this study: pseudo-static method, Newmark sliding block method, shear beam method and dynamic finite element method. Each of the methods, from simplest to most advanced, is searched on literature and presented by explaining the evaluation of the method, including number of studies on earth dams that use the above mentioned methods. Studies that contributed to different methods are mentioned in order to demonstrate the evaluation of seismic analyses of earth dams. The methods are explored and compared by means of their advantages and limitations. 
In the pseudo-static analysis, the most critical part is to determine the pseudo-static coefficient which is directly related to the accuracy of the results. Various works on determining the pseudo-static coefficient is presented in this study. Also, it is shown that pseudo-static method is an easy and simple approach to analyses that can give very conservative or non-conservative results. Newmark sliding block method makes it possible to obtain deformations by means of pseudo-static approach. Newmarks sliding block method is furthered by various works such as Makdisi-Seed simplified method mentioned in this study. This method has been used widely to obtain permanent deformations of earth dams caused by seismic excitation. 
Shear beam method is introduced as a common method that has been used to estimate the permanent shear deformation of earth dams. Although shear beam method has been widely used for analyses of earth dams, the method has its own limitations which makes it less desirable in comparison to dynamic finite element solutions. Some of the shear beam studies on earth dam deformations is shown in the figures with comparison to finite element method. Also, the researchers who made a significant contribution to evaluation of shear beam and finite element methods is shown in the tables as a list. Dynamic finite element method is the most widely accepted method of analysing earth dams as any structure in todays world. In this study, it is indicated that the finite element method is the most advanced method and gives the most satisfactory results compared to other methods for a number of reasons. The most significant one is that finite element method allows for the modeling of complex structure of soils and earthquakes far more closely to reality. 
Linear, equivalent linear and non-linear dynamic finite element solutions are explored in this study. Their advantages and limitations are demonstrated. It is shown that most advanced solutions can be made by non-linear analyses. In order to use non-linear finite element method on earth dams, a soil constitutive model is needed. Different types of soil constitutive models are investigated under the topics of non-linear elastic approach, elasto-plastic approach and critical state approach. Comparative studies on earth dams with different soil models is presented. 
Case studies are explored in the last part of this study in order to cite the real response of earth dams during earthquake. For this purpose the cases of Lower San Fernando, La Villita, El Infiernillo, Sürgü and Los Angeles earth dams are presented. The lessons learnt from the cases are mentioned by the means of many articles written on these well-known cases. The destructive effects of earthquakes on earth dams are demonstrated by a number of figures. The level of damage is stated by the above mentioned liquefaction, crack, sliding and settlement deformations.