Anakaya eğiminin zemin büyütmesine etkisi
The effect of the bedrock slope on the soil amplification
Yazar: HADİ KHANBABAZADEH
Danışman: DOÇ. DR. RECEP İYİSAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Geoteknik Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Doktora, 2014
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Depremler sırasında yerel etkiler nedeniyle meydana gelen zemin büyütmesi yeryüzünde önemli hasarlara neden olmaktadır. Birçok durumda yerel etkiler ve zemin büyütmesi yatay ve düşey yönde yayılan kayma dalgası varsayımına dayanan tek boyutlu dinamik analizlerle hızlı ve kolay bir şekilde hesaplanabilmektedir. Ancak, gerçekte üç boyutlu yayılan bir dalganın yanal süreksizliklerden dolayı yüzey dalgalarının bir bölgede odaklanmasına yol açmasından dolayı, zemin büyütmesinin tek boyutlu analizlerle değerlendirilmesi her koşulda doğru bir yaklaşım olmamaktadır. Bu durumda iki boyutlu dinamik analiz gerekli olmaktadır. İki boyutlu zemin büyütmesi, zemin tabakaları arasındaki empedans farklılığı ve onların yanal süreksizliğinden kaynaklanan yüzey dalgaların bir aradaki etkisinden meydana gelmektedir. 
Bu araştırmada farklı anakaya eğimi ve zemin cinsinin jeolojik düzensizliklere sahip ovaların dinamik davranışına etkisinin anlaşılması için numerik bir çalışma yapılmıştır. Bu amaçla farklı anakaya eğimi ve zemin tabakalarından oluşan ovaların dinamik davranışı varsayımsal modeller üzerinde ve farklı deprem yükleri altında bir ve iki boyutlu dinamik analizler ile incelenmiştir. Modellerin yüzeyinde farklı noktalarda zemin büyütmesi ve şiddet faktörü bulunmuş, sonuçlar karşılaştırılmıştır. Dinamik analizler sonlu farklar yöntemi esaslı FLAC yazılımıyla yapılmıştır. Kullanılan anakaya ivmesi kayıtlarından zemin tabakalarının etkisinin kaldırılması için kayıtların sert tabakalar üzerinde alınmış olduğuna dikkat edilmiştir.
İki boyutlu varsayımsal modellerin dinamik analizinin yanı sıra yatay yönde sınırlı ve eğimli anakaya sınırına sahip Dinar Ovası'nın sismik tepkisi de yapılmıştır. Bu araştırmanın sonucu iki boyutlu dalga yayılmasının etkin olduğu alüvyonel ovalar gibi yumuşak tabakaların dinamik davranışının anakaya eğimi ve zemin cinsinden etkilendiğini göstermektedir. Sonuçlar ovaların yüzeyinin farklı noktalarının farklı periyotlara duyarlı olduklarına işaret etmektedir. Etkilerin anakaya hareketinin kuvvetinin artmasıyla da devam ettiği görülmektedir. Ayrıca, tek ve tabakalı zeminlerden oluşan alüvyonel ovaların farklı özelliklere sahip anakaya hareketi etkisi altında gösterdikleri dinamik davranışının maksimum zemin büyütmesi ve rezonans periyodu açısından birbirinden farklı oldukları anlaşılmaktadır. Simetrik olmayan Dinar Ovası'nın iki boyutlu analizinden elde edilen sonuçlardan bu ovanın dinamik davranışının büyük ölçüde ikinci boyuttan etkilendiği ortaya çıkmaktadır.

Abstract:
Local geological conditions generate significant amplification of ground motion and concentrated damage during earthquakes. In some situations local amplification and site effect can be inferred reasonably using simple one dimensional shear models based on vertically propagating shear wave concept. However, lateral heterogeneity may give rise to focusing and to locally generated surface waves; therefore, the estimate of the local amplification using 1D models may be erroneous. In fact, two dimensional wave amplification occurs due to the combined effect of impedance ratio between soil layers and surface waves propagating due to the limited extent of the basin. In such situation the application of the two dimensional dynamic analysis is necessary.
The purpose of this study is to investigate the effects of the basin edge on the dynamic behavior of the basins. In this research, the one and two dimensional dynamic behavior of the alluvial basins with different soil types and stratification are investigated using hypothetic models under earthquake motions. To attain this aim, a range of bedrock inclinations at the valley sides from slighter 10◦ and 20◦ to steeper 30◦ and 40◦ slopes are selected. The total width of the basin is selected 2 km, which is long enough to provide opportunity to investigate the effective distance of the basin edge. Also, total depth of the all models is 100 m.
Since the material characteristics of the basin affect the behavior of the site, the behaviors of the two sandy and clayey sites are compared in this research. To make the results useful at engineering affaires, at selection of the material characteristics it was tried to select two groups of sandy and clayey materials belonging to the different class of soils with respect to the soil classification codes. The selected materials are classified as stiff, medium plasticity and soft clay, and dens, medium and loose sand with respect to Turkish earthquake code.
At the other part of this study the numerical study of the effects of the stratification and change in material type on the dynamic behavior of the clayey basins are investigated. For this purpose two kinds of single and double layer basin models with the total depth of 100m and width of 2 km have been selected. The single layer basins consist of soft, medium and stiff clayey materials, while double layer basins consist of a stiff sandy sub-layer overlaid by one of the soft, medium and stiff clayey soil layer. The bottom layer in all double layer basins is of dens sand.
The constructed models are subjected to the collection of 16 earthquakes with different PGA's of 0.1, 0.2, 0.3 and 0.4 g, four motions for each PGA. To provide the circumstances of the north Anatolian fault with right lateral strike-slip faulting mechanism, the accelerograms produced by SanAndereas fault with same faulting mechanism are included in this collection. For missing the effect of soil layers on selected accelerograms, the used accelerograms are chosen from among those recorded on stiff layers during real earthquakes, or deconvoluted to the correspondence bedrock motion. This collection includes records of different peak ground accelerations, frequency contents and durations. 
The performed dynamic analyses are done using FLAC3D code which utilizes explicit finite difference method. The combination of two advanced dynamic boundary formulation is used at the models boundaries. To model the real situation of interaction between bedrock and soil at the base of the model the flexible base logic which utilizes quiet boundary is applied at the horizontal boundaries, while free-field boundaries are set at the vertical boundaries to simulate the infiniteness of the media at sides. The verification of the modeling is tested using a solution that has been verified extensively. 
Along with the two dimensional analysis of the trapezoidal basins, the dynamic analysis of the Dinar basin model has been done in this study. Using the different geotechnical and geological field exploration data the two dimensional model of the basin have been estimated. To evaluate the dynamic behavior of the basin it was subjected to the collection of 16 earthquakes with different PGA's of 0.1, 0.2, 0.3 and 0.4 g, four motions for each PGA. 
Also, in this study by conducting 1D analyses along the modeled basin surface the aggravation curve for every case are evaluated and discussed. The amplification and aggravation factor of the different points on the modeled basins' surface have been estimated, and the results have been compared. 
The results of this study show that the dynamic behavior of the basins with two dimensional bedrock geometry is affected by the bedrock inclination and soil types. Also the important effect of the bedrock motion strength was observed. It was seen that different parts of the basin surface are sensitive to different periods. The effects of the different parameters on the basin dynamic behavior continue by increasing the PGA of the bedrock excitation.
Since the behavior of the basins is affected by the several parameters like strength and frequency content of the dynamic loads, stratification, soil specification and bedrock angle, a general rule that holds for all cases can not established. 
Based on results, it was seen that in the basins with 100 meters depth and 2000 meters width the effect of the two dimensional bedrock is limited to 500 meters from the outcrop. Also, the results indicate the differences in dynamic behavior of single and double layer basins from the aspect of amount and position of the maximum amplification and resonance period. The sensitivity of the shallower lateral parts to lower periods and inner parts to higher periods can be tracked at both single and double layer basins. Beside other differences the most important difference between single and double layer basins is the resonance period of the inner parts of the basins. The results show that almost in all cases the resonance period of the inner points of the single layer basins is lower than double layer basins. 
Since the aggravation factor presents only the ratio of the acceleration estimated by two and one dimensional analyses, the use of this factor in the investigation of the dynamic behavior of the basins would be misleading. In this regard, the results indicate that the maximum acceleration evaluated by one dimensional analysis is insufficient in evaluation of the dynamic behavior of the basin edges when compared with two dimensional analysis results. 
The results of the analysis of the nonsymmetric Dinar basin indicates that the dynamic behavior of the basin is broadly affected by the two dimensional bedrock. Also, the results indicates the higher effect of the 6° bedrock inclination at the basin east on the edge amplification with respect to the steeper 20° bedrock slope at the basin West. The results show the insignificant effect of the bedrock located at the level deeper than 150 meter on the amplification behavior of the Dinar basin.
At the nonsymmetric Dinar basin with 200 metres depth and 6000 meters width the effect of the 6° bedrock angle at the eastern parf continues untill 1500 meters from the outcrop, while at the western part with 20° bedrock angle the two dimensional amplification is affected until 700 meters from the outcrop.