Alüviyal sahalarda sahaya özel zemin davranış analizi = Site response analysis on the alluvial fields

Abstract

Sismik yer hareketi mühendislik yapılarının tasarımı için önemli olduğu kadar yapıların bulunduğu yerel zemin şartlarında da incelenmesi gereken önemli etkilere yol açmaktadır. Zeminlerde meydana gelen problemlerin çoğu deprem sırasında meydan gelen yerel zemin davranışı sonucunda taşıma gücü kayıplarının yaşanması, oturma, sıvılaşma ve zemin büyütme davranışları olarak gözlemlenebilmektedir. Deprem yükleri altında yapılarda oluşacak deformasyon seviyeleri yerel zemin koşullarına bağlı olarak değiştiği için zeminlerdeki dinamik davranış analizlerinin gerekliliği önem arz etmektedir. Türkiye Bina Deprem Yönetmeliği-TBDY 2018 kapsamında yapılan değişikliklerle tasarımlarda dikkat edilmesi gereken hususlar belirtilmiştir. Yönetmelik kapsamında yapılan değişikliklerden biri olarak kabul edebileceğimiz "ZF" yerel zemin sınıfı için Sahaya Özel Zemin Davranışı Analizi 'nin yapılması zorunluluğu ile TBDY-2018 16.5 bölümünde izlenmesi gereken adımlar belirtilmiştir. Yerel zemin sınıfı ZF dışında olan zeminler için ise analizlerin yapılması çalışma sahası özelinde gösterdiği zemin davranışına uygun olarak tasarımcıların inisiyatifine bırakılmıştır. Bu çalışmada, Adapazarı'nda bulunan çalışma alanı için toplam 85 metre derinliğe sahip 5 adet döner sondaj sonucu elde edilen değerler kullanılmıştır. Çalışma alanında yapılan geoteknik ve jeofizik çalışmalar sonucunda 140 m derinlik seviyesine kadar tabakalı zemin modeli oluşturulmuştur. TBDY-2018 2.2.2'ye göre standart tasarım deprem yer hareketi olarak adlandırılan Deprem Yer Hareketi Düzeyi-2 (DD-2) ve yerel zemin sınıfı ZC yerel zemin şartlarını sağlayacak şekilde deprem seçimi ve ölçeklendirme işlemi gerçekleştirilmiştir. Çalışma kapsamında TBDY-2018'de maddeleri göz önünde bulundurularak kuvvetli yer hareketi kayıtları için; deprem büyüklükleri Mw: 6.0-8 aralığında, Vs30 değeri ZC zemin sınıfını temsil eden 350-760 m/s aralığında seçilmiştir. Toplamda çalışma alanı ile uyumlu 11 adet kuvvetli yer hareket kaydı belirlenmiş ve PEER veri tabanından elde edilmiştir. Seçilen bu kuvvetli yer hareketi kayıtları SeismoMatch2022 programı kullanılarak ölçeklendirilmiştir. Ölçeklenen deprem kayıtlarının DeepSoil v.7 programına tanımlanması ile 56 adet tabakadan oluşan zemin profili çözümlenmiştir. DeepSoil analizi zaman tanım alanında doğrusal olmayan analiz yöntemi (1D Nonlineer-NL) kullanılarak gerçekleştirilmiştir. Zemin davranış spektrumlarının her iki bileşenin ortalaması alınarak doğrusal olmayan analiz tamamlanmıştır. TBDY-2018 2.4.1 maddesi kapsamında sahaya özel deprem yer hareketi spektrumlarının ordinatları için belirlenen değer aralığına uygun biçimde deprem yer hareketi spektrumu %90 DD2_ZD spektrum değerlerinde grafiklerde gösterilmiştir. Çalışma sonucunda ise DD-2 deprem düzeyi için üst yapı analizlerinde kullanılması önerilen birleştirilmiş sahaya özel deprem tasarım spektrumu verilmiştir. Ortalama yer hareketlerinin verildiği grafikten yararlanarak kısa periyot için yerel zemin etki katsayısı FS: 0.266 olarak hesaplanırken, 1,0 saniye periyot için yerel zemin etki katsayısı F1: 1.155 olarak hesaplanmıştır.

Seismic ground motion is not only important for the design of engineering structures, but also has significant effects on the local soil conditions where the structures are located. Most of the problems can be observed as loss of bearing capacity as a result of settlement, liquefaction and soil amplification behavior, as well as local soil behavior during earthquakes. Since the deformation levels that will occur in structures under earthquake loads vary depending on local soil conditions, the requirement for dynamic behavior analysis of soils is important. In sequence to determine the changes in the surface of local soil classes due to seismic movements, local soil behavior analysis is performed by considering the characteristic seismic properties of the site. The seismic waves can be observed on the surface under the effect of damping or amplification according to the dynamic properties of the soil. The amplified earthquake motion transmitted to building foundations can have destructive effects on structural elements at various deformation levels. Depending on the local soil conditions, the acceleration-time histories can reach high values in certain areas, resulting in very serious structural damage. With the issues made within the scope of Turkey Building Earthquake Regulation-TBDY 2018, the points to be considered in the designs are specified. TBDY 2018 requires site-specific response analysis to be performed in order to determine the soil behavior in accordance with the actual soil behavior, and thus, it is targeted to model the results of the actual acceleration effects in the building designs in the most accurate modeling. The necessary stages to be applied are specified in section 16.5 of TBDY 2018 with the requirement of performing a Site Response Analysis for the "ZF" local soil classification, which can be considered as one of the issues introduced within the scope of the regulation. The site response analysis for soils other than local soil class "ZF" is at the designers' discretion in accordance with the behaviors of the soils exhibited in the field. The behavior of the soil contains the parameters that are important for the structural calculation methods of the structure to be constructed. The soil behavior or the parameters to be used in the design of the structures vary depending on the site response. By taking into account the soil-structure interaction, both soil properties and structural properties are considered to be compatible with each other in the designs. It is significant to identify the local site response and design features of the constructions in order to evaluate the damage assessments that may occur as a be a consequence of seismic activities. The amplified earthquake motion transmitted to structure foundations can have devastating effects on structural elements, including different kinds of deformation degrees. Bedrock accelerations with very minor magnifications can be amplified several times in some regions due to the influence of local site conditions, producing very severe damage. Many earthquakes in the past have revealed the importance of soil-structure interaction and the necessity to investigate site-specific analyzes. The site response analysis are performed by taking into account the local seismic characteristics in order to determine the earthquake characteristics that will occur on the surface of local soil layers as a result of earthquake activities. As in Adapazari, it is important to evaluate the local soil behavior in order to determine the most optimum design parameters and adequate design limits for structures to be designed on soft soils (nuclear power plants, sports complexes, industrial structures, high-rise structures, storage sites, etc.) composed of deep sedimentary soils. For this reason, not only the performance of the construction elements but also the soil behavior in accordance with the local soil properties should be taken into consideration while examining the seismic behavior effect during design phase of the constructions. Considering the variety of studies carried out, the performances that will be revealed according to the type of analyzes to be carried out should be evaluated in accordance with the field within the scope of the regulations and improvements should be continued by taking into account the recommendations in this respect. In order to examine the soil and structure behavior under the influence of earthquakes effectively, the soil profile and seismic records should be determined in accordance with the reality. For site response; soil profile and geotechnical properties of the soils should be realistically representative and constructed. The selection of earthquake records should be made appropriate to the design spectrum and according to the method used in the selection. In the TBDY 2018 regulation, several amendments have been made in order to minimize the damages that may occur during an earthquake. Site-specific design acceleration spectrum parameters were updated by redetermining the local soil classes. Six soil classes are represented and the parameters (Vs)30, (N60)30 and (cu)30 are used to characterize them. According to this regulation, soils termed as soil class ZF are defined as the soil for which a site response analysis is to be mandatory. Design earthquake response spectrum curves are used to inspect the effect of earthquake loads on structural design. The design spectra exhibit the dynamic characteristics of the seismic recordings. Within the scope of the regulations, it is also recommended to use a normalized acceleration spectrum to examine the impact of seismic motions on designs. AFAD Turkey Earthquake Maps have provided the opportunities to use the design of spectrum in a site-specific manner. With the AFAD application, spectrum curves can be used for different earthquake levels and soil classes to be applied in the structures planned to be designed. In this study, the values obtained from 5 rotary drillings with a total depth of 85 meters were used for the study area located in Adapazari. With the help of the data acquired from the geotechnical and geophysical investigations conducted for the study area, a stratified soil model was constructed to a depth of 140 m. According to TBDY2018 2.2.2, the real earthquake records were selected and scaled in accordance with the standard design earthquake motion which is referred to as Earthquake Motion Level-2 (DD-2) and local soil properties. Within the scope of the study, in accordance with the seismicity of the site; earthquake magnitudes were assigned in the range of Mw: 6.0-8.0 and shear wave velocity (Vs)30 was assigned in the range of 350-760 m/s for the first thirty meters to represent the local soil class ZC soils. In total, 11 strong ground motion records compatible with the study area were selected and the actual acceleration recordings were obtained from the The Pacific Earthquake Engineering Research Center (PEER) database. These selected strong ground motion records were scaled using the SeismoMatch2022 program. By defining the scaled earthquake records in DeepSoil v.7 program, the soil profile which consists of 56 layers has been analyzed. The soil behavior analysis of DeepSoil has been performed in time-domain using the nonlinear analysis method (1D Nonlinear-NL). The nonlinear analysis has been completed by averaging both the components (North-South (EW), East-West (NS)) of the soil behavior spectrum. According to the value range specified for the ordinates of site response earthquake ground motion spectra according to TBDY-2018 2.4.1, the earthquake ground motion spectrum has been shown in the graphics for 90 % DD2_ZD spectrum values. At the conclusion of the study, a site-specific integrated earthquake design spectrum has been obtained for the DD-2 earthquake level, which is recommended to be used during the design of the structure. By using the mean ground motion graphic, the local soil impact coefficient has been calculated as FS: 0.266 for the short period, while the local soil impact coefficient has been calculated as F1: 1.155 for the period of 1.0 second.