Yerel zemin koşullarının yer hareketine etkisi üzerine bir çalışma
A Study on the effects of local soil conditionson ground motions
- Tez No: 100874
- Danışmanlar: PROF.DR. ATİLLA M. ANSAL
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Deprem, Sismik deneyler, Yer hareketleri, Zemin yapısı, Earthquake, Seismic tests, Ground motions, Soil structure
- Yıl: 2000
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
YEREL ZEMİN KOŞULLARININ YER HAREKETİNE ETKİSİ ÜZERİNE BİR ÇALIŞMA ÖZET Türkiye'de son yıllarda yaşanan depremler bir kez daha göstermiştir ki yapısal hasar, deprem ve yapılaşma özellikleri yanında yerel zemin koşullarından da etkilenmektedir. 17 Ağustos'da Kocaeli'nde Mw=7,4 büyüklüğünde meydana gelen depremin şiddeti de yerel zemin koşulları elverişsiz zeminler üzerindeki yapılarda olumsuz biçimde yaşanmıştır. Bir depremin zemin tabakaları içerisinde ilerlerken yapılarda olan yıkıcı etkisi esas olarak yayılan dalgaların genliklerindeki artımdan kaynaklanmaktadır. Zemin büyütmesi denilen bu olayın yanında aynı şekilde noktadan noktaya değişiklik gösterebilen zeminin hakim peryodu da meydana gelebilecek hasarda etkili olabilmektedir. Zemin tabakalarının deprem anındaki sismik davranışlarını belirlemek için arazi ve laboratuvar deneyleri oldukça faydalı bilgiler içermektedir. Bununla birlikte mikrotremor ölçümleri de gün geçtikçe zamandan kazandırmasından ve ekonomik avantajlarından dolayı daha da yaygınlaşmaktadır. Bu çalışmada bir inceleme sahasında SPT, CPT ve bunlara ek olarak yapılan laboratuvar deneylerinin sonuçlan ile arazide sismik deneyler ve mikrotremor ölçümleri yapılmış, veriler arasında korelasyon bağıntıları geliştirilmiştir. Mikrotremor kayıtlarının alındığı noktalarda daha önceden yapılmış SPT deneyleri sonucundaki darbe sayıları ve numuneler üzerindeki laboratuvar deneylerinden elde edilen su muhtevası, plastisite indisi ve zemin sınıflan ışığında eldeki bazı bağıntılardan sismik davranışta çok önemli bir parametre olan kayma dalgası hızına geçilmiş, bunlar yine bazı formüllerle büyütmelere dönüştürülmüştür. Benzer olarak CPT deneylerinden bulunan uç direnci, sürtünme ve boşluk suyu basmcı gibi parametrelerle mikrotremor büyütmeleri ve hakim peryotlan arasında bağıntılar geliştirilmiştir. Aynca 3 kuyuda kullanışlı bir sismik yöntem olan kuyu içi PS- Loggjng deneyleri yapılmış, elde edilen kayma dalgası hızının tabakalar boyunca değişimi ve SPT-N sayılanndan bulunan kayma dalgası hızlan arasında ilişkiler değerlendirilmiştir. Mikrotremor kayıtlarının spektrum analizlerinde yatay hareket bileşeninin düşeye oranlanması esasına dayanan Nakamura yöntemi kullanılmıştır. Bağıntılarla bulunan büyütmelerle mikrotremorlann bu spektral oranlanndan bulunan zemin büyütmeleri ile ilişkiler aranmış, bağıntılan etkileyen olası faktörlerin üzerinde durulmuştur. Bir bölge depremsel açıdan incelenirken arazi, laboratuvar ve sismik deneyler yarımda mikrotremorlann da kullanımı giderek gerekli hale gelmektedir. Mikrotremor ölçümleri Türkiye'de büyük oranda ihtiyaç hissedilen mikrobölgeleme çalışmalarında çok cazip bir yol olmaktadır. Kayıtlannın analizlerinde bu çalışmanın da amaçladığı gibi bazı korelasyonlardan faydalanarak değerlendirme yapmak daha anlamlı görünmektedir. Analizlerin dikkatli yapılması ve yeterli sayıda kayıt alınması durumunda zemin tabakalannın dinamik davranış özelliklerinin belirlenmesinde mikrotremorlann gün geçtikçe daha kullanışlı olacağı kesindir.
Özet (Çeviri)
A STUDY ON THE EFFECTS OF LOCAL SOIL CONDITIONS ON GROUND MOTIONS SUMMARY In recent earthquakes it is widely seen that local soil conditions are effective on the damage as well as earthquake and structural characteristics. The increase on the amplitude of seismic waves by the soil layers is the most important factor on the adverse results of the earthquake. It is known that this phenomenon defined in terms of soil amplification and predominant period of the local soil layers are both important in evaluating the response of structures. Kocaeli earthquake occurred at 03:02 a.m. on 17 August 1999 (Mw=7,4), and caused about 20.000 casualties and a significant damage over a wide area such as the city of Adapazari, Eskişehir, Bolu and some towns of Istanbul. Beside the fact that it has a large magnitude, unfavourable local soil conditions increase the damage in the structures located in the vicinity. The research field in Derince is located across the sea about 2-3 km far from the epicenter. The town of Derince had an extensive damage because of the earthquake, but observed minor damage in the investigated site comparing to the other parts of Derince. In order to mitigate earthquake hazard, the earthquake phenomenon must be thoroughly studied. The factors affecting the Ak, amplification factor and To. predominant period play the main role in the cyclic behaviour of soil layers. Vs, the shear wave velocity passing through the soil layers is an important data when it is used in correlations to estimate Ak, amplification factor. Although there are different methods to measure the Vs, in this study the suspension PS-Logging test and SPT-N correlations were utilised to define the shear wave velocity profiles. When there are some geotechnical in-situ and laboratory test results such as SPT- CPT, Wn, Ip, soil classification and shear wave velocity measurements, correlation can be evaluated with respect to Ak, amplification factor and To, predominant period obtained from microtremor records. So the aim of the study is to determine correlations among all of the results for the use of both theoretical and practical issues. SPT is a common field test in geotechnical engineering and there are reliable correlations between SPT-N blow counts and Vs in the literature. The correlation developed by İyisan (1996) as: Vs=51,5.N°'516(m/s) (1) XIwas used to convert to Vs, shear wave velocity from the SPT-N values in this study. The second common field test to obtain the engineering properties of soil layers is called CPT, Cone Penetration Test. The q", cone tip resistance, u pore pressure and fs friction can be measured digitally and continuously by means of electrical CPT devices. It can also be possible to derive correlations between CPT parameters and Ak, amplification factor and To, predominant period determined from microtremors. The third measurement in the field was suspension PS-Logging seismic wave velocity measurements through the soil layers. The records are kept in magnetic diskets that allows evaluation of the Vs data digitally conducted to determine variation of shear wave velocity. The last stage in order to study site response characteristics in the investigated area, microtremor measurements were conducted. It has many advantages such as economy and time. Ak, amplification factor and To, predominant period can be easily obtained after the analysis by using Nakamura (1989) method to develope the spectrums of recordings. Nakamura's method is based on the spectral ratio of horizontal to vertical component. 9 SPT, 12 CPT, 3 PS-Logging and 21 microtremor measurements were carried out in order to develop correlations among these measured parameters. The evaluation of the SPT-N blow counts indicate that Neq, equivalent N counts are varying between 25-65 and Vseq values are between 200-400 m/s. If they are compared to 3 Vseq determined by PS-Logginğ, it is seen that there is a good relationship between two shear wave velocities. This indicates that Equation (1), given by İyisan (1996) is a reliable correlation to obtaining Vs when it is not possible to carry out PS-Logging or the other Vs measuring tests in the field. Correlation coefficients of these relationship are given in Table 1, (No. 7 and 8). Careful assessment the SPT-N blow counts can indicate soft soil layer thicknesses for each borehole. H, soft soil thickness are varying between 2,00-20,00 m based on 9 borehole data. It is assumed that N values of 20-25 counts for 30 cm can be a criteria for the soft soil layer. The relationship obtained in this study can be seen in Table 1, (No.l). Beside the fact that the correlation coefficient is relatively high, it is also possible to conclude Ak, amplification factor is affected significantly by the surface soft soil layers laying on the bedrock. As a similar approach CPT cone tip resistances qc<l MPa can also be a criteria for defining soft soil layer thickness over the bedrock. In Table 1, correlation 2 was based on 12 CPT tip resistance qcs. Correlations 5 and 6 also explains the high correlation between the CPT parameters and Ak amplification factor and Tos obtained from microtremors. It is believed that this approach will be very useful for future research on this topic. Although there are so many correlations based on SPT values in the literature, CPT and microtremor relationships are rarely founded. XllTable 1. Correlations and coefficients CPT parameters such as u, fs etc. were also studied to obtain reliable correlations for the Ak, amplification factor and To, predominant period. As can be observed from Table 1, correlation 9 and 10, in spite of the fact that correlation coefficient were high, this relationship between CPT and microtremor results can not completely be explained. The laboratory test results such as Ip, plasticity index and Ak relationships has been reported several times by the different researchers. Each Ip and layers examining whole borehole and soil classifications and the other test results, it can be defined Ipeq, an equivalent plasticity index, what is representative of that borehole. The Ipeq values in this study varies between 15 and 35 in percent and was developed a correlation 3, given in Table 1 based on 9 borehole data. The objective of this study was to investigate and emphasize the requirements of correlations among geotechnical in-situ, laboratory tests and in-situ seismic and microtremor measurements. For this purpose different kind of tests were carried out and possible correlations obtained are presented Table 1. Based on the results of microtremor measurements and the other experimental studies, it is concluded that easily obtained microtremor measurements can give some significant parameters for the microzonation and site response information to a study regional earthquake hazard. The correlations obtained supports this approach. xiuAs a conclusion, the study demonstrates that if microtremor measurements are done correctly, reliable results concerning response characteristics of local soil conditions can be obtained. By means of correlations obtained in this study, preliminary estimates can be obtained concerning the behaviour of soil layers during earthquakes. But it should not be neglected that when a correlation is used, all the other conditions must be thoroughly examined. The two important advantages of microtremor measurements are that they require relatively short time and economically smaller budget. The other advantage is to be able to carry out a study in a larger scale that can be used for preliminary microzonation purposes. xiv
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