Zeytinburnu bölgesinde yerel zemin koşullarının zemin büyütmesine etkisi
Effects of local soil conditions on site amplification for Zeytinburnu region
Yazar: YASİN BAYRAKLI
Danışman: PROF. DR. ATİLLA ANSAL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Geoteknik Ana Bilim Dalı
Derece: Yüksek Lisans, 2000
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Zemin büyütme = Soil amplification ; Zemin yapısı = Soil structure ; İstanbul-Zeytinburnu = İstanbul-Zeytinburnu

Özet:
Depremler sonucu meydana gelen hasarın bir nedeni ve yer hareketinin yüzeye yakın zemin tabakalarında meydana getirdiği önemli bir sonuç olan zemin büyütmesi, yerel zemin koşullarına bağlı olarak önemli değişiklikler göstermekte; zemin türü, yeraltı su seviyesi, arazi ve anakaya topografyası, tabaka kalınlığı gibi yerel zemin koşullarından etkilenmektedir. Yerel zemin koşullarının deprem hasarına etkisi gerek depremler sonrası yapılan gözlemler gerekse de aletsel kayıtlarla ortaya konmuştur. Bu çalışma, bir araştırma sahasında yer alan 100 m derinliğinde ve özel kaplamalı sondaj kuyusu ve kuvvetli yer hareketi kayıt istasyonundan elde edilen veriler kullanılarak yerel zemin koşullarının zemin büyütmesi üzerindeki etkilerin belirlenmesi amacıyla yapılmıştır. Zemin büyütmesi ile İTÜ Zemin Mekaniği Laboratuarında yapılan deneylerden elde edilen tabaka özellikleri ve arazide yapılan kuyu içi ve aşağı kuyu yöntemleri sonucu elde edilen kayma dalgası hızı,Vs gibi dinamik zemin özellikleri arasındaki olası ilişkiler araştırılmıştır. Çalışmada sondaj sırasında alınan numuneler üzerinde tabaka özelliklerini belirlemek üzere yapılan laboratuvar deneylerinin ardından arazi çalışmalarına geçilmiştir. Arazi çalışmalarında ilk olarak kuyu çevresinde alınan mikrotremor ölçümleri alınmış ve bu kayıtlardan zemin büyütmesi ve zemin hakim peryodu belirlenmiştir. Kuyu profilini meydana getiren tabakaların kayma dalgası hız profilini belirlemek üzere önce kuyu içi yöntemi ile P ve S dalga hızlan belirlenmiştir. Ayrıca, aşağı kuyu yöntemi ile de ilk 25 m derinliğe kadar olan tabakaların P ve S dalga hızlan belirlenmiştir. Kuvvetli ve zayıf yer hareketleri sırasındaki zemin tabakalannın göstermiş olduklan davranış uzun yıllardan beri üzerinde çalışılan bir konudur. Zemin tabakalannın mekanik ve dinamik özelliklerinin belirlenlenerek bu özellikler arasında çeşitli ilişkilerin araştınlması sırasında aynı zemin koşullannda alınan kuvvetli ve zayıf yer hareketi kayıtlan çalışmadan elde edilecek sonuçlar açısından da önem arz etmektedir. Zeytinburnu Gözlem Kuyusu civannda bir noktaya yerleştirilen kuvvetli yer hareketi cihazı (ZYT istasyonu) ile alınan 17 Ağustos Depremi'nin anaşok ve artçı şok kayıtlarının analizi de bu çalışmanın önemli bir bölümüdür ve yerel zemin koşullannın zemin büyütmesine etkisi araştınlırken kullanılmıştır. Ayrıca, 1998 Adana Ceyhan Depremi'nin ana ve artçı şok kayıtlan incelenmiş ve bu yer hareketikayıtlanndan elde edilecek sonuçlann ZYT istasyonundan elde edilecek kayıtlann analizinde de kullanılabilirliği konusu üzerinde de durulmuştur. Laboratuvar deneylerinden elde edilen tabakalann mekanik özellikleri, arazi deneylerinden elde edilen zeminlerin dinamik özellikleri ve aynı zemin koşullannda alınan kuvvetli ve zayıf yer hareketi kayıtlanndan elde edilen sonuçlann kullanıldığı bu çalışmada değişik yöntemlerle bulunan zemin büyütmesi ve zemin hakim peryodu değerleri karşılaştınlmış ve bulunan sonuçlar arasındaki olası korelasyonlar araştınlmıştır.

Abstract:
Recent destructive earthquakes such as the 1989 Loma Prieta, 1994 Northridge, 1995 Kobe, 1992 Erzincan, 1995 Dinar, 1998 Adana, 17th August 1999 Izmit and 12th November 1999 Duzce Earthquakes have demonstrated that local site conditions can have a significant effect on the nature of ground motion, structural damage and casualties. Considering the complexity of the factors affecting earthquake induced damage, it is very difficult to separate these factors, if not impossible. However, separate evaluation of the local site conditions, which play an important role in site amplification, is more convenient, especially if there is a need for investigating a specific area for the site response. As a result, the studies in geotechnical earthquake engineering studies in the last decades have mostly been focused on local site conditions and their effects on site amplification. These studies showed that the degree of the amplification is affected by the soil parameters such as soil type, ground water level, site - bedrock topography and layer thickness. Amplification is the increase in the amplitude of the seismic waves during their propagation from rigid media, like intact rock, to more looser media such as soft soil layers near to the surface. Site amplification is one of the major reasons of the earthquake induced damage and an important result of ground motion in the upper soil layers. Site amplification, liquefaction, slope failures and landslides are the main outcome of the earthquakes, which are controlled by local soil conditions, topography, etc. Liquefaction, slope failures and landslides, are related to site conditions and topographical features, thus they may not take place during every strong ground motion event, while amplification can be observed in every earthquake. Significant variations were observed in earthquake damage level at very short distances as a primary result of the different characteristics of soil types in the upper part of the soil profile. In addition to this, the upper 30 meters of the soil profile was found to be effective in the earthquake induced damage, thus the equivalent shear wave velocity of the first 30 meters in the soil profile is chosen as the most appropriate parameter to represent the soil properties (Borcherdt, 1994). There are many uncertainties in evaluating the strong ground motion characteristics where the main obstacle in this evaluation is the nonlinear behaviour of the soil layers; this represents the main reason for the amplification difference between weak and strong ground motions. Regardless of the ground motion level, local soil conditions including physical properties of soils depending on soil type, like void ratio, plasticity, saturation, relative density, etc.; features like soil stratification, layer XIthicknesses, site - bedrock topography, ground water level; and dynamic properties like shear wave velocity, damping, dynamic shear modulus, define the main behaviour of soil under earthquake induced ground shaking. Detailed soil investigation program consisting of laboratory and field tests should be carried out to determine these parameters to calculate the site amplification and utilise the proper parameters for the design of structures as well as the microzonation purposes to mitigate earthquake hazards. As a result, the whole procedure includes various stages, which are not feasible in many projects due to the high cost. In this study, the effects of local soil conditions on site amplification were investigated to determine possible correlations between amplification and soil properties. For this purpose, a site with a 100 meter deep borehole in Zeytinburnu was investigated by performing detailed conventional laboratory and field tests as well as conducting new test methods like PS-Logging, downhole and microtremor measurements. A strong motion recorder had installed to measure the ground motions at the site, which has recorded one major earthquake occurred in 1999. Values obtained for soil conditions at the site and values from laboratory and field tests were compared. The aim of this study has been to investigate the effects of the local soil conditions on the site amplification in the Zeytinburnu Region. The data used in the investigation included laboratory analysis of the soil samples collected during the installation of 100 m deep well specially drilled for the study, field tests, microtremor recordings and ground motion data collected from previous destructive earthquakes. Laboratory tests performed on the soil samples included natural water content (39 tests), consistency indexes (45 tests), sieve analysis (14 tests), solid density tests (12 tests), consolidation tests (14 tests), unconfined compression tests (12 tests), direct shear tests (4 tests), triaxial compression test (1 test). Field tests were comprised of PS-logging (for 100 m), downhole (for upper 25 m) methods. Site amplification and predominant period were determined using four microtremor measurements performed at three points. Strong ground motion data collected during mainshock and aftershocks of 17th August Earthquke and mainshocks and aftershocks 1998 Adana Earthquake. The following conclusions were found for the Zeytinburnu Region following the analysis of the data: 1. It is known that strong correlation Coefficients exist between soil parameters and SPT-N30, including dynamic soil properties such as Vs shear wave velocity and Ak site amplification. It was found that a strong correlation existed between the corrected SPT-N30 and Vs like given as: Vs = 24.285N' 082 (m/sn) (1) Despite the fact that a relation between the liquidity index obtained using the Atterberg limits and Vs might be a better choice to be investigated. This relation was found to have correlation for R= 24 %. 2. It was postulated by Chin and Aki(1991) that amplification ratios obtained from weak ground motion would not be representative based on the nonlinear behaviour xuanalysis of strong ground motion of a site located 50 km away from epicentre of the 1989 Loma Prieta Earthquake. The analysis of the mainshock and aftershocks of the 17th August Earthquakes recorded at ZYT station resulted in Sa-sgm= 2.5 by using the average of the peak horizontal dround motion accelerations. The analysis was performed to identify the value Sa.Sgm resulting from a strong ground motion. The value of Sa-Wgm, which represents the mean value of the vertical component of the weak ground motions recorded at ZYT station, is 2.4. In this case, it was, therefore, found that spectral amplification ratio obtained from strong ground motion was in line with those obtained from weak ground motion records. 3. The spectral characteristic periods were determined as Ta= 0. 1 5 and Tb= 0.70 sec using acceleration spectrums based on the ground motions recorded at ZYT. These values are given as 0.15 and 0.60 sec, respectively, in 1997 Turkish Earthquake Code. This indicates that soil classification used in this code is in accordance with the results of this study. 4. Amplification values can be obtained from empirical relationships developed bey various researchers using Vs3o, equivalent shear wave velocity of for upper 30 m deep of soil profile, besides the calculations based on ground motion records and microtremor measurements. The equivalent shear wave velocity for upper 30 m deep of soil profile was found to be Vs3o=250 m/sec for Zeytinbumu investigation site. Amplification factors were calculated as 2.5 using the relationship by Midorikawa, 3.7 using the relationship by Iyisan and 2.7 using the relationship by Borcherdt. The amplification factors for the soil conditions in the investigation site was found to be the same using spectral amplification and the relation given by Midorikawa. 5. The average amplification, Ak,ave and the predominant period were found to be 2.2 and 1 sec, respectively, using the microtremor recordings collected in this study. 6. The value of spectral amplification ratio, Sa was, therefore, establihed as 2.5 using the microtremor measurements, ground motion records and the emprical relationships.