Dinar'da yapılan mikrotremor ölçümleri ile arazi deneylerinin karşılaştırılması
Mikrotemor measurements in Dinar and comparison with in-situ test
- Tez No: 66468
- Danışmanlar: PROF. DR. ATİLLA ANSAL
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Geoteknik Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 132
Özet
ÖZET Diğer doğal afetler gibi depremlerin de nerede ve ne zaman olacağım belirleyebilmek günümüz teknolojisiyle mümkün değildir. Depremler sırasında görülen hasarın büyüklüğü üç temel unsurun etkileşmesine bağlıdır. Bunlar deprem özellikleri, üst yapı özellikleri ve yerel zemin koşullandır. Dinar depremi gibi orta şiddetli bir depremin bu kadar ağır hasara yol açması, kötü yapılaşma ve elverişsiz zemin koşullan ile açıklanabilir. Depreme dayanıldı yapı tasarımında zeminlerin dinamik özelliklerinin bilinmesi şarttır. Günümüzde zeminlerin sismik yükler altındaki davranışları çeşitli arazi ve laboratuvar deneyleriyle yaygın olarak belirlenmektedir. Ancak bu deney sistemleri hem pahalı hem de sonuçların elde edilmesi uzun sürmektedir. İşte mikrotremorlar burada devreye girmektedirler. Çünkü kullanılan oldukça basit ve verilerin analizi çok kısa sürmektedir. Böylelikle hem iş gücü hem de maliyet açısından büyük kazançlar sağlamaktadırlar. Dinar'da yapılan mikrotremor kayıtlarının amacı, kent sınırlan içinde yerel zemin koşullarına bağlı olarak zemin büyütme katsayısının nasıl değiştiğini incelemektir. Bu nedenle 56 noktada kısa süreli ölçümler yapılarak her mahalle için zemin büyütme oranlan Nakamura ve sabit nokta yöntemleri kullanılarak hesaplanmıştır. Bu şekilde hesaplanan zemin büyütmesi ve deprem sonrası belirlenmiş hasar oranlan arasında doğrusal bir ilişki olduğu gözlenmiştir. Ayrıca 26 Eylül-6 Ekim tarihleri arasında Dinar Meteoroloji İstasyonunda alınan gerçek deprem kayıtlan ile aynı yerde alınan mikrotremor kayıtlan karşılaştırılmış ve elde edilen zemin büyütmeleriyle hakim peryotların birbirine çok yalan oldukları gözlenmiştir. Dinar depremi sonrasında bölgede geniş çaplı arazi ve laboratuvar deneyleri yapılmıştır. Dinamik sonda deneylerinden elde edilen vuruş sayılan, bazı formüller yardımıyla önce eşdeğer SPT-N darbe sayılarına ardından da kayma dalgası hızlanma çevrilmiştir. Daha sonra yine formüller yardımıyla zemin büyütmesi katsayılan hesaplanmıştır. Bulunan bu değerlerle mikrotremor kayıtlan karşılaştırılarak elde edilen sonuçlar değerlendirilmiştir.
Özet (Çeviri)
MIKROTEMOR MEASUREMENTS IN DINAR AND COMPARISON WITH IN-SITU TESTS SUMMARY Earthquakes compared to other natural disasters have a more dominant effect on the observed damages and on human beings. In order to mitigate earthquake hazard, the earthquake phenomenon must be thoroughly studied. The basic principle of a seismic design and construction against earthquakes is to determine how seismic waves are altered when they pass through soil layers and how they are affected by the local soil conditions. Earthquakes can be classified according to hipocenter distances. If the hipocenter distance is less than 70 kilometers they are called“shallow earthquakes”, between 70 to 300 kilometers called“medium deep earthquakes”and more than 300 kilometers called "deep earthquakes. Deep earthquakes are affective in wide ranges but shallow earthquakes are affective in the epicenter distances. So shallow earthquakes are more destructive than the others. Dinar earthquake occurred on 1 October 1995 at 17.57 p.m. and caused 90 deaths, 250 wounded and a damage of approximately 250 million dollars. The earthquake sequence that affected Dinar was composed of small to medium size foreshocks, main shock and aftershocks. The foreshocks started on September 26, 1995 and the main shock took place on October 1,1995. Nine strong motion records for earthquakes with Ml > 4 were recorded at the Meteorological Station located in the valley on the alluvium deposit between September 26 and October 6, 1995. The earthquake caused highly localized damage in Dinar Town. Damage in nearby villages and towns are mostly light to medium and rapidly attenuate with distance from Dinar. The building structures in Dinar town center range from one to five stories. There is no industrial facility in the town. Along the main streets, first stories of buildings are generally occupied for commercial purposes. Buildings with more than three stories are almost all reinforced concrete construction. Buildings with lower number of stories are partly reinforced concrete and mostly brick masonry. Stone masonry and adobe buildings are very few. During the main shock of October 1, 1995, most of four and five storey reinforced concrete apartment buildings were either heavily damaged or totally collapsed. Some three storey buildings suffered similar damage. One or two storey building collapses are rare. Nearly 40 percent of all the buildings have collapsed or were heavily damaged. Dinar is located partly on the hills and partly in a valley extending below the hills. The surficial geology of the hills to the east of the town consist of limestone, marn and schist. The flat zone is covered with alluvium deposit containing alternating layers of loose to medium dense silty sands and soft to medium stiff silty fat clays at some locations of organic nature. In order to determine the variations in local geotechnical site conditions in Dinar, detailed geotechnical investigation composed of insitu penetration tests, seismic wave velocity measurements, microtremor measurements and laboratory tests were XIperformed. Several borings in the depth of about 30 m were drilled in Dinar. In the most of these boreholes soil stratification consists of alluvium deposit containing alternating layer of silty clay and clayey sand. The ground water table is almost at the ground surface. Soil profile consists of a fill with thickness of about 0.5 m, underlain by a medium to stiff sandy brown colored clay layer of SPT-N value about 17 with thickness of about 2.5 m, underlain by a silty sandy and fine gravely with low to medium plasticity gray colored clay layers of N values between 8 to 17 with thickness of about 2 and 7 m, receptively. A fine gravely medium to dense sand layer of N value about 14 is located at depths of 12 to 15 m. Then, sandy, silty clay and clayey silty sand layers are located alternatively, and N values in these layers are relatively high. In every district the dynamic penetration tests with the type of LRS-10 motorized test equipment were conducted and dynamic penetration test results were converted to equivalent SPT-N blow counts. The type of dynamic penetration test was LRS-10 that utilize ten-kilogram free fall hammer from a height of fifty centimeters. The diameter of the cone is 3.56 centimeters and the cross section is ten square centimeters. The penetration values (Nio) are converted to the equivalent SPT-N blow counts by using the following formula (DIN 4094). SPT-N =10°-025*(N10)0-675 (1) Also observation pits were excavated near the footings of the collapsed buildings. Soil layers were classified according to the new 1996 Turkish Earthquake Code and characteristic periods were estimated. Soil conditions in Dinar is considered as group D that is defined as soft, thick alluvium layers of loose sand and soft clays with high water table. The characteristic predominant periods are between 0.20sec and 0.90sec. The design soil acceleration is specified as 0.40g. Laboratory test results performed on soil samples obtained from the pits indicate that plasticity index of soils varies between 13% and 46%, and soil groups were generally determined as CL. Shear wave velocity (Vs) of surface layers is very useful index properties for evaluating site amplification (Shima, 1978). Shear wave velocities indicate the dynamic characteristics of the local soil conditions under seismic loads such as earthquakes. Shear wave velocities were measured ranges of 175 and 217 m/sec. in Dinar. Shear wave velocity was calculated from SPT-N value using the correlation between Vs and N when insitu seismic measurements were not performed. In this study SPT-N blow counts were converted to shear wave velocity following equation (îyisan,1996). Vs=51.5*N°-516 (m/sec) (2) The soil amplification factors based on shear wave velocity were determined using following formula given by Midorikawa ( 1 987). Afc = 68*V8-°-6 (3) Soil Amplifications At estimated from shear wave velocity and SPT-N values are given in Table 1. XllTable 1. Soil amplifications from shear wave velocity The damage distribution in Dinar clearly indicates the effects of the differences in the geotechnical conditions. The buildings located on rocky hill slopes suffered relatively minor damage while heavy damage occurred in the valley. A detailed damage survey conducted by the General Directorate of Disaster Affairs showed large variations in damage ratios within different districts in the town of Dinar. Insufficient construction quality and unsuitable soil characteristics can be an example how a medium strong earthquake like Dinar earthquake with magnitude of 5.9 can cause a huge damage as observed. The investigation of dynamic characteristics of local soil layers by means of in-situ and laboratory tests is widely used. However, this requires time and the cost of seismic in-situ tests such as down-hole, cross-hole, seismic refraction and laboratory tests such as cyclic triaxial, cyclic simple shear tests are relatively high. Therefore these methods may be considered in major projects and for detailed investigation of limited areas. An alternative approach is to use microtremors or ambient seismic noise measurements to estimate site response under earthquake excitations. Microtremors are low amplitude oscillations of the ground surface produced by natural sources such as wind, ocean waves, geothermal reactions and small magnitude earth tremors. They can also be produced by artificial sources such as cultural noise. Wave modes contributing to their mechanism have been inadequately defined. While some researchers accept them as surface waves, the others believe that they are body waves. In general, it appears that surface sources such as wind, ocean waves and cultural noise act as surface wave generators, whereas extremely small magnitude, naturally occurring earth tremors act as body wave microtremors. These body wave sources occur deep in the Earth and contain vertically traveling components. In spite of the inadequately known wave characteristics, many researchers have been able to characterize local site response successfully using microtremor data (Nakamura, 1989). The potential for using microtremors in order to characterize earthquake site response is attractive because of the relative ease and economy of the method. There exists three different techniques commonly used to estimate site effects from microtremor measurements: xni1. Interpretation of Fourier amplitude spectra or power spectra. 2. Computation of spectral ratios relative to a firm reference station. 3. Computation of spectral ratios of horizontal components relative to the vertical component of ground motion (Nakamura's method). During 20 to 22 August 1996 microtremor measurements were taken in Dinar and later were analyzed. The aim of these measurements was to obtain soil amplification ratios and predominant periods at different locations within the city of Dinar. The measurement locations were selected as the nearest position to previously performed dynamic penetration in order to establish a correlation with respect to soil conditions. After analyzing the microtremor tests records using above mentioned three methods it appeared that amplification ratios computed using Nakamura's technique varied in a relatively narrow range of 2.6 to 3.4. But the amplification ratios computed with respect to the firm reference station was observed to be scattered in a relatively wide range of 5.8 to 34. One possible reason for this may be different local source effects. The reference station was located in the slopes of the hill away from the city center in a relatively quite area. On the other hand the city was very noisy. In chapter five, the analyzed recorded measurements are explained in detail. Table 2 shows the results of Nakamura and Reference point methods together. Table 2. Amplification Ratios and Predominant Periods For Dinar The actual acceleration records recorded at the Dinar Meteorological Station and the microtremor measurements at the same point are compared. A relatively good agreement was observed between actual strong motion records and microtremor measurements. They have nearly the same amplification ratios and predominant periods. During Dinar Earthquake sequence large number strong motion records were obtained at the Meteorological Station. The normalized elastic acceleration response spectra calculated for these records. The site amplification was in the range of A=3-5 and predominant periods were in the range of T=0. 15-0.6 sec. In order to make a comparison, microtremor measurements were taken at the same place right by the strong motion instrument. The amplification ratio calculated for these microtremor records using Nakamura method. The amplification thus calculated is in the range of xivA=2.5-3 and predominant periods were in the range of T=0.6-1.0 sec. The difference observed between microtremors and strong motion records is most likely due to the inelastic nonlinear behavior of soil layers that becomes more dominant during earthquakes. The soil amplification ratios thus were computed by two different methods (a) from microtremors and (b) from shear wave velocities. The correlation between damage ratios and amplification coefficients obtained by Nakamura method are illustrated in chapter seven. From these results a linear relationship were observed as A^c = 2.7028 DR + 1.7484 (4) where A^ represents the soil amplification and DR shows the damage ratio. This investigation clearly demonstrates that if microtremor measurements are done correctly reliable results of local soil conditions can be obtained. 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. xv
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