SEISMIC AMPLIFICATION AT AVCILAR, ISTANBUL

Prof.Dr.Semih S. Tezcan (1), Erdem Kaya ⁽²⁾, İ. Engin Bal ⁽²⁾ and Zuhal Özdemir <sup>(2)

(1) Professor of Civil Engineering, Boğaziçi University,Istanbuıl</sup>
(2) Research Assistant, Higher Education and Research Foundation

ABSTRACT

Based on the soil data established previously by a team of researchers at the Technical University of Istanbul, a wave amplification study is conducted for the town of Avcılar, Istanbul, located at about 120 kilometers west of the epicenter of the Kocaeli earthquake of August 17, 1999. It is determined, through the use of well known computer program SHAKE, that the three major predominat periods of the ground are, 1.60 sec, 1.00 sec and 0.70 sec. Thus, the reasons of extensive damages occured to 5 to 8 storey high residential buildings in the region, may be attributed to both the long distance effects of the high period waves of the earthquake and soil amplification.

Introduction

While propagating upward through a layered soil medium, the frequency content and the amplitudes of the earthquake motion may be greatly modified. Density, rigidity, thickness and other physical properties of the soil strata as well as the intensity of seismic motion are the prime factors affecting the characteristics of the seismic waves. A soil amplification study may be performed following one of the three methods of analysis; (a) Lumped mass idealization, (b) Solution of differential wave equations, and (c) Finite element idealization.

The first two methods are used for horizontally layered soils idealized into one dimensional mathematical models, while the finite element procedure is preferred for two or three dimensional problems. In addition, there are several studies representing the case, when layering is not horizontal but inclined [1], and also the effect of incidence angle on the surface wave generation, thus on soil amplification [2]. Details of different methods of analyses as mentioned above, are available in the literature [3], [4], [5] and [6].

Nonlinear wave propagation technique has been successfully employed [3] to study the earthquake response of horizontally layared soils. The computer program SHAKE developed is a sophisticated and versatile tool to determine the effects of local soil conditions on ground response. In this presentation, a series of soil amplification analyses have been performed for eight different types of soil profiles at Avcılar, Istanbul.

Examples of Soil Amplification in Past Earthquakes

The existence of soil amplification was amply demonstrated in many past destructive earthquakes. It seems clear from studies of recent earthquakes that the relationship between the periods of vibrations of a structure and the predominant periods of the supporting soil is profoundly important regarding the seismic response of a structure. In some instances, such as the Gediz Earthquake, Türkiye (1970), the Romanian Earthquake (1979), the Mexico City Earthquake (1985), the surface accelerations may be as large as 4 to 5 times those of the base rock accelerations [7], [8], [9], [10], [11], [12],and [13] .

During the 1970 earthquake at Gediz, Türkiye, for instance, the paint workshop building of the Tofaţ/Fiat automobile factory was demolished in Bursa, 135 km away from the epicenter, while no other buildings in Bursa were damaged. Subsequent investigations revealed that the fundamental period of vibration, T=1.2 seconds, of the paint workshop building was approximately equal to that of the underlying soil.

Further evidence about the importance of predominant periods of vibration of soils was derived from the medium sized earthquake of Caracas (Venezuella) in 1967, which completely destroyed four buildings and caused extensive damages to many others. The pattern of structural damage has been directly related to depth of soft alluvium overlying the bedrock. Extensive damage to medium-rise buildings (5-9 storeys) was reported in areas, where depth to bedrock was less than 100 meters, while in areas where the alluvium exceeded 150 meters, the damage was greater in taller buildings (over 14 storeys).

The town of Avcılar of the City of Istanbul, is located at a distance of 120 kilometers to the epicenter of the August 17, 1999 Kocaeli Earthquake. Despite such a long distance to the epicenter, surprisingly heavy and extensive damages occured to many buildings at Avcılar. The casualties to life were 273 dead and 630 wounded. A total of 158 apartment buildings either totally collapsed or heavily damaged beyond repair. About 526 buildings suffered medium damages and 800 buildings suffered minor damages.

Such an extensive damage toll, at such a long distance to epicenter, has been a great surprise to all concerned, since there were practically no heavily damaged or even moderately damaged buildings in the City of Istanbul, which is 25 kilometers closer than Avcılar to the epicenter. A few examples of damaged buildings at Avcılar are shown in Fig.1.


Figure 1

It is also a strange phenomenum that the maximum ground acceleration recorded during the main shock of the Kocaeli earthquake at the Ambarlı Thermal Power Plant (ATS), near Avcılar is 0.25 g, while the peak ground acceleration on bedrock is only 0.04 g at the Public Works Building, Barbaros Boulevard, Besiktas, at the heart of the City of Istanbul. It is seen that the seismic waves must have been amplified greatly, by at least 5 to 6 times, at Avcılar. The peak horizontal ground acceleration values recorded at various stations, during the Kocaeli earthquake of August 17, 1999, are shown in Fig. 2.


Figure 2

The distribution pattern of damaged buildings in the town of Avcılar, is indicated on a map given in Fig. 3 .


Figure-3

Effects of Soil Conditions

The depth of alluvium directly affects the predominant periods of vibration of the ground. Considering shear waves traveling vertically upward through a single soil layer of depth H above bedrock, the predominant period of horizontal vibration of the ground is given by

where n is an integer, 1, 2, 3, ... representing the various modes of vibration, and v_s is the velocity of the shear wave. The nature of the sub-soil may influence the seismic response of structures by way of soil amplification in which the seismic excitation at bedrock is modified during transmission through the overlying soils.

This may cause attenuation or amplification effects. It follows that the soil amplification will be influenced by the presence of the structure, as the effect of soil-structure interaction is to produce a difference between the motion at the base of the structure and the free-field motion which would have occured at the same point in the absence of the structure. In practice however, this refinement in determining the soil amplification is seldom taken into account, the free-field motion generally being that which is applied to the soil-structure model.

The scope of this study is restricted to determining the nature of soil amplification at free surface, for the town of Avcılar, 25 km west of Istanbul.

Physical and Dynamic Properties of Soil

The physical and mechanical properties of the subsoil layers play an important role in the dynamic response of the surficial layers. All pertinent data about the subsoil conditions should be determined by means of both insitu and laboratory testing. The following information about the subsoil layers is considered to be the most essential; layer thickness, angles of inclination and general stratigraphy, strength properties, grain size distribution, consolidation data, mineralogy, natural moisture content, Atterberg limits, unitweights, shear strength, relative density, overconsolidation ratio, ion exchange capacity, sensitivity, swelling, shear modulus, damping, Poisson’s ratio, bulk modulus, cyclic shear strength, seismic wave velocities, intensity of cracks, permeability, etc.

It is always advisable to determine most of these patameters by more than one measuring technique for the purpose of correlation and realistic evaluation. In the following sections, the variations of shear modulus, G, and critical damping ratio, β, by the amount of shear strain are discussed.

The shear modulus of soil may be estimated easily from shear wave velocity test. An explosive charge or a hammer is used to produce waves in the soil. The velocity is measured by applying the excitation at one borehole and measuring the velocity at another borehole or by applying an excitation on the ground and measuring the velocity at a borehole [14] . The fundamental period of ground is an important property for the earthquake resistant design of structures. It can be estimated by means of an analytical study or from measurement of small earthquake disturbances.

The moduli E and G of soils can be determined by applying axial and torsional vibrations to the cylindrical sample through the “resonant column” testing procedure [15]. There are various field and laboratory methods avaiable for finding the shear modulus, G of soils. Field tests concentrate on

finding the shear wave velocity, v_s, and calulating the shear modulus from the relationship given by

in which, = mass per unit volume. When the shear wave velocity is not measured, the standard penetration blow count, N, may be used to determine the shear wave velocity, v_s, by means of the empirical expression

as applied by Fujiwara [16] and illustrated in Fig. 4 .


Figure-4

Laboratory methods generally measure G more directly from stress-strain tests. It is important that the level of strain at which G is measured must be known. Variation of shear modulus with shear strain, as used in our computations are shown [17], for clay and sand, in Fig. 5 . The values assumed in our computer analyses are also listed in Table 1. Although, the shear modulus and damping of soils may be determined by experiments as described above, empirical expressions are essential for theoretical analysis purposes. In fact, extensive design equations and charts have been proposed by Hardin-Drnevich [18] and Pitilakis et. al. [19]. The second key dynamic parameter for soils is the critical damping ratio. Two fundamentally different damping phenomena associated with soils, namely material damping and radiation damping, will be explained below.


Table-1

a) Material damping

Material damping in a soil occurs when any vibration wave passes through the soil. It can be thought of as a measure of the loss of vibration energy resulting primarily from hysteresis in the soil.

Damping is conveniently expressed as a fraction of critical damping, in which form it is referred to as the critical damping ratio.
Published data on critical damping ratios are sparse, and consist only of values deduced from tests on small samples, or theoretical estimates. It should be appreciated that to date no insitu determinations of material damping have been made, and that damping ratios can only be used in analyses in a comparative sense. Accepting a philosophy that dynamic analysis will be warranted for some projects, at least for its qualitative information, a means of choosing values of material damping is required. Some material damping values for varying shear strains are given in Fig.5 .


Figure-5

These represent average values of laboratory test results on sands and saturated clays as presented by Seed et. al [20]. In absence of any other information it may be reasonable to take the damping of gravels as for sand.

The material damping ratio, β, is computed normally from the hysteretic curve of the material on cyclic testing. The calculation of is given by [17]

β = ( 4 / ) A_h / A_r (4)

where, A_h = the total area inside one complete hysteretic loop, A_r = the total area of the rectangle enveloping the full hysteretic loop.

b) Radiation damping

In considering the vibration of foundations, the radiation damping is present as well as the material damping. Radiation damping is a measure of the energy loss from the structure through radiation of waves away from the footing, i. e. it is a purely geometrical effect. Like material damping, it is very difficult to measure in the field. The theory for the elastic half-space has been used to provide estimates for the magnitude of radiation damping. A detailed account of radiation may be found elsewhere [17].

Soil Conditions at Avcılar, Istanbul

The geological and geotechnical data of the soil conditions under the urbanised section of the Avcılar Municipality, have been extensively investigated by a team of researchers at the Technical University of Istanbul [21]. The township of Avcılar is located at about 25 kilometers west of Istanbul, between the Küçükçekmece and Büyükçekmece Lakes, bounded by the Sea of Marmara on the south, and European Motorway (E5) on the north, as shown in Fig.3.

Figure-6

The typical geological formations existing in the area are indicated in the soil profiles given in Fig. 6. Since, the thicknesses of especially the three uppermost formations are variable, eight different combinations of soil layers, with extreme values of layer thicknesses have been considered. The top soft clayey layer is named Güngören formation with a thickness varying between zero and 10.00 m. There is a relatively strong limestone (Bakırköy) formation of upper Miocene age underneath this clay, with a thickness varying between 7.5 m and 15.0 m.

The third typical layer below the surface is again the same clayey formation (Güngören) with a thickness varying between 4.0 m and 15.0 m. It is underlain by a 15.0 meter thick fine dense sand formation (Çukurçeţme) of Pliocene age, which is unconsolidated and partially saturated. The SPT values at this sand layer average at N₆₀ = 25 .

The grain size distribution of some of the sand samples taken from the Çukurçeşme formation, falls well within the highly liquefiable fine sand category. Some other sand samples however, do not exhibit such a high liquefaction potential in their particle size distribution. Nevertheless, for any future construction at Avcılar, a proper liquefaction hazard risk analysis must be performed, using both experimental and analytical means, especially when the top of the sand layer is less than 12 m to 15 m below surface.
The ground water table is located at 6.0 m to 16.0 m below the surface. There is a hard clayey layer (Gürpınar) of about 300 meters thick overlain by the Çukurçeşme sand formation. Beneath the Gürpınar hard clay layer, a strong tuffaceous bedrock formation exists. The typical geotechnical parameters of these fıve distinct soil layers are summarised in Fig. 6.

Amplification spectra

One dimensional shear wave propagation analyses have been conducted, from bedrock to surface, for all eight different types of soil profiles using the SHAKE computer program. The time history motion assumed at bedrock level is the NS-component of the El Centro earthquake of 1940, except that the time spacing reduced to D t = 0.005 seconds, in order to increase the predominant frequency content of the record. Further, the amplitudes of the El Centro record are scaled down to a small value to correspond to the estimated bedrock peak acceleration of 0.03 g., at Avcılar, during the main shock of the Kocaeli, Turkey earthquake of August 17, 1999. The response spectrum curves at the surface for soil profiles No.1 and No. 2, for 5, 10 and 15 percent damping values, are shown in Fig. 7, together with the elastic design spectrum curve of the 1998 Turkish Earthquake Code.


Figure-7

It is seen that, for 5 percent damping case, there is a marked exceedance beyond the maximum 2.5 magnification of the 1998 Turkish Earthquake Code. The amplification spectra of the surface motion, have been also determined for the same soil profiles No. 1 and No. 2 and shown in Fig. 8.


Figure-8


It is seen that when the peak acceleration of the bedrock motion is 0.03 g, there are distinct peaks at periods T₁=1.60 sec, T₂ = 1.00 sec, and T₃ = 0.70 sec, with amplification factors as high as AF = 3 to 7. Hence, buildings with natural periods of vibrations close to these values are very much susceptible to heavy damages. In fact, 158 apartment blocks 5 to 8- storey high with periods falling into the range of T = 0.7 sec to T = 1.0 sec, either totally collapsed or heavily damaged beyond repair at Avcılar, during the Kocaeli earthquake of August 17, 1999.

The existence of soil amplification at Avcılar, has been also proven by Meremonte et. al. [22], through an array of seven seismographs, installed to record the aftershocks of the Kocaeli earthquake.During one particular aftershock of M = 5.2, the records taken at the damaged neighborhood of Avcılar displayed unusually large amplitudes, while other records taken at undamaged areas of Istanbul, showed very little or practically no motion.

As an alternate study, mainly for the purpose of investigating the changes in soil amplification, with increase of intensity of shaking, the peak acceleration of the El Centro record, assumed to exist at bedrock level, is increased from 0.03g to 0.20 g. In this case, no soil amplification is detected (Fig. 8). Infact, the amount of amplification is greatly reduced to normal levels of AF = 2 to 3. It can then be concluded that the amplification occurs only when the intensity of shaking is very small, that is only during distant strong earthquakes, or during mild nearby earthquakes[8].

Conclusions

1. The peak ground acceleration measured at Avcılar ( actually at Ambarlı Thermal Power Plant, only two kilometers west of Avcılar) is 0.25 g. This is six to seven times greater than the peak ground acceleration recorded at bedrock right at the center of the City of Istanbul during the August 17, 1999 Kocaeli, Turkey earthquake. The reason for such a high value of amplification is determined to be the shear wave amplification through the soft soil layers above the bedrock.

2. The unusually high rate of soil amplification is a consequence of not only the unfavourable existence of a variety of soft sandy and clayey layers, but also of the intensity of shaking at bedrock level being very low, on the order of 0.03 g.

3. It is shown that when the intensity of shaking at bedrock becomes relatively large, on the order of 0.20 g for example, during a future nearby earthquake, practically no soil amplification is expected.

4. For mild nearby earthquakes, or for long distance strong earthquakes occuring within an epicentral distance of about 120 kilometers, there are three distinct predominant periods of the ground as T=1.60 sec, T=1.00 sec and T=0.70 sec. Buildings at Avcılar, with natural periods of vibration close to anyone of these peak ground periods, are expected to experience relatively heavier damages due to soil amplification.

5. A proper liquefaction hazard analysis is recommended for any new consruction site at Avcılar since, the Çukurçeşme sand formation, from place to place, is susceptible to liquefaction, especially when the depth of sand is less than 12 m.
   
   

References

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[3] Schnabel, P. B., Lysmer, J., and H.B. Seed, “SHAKE: A Computer Program for Engineering Response Analysis of Horizontally Layered Soils”, University of Berkeley, Earthquake Engineering Research Center, Report No. EERC 72-12, California, USA, December, 1972.

[4] Idriss, I.M., and H.B. Seed, “ Seismic Response by Variable Damping Finite Element ”, Journal of Geotechnical Engineering Divison, ASCE, Vol. 100, No. GT 1, January, 1974.

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[7] “The Mexican Earthquake of 19 September 1985, A Field Report by EEFIT” ( Earthquake Engineering Field Investigation Team ), London, September, 1986.

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[10] Whitman, R. V., Christian, J. T., Yegian, M.K., and S.S. Tezcan, “Ground Motion Amplification Studies, Bursa, Türkiye,” M.I.T., Department of Civil Engineering Report, 1974.

[11] Tezcan, S.S., Seed, H. B., Whitman, R.V., Serff, N., Christian, J.T., Durgunoğlu, H.T., and M. K., Yegian, “ Resonant Period Effects in the Gediz, Turkey Earthquake of 1970,” Earthquake Engineering and Structural Dynamics, Journal of the International Association for Earthquake Engineering, Vol. 5, No. 2, April-June 1977.

[12] Tezcan, S. S., Yerlici, V., and H. T. Durgunoğlu, “A Reconnaissance Report for the Romanian Earthquake of March 4, 1977.” Earthquake Engineering and Structural Dynamics, Vol. 6, pp. 379-421, 1978.

[13] Cassaro, M. A., and E. M. Romeo (editors), The Mexico Earthquakes-1985, ASCE, New York, 1987.

[14] Duke, C. M., “Techniques for Field Measurement of Shear Wave Velocity in Soils”, Proceedings of the Fifth WCEE, 1969, Santiago, Chile.

[15] Wakabayashi, M., Design of Earthquake Resistant Buildings, McGraw-Hill, New York, 1986.

[16] Fujiwara, T., “Estimation of Ground Movements in Actual Destructive Earthquakes,” Proceedings of the Fourth European Symposium on Earthquake Engineering, London, September 5-7, 1972.

[17] Dowrick, D. J., Earthquake Resistant Design, John Wiley & Sons, 1977

[18] Hardin, B. O., and V. P. Drnevich, “Shear Modulus and Damping in Soils: Design Equations and Curves,” Journal of Soil Mechanics and Foundations Division, ASCE, Vol. 98, SM 7, July, 1972

[19] Pitilakis, K. D., Anastasiadis, A. J., and F. Veldemiri, “Natural Soil Deposits: Design G-D Curves and their Seismic Response,” Earthquake Geotechnical Engineering, pp. 447-453, Balkema Rotterdam, 1995

[20] Seed, H. B., and I. M. Idriss, “ Soil Moduli and Damping Factors for Dynamic Response Analyses, ” Earthquake Engineering Research Center, Report No. EERC 70-10, University of California, Berkeley, USA, 1970.

[21] Yüzer, E., Öztaş, T., Vardar, M., and Eyidoğan, H., “Engineering Geology and Geotechnical Aspects of Avcılar, İstanbul ”,İstanbul Technical University Foundation, Vol. I. , II. , and III. , February, 1997 (in Turkish)

[22] Meremonte, M., Özel, O., Cranswick, E., Erdik, M., Şafak, E., Overturf, D., Frankel, A., and Holzer, T., “ Damage and Site Response in Avcılar, West of Istanbul, ” The 1999 Izmıt and Düzce Earthquakes: Preliminary Results, edited by Aykut Barka et. al., Istanbul Technical University Press, ITU Foundation 2000, Istanbul, Turkey, pp. 265-276.