روش شبه‌استاتيك براي تحليل سدهاي خاكي با در نظر گرفتن اثر پي(مطالعه موردي: سد مسجدسليمان)

Development of Pseudo-Static Method for Stability Analyses of Embankment Dams with egard to Foundation Effect

روش شبه استاتيك روشي كاربردي براي تحليل لرزه اي سدهاي خاكي است. اولين گام براي رسيدن به نتايج دلخواه با روش شبه استاتيكي، انتخاب ضريب لرزه اي مناسب است. نظرات زيادي بدون توجه به مشخصات هندسي، مشخصات ديناميكي سازه و همچنين آثار ساختگاه، براي تخمين ضريب شتاب لرزه اي ارايه شده است. در اين پژوهش، سد مسجدسليمان براي مطالعه ي موردي انتخاب شده و با بهره گيري از نتايج كاوش استاتيكي، شبه استاتيكي و ديناميكي با روش اجزا محدود، با مجموعه نرم‌افزاري Geosudio-Geoslop 2007، اثر پي و اثر زلزله هاي حوزه ي دور و نزديك بر ضريب لرز ه اي شبه استاتيك، بررسي و روشي نوين براي به‌دست آوردن ضريب شبه استاتيك پيشنهاد شده است. با توجه به اين كه در روش جديد نياز به استخراج دو شاخص ? و B است، در اين پژوهش، در حالت هاي گوناگون مشخصات پي (سختي، نسبت ميرايي و ابعاد هندسي) اين دو شاخص استخراج شده است. نتايج، نشان دهنده ي تاثير غير قابل انكار شرايط پي در روش پيشنهادي است؛ البته تاثير شاخص B محسوس بوده اما شاخص ?، روند نسبتاً ثابتي داشته است.

Abstract: Pseudo-static method is one of the oldest and simplest techniques for seismic stability analysis of embankment dams. Selection of appropriate seismic coefficients is the foremost part of analysis in Pseudo-static method. Previous researchers and design manuals often suggest constant values for selecting the seismic coefficient, regardless of geometry and stiffness of foundation and the dynamic characteristics of the structures. In the proposed method, the seismic coefficient is a function of parameters ? and B. Parameter B is indicative of acceleration amplification in the direction of the dam height, and is related to the geometrical specifications and material properties of the dam body and foundation. Therefore, B can be obtained through dynamic analysis. In this research, in order to obtain the effect of foundation on this parameter, a geometrical model of the Masjed Soleiman Dam has been analyzed dynamically using seismographs of earthquakes that occurred in different sites. Dam consultants used this value for the MDE of the Masjed Soleiman Dam site. Parameter was obtained by assessing the way in which the maximum acceleration varied at different points on the height of the dam. Also, the effect of far and near field records of earthquake are evaluated. In this research, the safety factor in a wedge corresponding to the seismograph that causes the allowable displacement in that wedge is assumed to be equal to one. Thus, available seismographs were scaled to peak acceleration values and, using them, the wedge displacement values were calculated. The safety factor changes were then calculated using the seismograph that caused the allowable displacement in the wedge. These safety factors were assumed to be equal to one, and the dynamic safety factor for each wedge was determined by comparison of the results obtained from the original seismograph. After comparison of the dynamic and pseudo-static safety factors, the desired safety factor was determined. The distribution of the horizontal acceleration corresponding to the safety factor was compared with the linear distribution of horizontal acceleration proposed in this article and the values of B were determined. Finally, a new technique to estimate the pseudo-static seismic coefficient is presented. Different conditions for foundation are assumed and results of analyses are evaluated. Results of this research imply that geometry, stiffness and damping of foundation have sufficient effect of seismic coefficient of earth dams. By increase in the thickness of foundation of dam, (B) increased and thus the magnitude of seismic coefficient is increased.