بررسي عددي اثر مولفه قايم زلزله‌هاي حوزه نزديك بر رفتار ديناميكي سدهاي خاكي: مطالعه موردي سد البرز

Numerical Evaluation of the Effect of Vertical Component of Near-Field Earthquakes on the Dynamic Behavior of Embankment Dams, A Case Study: Alborz Dam

در اين مقاله اثر مولفه قايم زلزله‌هاي حوزه نزديك بر رفتار ديناميكي سدهاي خاكي مطالعه و ارزيابي مي‌شود. بيشتر پژوهش‌هاي انجام شده در رابطه با تاثير زلزله‌هاي حوزه نزديك بر رفتار ديناميكي سدهاي خاكي، شامل اثر مولفه افقي آن‌ها است. يكي از ويژگي‌هاي مهم زلزله‌هاي حوزه نزديك، دارا بودن مولفه قايم قابل توجه با محتواي فركانسي بالاست كه در بسياري از موارد از مولفه افقي همان زلزله به مقدار قابل توجهي تجاوز مي‌كند. تاكنون مطالعات كمي در اين زمينه صورت گرفته است. براي بررسي اثر مولفه قايم زلزله‌هاي حوزه نزديك بر رفتار ديناميكي سدهاي خاكي، مدلي دو بعدي از سد البرز به روش تفاضل محدود و با كمك نرم‌افزار FLAC 2D در نظر گرفته شد. در اين مطالعه شتاب نگاشت‌هاي حوزه نزديك و دور از يك زلزله انتخاب شد تا امكان مقايسه بهتر و دقيق‌تر فراهم شود. نتايج تحليل‌ها بيانگر اثر قابل توجه مولفه قايم زلزله‌هاي حوزه نزديك بر رفتار ديناميكي سدهاي خاكي شامل افزايش نشست تاج سد تا حدود 45 درصد، افزايش تغيير شكل هاي افقي محور سد، كاهش ضريب بزرگ‌نمايي تاج سد و همچنين لزوم توجه به اثر مولفه قايم زلزله، به ويژه در نواحي نزديك به گسل كه احتمال وقوع زلزله‌هاي حوزه نزديك وجود دارد، است، كه اين مسيله بايد در جانمايي سدهاي خاكي به لحاظ توان لرزه‌اي و فاصله از گسل‌هاي موجود در منطقه و همچنين در طراحي آنها در نظر گرفته شود.

Most of the research has been concentrated on the effects of the horizontal components of near-field earthquakes on the dynamic behavior of the embankment dams. In this paper, the effects of the vertical components will be considered. One of the important characteristics of the near-field ground motions, is the noticeable vertical component with the high frequency content that can exceed considerably, in many cases, the horizontal component of the same earthquake. So far, few studies have been done in this area. In order to investigate the effect of the vertical ground motion on the dynamic behavior of embankment dams, a two dimensional numerical model of the Alborz dam is analyzed by using finite difference method which is used in FLAC2D code. It should be noted that the Alborz dam is a rockfill type with clay core and a maximum height of 78 m located on the Babol River in the north of Iran. The Mohr Coulomb elastic perfectly plastic constitutive model was used to simulate the stress-strain behavior of the dam body and its foundation during the static and dynamic loading. Steps of modeling are as follow: At first stage, construction was carried out in 16 layers. At this step, coupling analysis were done in order to simulate the consolidation and build up of pore pressure in clayey core, with respect to the real time of construction for each layer. Then the analyses were continued to modeling of the impounding. So at this stage the reservoir was raised to the normal water level and the model were analyzed to the steady state seepage condition. Records of near-field and far-field were selected from the same earthquake to provide better and more accurate comparison. Before applying the earthquake records to the base of the foundation in the model, they must be modified. So deconvolution analyses were done by using SHAKE2000 code in order to get the target motion with peak ground acceleration of 0.52g at the surface of the foundation (maximum credible earthquake level at the site of Alborz dam). In addition filtering process, baseline correction and conversion the acceleration time history to the stress time history were done. Results of analysis show that the vertical component of near-field ground motion has considerable effect on the magnitude of strains and deformations including: increasing the settlement of the dam crest to about 45 percent, increasing the deformation of the horizontal axis of the dam, reduction of the magnification factor of the dam crest and especially in the case of near-fault, which the occurrence of near-field earthquakes is more probable. Therefore, this issue should be considered in locating the embankment dams regarding the seismic potential and the distance from the fault, and in the design of them