ارزيابي مبتني بر عملكرد رفتار سدهاي خاكي تحت بارگذاري زلزله

Based Performance Evaluation of Earth Dams under Seismic

در اين پژوهش، رفتار يك سد خاكي نوعي در دو سطح عملكردي، موسوم به سطح عمكرد مبنا و مطلوب ، با انجام تحليلهاي ديناميكي غيرخطي ارزيابي شده كه، در هر سطح عملكرد، سطوح لرزهاي معين و سطوح تخريب مجاز متناظر تعريفشدهاست. همچنين، رفتار سد براي عواملي مانند شتابنگاشت ورودي، مدل رفتاري و عدد نفوذ استاندارد پوسته حساسيت- بررسيشدهاست. نتايج نشان ميدهد كه الزامات (SRF سنجي و پايداري سد با انجام تحليلهاي كاهش پارامترهاي مقاومتي (تحليل عملكرد مبنا براي سد مطالعه شده با توجه به ويژگيهاي هندسي و ژيوتكنيكي آن قابل تامين است. اما تامين عملكرد مطلوب براي اين سد، مستلزم ايجاد تغييرات در ساختار سد و يا بهسازي لرزهاي آن است. وجود ضرايب اطمينان پايين در سطوح لرزهاي مطالعه شده، اين مطلب را تاييد مي كند. تحليلهاي حساسيتسنجي به شتابنگاشتهاي فريولي و ساكاريا، باوجود تفاوت در پاسخ طيفي و چگالي انرژي آنها، نتايج يكساني را ارايه مينمايد. مدل فين بسته به تراز شتاب بيشينه اعمالي، فشار آب حفره اي اضافي ايجاد شده در بالادست سد را 20 درصد بيشتر از مدل رفتاري مور- كولمب پيشبيني مينمايد. با تغيير عدد نفوذ استاندارد پوسته و تحليل سد در شتابهاي بيشينه مختلف مشاهده مي شود كه مدل فين در اعداد نفوذ استاندارد اصلاح شده كمتر از 35 و در 0/7 ، وقوع پديده روانگرايي را در خاكريز بالادست سد پيشبيني مينمايد.

Abstract: Dams shall safely retain the reservoir and any stored solids, and pass environmentally acceptable flows, as required for all loading conditions, ranging from normal to extreme loads, commensurate with the consequences of failure. The new trend for performance-based design is to consider 2 levels of seismic actions and analyze the situation where the limit of force balance is exceeded for high intensity ground motions, associated with a very rare seismic event. For the design, two basic requirements are defined: (i) Non-collapse requirement (ultimate limit states), i.e. after the occurrence of the seismic event, the structure shall retain its structural integrity, with respect to both vertical and horizontal loads, and adequate residual resistance, although in some parts considerable damage may occur, (ii) Minimization of damage (serviceability limit state) , i.e. after seismic actions with high probability of occurrence, during the design life of the structure, some parts can undergo minor damage without the need of immediate repair. This study evaluates the behavior of a typical earth dam by nonlinear seismic analyses, in two performance levels, named “Base Performance Level” and “Desired Performance Level.” The level of seismic action and related acceptance level of damage are defined for each performance level. In “Base Performance Level,” with seismic levels of OBE (0.3g) and MDE (0.5g), the structure shall be serviceable and repairable and in “Desired Performance Level”, with seismic levels of MDE (0.5g) and MCE (0.7g), the structure shall be serviceable and repairable, respectively. Also, the stability of dam has been assessed by the “Strength Reduction Analysis.” The analyses are nonlinear and the constitutive law of the materials was assumed to follow Finn and Mohr-Coulomb models, incorporated into “FLAC 2D” finite difference analysis program. The factors such as initial shear modulus, variation of shear modulus versus shear strain, generation and dissipation of pore pressure and hysteretic damping are considered in this study. In addition, using the scaling method of applying maximum acceleration, the response of dam is investigated in different maximum accelerations. The results show that the dam needs to be changed in geometrical specifications or seismically improved in “Desired Performance Level”, in contrast with “Base Performance Level.” Results are confirmed by low amount of safety factors of stability in dam, which are calculated for different seismic loads. Also, the behavior of dam is examined by sensitivity analysis for type of accelerograms, constitutive model and the standard penetration number in shell of dam. Two accelerograms, including “Friulli” and “Sakaria” are considered. Maximum acceleration and duration of both of them are equalized and frequencies more than 5Hz are filtered. Sensitivity analyses of “Friulli” and “Sakaria” accelerograms, despite the difference in response spectra and specific energy density, show approximately similar results. “Finn” model predicts the amount of excess pore water pressure to be more than Mohr- Coulomb up to %20, and shows the occurring of liquefaction in SPT more than 35 and acceleration more than 0.7g, in shell of upstream of dam.