مدل سازي عددي رفتار لرزه اي شمع ها در خاك هاي مستعد روانگرايي

Numerical Modeling of Seismic Behavior of Piles

در جريان زلزله‌هاي گذشته روانگرايي خاك و تغييرشكل‌هاي ناشي از آن، آسيب‌هاي گسترده‌اي را به پي‌هاي شمعي وارد آورده است. اين آسيب‌ها بيشتر در مناطق ساحلي و يا زمين‌هاي شيبدار مشاهده شده است. بررسي مسيله روانگرايي براي گسترش بنادر شمالي و جنوبي و اجراي سازه‌هاي ساحلي و فراساحلي در كشور ايران به دليل قرارگرفتن در ناحيه‌اي با خطر لرزه خيزي بالا و وجود خاك هاي مستعد روانگرايي در مناطق ساحلي از اهميت ويژه‌ايي برخوردار است. اگرچه در سال‌هاي اخير مطالعات زيادي به منظور شناخت جنبه‌هاي مختلف اين پديده صورت گفته است، اما هنوز عدم بسياري از قطعيت‌ها درباره تغييرشكل‌هاي جانبي خاك و آثار آن بر پي‌هاي عميق وجود دارد. در اين پژوهش سعي شده با استفاده از تحليل سه‌بعدي همبسته ديناميكي در حوزه زمان و به كارگيري مدل چندصفحه‌اي دراگر- پراگر براي خاك، رفتار گروه شمع‌هاي مختلف بررسي مي شود. بدين منظور با استفاده از نرم‌افزار Opensees SP رفتار شمع تكي و گروه شمع (1×2، 1×3، 2×2 و 3×3) در پروفيل خاك دو لايه تحت نيروهاي زلزله و فشار جانبي ناشي از گسترش جانبي خاك، تغييرات فشار آب حفره اي و فشار اندركنشي ميان شمع و خاك بررسي و تحليل مي‌شود. نتايج نشان مي دهد كه بيشتر عوامل تاثيرگذار بر رفتار شمع و خاك در آيين‌نامه‌هاي طراحي موجود مانند آيين نامه راه ژاپن (Japanese Road Association) لحاظ نشده است و اين موارد لزوم بازنگري روش‌هاي تحليل و طراحي كنوني را آشكار مي‌سازد. همچنين درگروه شمع‌ نيروي جانبي وارد به هر شمع به موقعيت شمع در گروه بستگي دارد كه اين رفتار ناشي از آثار سايه و همسايگي (اندركنش) در گروه شمع است

The lateral spreading of mildly sloping ground and the liquefaction induced by earthquakes can cause major destruction to foundations and buildings,mainly as a result of excess pore water pressure generation and softening of the subsoil. During many large earthquakes, soil liquefaction results in ground failures in the form of sand boils,differential settlements, flow slides, lateral spreadingand loss of bearing capacity beneath buildings. Such ground failures have inflicted much damage to the built environment and caused significant loss of life. The risk of liquefaction and associated ground deformation can be reduced by various ground improvement methods, including densification, solidification (e.g., cementation), vibro-compaction, drainage, explosive compaction, deep soil mixing, deep dynamic compaction, permeation grouting, jet grouting,pile group and gravel drains or SCs.Nowadays, using pile foundation is one of the popular solution for soils vulnerable to liquefaction. The pile with enough length more than liquefiable soil depth can reduce the large deformation and unacceptable settlements. Liquefaction and lateral deformation of the soil has caused extensive damage to pile foundations during past earthquakes. Several examples of significant damages in pile foundation have been reported in the literature from the 1964 Niigata, 1983Nihonkai-Chubu,1989 Manjil and 1995 Kobe earthquakes. These damages have been observed mainly in coastal areas or sloping ground. Evaluation of liquefaction in order to develop the northern and southern ports and implement coastal and offshore structures in Iran is of particular importance due to locating in a high seismic hazard zone and Liquefactable soil in coastal areas. Although, in recent years, many studies have been conducted to understand the various aspects of this phenomenon, yet a lot of uncertainties have remained about the lateral deformations of the soil and its effects on deep foundations. In this study, the behavior of pile groups (2 × 1, 1 × 3, 2 × 2 and 3 × 3) was evaluated using fully coupled three-dimensional dynamic analysis. Therefore, the influence of effective parameters such as number of piles, ground slope angle of soil and pile behavior has been studied using the finite element software Opensees SP v2.4. Results indicate that most of the factors affecting the behavior of the pile, soil is not considered in the current design codes (such as JRA 2002) and these issues indicate the need to revise the current design and analysis methods. Lateral Pressures compared to that of JRA regulations show that these regulations cannot exactly predict pressures on pile and pile groups. Altogether comparing the results of numerical models of this research to various laboratory observations indicate that the use of numerical method can be reliable to predict the behavior of the soil and pile qualitatively and quantitatively using the appropriate constitutive model and parameters for soil and pile