مدلسازي عددي سه بعدي پاسخ لرزه‌اي پي – شمع در ماسه اشباع به منظور بررسي اثرات ساختگاهي

Three-dimensional modeling for seismic response of pile-raft in saturated sand to investigate local site effect

يكي از مهم ترين مباحث ژئوتكنيك لرزه اي مسئله اثرات محلي ساختگاه است كه در مناطق پوشيده از خاك هاي مستعد روانگرايي از اهميت ويژه اي برخوردار است. از راه هاي كاهش خرابي سازه ها در اين مناطق، استفاده از پي هاي عميق متكي بر لايه محكم مي باشد. در چنين مواردي به علت كمبود ظرفيت باربري سطحي از سيستم پي -شمع استفاده مي شود. بزرگ نمايي امواج براي سازه متكي بر پي -شمع به علت اندركنش هاي موجود بين اين سيستم و خاك شرايط پيچيده اي داشته و وجود اين سيستم مي تواند بر مشخصات دامنه و محتواي فركانسي موج تاثير گذارد. چنين موضوعي در طراحي معمول سازه هاي متكي بر پي شمع لحاظ نمي شود. در اين تحقيق با استفاده از تحليل سه بعدي همبسته ديناميكي در حوزه زمان، ميزان تاثير حضور پي ndash; شمع بر بزرگ نمايي امواج بررسي شده و نتايج حاصل با نتايج شرايط ميدان آزاد مقايسه شده است. بر اساس نتايج كلي حاصل از اين تحقيق وجود ماسه اشباع و شرايط روانگرايي سبب كاهش ضريب بزرگ نمايي در شرايط ميدان آزاد و همچنين ساختگاه با فونداسيون پي شمع شده است. اما ميزان كاهش ضريب بزرگ نمايي در فونداسيون پي شمع در مقايسه با شرايط ميدان آزاد كمتر است. همچنين افزايش سختي سيستم پي شمع متكي بر لايه محكم و قوي تر شدن حركت ورودي (افزايش دامنه و يا پريود حركت ورودي) سبب افزايش ضريب بزرگ نمايي سيستم پي شمع شده است.

Saturated loose soils have constituted superficial layers of the ground in vast regions of the country. For instance, geotechnical site investigations have revealed that shoreline of the Mazandaran Sea involves thick layers of uniform sand mixtures. Presence of such soil deposits in the northern and southern Iran, which are prone to seismic activity, may produce severe damages due to liquefaction occurrence. To prevent earthquake damages to the structures relied on liquefiable soils two strategies might be preferred: (1) improvement of liquefiable soil and ceasing liquefaction, and (2) bypassing the liquefiable layer via deep foundations. The latter strategy aims to transfer the superstructure load to the underlying stiff layer by end-bearing piles while raft foundation is also required because the superficial liquefiable soil may be unable to provide sufficient bearing capacity due to seismic pore pressure generation. In pile-raft systems passing through the liquefiable layer it seems that the liquefiable layer has less influence to the response of the system. However, several interactions in the environment such as pile-liquefiable soil, pile-pile, pile-raft, and raft-liquefiable soil could result in a sophisticated problem; affecting the amplification of the upward propagating seismic waves. Amplification of seismic wave denotes variations of amplitude and frequency content of upward propagating wave passing through the reinforced liquefiable soil layer. It is expected that the pile-raft system in conjunction with the liquefiable layer considerably change seismic response of the ground compared with the free-field liquefiable ground in the absence of pile-raft system. In the design of routine projects for which the national seismic building code is employed, there is no clear recommendation to account for the influence of pile-raft on the site amplification factors. The currently used building codes have poorly addressed the problem; and thus, considerable researches might be required. The aim of this paper is to study the characterization of seismic wave amplification by considering the presence of piled raft. To achieve this goal three-dimensional numerical modeling of piled raft and free-field in both liquefied and dry sand deposit is used. Results of some centrifuge experiments of a piled raft structure on liquefied sand are used to evaluate the predictive capabilities of the numerical model constructed in OpenSees, as a state-of-the-art numerical tool. Fully-coupled solid-fluid 3D nonlinear numerical simulations were performed in OpenSees, in combination with the pressure-dependent-multiyield soil constitutive model that enables dynamic effective-stress modeling of soil liquefaction in addition to embedded pile and superstructure elements. The numerical simulation results demonstrated reduction of seismic wave amplification in liquefied sand versus dry sand due to reduction of soil strength and increase damping. In both liquefied and dry state, the presence of piled-raft increases the soil stiffness and seismic wave amplification. The level of site amplification depends on many factors such as lateral stiffness of the pile-raft system and characteristics of input motion. Parametric study was then carried out to address these factors. Results of this study indicate that amplification factor decreases due to the presence of liquefied soils. However, the decrease of amplification factor at the free-field is larger than pile-raft foundation. Furthermore, amplification factor increases due to increase of pile stiffness, amplitude and period of input motion. Therefore, the site-specific analysis might be necessary to account for the presence of piled-raft system in the sites involving thick sand layers.