تعيين ضريب شبه ‌استاتيكي خاك ميخكوبي‌شده در آبرفت هاي شهر تهران براساس سطوح عملكرد لرزه‌يي

EVALUATION OF PSEUDO-STATIC COEFFICIENT FOR SOIL NAILED WALLS IN COARSE GRAINED ALLUVIUM OF TEHRAN ON THE BASIS OF SEISMIC BEHAVIOR

در اين تحقيق در راستاي اصلاح روش شبه ‌استاتيكي بر مبناي عملكرد لرزه‌يي سيستم خاك ميخكوبي‌شده، سعي شده است تا ضريب شبه ‌استاتيكي به‌منزله‌ي تابعي از پارامترهاي اصلي زلزله نظير دوره‌ي تناوب غالب و ويژگي‌هاي هندسي نظير ارتفاع سازه معرفي شود. به‌منظور انتخاب ضريب شبه ‌استاتيكي متناسب با شرايط لرزه ‌يي و هندسي مشخص، از برابر قراردادن عملكرد لرزه‌يي سيستم ميخكوبي در قالب تغييرمكان‌هاي افقي رويه در دو حالت ديناميكي و شبه استاتيكي استفاده شده است و در انتها ضريب شبه استاتيكي به‌منزله‌ي تابعي از دوره‌ي تناوب غالب، ارتفاع سازه، و سطح عملكرد معرفي مي ‌شود. نتايج حاصله بيان‌گر تاثير چشم گير ارتفاع سازه و دوره‌ي تناوب غالب زلزله بر ضريب شبه استاتيكي است، به‌گونه ‌يي كه افزايش ارتفاع با كاهش ضريب شبه استاتيكي همراه بوده است و بيشترين ضريب شبه استاتيكي و عمده‌ي تغييرات آن در محدوده‌ي دوره‌ي تناوب طبيعي سيستم حاصل مي شود.

Soil nailing is an in situ soil reinforcement technique used for enhancing the stability of slopes, retaining walls and excavations. The technique involves installation of closely spaced, relatively slender structural elements, i.e., soil nails, into the ground to stabilize the soil mass. Flexibility, ease of construction, and economical justification of soil nailing, with respect to other stabilizing methods, has gained much attention during the last three decades. Moreover, the increase in application of this method in seismic prone areas for permanent stabilization has established a need to investigate the seismic behavior of soil nailed walls. While time history dynamic analysis consumes lots of time and money, the pseudo-static method achieved a high and specific rank, despite its inherent lack of accuracy. The independency of the pseudo-static method from factors affecting the seismic behavior of structures, and the dependency of the seismic behavior of geotechnical structures, specifically soil nailed walls, to the primary parameters of earthquakes, geotechnical parameters and geometrical configurations are the main deficiencies of this method. In this study, in order to modify the pseudo-static method, on the basis of seismic behavior of soil nailed walls, it has been endeavored to define the pseudo-static coefficient as a function of main parameters of earthquakes, soil mass, and geometry of the structure. To do so, among the important factors of earthquakes (maximum acceleration (amax), predominant period (Tp), effective time (t), magnitude (M)), the predominant period, and, among geotechnical and geometrical characteristics, (height of structure (H), length of nails (L), horizontal and vertical distance between nails (SH & SV), geotechnical properties of soil (FS), angle of slope (?)), the height of the structure, have been picked up as primarily parameters. To choose appropriate coefficient for specific earthquake records, and geotechnical and geometrical characteristics, the behavior of soil nailed walls in two modes of dynamic and pseudo-static seismic behavior have been synchronized for wall horizontal displacements. Afterwards, the pseudo-static coefficient is defined as a function of predominant period and height of structure. Obtained results demonstrate the great impact of the height of structures and the predominant earthquake period on the pseudo-static seismic coefficient. They include the fact that an increase in height will bring a decrease in the pseudo-static coefficient, and the highest of coefficients and its variation yields, by the natural period of the structure. This highlights the importance of investigating the seismic behavior of soil nailed walls for their natural periods.