تاثير مشخصات بار لرزه‌اي بر نشست دايمي پي‌هاي سطحي مستقر بر ماسه مستعد روانگرايي

Role of Seismic Load Characteristics on Permanent Settlement of Shallow Foundations on Liquefiable Sand

وقوع پديده روانگرايي در زلزله‌هاي مخرب گذشته، باعث بروز خسارت‌هايي به سازه‌ها شده است. در اين ميان، نشست‌هاي ماندگار سازه‌هاي مستقر بر پي‌هاي سطحي كه بر خاك‌هاي روانگرا بنا شده بودند، خسارت‌هاي چشمگيري را به همراه داشته است. تحقيقات زيادي به‌منظور ارزيابي مقادير نشست پي‌هاي سطحي بر خاك‌هاي روانگرا، بررسي پارامترهاي تاثيرگذار و ارايه راهكارهايي جهت مقابله با آن انجام شده است. يكي از تاثيرگذارترين پارامترها، مشخصات زلزله ورودي مي‌باشد كه كمتر مورد بررسي قرار گرفته و معمولاً اثر زلزله ورودي را با شتاب بيشينه آن لحاظ مي‌كنند. در اين تحقيق، به بررسي اثر مشخصات زلزله ورودي با استفاده از مدل‌سازي عددي پرداخته شده است. براي اين منظور، ابتدا مدل عددي ساخته‌شده با نرم‌افزار تفاضل محدود FLAC، با نتايج يك آزمايش سانتريفيوژ، صحت‌سنجي شده و بعد از آن، نشست سازه مستقر بر خاك روانگرا تحت بارگذاري‌هاي مختلف بررسي مي‌شود. نتايج اين تحقيق نشان مي‌دهند كه پارامتر شتاب بيشينه زلزله، به‌تنهايي نمي‌تواند اثر زلزله ورودي را در نظر بگيرد و تاثير پارامترهاي نشان‌دهنده محتواي فركانسي زلزله مانند پريود ميانگين، قابل‌توجه است. در مقابل، پارامترهايي كه بر حسب سرعت حركت زمين هستند، نظير سرعت بيشينه زلزله ورودي، به‌تنهايي همبستگي بسيار خوبي با نشست پي‌هاي سطحي مستقر بر خاك مستعد روانگرايي دارند.

In this study, the role of input motion characteristics on the amount of residual settlement of shallow foundations relied on liquefiable soil is investigated. First, two-dimensional numerical model is created using finite difference software FLAC-2D, and the UBCSAND constitutive model. After verification of the numerical model with the results of a centrifuge test, behavior of foundation is investigated under different earthquake loadings. To evaluate the effect of the input motion characteristics on foundation settlement, three records from Irpinia 1980, Northridge 1994, and Landers 1992 earthquakes are used. The frequency contents of these records are considerably different. These records are scaled to five levels of PGA (including 0.05g, 0.1g, 0.2g, 0.3g, 0.45g) to be used in the analyses. The results of the analyses demonstrate that the amounts of foundation settlement for the same PGA in three records are considerably different. Therefore, the use of PGA alone might be insufficient to predict the amount of foundation settlements. This fact is shown in Figure (1a). Figure (1b) illustrates the amounts of foundation settlement against the mean period of records at different PGA levels. It is observed that the mean period of records which indicates their frequency content, has a significant influence on the values of foundation settlements. It is concluded from the numerical analyses that the seismic parameters associated with ground velocity have better correlation with settlements of foundation. For example, the amounts of foundation settlements against the peak ground velocity (PGV) of the input motion are shown in Figure (2). This result reveals a high correlation between the ground velocity and the residual settlement of foundation, irrespective of frequency content and PGA of the earthquake records. It can be concluded from the results that both PGA and mean period of input motions have significant influence on the amounts of the foundation settlement, but they cannot be a good criteria for evaluating the amounts of the foundation settlements alone. In contrast, velocity-type parameters that are called intermediate-frequency intensity measures can be appropriate for prediction of foundations settlement in liquefaction condition. These results have been achieved for conditions of the calibrated foundation with its certain inherent parameters such as natural frequency content. For other conditions such as larger foundations’ width or sand density, more detailed studies are required.