بررسي تاثير مشخصات هندسي لايه روانگرا در رفتار پي هاي شمعي

Investigation of effect of the liquefiable layer geometry in behavior of pile Foundations

جابه جايي هاي ناشي از روانگرايي باعث خسارت هاي عمده اي به زيرساخت ها، مانند ساختمان ها، پل ها، لوله هاي مدفون و ... مي‌شود. وقتي سطح زمين داراي شيب باشد امكان وقوع پديده گسترش جانبي در خاك هاي روانگرا وجود دارد كه باعث جابه جايي هايي تا حد چند متر نيز مي شود. با توجه به كاربرد وسيع پي هاي شمعي، ايمني شمع ها در زمان زلزله از اهميت ويژه اي برخوردار است. مطالعات پس از وقوع زلزله در خاك روانگرا نشان داده است كه هم نيروي ناشي از سازه و هم اندركنش سينماتيكي بين شمع و خاك نقش مهمي در رفتار مكانيكي شمع ها دارد، در نتيجه بايد تحليل براساس اندركنش شمع – خاك – سازه صورت گيرد. در اين پژوهش از روش تفاضل محدود (FDM)، براي بررسي تاثير ضخامت لايه روانگرا، شيب لايه روانگرا و تراز آب زيرزميني بر پي هاي شمعي استفاده شده است. نتيجه آن است كه با افزايش شيب لايه روانگرا، لنگر خمشي ماكزيمم، افزايش مي يابد ولي شيب اين نمودارها در ترازهاي آب پايين (نزديك به سطح زمين) بيشتر است. اين نوع رفتار در نيروي برشي ايجاد شده در شمع-ها نيز مشاهده شده است.

For saturated sand deposit subjected to earthquake excitations, if the pore pressure were instantly generated and equal to the effective stress of soil, the liquefaction would occur. Liquefied soils can cause considerable loss of the ground strengths to yield large settlement via to result in lateral spreading. For superstructures supported on pile foundations at that site, these effects are significantly important. Pile foundations are primarily designed to transfer vertical loads from the superstructure to the bearing stratum. For this reason, piles are relatively vulnerable to lateral loads such as those imposed by ground shaking during strong earthquakes It is known from previous earthquakes that liquefaction can cause very large loads on pile foundations, both from inertial loads from the superstructure and from lateral displacements of liquefied soil. Extensive damage to pile-supported bridges and structures in areas of liquefaction and lateral spreading has been observed in many earthquakes around the world. Predicting the behavior of a pile foundation in liquefying ground during an earthquake requires consideration of design motions, free-field site response, superstructure response, and soil-pile-superstructure interaction. Evaluating pile performance requires consideration of the inertial and kinematic loads imposed on the piles and their pile-cap connections, transient or permanent deformations of the pile foundation, the influence of the pile foundation on the dynamic response of the superstructure, and the performance criteria for the pile foundation. The mechanisms involved in soil-pile structure interaction in liquefying soil are not yet well understood although research in recent years has begun to clarify certain aspects of behavior. In particular, when the ground surrounding a Structure liquefies due to seismic excitation, the behaviour of the piles becomes more complicated. Considering the widespread use of pile foundations, their safety in the occurrence of earthquake has a special importance. Many structures were damaged during Earthquake. It was found from the field investigations after the earthquake that not only the pile heads but also the lower parts of the piles had cracked or failed. These results have shown that both the force due to structure and the Kinematics interaction between the pile foundations and the soil play an important role in mechanical behavior of piles. Since the effect of the superstructure on the pile-soil interaction analysis is significant; the analysis should be done based on the interaction axis of pile-soil-structure. Liquefaction may cause damage to the pile foundation due to the lateral flow of liquefied soil and/or pile failure may occur at the boundary between the two different soil layers of which one liquefies while the other does not. A series of numerical simulations of the seismic behaviour of a pile foundation constructed in a two-layer ground is carried out. In this study, finite difference method (FDM) has been used to investigate the effect of the thickness of liquefied layer, slope of liquefied layers and the underground water level on behavior of pile foundations. Results indicate that with an increase in the slope of liquefied layers, the maximum bending moment raises but the slope of this graph for low underground water level (near the surface) is higher. This type of behavior also is observed in the shear force created in the pile foundation.