تحليل لرزه‌اي غيرخطي گروه شمع واقع در خاك‌هاي لايه‌اي ناشي از اثرات اندركنش كينماتيكي

Nonlinear Seismic Analysis of Pile Groups in Layered Soils due to Kinematic Interaction Effects

با توجه به‌ كاربرد وسيع پي‌هاي شمعي، ايمني شمع‌ها در زمان زلزله از اهميت ويژه‌اي برخوردار است. تحقيقات نشان داده است كه علاوه بر نيروي ناشي از روسازه، اثرات اندركنش كينماتيكي خاك- شمع باعث اعمال تقاضاي لرزه‌اي قابل توجهي به شمع‌ها در محيط‌هاي با خاك لايه-اي مي‌شود. در ‌اين مقاله از نرم‌افزار اجزا‌ محدود سه‌بعدي ABAQUS براي تحليل لرزه‌اي اندركنش كينماتيكي گروه شمع‌ در خاك‌هاي لايه‌اي داراي رفتار غيرخطي استفاده شده است. پس از ارزيابي صحت نتايج روش تحليل عددي تحت بارگذاري‌هاي استاتيكي و ديناميكي، اثرات اندركنش كينماتيكي روي رفتار مكانيكي گروه شمع‌ واقع در خاك دو لايه‌اي (لايه‌ي نرم بر روي لايه‌ي سخت) بحث و بررسي مي‌گردد. مطالعات پارامتري شامل عوامل موثر بر پاسخ سيستم خاك- شمع نشان مي‌دهد كه نسبت فاصله به قطر شمع،‌ تعداد شمع‌ها درگروه، قطر شمع، نحوه‌ي اتصال شمع به كلاهك و تغيير مشخصات لايه‌هاي خاك، تاثير قابل‌ملاحظه‌اي در رفتار گروه شمع در برابر زلزله دارد.

During earthquakes, piles undergo stresses due both motion of the superstructure (i.e. inertial interaction) and that of the surrounding soil (i.e. kinematic interaction). In practice, structural engineers commonly take into account stresses induced by the inertial interaction, which is responsible for pile head failure, but they neglect the effects of the kinematic interaction that is responsible for failures along pile’s length in the case of layered soils with highly contrasting mechanical characteristics even in the absence of the superstructure. Thus, the evaluation of kinematic forces developing in piles during earthquakes has been receiving increased interest from the researchers. Numerical methods for the analysis of kinematic soil-pile interaction can be classified into two groups; continuum-based approaches and Winkler methods [1-3]. It has been customary in professional engineering and research practices to assume a linear behaviour for the soil and the pile foundation. However, under strong excitation, the nonlinear behaviour of soil media at the soil-pile interface has a strong influence on the response of the pile foundation. The aim of this study is to investigate the influence of soil nonlinearities on the kinematic interaction forces of pile groups embedded in layered soil deposits during seismic actions. Figure (1) shows the assumed soil-pile group case with 5 by 5 piles embedded in two layer subsoil profile. The pile has been considered as an elastic beam, while the soils have been modelled using the elastic-plastic solid element. The corresponding 3D finite element mesh has been shown in Figure (2). Based on the symmetry, only half of the model is meshed. Dynamic numerical analysis has been performed using the FE program ABAQUS [4]. Necessary parameters to simulate the examined cases are listed in Table (1). It is worth noticing that the comparison with available experimental and theoretical results in the literature has been made to validate the numerical model. The maximum displacements and the envelopes of kinematic bending moments and axial forces along the piles depth have been reported in Figure (3) due to the 1978 Tabas, Iran earthquake ground motion at the Dayhook station. As shown, it can be observed that the kinematic force distributions present relative maximum values very close to the layer interface. On the other hand, the diagram of the maximum displacements is characterized by a shape very similar to the first vibration mode of the soil deposit with maximum value at the piles head and almost zero value near the bedrock. In addition, the effects of kinematic group interaction lead to a decrease of bending moments at the pile head and also the layer interface as compared to the results from the single pile. Finally, the influence of main parameters governing the seismic response of piles like the space-diameter ratio, number of piles in the group, pile dimater, pile-to-cap fixity condition and the variation of soil layers properties are discussed.