مدلسازي عددي اندركنش شمع و خاك روانگرا با استفاده از روش فنر غيرخطي

Numerical Modeling of Pile and Liquefied Soil Interaction Using Non-Linear Spring Method

كاهش مقاومت و سختي خاك در اثر روانگرايي باعث ايجاد لنگرهاي خمشي و تغييرشكل هاي جانبي زيادي در شمع هاي واقع در خاك روانگرا مي شود. در اين تحقيق براي بررسي رفتار شمع در محيط روانگرا از آناليز ديناميكي همبسته اندركنش خاك-شمع-سازه (SPSI) استفاده شده است. اندركنش شمع-خاك با استفاده از روش فنر غيرخطي ديناميكي p-y شبيه سازي شده است. تاثيرات روانگرايي با استفاده از يك ضريب كاهنده به مقاومت جانبي شمع اعمال مي گردد. رفتار خاك روانگرا با استفاده از مدل چند تسليمي وابسته به فشار مدلسازي مي شود. براي شبيه سازي اندركنش فازهاي جامد و سيال خاك اشباع بر مبناي تئوري محيط متخلخل اشباع از فرمولاسيون همبسته u-p استفاده شده است. به منظور تاييد صحت مدل عددي، نتايج مدل عددي با داده هاي دو آزمايش سانتريفيوژ مقايسه شده است. پس از صحت سنجي مدل عددي، براي بررسي تاثير پارامترهاي مختلف بر پاسخ ديناميكي شمع و فشار جانبي وارد بر شمع، آناليز پارامتريك انجام شده است. پارامترهاي مورد بررسي شامل ضخامت لايه روانگرا، فركانس تحريك ورودي، گيرداري سر شمع، سختي شمع، شتاب ماكزيمم ورودي و دانسيته نسبي خاك روانگرا مي باشند. نتايج مطالعه پارامتريك نشان داد كه فشار جانبي وارد بر شمع تقريباً در عمق لايه روانگرا شده ثايت بوده و حدود 7 الي 10 درصد فشار سربار كل در پايه لايه روانگرا مي باشد.

Decrease in the strength and stiffness of soil due to liquefaction may cause large bending moments and lateral deformations in piles located in this type of soils. For reliable design of pile foundations in the liquefaction-susceptible soils, it is necessary to have an accurate evaluation of the lateral pressure. However, the pressure is exerted on the pile if the subsurface layers experience liquefaction and lateral spreading in the course of earthquake. In this study, a coupled Soil-Pile-Structure Interaction (SPSI) analysis method is used to investigate the behavior of piles in liquefiable soils. Interaction of soil-pile is simulated by using nonlinear p-y springs. The liquefaction effects are taken into account by introducing a degradation multiplier to the lateral resistance of soil. The degraded lateral resistance of liquefied soil is considered equal to 5% of its initial value for loose sand and 10% for medium sand. Fully coupled dynamic analysis of a soil column in free-filed condition is performed in OpenSEES (Open System for Earthquake Engineering Simulation). For simulating the interaction of solid-fluid phases based on the theory of saturated porous medium, u-p formulation is used. Liquefied soil behavior is modeled using “pressure dependent multi yield material model”. From the coupled analysis, time histories of excess pore pressure ratio at different levels are obtained. The values of excess pore pressure ratio (between 0.0 to 1.0) are used to interpolate the transient lateral resistance of soil from its initial value in the static condition (excess pore pressure ratio equal to 0.0) to its final degraded value in the fully liquefied condition (excess pore pressure ratio equal to 1.0). In order to verify the numerical model, results are compared with those of two centrifuge experiments. Both experiments include two soil layers and the pile is extended into the lower layer. In the first experiment, the loose sand layer is located above the medium dense layer and in the second experiment the medium dense sand layer is located above the dense layer. After verification of the numerical model, parametric analysis is performed to study the effect of various parameters on the dynamic response of piles and applied lateral pressure from the spreading liquefied soil to pile. Investigated parameters are thickness of the liquefaction layer, frequency of the input excitation, fixity of the pile cap, pile stiffness, maximum input acceleration and the relative density of liquefiable soil. The results show that the maximum bending moment in the case of fixed head occurs at the top of pile and in the case of free head at the depth of 1 ~ 3 meters. The maximum bending moment of pile is also greater in the case of fixed head pile; however, its lateral deformation is lower. Increasing the frequency of input motion and soil relative density or decreasing the liquefied soil thickness may lead to decrease of maximum bending moment and deformation of pile. Regarding the lateral pressure exerted on the pile, the results of analysis indicate that the lateral pressure is relatively constant at the depth of liquefied layer and is equal to 7 to 10 percent of the total vertical pressure at the base of liquefied layer.