کاهش ارتعاشات در زمین های همگن و لایه‌لایه خشک و اشباع متخلخل با سطح آب زیرزمینی متغیر با استفاده از موانع موج

در مقاله حاضر به بررسی کاهش ارتعاشات زمین تحت اثر بارهای دینامیکی با استفاده از موانع موج نرم و سخت پرداخته شده است. برای درنظرگرفتن شرایط واقعی، زمین علاوه بر حالت همگن به صورت لایه‌لایه نیز مدل شده است. یکی از مهم‌ترین مواردی که در ارتعاشات زمین باید لحاظ گردد اثر سطح آب زیرزمینی است. بدین منظور چند تراز مختلف برای سطح آب زیرزمینی در نظر گرفته شده است. برای مدل‌سازی پدیده انتشار امواج در خاک اشباع از تئوری پوروالاستودینامیک بیو (biot) و مدل‌سازی پیشرفته اجزا محدود استفاده شده است. موانع موج به دو صورت ترانشه های توخالی به‌عنوان موانع نرم و موانع بتنی به‌عنوان موانع سخت در نظر گرفته شده اند. باتوجه‌به حالت های بسیار متعدد، برای تعیین موقعیت و هندسه موانع نرم و سخت از الگوریتم بهینه‌سازی cma-es استفاده شده است. برای این منظور و به‌دست‌آوردن تابع بهینه‌سازی، مدل اجزا محدود پوروالاستودینامیک به‌عنوان تابع هدف در الگوریتم بهینه‌سازی گنجانده شده است. نتایج به‌دست‌آمده نشان می دهند که ترانشه های توخالی بیشتر از موانع بتنی میزان ارتعاشات را کاهش می دهند. علاوه ‌برآن در خاک های همگن، موانع موج بهینه بتنی در برخی حالت ها به صورت دال افقی قرار می گیرند درحالی‌که در خاک های لایه‌لایه این موانع به شکل ستونی قرار گرفته و مرز بین دولایه بالایی را قطع می کنند.

using wave barriers for vibration mitigation in dry and saturated homogenous and layered grounds with variable groundwater level table

this paper investigates the mitigation of vibrations in grounds subjected to dynamic loads using soft and hard wave barriers. in order to consider the real problems, layered grounds are also modeled in addition to homogenous grounds. one of the important factors that needs to be considered in the groud vibration analysis is the effect of the groundwater table. within this context, different levels of groundwater level are considered. due to the difference between the impedance values at the interface of the dry and saturated parts of the ground, the upcoming incident waves experience refraction phenomenon, in which part of the wave reflects back to the medium from which it is propagated, while the other part transmits to the medium on which it impinges on. the amplitude of the applied loadings is small and therefore, the assumption of linear material behavior holds on. biotchr(’39’)s poroelastodynamic theory and advanced finite element models are used for simulation of the wave propagation phenomenon in the saturated soil. soft wave barriers are considered to be as open trenches while hard barriers are filled with concrete. considering the very large number of solution space for finding the position and geometry of the soft and hard barriers, cma-es optimization algorithm is used. to find the optimization function, the poroelastodynamic finite element model is coupled to the optimization algorithm. this is performed using developed robust scripts by which the whole finite element model including the geometry, loading, boundary conditions, and assigning poroelastodynamic constitutive relation parameters are defined, at each step of optimization, without implementing graphic user interface (gui). the soil domain is considered as homogeneous and layered unbounded half spaces. to model the unbounded soil medium in finite element simulations, low reflecting boundary conditions are applied around the model. one of the important parameters that affects the properties of the wave barriers is the frequency of loading. this is related to the dimension of the wavelength generated by the dynamic loading at a specific frequency. to consider this effect, the optimizations are performed for dynamic loadings with two different frequency values of 10 and 20 hz. the obtained results indicate that open trenches are more effective than the concrete barriers. this is attributed to the very large impedance mismatch between the soil and air. the shape of optimal barriers is different in homogeneous and layered grounds and also water level table has a significant effect on the optimal barrierschr(’39’) shape. in addition, in the homogenous ground, optimal trenches sometimes take a slab-like geometry while in the layered ground, these barriers have a vertical column geometry and intersect the boundary between the two upper soil layers. all of the optimizations are performed by assigning a constraint for the maximum allowable volume to the barrier. this is performed by defining an appropriate penalty function. it is found that optimal barriers do not necessarily occupy the whole allowable barrier volume and in some cases their volume is less than the defined maximum constraint. this observation indicates that there is always no need to make the barriers as large as possible, which helps saving construction material and reducing the amount of earthwork