تاثیر استفاده از شمع‌های مسلح کننده در کاهش حرکت لرزه‌ای سطح زمین

یکی از روش‌های افزایش سختی و بهسازی خاک به ویژه در زمین‌های سست، استفاده از شمع‌های مسلح کننده (تقویت کننده) می‌‌باشد. از این نوع شمع‌ها می‌توان در محل پایه و زیر سازه ها به منظور کاهش پاسخ لرزه‌ای زمین و سازه استفاده نمود. در این مقاله به بررسی تاثیر تغییرات پارامترهای هندسی شمع‌های تقویت کننده نظیر قطر، طول، فاصله بین آنها و سرباره وارده بر پاسخ لرزه‌ای سطح زمین بر مبنای مدل پایه پل ازمیت ترکیه به‌عنوان مطالعه موردی، پرداخته شده است. عمق تاثیر با مقایسه طیف پاسخ شتاب سطح زمین مدل دو‌بعدی با حضور شمع‌های مسلح کننده با کمک نرم افزار flac2d به روش غیرخطی مدل هسترزیس، با عمق معادل طیف پاسخ شتاب مدل یک بعدی میدان آزاد به‌دست آمده است. نتایج به‌دست آمده نشان می‌دهد که با افزایش نسبت فاصله به قطر شمع‌های تقویت کننده (s/d) میزان عمق تاثیر به علت تقلیل سختی سیستم پیشمع مسلح کننده کاهش می‌یابد و پس از رسیدن به نسبت 5 به مقدار ثابتی رسیده است. به عبارت دیگر با افزایش سختی سیستم خاکشمع، اندرکنش سیستماتیکی سیستم خاکشمع افزایش می‌یابد. همچنین با افزایش نسبت طول به قطر شمع های مسلح کننده (l/d)، میزان عمق تاثیر ابتدا افزایش یافته وسپس به مقدار ثابتی خواهد رسید که بهینه‌ترین بازه برای نسبت طول به قطر شمع‌ها در محدوده 15 تا 30 می باشد. علاوه بر این، با افزایش میزان نسبت سربار وارده در بالای شمع های مسلح کننده (q ̅)، میزان عمق تاثیر به‌صورت خطی افزایش می‌یابد.

Effect of Inclusion Piles on Mitigation of Seismic Surface Ground Motion

The need to construct structures on soft and unstable soils due to the appropriate technical and economicconditions has led to the development of various soil remediation methods. Moreover, the experience obtained fromrecent earthquakes has indicated the influence of sites’ stiffness on the surface seismic ground response. One of theways to increase the stiffness to improve the soil, especially in soft soils, is to employ inclusion piles. These types ofpiles can be used at the bridge’s piers to reduce the seismic response of the aboveground structures. In this regard,the role of the geometry characteristics of the inclusion piles can be significant. This paper investigates the effect ofchanges in the geometric parameters of inclusion piles such as diameter, length, the distance between them, andsurcharge on the ground seismic response based on the offshore Turkish Izmit Bridge as a case study and basemodel. The effective depth was obtained by comparing the ground response spectrum of the twodimensional modelwith inclusion piles using FLAC2D software based on the nonlinear hysteresis model, with the depth equivalent tothe acceleration response spectrum of the freefield model. The geotechnical subsurface conditions at the NorthTower Izmir bay bridge consist of 10 meters of loose to medium dense sand layers with silt, underlain by 127 metersof dense sand and hard sand clay. Bedrock lies approximately 144 meters below the mudline datum. The 1Dresponses obtained from the FLAC 2D and DEEPSOIL 1D software have been compared using the nonlinear soilbehavior to verify the numerical modeling results. Then, with the calibration of soil parameters and lateral andbottom boundaries, inclusion piles have been added to the validated freefield model in FLAC2D software.In this study, the 2D modeling process includes introducing soil layers’ characteristics and determining thelateral freefield boundaries and the quiet boundary as the bottom boundary subjected to the seven earthquakeexcitations is performed. The inclusion pile was modeled using the beam and cable combine elements in theFLAC2D. Besides, inclusion piles are twodimensional elements with 3 degrees of freedom (two displacements andone rotation) at each end node. Piles interact with the FLAC grid via shear and normal coupling springs.The obtained results indicated that by increasing the ratio of distance to the diameter of inclusion piles (S/D), theeffective depth decreases due to reducing the stiffness of the inclusion pile system, and after reaching a ratio of 5, ithas reached a constant value. In other words, with increasing stiffness of the soilpile system, the effect of kinematicinteraction on the soilpile system increases. Moreover, by increasing the length to diameter ratio of inclusion piles(L/D), the effective depth will first increase and then reach a constant value, in which the optimal range for thelength to diameter ratio of piles is 15 to 30. Also, the effective depth increases linearly with an increasing surchargeratio above the inclusion piles ( q ).Finally, it should be noted that the soil improvement using inclusion piles due to the kinematic interaction canapply a new foundation input motion altered from the freefield ground response. This interaction increases theeffective depth of the equivalent freefield model, which can reduce responses of the aboveground structures (e.g.,buildings or bridges, etc.). Therefore, the use of this type of piles due to having more stiffness than traditional soilimprovement approaches such as stone columns or deep soil mixing, etc., can be effective in order to optimallydesign structures located on loose or soft saturated soils.