بررسی اثرات سازه زیر زمینی بر روی تفرق امواج لرزه ای رسیده به سطح زمین

شناخت و درک واقع‌بینانه از تفرق امواج لرزه‌ای ناشی از سازه‌های زیرزمینی که تحت عنوان پدیده بزرگ‌نمایی در سطح زمین شناخته می‌شود، می‌تواند منجر به طراحی ایمن و مقاوم سازه‌های ساخته شده در سطح زمین شود. در پژوهش حاضر، با استفاده از نرم‌افزار تفاضل محدود flac مدل‌های مختلفی از ایستگاه مترو دایکای که تحت زلزله کوبه در سال 1995 به‌شدت آسیب دید، مورد بررسی قرار گرفت. برای مطالعه پدیده بزرگ‌نمایی، پاسخ‌های سطح زمین در دو حالت بدون سازه و با وجود سازه بررسی شد. نتایج نشان می‌دهد که بیشترین مقدار بزرگ‌نمایی به مقدار 2/2 در نقطه میانی سازه و بیشترین پیک شتاب در نقطه مجاور لبه سازه ثبت شد. در بین نقاط مختلف روی لایه‌های خاک، نقاطی که در تماس با سازه قرار داشتند بیشترین مقدار افزایش نسبت به حالت بدون سازه را تجربه کرده‌اند. در ادامه مدل‌های مختلفی از سازه در ضخامت‌های 10 تا 70 سانتی‌متری ستون مرکزی تهیه و تغییر مکان‌های جانبی سازه مورد بررسی قرار گرفت. نتایج نشان می‌دهد که حداکثر تغییر مکان جانبی سازه بین کمترین و بیشترین ضخامت کمتر از 10 میلی‌متر می‌باشد.

Investigating the Effects of Underground Structures on the Scattering of Seismic Waves Reaching the Ground Surface

In the present study, the FLAC Software is applied to investigate the different models of Daikai Subway Station, which was heavily damaged in the 1995 Kobe earthquake. The responses of the ground surface were analyzed with and without the presence of the structure to examine the phenomenon of amplification. The maximum amplification of 2.2 was obtained in the middle point of the structure, while the maximum acceleration was obtained in the point adjacent to the edge of the structure. The points located on the contact surface of the structure were experienced the highest amplification in comparison with the case of without structure. In the next step, different models of the structure with the central column thicknesses of 1070 cm were developed to investigate the lateral displacements of the structure. The results indicated that the maximum lateral displacement of the structure between the maximum and minimum thicknesses was smaller than 10 mm.1. IntroductionToday, the importance of underground structures such as subways is more than ever because of their no doubt critical roles in solving traffic problems in metropolises. Since subways are typically constructed in populated cities and pass beneath major commercial and economic centers, they can impose considerable casualties and economic losses if damaged or destroyed [1]. For many years, the performance of underground structures was believed to be better than that of ground structures when subjected to the earthquakes. However, the experience of some earthquakes demonstrated that underground structures might undergo large deformations or even major failures [23]. The present study models a real underground structure, which experienced serious damages during the earthquakes, by the finite difference method (FDM) using the FLAC Software. The present study aims to investigate the effects of the structure on the scattering of the seismic waves reaching the ground surface. The quantity and pattern of relative displacements for different sizes of the central column (destroyed by the earthquake), the relationship between the recorded accelerations on the ground surface and the effects of the presence of the structure on the acceleration response are investigated.2. Numerical ModelingThe numerical modeling was performed by the Finite Difference Method using FLAC 3D software. The static mode of the gravity loading was employed to define stresses induced by the soil weight. Roller supports were used for vertical boundaries, while pinned supports were applied to the bottom of the model. These boundaries reflect waves into the model and thus they cannot be applied to the dynamic mode.ConclusionThe present study employed the FDM to investigate the seismic responses of a real subway station. The obtained results are as follows: 1. Comparison of the amplification induced by the effects of the structure on the scattering of seismic waves indicated that the scattering effects increased near the edges of the structure, leading to the large amplification factors at the end area.2. The maximum amplification of 2.2, and a 25 percent increase was obtained in the maximum acceleration of Daikai under the 1995 Kobe earthquake.References1. Dowding, C.H. and Rozan, A. (1978) Damage to rock tunnels from earthquake shaking. ASCE J. Geotech. Eng. Div., 104, 17591.2. JingMing, W. and Litehiser, J.J. (1985) The distribution of earthquake damage to underground facilities during the 1976 TangShan earthquake. Earthquake Spectra, 1, 74157.3. Sharma, S., and Judd, W.R. (1991) Underground opening damage from earthquakes. Eng. Geol., 30, 26376.