بررسي پاسخ لرزه اي سطح زمين در حضور تونل، مطالعه موردي خط 7 متروي تهران

An Investigation on Seismic Response of the Ground Surface due to the Metro Tunnel– Case Study: Line 7, Tehran

امروزه با توجه به توسعه محيط هاي شهري و افزايش روزافزون جمعيت و مشكلات ترافيكي شهرهاي بزرگ، همواره ساخت فضاهاي زيرزميني به عنوان يك راهكار مدنظر مي باشد. در اين تحقيق تلاش شده تا مخاطرات ناشي از ساخت اين فضاها و تاثير آن بر سطح زمين در حين زلزله مورد بررسي قرار گيرد. بدين منظور خط 7 مترو تهران با توجه به عبور از مجاورت مناطق مسكوني و آبرفت هاي مختلف، در مناطق بحراني به عنوان مطالعه موردي انتخاب گرديده است. اندركنش ديناميكي تونل با سطح زمين به روش عددي و نرم افزار تفاضل محدود تحت محرك هاي ورودي مختلف انجام گرديده است. تحليل ها در دو حالت حوزه پاسخ آزاد زمين و نيز حوزه تونل انجام گرديده است. مدل سازي براي سه حالت مختلف از شرايط بستر و نيز هفت روباره مختلف تحت چهار زلزله با محتواي فركانسي متفاوت انجام شده است. مدل عددي صحت سنجي شده و نتايج مورد ارزيابي قرار گرفته اند. تحليل ها نشان مي دهد كه همواره در بحراني ترين حالت، حضور تونل باعث افزايش 33 درصدي مقادير شتاب در سطح زمين مي گردد. اين افزايش مقادير مي تواند شتاب مبناي طراحي ساختمان ها را 0/35g به 0/46g تغيير دهد. همچنين بيشترين تاثير حضور تونل بر مقادير شتاب سطح در فاصله 0/5D تا 1/5D از محور تونل مي باشد كه در اين محدوده نيز در مسير خط 7 مترو ساختمان هاي مسكوني قرار دارد. در تحليل هاي با عمق دفن هاي مختلف نيز مشخص گرديد كه افزايش روباره تونل باعث افزايش شعاع تاثير روي سطح زمين مي شود، درصورتي كه اثر افزايشي آن كاهش مي يابد.

In order to develop the underground structures such as subways, metro tunnels and other buried facilities in big cities, the dynamic interaction of these structures and their environment should be considered. In this study, all metro lines in Tehran are investigated and the 7th line with 28 km long and its environmental conditions in all length have been used as a case study. This line is selected because it passes through all kinds of geotechnical alluvium and also the plan and profile of this line are suitable for interaction with the buildings. A numerical method in this study was analyzed with finite difference code, FLAC, in dynamic cases. The models are performed for 8, 10,12,13,15 and 20 meter overburden and three different alluviums are used. In dynamic interactions, previous studies showed high amplification of displacement in the vicinity of the tunnel in the surface. The parameter of acceleration is also investigated in some physical model tests. Both acceleration and attenuation of acceleration are reported in different locations and under the different seismic motions. For choosing the critical sections in selected line, the buildings with overburden less than 20 m and the maximum surface distance up to 5D are selected. In this study, two conditions of problem such as free-field, and tunnel field are analyzed. In tunnel-filed models, results show that the presence of tunnel can increase the surface acceleration up to 33% in initial part of line with 8 m overburden. This result is accrued under Chi Chi earthquake with the dominant frequency of 1.68. This increment of acceleration in critical cases can change the design acceleration of the building to 0.46 g instead of 0.35 g that is proposed by Iranian Code. By considering the results of all cases, it can be seen that the most effective case happened when the dominant frequency of motion is so close to the natural frequency of medium. The frequency content of medium is predicted by 1D propagation of wave with equivalent linear method in frequency content. The region between 0.5D to 1.5D from the center of the tunnel on the surface is most affected by the interaction. The effect of overburden of tunnel is also studied that shows that the deeper tunnel has less effect on surface but the influence zone on surface is greater. Both amplification and attenuation happened in the models for acceleration of the surface. The output data show that the variation of acceleration in surface depends strongly on the frequency content of input motion, the surface distance from axis of the tunnel and overburden of the tunnel. The most important factor that can describe the behavior of the surface is the variation of wave propagation around the tunnel. The presence of the tunnel under SH wave based on the mode of deformation can amplify the motions around the tunnel and not top of it in the surface. The tunnel in the medium can also prevent the propagation of waves to top of it and this may cause attenuation of motion. The color contour of the acceleration distribution around the tunnel and near of the surface during the dynamic analysis of model can better describe these phenomena.