تحلیل عددی پاسخ لرزه‌ای تونل زیرزمینی مترو تبریز

در این مقاله شبیه‌سازی عددی و تحلیل تونل خط 2 مترو تبریز، واقع در ناحیه با فعالیت لرزه‌ای بالا، تحت تحریکات زلزله‌های مختلف بررسی شده است. تحلیل‌های دینامیکی تاریخچه زمانی سیستم به هم بسته خاکتونل با استفاده از نرم‌افزار flac2d انجام شده‌اند. رفتار غیرخطی خاک و اندرکنش خاک و سازه با در نظر گرفتن مشخصات مربوط به تونل زیرزمینی مدل شده است. عملکرد لرزه‌ای مدل به‌وسیله مدل رفتاری ubchyst برای خاک ارزیابی گردیده است. این مدل رفتار سیکلی غیرخطی شامل کاهش مدول برشی با کرنش برشی و نسبت میرایی وابسته به کرنش می‌باشد. پارامترهای مدل ubchyst به‌وسیله مقایسه منحنی‌های کاهش مدول و میرایی حاصل از شبیه‌سازی مدل با منحنی‌های دارندلی کالیبره شده و سپس مدل تحت زلزله‌هایی با ماکزیمم شتاب زمین و فرکانس‌های غالب مختلف قرار گرفت. نتایج تحلیل‌های دینامیکی برحسب جابه‌جایی، تنش موثر و نیروهای داخلی در طی زلزله ارائه شده است. بر اساس تحلیل‌ها، افزایش ماکزیمم شتاب زمین و ماکزیمم جابه‌جایی افقی منجر به افزایش نیروهای داخلی دینامیکی در پوشش تونل، افزایش نشست دائمی خاک و کاهش تنش موثر در خاک می‌شود. به‌علاوه، پس از هر زلزله مقادیر باقیمانده قابل‌توجهی برای لنگر خمشی دینامیکی در پوشش تونل در اثر کرنش‌های تجمعی مشاهده شد درصورتی‌که مقادیر باقیمانده پس از زلزله برای نیروهای محوری در پوشش تونل کوچک‌تر است.

Numerical Study of the Seismic Response of Tabriz Subway Tunnel

In this paper, the seismic response of Tabriz subway tunnel line 2, located in the high seismic activity area in the northwest of Iran, was studied by using the twodimensional finite difference program FLAC 7.0. The dynamic time history analyses of the coupled tunnelsoil system were carried out under plane strain conditions. The tunnel lining is made of segments of precast concrete with a thickness of 0.35 m and length of 1.50 m installed behind the shield of the earth pressure balance TBM. In the study area of the path, the type of the soil is mostly GM and SM, and the water level is approximately 10 m under the ground surface. By considering the location of Tabriz subway tunnels that are close to the Tabriz north fault, the characteristics of three nearfield earthquakes, Kocaeli, Kobe, and ChiChi, were used for the numerical analysis. In this study, the tunnel was meshed by linear elements, whereas the soil was modeled with quadratic planestrain elements. The linear elastic model was assumed for the tunnel behavior, while the combined hysteretic totalstress constitutive model (UBCHYST) was used to model the soil behavior. This constitutive model simulates nonlinear cyclic behavior containing degradation of shear modulus with shear strain and straindependent damping ratio. Modulus degradation and damping curves proposed by Darendeli were used for calibrating the UBCHYST model parameters. Based on the sensitive analysis, the width and height of the model were selected 80 m and 50 m, respectively. The freefield boundary was assigned to the lateral boundaries so that it supplies similar conditions to that of an infinite model. The interaction effects of soillining were also taken into account by applying interface elements. In addition, in dynamic analyses, seismic ground motions related to the shear waves were applied to the base of the models as a function of time. Generally, the model analysis was conducted in three phases. First, the soil elements were loaded under a geostatic condition to obtain the natural steady state. These values were used as initial stress for the next calculations. Then, the concrete lining was placed in the soil, the interface properties were applied and thus, the model was analyzed again. In the static analysis, the soillining system was under gravity loading only, the base boundary was fixed in all directions and the side boundaries were fixed in the xdirection. Finally, the seismic analysis was carried out. According to the performed analyses, the natural frequencies of the soil layers and the soiltunnel system were obtained 3.50 and 3.46 Hz, respectively. Therefore, the existence of the tunnel inside the soil decreases the natural frequency of the system. Also, the ratio of calculated acceleration at the different depths to the base acceleration along the tunnel is generally larger than one, which indicates the tendency of the model to amplify the base signal moving toward the ground surface. The Kobe earthquake with two peak ground accelerations (PGA=0.175g and 0.35g) was chosen as the input seismic load applied to the model base for evaluating the impact of the earthquake maximum amplitude. According to the analysis results, by increasing the earthquake maximum amplitude, the ground surface settlement increase as well. In addition, dynamic axial force and bending moment caused in the lining tunnel increase by increasing peak ground acceleration and ground surface displacement during earthquake. It should be noted that a dependence of the increment of bending moment and axial force on the ground surface settlement was observed. Also, at the end of the analysis, the residual axial force remaining in the lining is the same for both earthquakes. In order to study the effects of earthquake frequency content, acceleration time history of three earthquakes (Kocaeli, Kobe, and ChiChi) with different predominate period (1.40, 0.16 and 0.93sec, respectively) and the constant PGA (0.35g) were applied to the model. Based on the analysis results, for a given PGA, not only decreasing the predominate frequency of earthquake has the effect on increasing of internal forces, but also other earthquake characteristics such as ground displacement and energy density of earthquake have their own effects. Furthermore, large residual values were observed after each earthquake for dynamic bending moments because of cumulative strains during the earthquake, but smaller residual values were observed after each earthquake for the dynamic axial forces. Moreover, the peak values of the dynamic bending moment are shown near the crown and the shoulders of the tunnel.