بررسي اثر انعطاف‌پذيري پوشش سازه‌ي زيرزميني در پاسخ شتاب سطح زمين

Effect of Underground Structure Lining Flexibility on Acceleration Response at Ground Surface

در اين نوشتار پس از صحت‌سنجي يك مدل عددي با نتايج آزمايش‌هاي سانتريفيوژ انجام‌شده بر روي يك تونل مترو، اثر انعطاف‌پذيري پوشش تونل در پاسخ شتاب سطح زمين مورد مطالعه قرار گرفته است. نتايج مطالعه‌ي پارامتري نشان مي‌دهد كه دو سيستم با ضريب انعطاف‌پذيري يكسان، ولي با جنس متفاوت منجر به پاسخ يكسان شتاب در سطح زمين مي‌‌شوند. پارامتر ضريب انعطاف‌پذيري، پاسخ شتاب در سطح زمين را تحت تاثير قرار مي‌دهد و اين مسيله تحت تاثير محتواي بسامدي حركت ورودي و بسامد طبيعي سيستم است. پوشش سخت‌تر (با ضريب انعطاف‌پذيري كمتر) باعث بزرگ‌نمايي بيشتر پاسخ در دوره‌هاي تناوب پايين نسبت به پوشش‌هاي منعطف‌تر شده است، كه مي‌تواند براي ساختمان‌هاي كوتاه مجاور تونل مسيله‌ساز باشد. در حالي كه پوشش منعطف‌تر منجر به پاسخ بيشتر شتاب در دوره‌هاي تناوب بلندتر نسبت به پوشش‌هاي سخت‌تر مي‌شود، كه سازه‌هاي بلند را تهديد مي‌كند.

Fast population growth in urban areas has resulted in a significant shortage of aboveground spaces and has led to an increase in underground structures. Different types of underground spaces and structures, including underground utility tunnels, subway tunnels, subway stations, underground shelters, culverts and etc., may exist in any large city. The presence of subsurface structures has static and dynamic interaction with their environment and adjacent constructions. Recently, the effect of underground structures on the acceleration response of the ground surface has attracted the attention of researchers, since it has been concluded that the presence of these subsurface structures also affects the seismic response of nearby ground. There are many parameters affecting the presence of underground structures on the acceleration response at the ground surface. These parameters include soil medium characteristics, excitation frequency and amplitude, tunnel diameter, depth of construction, and flexibility ratio of the lining. In this paper, a nonlinear numerical model was first verified against dynamic centrifuge test results performed at the Korean Advanced Institute of Science and Technology (KAIST) on an underground subway tunnel and, then, the effect of underground structure lining flexibility on the acceleration response at the ground surface was investigated. In this regard, the acceleration response at ground surface for three different materials of the lining was studied. From parametric studies, it was concluded that the same values of flexibility ratio for two tunnel systems with different lining material resulted in the same acceleration response at the ground surface. The flexibility of the lining affects the acceleration response at the ground surface; however, it depends on the frequency content of the input motion and the natural frequency of the system. Stiffer lining resulted in amplification of motions with short periods, which threatens short buildings, whereas, softer lining amplifies motion with longer periods, which can be dangerous for tall buildings. Therefore, the flexibility of the lining plays an important role in acceleration response at the nearby ground acceleration response.