مطالعه‌ي عددي مكانيزم‌هاي موثر در رفتار لرزه‌يي سازه‌ي زيرزميني مدفون در خاك‌هاي‌ ماسه‌يي سُست با پتانسيل روان‌گرايي

Numerical Study of Effective Mechanisms on Seismic Response of Underground Structures Buried in Liquefiable Sandy Soil

مشاهدات حاصل از زلزله‌هاي پيشين، مويد اعمال خسارت به سازه‌هاي زيرزميني واقع در خاك‌هاي با ‌پتانسيل روان‌گرايي است؛ با وجود اين، در پژوهش‌هاي پيشين توجه كمتري به مكانيزم‌ها و عوامل موثر در خسارت‌ها صورت گرفته است. در اين پژوهش، پاسخ‌هاي لرزه‌يي سازه‌ي زيرزميني سطحي واقع در خاك ماسه‌يي مستعد روان‌گرايي با استفاده از نرم‌افزار المان محدود COM3 ارزيابي مي‌شود. نتايج پژوهش نشان مي‌دهد كه توزيع غيريكنواخت اضافه‌ي فشار آب حفره‌يي در زير سازه و تغييرشكل جانبي سازه به‌دليل تغييرمكان جانبي خاك روان‌گراي پيرامون، از عوامل موثر و لازم در تغيير حجم خاك روان‌گراي فشرده‌شده به زير سازه به‌عنوان مكانيزم متداول بلندشدگي سازه است. در شرايط زهكشي‌شده، تغييرشكل جانبي سازه تاثير چشمگيري در ميزان نشست آن داشته است كه با نيروي اينرسي سازه تحت تحريك لرزه‌يي مرتبط است. افزايش وزن سازه، تاثيرات مثبت‌‌ يا ‌‌‌منفي و افزايش تراكم تاثير مثبت در بهبود رفتار لرزه‌يي سازه در شرايط مختلف خاك ماسه‌يي سُست دارند.

Nowadays, population growth and civil facility development requirements have caused increases in the construction of underground structures in urban residential zones. Previous earthquake observations show that severe damage may occur to underground structures such as pipelines, manholes and even large underground structures, including subway tunnels. However, the seismic performance of subway tunnels buried in loose sandy soil with liquefaction potential needs further investigation, in order to recognize common contributed mechanisms to the seismic responses of underground structures from performance based seismic schemes. In this paper, the seismic responses of a subway tunnel buried in loose soil with liquefaction potential are numerically studied with a three-dimension finite element program called COM3. According to numerical studies, the subway tunnel shows upward movement in the liquefied soil with simultaneous occurrence of two conditions. These include non-uniform excess pore pressure distribution under the subway tunnel and lateral deformation of the structure due to lateral displacement of the surrounding soil. These are necessary and sufficient parameters that affect the amount of liquefied soil moved to the bottom of the structure, as a common mechanism in the uplift response of a shallowly buried subway. Although the subway tunnel is symmetrically simulated, the amount of liquefied soil moved from each bottom side of the structure is not identical because of seismic excitation characteristics. The lateral deformation of the subway tunnel causes the downward movement of the structure in the un-saturated soil condition, due to its inertial force under seismic excitation. Increasing the unit weight of a subway tunnel has a contrary effect on the seismic response of structures in loose soil conditions, and it is rational to consider the influence of both soil conditions on the seismic behavior of subway tunnels in the design stage. The reinforcement ratio variation of a subway tunnel has no significant effect on the seismic behavior of the subway tunnel under a saturated condition, but the reduction of reinforcement ratio may increase lateral deformation and settlement of the structure in an un-saturated condition. The loose soil relative density growth improves the seismic responses of subway tunnels under both soil conditions, and the traditional remedial method, including densification, is numerically verified as a useful procedure.