بررسي رفتار لرزه‌اي تونل مترو دايروي در ماسه خشك توسط آزمايش‌هاي ميز لرزه

Experimental Study of the Seismic Response of Tabriz Subway Tunnel in Dry Sand

يك سري آزمايش‌هاي ميز لرزه g1 براي بررسي پاسخ مدل تونل متروي تبريز كه يك تونل دايروي مدفون در ماسه خشك مي‌باشد، تحت بارگذاري نامنظم با دامنه كم و امواج هارمونيك با دامنه بالا انجام شد. اثرات پارامترهاي مختلف شامل شتاب ماكزيمم زمين و محتواي فركانسي حركات ورودي روي رفتار تونل بررسي شد. آزمايش‌ها در دو شتاب ماكزيمم زلزله g0.35 و g0.50 و فركانس‌هاي 1، 3، 5 و 8 هرتز انجام شدند. نتايج آزمايش‌ها برحسب نيروهاي ديناميكي پوشش تونل و شتاب نقاط مختلف مدل ارائه شده است. نتايج نشان مي‌دهد رفتار ديناميكي تونل‌هاي دايروي مي‌تواند به دو مرحله تقسيم‌بندي شود: مرحله ناپايدار در طي چندين سيكل اول و مرحله سيكل‌هاي پايدار كه در طي آن نيروها در پوشش تونل حول مقدار ميانگين نوسان مي‌كند. به‌علاوه، اندازه‌گيري‌هاي آزمايش‌هاي ميز لرزه نشان مي‌دهد با افزايش فركانس بارگذاري براي شتاب A=0.35g، لنگر ماكزيمم ايجاد شده در پوشش تونل ثابت مانده يا اندكي كاهش مي‌يابد اما براي A=0.50g، لنگر ماكزيمم به‌شدت كاهش مي‌يابد. شتاب افقي ثبت شده در چندين تراز نشان مي‌دهد كه تقويت سطحي به‌وسيله حفره تونل انجام مي‌گيرد كه به‌عنوان مانع در جهت گسترش امواج برشي به سمت بالا عمل مي‌كند.

A series of 1 g shaking table tests were performed to investigate the response of Tabriz subway tunnel, a circle-type tunnel embedded in dry sand, under sinusoidal excitations. In prototype, the subway tunnel with 9.2 m diameter and 0.35 m thickness was embedded in a soil layer. Two reduced-scale 1 g shaking table models, designated as FF and SF, were constructed in 1/45 scale. The FF was constructed to study the seismic response of the soil layer in free field condition, while the SF model includes a subway tunnel to study its seismic response during different excitations. The shaking table of Tabriz University with a platform of 3m×2m and one-degree of freedom was used to induce the desired excitations to models. The table can carry up to 6 tones and can reach acceleration levels up to 1.5 g with peak displacements of ±100 mm. A laminar shear box was designed in Tabriz University that includes 20 aluminum frames with dimensions of 1320×814×860 mm (L×H×W). In order to reduce the friction between the layers and simulate the displacement of soil layers, ball bearings were used between two adjacent frames. In this box type, the lateral boundary effect on the seismic response of the soil layer is reduced. The simulation laws for 1 g shaking table tests were utilized in the current study. Based on the simulation laws and the size of the laminar shear box, the prototype to model scale factor was considered to be 45. Therefore, the tunnel model was constructed by aluminum alloy with a diameter of 195.5 mm and thickness of 1.5 mm. Uniform dry sand provided from Qomtapeh was used in this study. During the construction, the tunnel and all the embedded instruments were placed in the model. To avoid any interaction of the tunnel with the laminar shear box, the tunnel was selected shorter than the box width. Two PVC circular plates were placed at both the tunnel ends to avoid the sand entrance into the tunnel model. To simulate the effects of friction on the soil–tunnel interaction, the outside surface of tunnel was covered by sand particles using epoxy coating. For reaching the same target relative density (Dr=65%) during the construction of models, the bulk unit weight was controlled to be constant for all layers. Seven strain gauges were installed on the tunnel surface to monitor the behavior of the tunnel. Five accelerometers were placed in different levels of the model to record the acceleration in the soil. Besides, two LVDTs were placed on the top of the model to measure the soil surface settlement. A 32-channel dynamic data logger was used to record and transfer all the measured data to a personal computer.