مكانيزم ديوار جداكننده در يك تونل سطحي واقع در خاك مستعد روان‌گرايي

Mechanism of cutoff wall on shallow tunnel in liquefied soil

خاك‌هاي روان‌گرا در حين زلزله سبب خرابي سازه‌هاي زيرزميني مي‌شوند. سازه‌هاي بزرگ زيرزميني كه در معرض اين نوع خاك‌ها قرار مي‌گيرند، دستخوش خرابي ناشي از روان‌گرايي خاك مانند بلندشدگي، گسترش جانبي و نشست‌هاي زياد مي‌شوند. با توجه به اينكه بلندشدگي تونل و افزايش نيروهاي داخلي پوشش تونل از خطراتي است كه ممكن است براي سازه‌هاي بزرگ زيرزميني در خاك‌هاي روان‌گرا پيش آيد، لذا در اين نوشتار به ارزيابي بلندشدگي و نيروهاي داخلي پوشش تونل با استفاده از نرم‌افزار Flac2D پرداخته شده است. سپس براي جلوگيري از بلندشدگي تونل، به‌منزله‌ي يك روش اصلاحي از ديوار جداكننده استفاده و به تاثير آن در تونل و خاك روان‌گرا پرداخته شده است و در نهايت، پارامترهاي طراحي ديوار يعني ضخامت، طول، و فاصله‌ي ديوار از لبه‌ي تونل مورد ارزيابي قرار گرفته است.

Gradual increments of excess pore pressure in saturated soil layers during earthquakes lead to a decrease in effective stress and soil liquefaction. The damage caused by soil liquefaction is divided into three groups: surface damage, settlement damage, and underground structural damage. Generally, the first two cases are visible after the earthquake, regarding which extensive studies have been carried out. But, in the third group, because of the shortage of instrumented field evidence and the complexity of model investigation into underground structural damage caused by ground liquefaction, fewer studies have been implemented. The damage resulting from liquefaction may cause underground structures to be buoyant and settled due to decreases in the bearing capacity and increases in the lateral earth pressure. According to field evidence, linear structures such as tunnels may be destroyed by the liquefaction of ambient soils and uplift pressure. Thus, this phenomenon should be taken into consideration in the designing stage of tunnels. When the surrounding soil of the tunnel is liquefied, large deformations occur in the area, leading to an increase in the internal stresses and deformations of the lining. Thus, if liquefaction occurs in the surrounding soil of the tunnel, by improving its surrounding area, stresses and deformations can be controlled and their magnitudes can be reduced to an allowable limit. Therefore ,evaluation of the liquefaction effects on the tunnel lining should be regarded as an important issue. In this research, the tunnel in the liquefied soil is modeled with Flac2D software, and in the liquefaction state, tunnel uplift and maximum internal forces of the tunnel lining are studied. In another model, cutoff walls are modeled on both side of the tunnel and their effects are studied. The studies show that a tunnel with cutoff walls reduces tunnel uplift and increase the internal forces of the tunnel lining. Then design parameters of cutoff walls, like wall thickness, wall length, and wall distance from the tunnel are evaluated.