مطالعه‌ي عددي پديده‌ي جوشش ماسه‌هاي روان در مجاورت ديواره‌ي آب بند

Numerical Study of Boiling Smooth Sand near the Cutoff

حفاري‌هاي عميق جهت ساخت وساز، جز لاينفك شهرسازي محسوب مي شوند. حفاري عميق، لازمه‌ي احداث تونل‌ها و پاركينگ‌هاي زيرزميني و سازه‌هايي از اين قبيل است. معمولاً نيروها در محل حفاري توسط انواع ديوارهاي آب بند در جهت قايم و افقي مهار مي شوند. سپر فولادي، نوعي ديواره‌ي آب بند است كه سطح آب در اطراف آن در تعيين عمق حفاري نقش اساسي دارد. نشت آب در خاك باعث كاهش پايداري خاك اطراف سپري مي شود و درنهايت، سبب ايجاد بالازدگي يا جوشش در كف محل حفاري مي‌شود. در اين پژوهش با استفاده از نرمافزار FLAC-2Dمبتني بر روش تفاضل محدود صريح، اثر عوامل مختلف در مكانيزم وقوع گسيختگي و ضريب اطمينان در برابر جوشش در مجاورت سپري فولادي مورد بررسي قرار گرفته است. مقايسه‌ي نتايج حاصل از مدل با نتايج آزمايشگاهي نشان مي‌دهد كه FLAC مي‌تواند براساس آناليز تنش، پديده‌ي جوشش را به‌طور مناسبي مدل‌سازي كند.

Deep drilling is a major part of construction in urban planning and is necessary for establishment of tunnels, underground parking lots and structures with deep excavations. Usually, vertical and horizontal forces acting on the location of the excavation are braced by various types of sheet pile and cut-off. The water level around the sheet pile is an essential element to determine the excavation depth. Seepage flow through soil reduces the stability of the soil body around the sheet pile, and, finally, causes boiling and heaving phenomena. In this study, by means of software based on the explicit finite difference method, the effects of various factors on the occurrence of failure mechanisms and safety factors against boiling near the sheet pile are investigated. Comparison of experimental data with numerical results indicates that FLAC can model and simulate, properly, the boiling phenomenon, which is based on the stress-strain analysis. Boiling and heaving near hydraulic structures with an alluvial foundation are so destructive and undesirable that remedial issues at the downstream part of such structures are vital. Proper modeling of such hydraulic structures (i.e. coastal dikes, levees etc) before implementation can be predicted using safe models and software such as FLAC. Good adaptation of numerical results with experimental data can prescribe accurate models for practical application, specifically the modeling of critical phenomenon such as boiling and heaving. Numerical analysis was performed to show the mechanism of failure under various conditions. Comparison of the computational results with experimental data and observations shows that FLAC can model correctly the mechanism of the boiling phenomena based on stress-strain analysis. The effects of the internal friction angle, the angle between the soil and the wall, and, finally, the dilation angle, on the behavior of soil, were investigated in detail. The increase in the internal friction angle caused the safety factor to rise significantly. The results of this study indicate that dilation angle is important in occurring the type and shape of heaving and boiling. Up to 15 degrees, the dilation angle created an increasing safety factor, while, after 15 degrees, the safety factor is decreased.