مدل‌سازي اثر ساخت سد خاكي بر روي مقاومت پي آبرفتي در برابر روانگرايي

MODELING THE EFFECT OF EMBANKMENT DAM CONSTRUCTION ON THE LIQUEFACTION POTENTIAL OF AN ALLUVIUAL FOUNDATION

درصورتي‌كه سازه‌هاي ژيوتكنيكي بر روي مصالح آبرفتي مستعد روانگرايي احداث شوند، مقاومت در برابر روانگرايي پي بعد از ساخت سازه تغيير مي‌كند؛ اين مسيله به‌علت تغييركردن بارهاي لرزه‌يي وارد بر پي آبرفتي، ميزان سربار وارده و تراكم مصالح در آبرفت پس از ساخت سد است. در اين نوشتار، روشي جديد براي محاسبه‌ي تغيير تراكم مصالح پي آبرفتي پس از ساخت سد ارايه شده است كه با استفاده از آن مي‌توان با محاسبه‌ي مقاومت در برابر روانگرايي در پي و مقايسه‌ي آن با تنش‌هاي برشي حاصل از زلزله، ضريب اطمينان در برابر روانگرايي را محاسبه كرد. براي مقايسه، سه نوع بافت تراكمي ضعيف، متوسط و قوي در آبرفت ارايه شده و ضريب اطمينان در برابر روانگرايي، قبل، و پس از ساخت سد مقايسه شده است.

Loose saturated sand and alluvial strata are susceptible to liquefaction during earthquakes. A simple and empirical method for evaluating their resistance against liquefaction is based on the results of standard penetration tests (SPT). One of the most important parameters to affect liquefaction resistance is initial stress. Standard procedures for considering this effect require correcting penetration resistances to an equivalent overburden stress of one atmosphere and adjusting the cyclic resistance ratio for the effects of overburden. Liquefaction potential in alluvium will change after heavy construction on them, which is due to changes in earthquake load, overburden pressure and soil densification. In this paper, a new methodology is explained to calculate changes in the compaction of an alluvial foundation after dam construction, which influence both shear strength and dynamic shear stress. With this new formula, one can determine the shear strength of alluvium against liquefaction and compare its values with induced dynamic shear stresses to evaluate the safety factor. The effect of dam construction on the liquefaction potential of alluvium is investigated through dynamic analysis based on an equivalent linear approach. The liquefaction potential of an alluvial foundation without dam construction is compared to that potential after construction, in two ways. In the first method, the effect of induced stresses on the liquefaction potential is considered in the numerical analysis, based on new stress states and related functions, and, in the second method, in addition to these changes, the relative density of the alluvium is corrected due to construction. For instance, a method is described to update alluvium relative densities and update material parameters. To clarify the effectiveness of the dam body on the liquefaction potential of the foundation, three different compressibility conditions, i.e. weak, medium and stiff states, are considered in the alluvium to discover changes in the safety factor after dam construction. Results of analyses show that the liquefaction potential of the alluvial foundation decreases significantly after construction in various alluvial strata. Considering results derived from this study, if a site selected for dam construction is prone to liquefaction, engineers might have an economic project by keeping the alluvium beneath the dam instead of total excavation.