تاثير بهسازي خاك در رفتار لرزه‌يي ديوارهاي ساحلي سپري در ساخت‌گاه مستعد روان‌گرايي

Effects of Soil Improvement on Seismic Behavior of Anchored Sheet Pile Quay Walls Embedded in Liquefiable Sites

پاسخ ديناميكي ديوار ساحلي سپري مهارشده‌يي كه لايه‌ي مستعد روان‌گرايي بخش مدفون ديوار را در بر گرفته است، به‌طور عددي تحليل، علل بروز ناپايداري شناسايي، و اثربخشي بهسازي مناطق مختلف پي در كاهش جابه‌جايي ديوار و تغيير مد گسيختگي بررسي شده است. پس از تشخيص منطقه‌ي بهسازي مناسب، به مطالعه‌ي تاثير طول اين ناحيه‌ پرداخته و كمينه و بيشينه‌ي طول موثر عمليات بهسازي معرفي شده است. تحليل با نرم‌افزار DIANA و مدل رفتاري توهاتا- يايي انجام شده است. ناحيه‌ي جلوي ريشه‌ي ديوار در سمت دريا، مستعدترين منطقه براي شروع روان‌گرايي است. تراكم اين منطقه جابه‌جايي‌ها را كاهش مي‌دهد، اما از مد گسيختگي پاي ديوار جلوگيري نمي‌كند. بهسازي خاك پشت ديوار از وقوع اين مد پيشگيري مي‌كند و جابه‌جايي‌ها را به‌طور قابل ملاحظه‌يي كاهش مي‌دهد. كمينه‌ي طول موثر براي بهسازي برابر مجموع طول گوه‌ي گسيختگي در لايه‌ي سُست و ناحيه‌ي مرزي بين خاك سُست و متراكم است.

The seismic performance of quay walls is found to be strongly dependent on liquefaction occurrence. Besides, in some existing walls that were designed without consideration of the liquefaction hazard, due to the relatively long length of quay walls, the orientation of soil strata probably caused the liquefiable layers to appear unavoidably neighboring the wall roots. In this paper, the dynamic response of anchored sheet pile quay walls embedded in liquefaction susceptible soil was investigated numerically, utilizing the strain space plasticity model for cyclic mobility available in the DIANA finite element program. Based on the results, the extension of liquefiable soil around the wall root leads to the "failure at embedment" mode. Deformed mesh indicates that the most visible deformations are localized in loose soil around the embedded section. Beside the noticeable heave of the seabed, its seaward displacement causes a significant reduction in its supporting role for the embedded section, and leads to the large tilt of the wall. Consequently, an active wedge, extending from the embedded section to the back of the anchors, is formed. Moving along this wedge, the anchors endure significant overturning. The mentioned deformation shape was previously observed in shaking table tests conducted by the authors. The leeward section of the loose layer is recognized as the most vulnerable zone against liquefaction. Besides the effect of soil softening in this zone, dynamic active pressure behind the wall causes the bottom of the wall to experience higher displacement than its top. The time history of monotonic bending moment infers that considerable moment is applied to the wall root by the loose section behind the root. This moment is the main reason for the "escape" of the wall root. The remediation method, by deep vibro-compaction of the weak area, is considered as a liquefaction countermeasure. The effectiveness of soil improvement in zones adjacent to the embedded section is discussed, based on analytical dynamic responses. Implemented countermeasures are found to considerably reduce deformations in the wall; however, in order to prevent failure at embedment, improvement of soil located behind the wall root is more effectual. The compacting of this section not only reduces driving moment applied to the wall root, but also creates resistant moment against root escaping. In addition to the impact of base acceleration amplitude, the optimum extension of improved zones is introduced.