ارزيابي پتانسيل روان‌گرايي در استان مازندران با كمك مطالعات صحرايي و آزمايشگاهي با نگاهي بر معيار ايشيهارا

EVALUATION OF LIQUEFACTION POTENTIAL IN MAZANDARAN PROVINCE USING IN-SITU TEST DATA WITH REVIEW OF ISHIHARA CRITERION

عموماً روان‌گرايي در خاك‌هاي ماسه‌يي ريزدانه و اشباع رخ مي‌دهد كه مي‌تواند منجر به جابجايي‌هاي مخرب زمين در حين زلزله شود. پتانسيل رخداد اين پديده‌ تابع جنس خاك، سطح آب زيرزميني، زلزله‌خيزي منطقه و همچنين عمق لايه‌ي سربار است. اين نوشتار ضمن معرفي اثر لايه‌ي سربار غير روان‌گرا در توانايي جلوگيري از روان‌گرايي لايه‌ي روان‌گراي زيرين در قالب معيار ايشيهارا، ضريب اطمينان تصحيح‌شده در برابر روان‌گرايي را براي برخي نواحي استان مازندران محاسبه مي‌كند و نهايتاً به ارايه‌ي پهنه‌بندي روان‌گرايي در بخش‌هاي مركزي و غرب و همچنين شرق استان مازندران مي‌پردازد. پهنه‌بندي ارايه‌شده فقط كلي و كيفي است و با توجه به كم‌بودن تعداد گمانه‌هاي مربوط به آزمايش‌هاي صحرايي در سطح استان مي‌تواند فقط به‌منزله‌ي راهنماي كلي جهت ضرورت مطالعات بيشتر در بخش‌هايي از نوار ساحلي تلقي شود. در تعدادي از شهرهاي نوار ساحلي همچون محمودآباد و بابلسر به‌نظر مي‌رسد مهندسان طراح بايد توجه ويژه‌يي به پديده‌ي روان‌گرايي داشته باشند و با درنظرگرفتن تمهيداتي چون پي‌هاي گسترده و عميق، چاه‌هاي شني، زهكش‌هاي فتيله‌يي و غيره به مقابله با اين پديده‌ي ويرانگر بپردازند. بديهي است اظهارنظر قطعي در مورد وقوع اين پديده و پتانسيل رخداد آن ملزم به ارايه‌ي طرح آزمايش‌هاي برجا به‌خصوص براي پروژه‌هاي با درجه‌ي اهميت بالا در آيين‌نامه‌ي زلزله‌ي ايران است.

Liquefaction-induced damage is more highlighted in saturated fine-sand deposits, where huge pore pressure buildup will cause premature strength loss within the stratum underneath. The potential for occurrence of liquefaction is directly dependent on factors like soil type, seismicity of the region, location of the water table and the overlying non-liquefiable stratum thickness. The current paper will focus on the effect of the overlaying non-liquefiable stratum on the liquefaction potential of the underlying layer, which is inherently liquefiable. The Ishihara method was invoked for this aim, and a corrected safety factor was proposed which considers the effect of liquefiable underlying and non-liquefiable overlying layer depths. In order to implement the suggested liquefaction potential correction scheme on a case in the real world, the Mazandaran province in Northern Iran was selected to investigate the liquefaction potential in its different regions. The liquefaction potential was sought by adoption of different criteria. In all the employed criteria in the standard penetration test, SPT data was used to calculate the cyclic resistance ratio, and the cyclic stress ratio was also estimated according to the seismicity of the region. The liquefaction potential is then calculated from different methods. Correction is finally applied to the calculated safety factors that reflect the liquefaction potential in different regions. A tentative liquefaction zoning map was then provided for Mazandaran province, which enables the engineers to avert uncalled-for situations by emphasizing the necessity of detailed study in zones with high liquefaction potential. The proposed zoneation map is more or less qualitative, due to the lack of sufficient test boreholes, and also the need for more sophisticated in-situ test schemes. However, it can pave the way towards an initial understanding of the liquefaction potential in different urban areas. To be more specific, it is crucially important to pay more attention to the liquefaction issue in some coastal cities, like Babolsar and Mahmoudabad, which are heavily populated and potentially liquefiable at the same time. Measurements like deep and mat foundations, gravel or stone columns and wick drains are alternatives in case remedial or improvement provisions are necessary. Microzoneation will, however, need more densely-allocated in-situ test schemes and detailed investigation into the liquefaction potential, which is followed according to the importance of the structure and the construction location.