مقايسه‌ي اثر نوع ريزدانه ها بر خصوصيات فشار آب حفره‌يي ماسه هاي اشباع

THE EFFECT OF FINES TYPE ON THE PORE PRESSURE GENERATION CHARACTERISTICS OF SATURATED SAND

ماسههاي موجود در طبيعت معمولاً به حالت خالص وجود ندارند و معمولاً به ‌همراه مقادير متفاوتي ريزدانه هستند كه اين ريزدانهها ممكن است چسبنده يا غيرچسبنده باشند. مطالعات انجام‌شده نشان ميدهند كه رفتار ماسههاي لايدار و رس‌دار اشباع از نظر پاسخ آنها به بارهاي سيكلي متفاوت است. در اين تحقيق نتايج آزمايش‌هاي سه محوري سيكلي كنترل كرنش بر روي نمونههاي ماسه‌يي حاوي لاي و كايولينيت، از ديدگاه فشار آب حفره‌يي مورد توجه قرار گرفته است. نتايج نشان ميدهند كه با افزايش مقدار لاي تا حدود 15? فشار آب حفره‌يي نسبت به ماسه‌ي تميز افزايش يافته است و در مقدار لاي 30? به حدود ماسه‌ي تميز ميرسد. همچنين افزايش ريزدانههاي كايولينيت تا 20? باعث افزايش قابل‌توجهي در فشار آب حفره‌يي نسبت به ماسه‌ي تميز مي‌شود و در مقدار ريزدانه 20 تا 30 درصد، به دليل غلبه‌ي ماتريس رس بر ماتريس ماسه در نمونه، باعث كاهش قابل‌توجهي در فشار آب حفرها‌يي نسبت به ماسه‌ي تميز ميشود. اين مطالعه همچنين نشان ميدهد، استفاده از معيارهاي روانگرايي در عمل بايد ناظر بر درنظرگرفتن نوع ريزدانه باشد، زيرا ارزيابي روانگرايي با توجه به معيارهاي متداول مانند روش استفاده از عدد نفوذ استاندارد (SPT) ممكن است درخلاف جهت اطمينان باشد.

Sands in nature generally contain plastic and non-plastic fines. Previous research has been mainly focused on the cyclic behavior of saturated clean sand. Recent studies, however, reveal that the cyclic behavior of saturated sand containing plastic fines is quite different from that including non-plastic fines. Moreover, most liquefaction studies have concentrated on stress-controlled cyclic triaxial testing of clean sand, while strain-controlled testing allows a more fundamental assessment of pore pressure generation. It is due to the strong relationship between shear deformation and pore pressure generation. In this study, the effects of plastic and non-plastic fines on the pore water pressure behavior of saturated sand are studied by means of a strain controlled cyclic triaxial tests. To this aim, all specimens are prepared by moist tamping using the undercompaction technique. Specimens are subjected to 50 sinusoidal cycles of axial strain at a loading rate of 0.1 Hz. Furthermore, shear strain levels are applied in the range of 0.003% to 0.3%, and pore pressure is directly measured in the specimens. The results demonstrate that the specimens with up to 15% non-plastic fine (silt) content generate a lower value of pore water pressure than that of clean sand. Increasing silt content to 30% leads to a pore pressure close to that of clean sand. On the other hand, in sand specimens with up to 20% plastic fines (kaolinite), generated excess pore water pressure is more than that of clean sand. Up to this plastic fine content, sand grains still get in touch with each other, and clay particles play a role like a lubricant film between them. The result is a more collapsible fabric as the sand–skeleton tries to rearrange itself into a more stable structure. By increasing plastic fines up to 30%, the excess pore water pressure noticeably decreases to a level that is less than that of clean sand. This could be attributed to the fact that at larger fine content, the sand-skeleton void ratio is larger than the maximum void ratio for clean sand. Therefore, the sand grains are no longer in contact, floating in the kaolinite matrix. Thus, the specimen shows a clay-like behavior. Consequently, the behavior of the specimen is controlled by the fines, so that the specimen indicates cyclic mobility behavior. This investigation reveals that the conventional criteria for liquefaction assessment, such as the standard penetration test (SPT), do not consider the effect of the plasticity of fines. Therefore, to evaluate the liquefaction potential of sands containing fines, one should resort to other precise tests, such as cyclic triaxial tests.