بررسی تغییرات متوالی درجه اشباع برخصوصیات فیزیکی، مکانیکی هسته سد خاکی (مطالعه موردی: سد دوستی)

یکی از عوامل تاثیرگذار بر مشخصه های فیزیکی و مکانیکی در سدهای خاکی، اثر تر و خشک شدن متوالی خاک هسته است که ناشی از نوسانات زیاد تراز آب مخزن در دوره های خشک طولانی مدت و آبگیری های مجدد است. در این پژوهش اثر تر و خشک شدن متوالی هسته رسی سد دوستی مورد بررسی قرار گرفت. پس از نمونه‌برداری از محل قرضه‌ سد، آزمایش‌های تراکم استاندارد، تک محوری، برش مستقیم، هدایت هیدرولیکی و منحنی مشخصه آب خاک انجام و شش مرحله تر و خشک شدن متوالی مورد بررسی قرار گرفت. نتایج نشان داد که پس از شش چرخه تر و خشک شدن مقاومت فشاری نمونه‌ها 32 درصد کاهش و همچنین میزان نفوذپذیری هسته سد حدود 9/1 برابر افزایش یافت. با انجام آزمایش منحنی مشخصه آب خاک که بیانگر رابطه میان درصد رطوبت و مکش در خاک های غیر اشباع است مشخص شد که با افزایش تعداد چرخه، منحنی مشخصه آب - خاک به سمت پایین تغییرمکان پیدا می‌کند و قابلیت نگهداشت آب توسط خاک کاهش می‌یابد. در نهایت با استفاده از نرم افزار geostudio یک مدل عددی از سد دوستی ساخته و برای صحت‌سنجی مدل عددی از نتایج ابزار دقیق موجود استفاده گردید. سپس با استفاده از داده‌های حاصل از آزمایشات در مدل عددی نشست هسته سد پس از 6 چرخه تر و خشک شدن پیش‌بینی شد. نتایج مدل سازی عددی نشان داد در حالت پس از اعمال شش چرخه تر و خشک شدن در مقایسه با حالت بدون اعمال چرخه ، افزایش 3/38 درصدی نشست هسته سد بوجود می آید.

Investigation of Frequently Changes in Degree of Saturation on Physical and Mechanical Characteristics of Core of Soil Dam (Case Study: Doosti Dam)

Today, the embankment dams are considered more prominent than concrete dams because of its formation and lower cost. According to the official site of the International Commission on Large Dams, embankment dams account for about 64 percent of the worldchr('39')s total dams. The results of the statistical analysis of the International Commission on Large Dams showed that the main cause of damage of half of the embankment dam is erosion that usually occurs during the first impounding of the dam reservoir. Therefore, the stability and leakage conditions of the embankment dam should always be examined. Moreover, the analysis showed that drying of the embankment dam due to drought and rewatering the reservoir is similar to the first dam impounding and therefore it is necessary to investigate it. One of the factors affecting the physical and mechanical properties of embankment dams is the effect of drying and wetting cycles of the core soil due to high fluctuations in reservoir water levels during prolonged dry periods and rewatering. In this study, the effects of frequent wetting and drying cycles of Doosti dam clay core were investigated. Investigation of the effect of frequent cycles of wetting and drying on compressive strength of soil showed that by applying drying and wetting cycles on specimens prepared from Doosti dam core borrow area, the compressive strength of specimens decreased. Increasing the number of wetting and drying cycles of the specimens increases the intensity of the compressive strength reduction of the specimens so that the greatest decrease in compressive strength occurred in the third cycle. Application of wetting and drying cycles on the specimens showed that application of such cycles reduced the amount of soil cohesion. The highest decrease in cohesion of the specimens occurred in the first three cycles and in the next four cycles, the intensity of cohesion parameter decreased. The cohesion of the specimens was reduced by 50% by applying six drying and wetting cycles. Results showed that with increasing number of drying - wetting cycles, there is no significant change in internal friction angle. Furthermore, the results of the hydraulic conductivity test showed that the application of six cycles of drying and wetting increased the hydraulic conductivity by 1.9 times. Furthermore, the characteristic curves obtained in this study showed different drying and wetting cycles, indicating that the percentage of volumetric water content in all cycles decreased due to increased suction. The soilwater characteristic curve also shifts downward with increasing number of wetting and drying cycles, decreasing soil water retention capacity and increasing the possibility of internal erosion and dam instability. Finally, Doosti dam numerical modeling was performed using GEOSTUDIO software in end of construction conditions and in two cases without drying and wetting cycle and with six drying and wetting cycles. The results obtained for numerical modeling in general were very consistent with Doosti dam instrument readings. Moreover, the results of the core settlement after the six drying and wetting cycles compared to the noncycled state in numerical modeling showed a 38.3% increase in the dam core settlement.