مقايسه‌ي نتايج تحليل سه ‎بعدي و دو بعدي براي سدهاي خاكي واقع‌شده در دره‎هاي تنگ (مطالعه‌ي موردي سد ونيار)

A Comparison of Three and Two Dimensional Analyses of Rockfill Dams in Narrow Valleys; Case Study: Vanyar Dam

سد ونيار در يك دره‎ي تنگ با نسبت طول تاج به ارتفاع برابر 3 احداث شده است. در اين نوشتار، تحليل عددي سه‎ بعدي سد ونيار انجام و نتايج حاصله با نتايج به‌دست‌آمده از تحليل عددي دو بعدي و داد‌ه‌هاي ثبت‌شده توسط ابزارهاي نصب‌شده در بدنه‌ي سد مقايسه شده‌ است. اين بررسي‌ها براي بزرگ‌ترين مقطع عرضي انجام و نتايج حاصل از تحليل شامل: نشست، فشار آب حفره‎‌يي و تنش عمودي كل با يكديگر بررسي شده است. نتايج حاصله نشان مي‎دهند كه نشست‌هاي ايجادشده در نيمه‎ي بالايي ارتفاع سد در شرايط تحليل دو ‎بعدي و سه ‎بعدي با قبول خطايي اندك بر يكديگر منطبق هستند، ولي در نيمه‎ي پاييني ارتفاع سد اختلاف نتايج زياد شده است، به‌طوري كه تحليل دو بعدي مقادير نشست را بيشتر از تحليل سه ‌بعدي و ابزار دقيق نشان مي‎دهد. اما در بخش تنش كل قايم و فشار آب حفره‏‌يي مي‎توان گفت كه در بيشتر نواحي مقطع مورد مطالعه، تحليل دو بعدي مقادير بيشتري را نسبت به تحليل سه ‎بعدي نشان مي‎دهد.

A two dimensional analysis on the largest cross-section of earth dams gives accurate results, provided that the dam is located in a U-shape valley. The Vanyar Dam is constructed in a narrow valley. According to the geometry of the valley, there is not a plane strain condition throughout the long side of the dam. In this research, the results of three and two dimensional numerical analyses of the Vanyar dam are compared, in terms of settlement, pore water pressure and total vertical stress. These analyses are carried out by the finite difference method, using FLAC-3D and FLAC-2D software packages. The numerical results obtained for the settlements are verified by comparing them with measured settlement data. The settlements obtained by three and two dimensional analyses in the upper half of the height of the dam are in good agreement. Moreover, it is shown that for the lower half of the height of the dam, the settlements obtained by two dimensional analyses are generally greater than those obtained by three dimensional analyses. In the core axis at cross section C, the maximum settlement of 88.14 cm is recorded at the height of 48 m from the bedrock ( ), which is about 1% of the dam height. Furthermore, the settlements obtained by three dimensional numerical analyses are in better agreement with measurement data, compared to two dimensional analyses. It indicates that the results of two dimensional numerical analyses of the earth dams located in the narrow valley are not sufficiently accurate. The total vertical stresses of the three and two dimensional numerical analyses are compared with those recorded by the pressure cells. To prevent damage to the pressure cells, light compaction is performed around the instruments. Therefore, a low stiffness area is formed around the pressure cells. This phenomenon is called local arching, which causes the vertical stresses recorded by pressure cells to be less than those numerically calculated. Moreover, the pore water pressures recorded by the piezometers are consistent with numerical results.