تعيين ابعاد بهينه زهكش پنجه‌اي بر اساس بهترين خط نشت در سد‌هاي خاكي همگن با استفاده از مدل آزمايشگاهي

Determination of the optimum size of a toe drain based on the best phreatic line in homogeneous earth dams by using vitro model

در اين مقاله تاثير زاويه و ابعاد زهكش پنجه‌اي بر خط نشت آزاد آب در بدنه سدهاي خاكي همگن با استفاده از مدل نرم آزمايشگاهي بررسي شده است. با توجه به خلا مطالعاتي در مورد ارتفاع بهينه زهكش پنجه‌اي سدهاي خاكي و عدم وجود استاندارد معروف و ملزوم، در اين پژوهش به اين موضوع مهم پرداخته و روش كار بحث شده است. بدين منظور مدل آزمايشگاهي مناسبي ايجاد شد. آزمايش‌ها با ارتفاع‌هاي مختلف آب در پشت ديواره سد خاكي همگن انجام شد و با استفاده از داده‌هاي پيزومتريك، وضعيت خط نشت آزاد آب در حالت‌هاي مختلف زهكش پنجه‌اي مقايسه شد. در پنجه سد سه نوع زهكش با زاويه‌هاي 45، 60 و 90 درجه در ارتفاعات مختلف ساخته شدند و روند خط نشت در بدنه سد خاكي در حالت‌هاي مختلف آزمايش و در حداكثر ارتفاع آب سد خاكي با استفاده از هفت چاهك مشاهداتي در بدنه سد و 30 پيزومتر در ديواره فلوم برداشت شد. زهكش‌ها در سه مقطع با سه ارتفاع ساخته شدند كه در ادامه 9 مدل آزمايشگاهي آماده شد و داده‌هاي نشت و دبي برداشت گرديده شد. منحني‌هاي برداشت شده در نقاط انتهايي خط نشت از روند خطي تبعيت مي‌كردند اين امر موجب شد معادلات رياضي خط نشت در انتهاي بدنه سد و ابتداي ورود به زهكش ترسيم گردد. و محل برخورد خط با بدنه زهكش از حل عددي معادلات رياضي خط نشت و خط رياضي در يك دستگاه حل معادلات برداشت شد. لازم به ذكر است مبدا مختصات در پاشنه سد قرار داده شده است و تمامي معادلات در اين محور مختصات ترسيم شده بر اساس اين مبدا مي‌باشند. در‌ادامه نقطه برخورد خط نشت با زهكش به صورت نقطه A معرفي شد. كه اختلاف ارتفاع اين نقطه با حالت شاهد (بدون زهكش) ميزان افت حاصل از جا نمايي زهكش معرفي شد و با مقايسه اين افت‌ها بهترين زهكش كه زهكش 45 درجه مي‌باشد، معرفي مي‌گرديد.

In this study, first an earth dam was embanked homogeneously in a laboratory flume with 6 m length, 60 cm width and height of 1 m. The physical model was constructed with a height of 1 m and a length of 4.5 m in the foundation. The upstream and downstream slopes of the earth dam were (2.5-1), which are conventional slopes in a homogenous earth dam. In order to check and obtain the hydraulic conductivity in the range of the homogenous earth dam, two kinds of soil were blended and finally K was compared with the references. The obtained K was equal to 4 meters per day, which conform to previous studies. Afterwards, the effect of the angle and dimensions of the toe drain on the phreatic line was investigated in the homogenous dams. Lack of enough studies about the optimal size of toe drain and its applied standard is the main aim of this new research; that is why the applied lab model was constructed at Shahrekord University. First, the model was embanked from homogenous material and the data were collected from this model with and without toe drain in 3 situations. In every situation, 3 toe drains were used whose sizes were variable. To plot the seepage line, 30 piezometers were set up and their heads were measured. In addition, observation wells were dug in the body of the model. Therefore, the observation wells and piezometers were used to monitor the phreatic line. After that, the obtained results were compared with each other using the Excel software and the tolerance was calculated. Eventually, the 10 situations were plotted. One of them had no toe drain and 9 of them had a toe drain. For example, in the first experiment, a toe drain with an angle of 45 degrees was used. Subsequently, the other toe drains angles were also examined. For solving the problem of dimension used (p/h) index that p was the height of the drain and h was height of water in reservoir of earth dam. Location of data sampling had been at behind of the homogenous earth dam wall on different height of water. Height of water fixed in 3 states in physical model (30, 60 and 80 centimeter). In state one and state two the phreatic line never coincide the downstream side of the earth dam body. Finally, it has decided to use the data of 80 centimeters. Situation of phreatic line will be monitored by using piezometers installed in different locations of toe drain. Three types of drain with constant angle (45, 60 and 90 degree) were instructed at different heights and the phreatic line was monitored for different experiments on the homogenous earth dam at maximum height of water. The best formation and dimension of stony toe drain is gotten by collation of different equations on phreatic line (based on comparison by experiment without drain). The results showed that the homogenous earth dams can be obtained highest loss in phreatic line when upward of toe drain angle and its top height had 45 degrees and 0.35 maximum height of water in the reservoir. Also in 90 degree toe drain the effect of toe drain on seepage was very well. It means that this toe drain have much head loss in seepage line, but the defect of this mater shown in this way when the toe drain created with big size (p/h= 0.35) and (90 degree) the flux of seepage was too much. So for a solution for this problem was using a form of toe drain which have a perfect head loss in seepage line and have optimum flux that exit in the toe drain that constructed in down slope of the dam. If it decided to show the effect of another toe drain like (p/h=0.35 and 60 degrees) this case, the toe drain with 90 degree has more flux than toe drain with 45 degree, but it hasn’t optimum head loss. Finally, by comparing all of states, the best toe drain that has the optimum loss head and seepage flux was introduced.