بررسي نواحي مختلف آبشستگي نسبت به سرعت جريان در پايه پايين دست در جفت پايه ها و تاثير روش حفاظتي كابل بر اين نواحي

چكيده- همه ساله پل‌هاي زيادي در سراسر جهان تخريب مي‌شوند. بسياري از پل‌ها به دليل ملاحظات سازه‌اي و ابعاد بزرگ آن‌ها روي گروه پايه‌ها استوار هستند. مكانيزم‌هاي ايجاد شده در فرآيند آبشستگي در گروه پايه‌ها باعث پيچيده‌تر شدن آن نسبت به تك پايه مي‌شود. تحقيق حاضر در شرايط آبشستگي آب‌زلال و بر روي پايه‌هاي استوانه‌اي انجام گرديد. در اين تحقيق روند آبشستگي پايه پايين‌دست بر اساس سرعت جريان مورد بررسي قرار گرفت. عمق آبشستگي موضعي اطراف پايه پايين‌دست را مي‌توان بر اساس مقدار سرعت جريان به چهار ناحيه تقسيم كرد: 1- ناحيه بدون آبشستگي 2- ناحيه آبشستگي هم‌زمان 3- ناحيه انتقالي 4- ناحيه انحرافي. در ادامه با اعمال كابل پيچيده شده در پايه‌ها به تأثير اين روش در كاهش آبشستگي در دو پايه و همچنين تأثير كابل در نواحي بوجود آمده در پايه پايين‌دست پرداخته شد. نتايج كاهش آبشستگي در اطراف پايه‌ها را با اعمال كابل نشان مي‌داد. همچنين عمق آبشستگي در پايه پايين‌دست كمتر از پايه بالادست بود و با به كارگيري كابل درصد كاهش آبشستگي پايه پايين-دست نسبت به بدون كابل در پايه پايين‌دست با فاصله D3 و D5 بترتيب 7 و 26 درصد مي‌باشد. بر اين اساس با كاهش عمق آبشستگي در سرعت‌هاي مختلف با به‌كارگيري كابل، نواحي به وجود آمده در پايه پايين‌دست دچار تغييرات به ميزان قابل توجه گرديد به نحوي كه ناحيه آبشستگي همزمان در بازه سرعت موجود از بين رفت.

Bridge failure is a common phenomenon all around the world. Bridges are one of the most important structures which are under attention from many years ago. Bridge is a structure to cross over obstacles such as rivers or valleys. Investigation of scouring in water structures especially in bridges is absolutely important in river engineering. Failure of several structures in all over the world are usually due to structural consideration and giant scales on piers. Created procedure of scouring by group of piers are more complicated than one single pier. Increasing the resistance of bed materials and decreasing the power of erosion factors are the ways to stand against local scouring. To decrease the power of erosion factors (horseshoe and wake vortex), equipment such as collars, submerged vanes and etc. are being used. This study was conducted with freshwater on cylindrical piers. The experiments were with a constant discharge during 6 hours in hydraulic laboratory of Ferdowsi University of Mashhad. In order to perform this study was used a channel that width, height and length were respectively equal to 0.3, 0.4 and 10 meters. In this research, Sands with median diameter of 0.72, special gravity of 2.65 grams per cubic centimeters and geometric standard deviation of 1.12 are used in the experiments. Range of flow rate was from 8 to 18 Liters per second. An adjustable weir in the downstream regulates the water depth in the channel. The area for conducting the experiments in the channel has one meter length and 10 cm bed height, which is 6 meters away from the beginning of the channel. Scouring procedure of downstream pier was investigated base on flow velocity. Results show that depth of local scouring of downstream pier can be categorized in 4 zone based on flow velocity: 1- No scouring occurrence zone, 2- Synchronized scouring zone, 3- Transitional zone, and 4- Deviance zone. Following previous investigations, effects of application of rolled cable over piers on reduction of scouring around two piers and zones of downstream pier (zones related to velocity) were examined. In this study, the efficiency of using rolled cable around the piers as one way of controlling and reducing local scouring, to decrease scouring is investigated. Results show that rolled cable, scouring will decrease. In fact, rolled cable around the piers will decrease the power of down flow, horseshoe and wake vortexes. Results show that scouring was reduced around piers due to application of rolled cable. So that cable at its best state reduces the maximum scour depth by 50 percent for downstream pier in the situation distance 3D and 54 percent for downstream pier in the situation distance 5D. Reduction of scouring depth of downstream pier was less than upstream piers. Scouring depth of downstream pier with rolled cable was 7 and 26 percent reduced in comparison with piers (No rolling cable was used) located at 3D and 5D from it. Because of scouring reduction as consequences of rolled cable, downstream zones were significantly changed that finally caused synchronized zone removal.