بررسی آزمایشگاهی تأثیر استفاده از طوق در کاهش عمق و تأخیر در شروع آبشستگی در اطراف تکیه‌گاه مستطیلی پایه پل

Scouring phenomena around bridge abutment is one of the most significant causes of bridges failure. Investigation on erosion around bridge abutments and present a reliable method to protect them against scouring is necessary because of significant and vital rule of these structures in transportation. There have been many studies to reduce the maximum depth of scouring at abutments such as using a protective collar, riprap mattresses, Geo bag, etc. But most of them are not economical and efficient to countermeasure the abutments with different shapes and length. Using a protective collar at abutment is one of the simplest and most applicable techniques to countermeasure of abutments in comparison with other protective methods. Simple application and appropriate performance of collar, lead to widespread investigations on it. To study the performance of collar, shapes and lengths (La) of the collar are the most effective parameters in reduction of maximum depth of scouring. While, the performance of collar varies for different flow intensities. For smaller flow intensities, using collar may lead to diminish the scouring at the tip of the abutments. But for higher flow intensities (near the threshold of the bed material), the efficiency of collar reduces and flow can be able to scour below the collar and reach to the tip of the abutment. As a whole, there are two types of abutment (short and long abutments) in terms of length. In short abutments, the ratio of the length of the abutment to the flow depth is less than one ( ), while for long abutments this ratio is more than unit ( ). In the previous investigations, the applicability of using collars in reduction of scour depth at short abutments has not been studied widely. Also, there are no many studies on the effects of various flow conditions on the efficiency of collar for both short and long abutments and no comparing between them. In this study, scouring around two kinds of rectangular abutments (short and long abutments) were investigated. All experiments were conducted in a 1 m width, 1 m depth and 14 m length rectangular flume located at the Porous Media Laboratory of Amirkabir University of Technology. The test region, where the abutments were placed in the flume, was selected in a way that a uniform flow was established and fully developed turbulent flow was reached. The bed materials are uniform with a median size of 0.91 mm and a geometric standard deviation of 1.38. The experiments were conducted under the clear water and various flow conditions (U/Ucr=0.95, 0.85, 0.75). The discharge of the flow was adjusted by an inlet valve and measured by a rectangular weir at the end of the flume. In this investigation, long duration (70 hour) tests were conducted to determine the equilibrium depth of scouring. Equilibrium depth of scouring was defined as a depth with less than 1 mm change after 2 hours of experimental run. According to the conducted tests and obtained time duration data, 40 and 50 hours were adopted as the equilibrium time for short and long abutment experiments (with and without collar), respectively, in which 95% maximum depth of scouring was occurred. The experiments were continued until reach the relative equilibrium state. The experimental results for scouring of the bed materials for the various flow intensities of U/Ucr=0.95, 0.85 and 0.75 showed that the use of collar with 2La in length for short abutment and La for long abutment at bed level, causes the reduction of scouring about 46%, 57% and 100% for short abutment and 26%, 24% and 60% for long abutment, respectively. The start time of scouring at the tip of the long abutment in the same flow condition and using collars with the same sizes, took place 405, 490 and 760 minutes after experiments run, respectively. Also, the start time of scouring at the tip of the short abutment protected by collars at bed elevation took place 300 and 255 minutes for U/Ucr=0.95, 0.85 flow intensity and no scouring on the tip of abutment during 40 hours of test run. Furthermore, applying collar on abutment led to reduce the scouring volume around the abutment equal to 37%, 64% and 25% for short and 34%, 43% and 44% for long abutment in U/Ucr=0.95, 0.85 and 0.75 flow intensities, respectively.