بررسي آزمايشگاهي مدل‌هاي تركيبي سكو، شيب جانبي و شكاف در كاهش فرسايش بستر پيرامون پايه هاي پل رودخانه‌اي

Experimental Study of Combined models of berm, side slope and slot in decreasing the bed erosion around bridge piers

در اين مقاله نتايج آزمايش 12 مدل پايه پل مركب براي كاهش عمق آبشستگي، در شرايط آب زلال ارايه شده است. گروه هاي آزمايشي به ترتيب عبارتند از 1) پايه سكويي و پايه نيمه مخروطي 2) تركيب پايه سكويي با مقاطع نيمه مخروطي با قطر متغير در بستر 3) تركيب پايه سكويي با مقاطع نيمه مخروطي با قطر ثابت در بستر و 4) تركيب گروه سوم با شكاف ذوزنقه اي. در پايه سكويي كاهش قابل توجه عمق آبشستگي (تا حد 46 درصد) نسبت به پايه استوانه اي مرجع مشاهده شد. اين كاهش در مقطع نيمه مخروطي نيز وجود داشته ليكن مقدار آن بسيار كمتر از پايه سكويي است. در مدل پايه هاي گروه دوم، پايه مركب نيمه مخروطي با شيب 2 درجه، كمترين فرسايش بستر را نسبت به دو مدل ديگر داشته است. در اين مدل عرض سكو بزرگتر و قطر پايه در تراز بستر نيز كمتر است. در اين گروه، عمق آبشستگي نسبت به مدل پايه سكويي افزايش داشته است ليكن مقدار آن نسبت به پايه استوانه اي مرجع كاهش يافته است. در مدل پايه هاي گروه سوم، با ثابت نگه داشتن قطر پايه در عمق بستر، كاهش بسيار اندكي در عمق آبشستگي مشاهده شده است. از اين رو در پايه هاي مركب نيمه مخروطي، افزايش قطر پايه در تراز بستر، تشديد فرسايش بستر را به همراه داشته است. در گروه چهارم نيز اعمال شكاف در پايه هاي مركب، نقش موثري در افزايش نقش محافظتي آن ها نداشته است. بنابراين در مجموع مي توان نتيجه گرفت تاثير مدل پايه سكويي در كاهش عمق آبشستگي به مراتب بيشتر از ساير پايه ها بوده و تركيب ابزارهاي مختلف بازدارنده فرسايش بستر، نمي تواند در تمامي موارد تاثير مثبتي در كاهش عمق آبشستگي داشته باشد. در تمامي مدل ها مقايسه بيشينه عمق آبشستگي، نسبت به پايه استوانه اي مرجع و پايه سكويي انجام گرفته است.

When a structure is placed on an erodible bed, it causes an increase in local sediment transport capacity and consequently this leads to scour in the vicinity of the structure. This is referred to as local scour in the literature. Local scour has been identified as one of the key factors that cause failure of structures in the bridges, Jetties and offshore platforms. The complexities of bridge scour mechanism have caused the scour is to be one of the most active topics in civil engineering researches. Numerous studies have been reported on local scour around bridge piers in steady currents in the last decades. A majority of these studies deal with laboratory model studies. A brief literature review can reveal the importance of this subject. Several methods have been proposed by researchers to control the scour around the bridge piers. These methods can be divided in two main categories: the first is the armoring method or increasing the resistant of the bed material around the pier as rip rap, and altering of the flow pattern around the pier is the next one. The combined effect of some tools has been studied in recent years. Using just one bed erosion controlling tool does not appear to be adequate in many cases according to conclusions of these studies. On the other hand, combining different tools could not always lead to gain better results than using a single one of these tools. In this paper, the test results of 12 combined bridge pier models to reduce the maximum score depth in clear-water condition have been presented. Experimental models are respectively: (1) berm and semi-conical piers, (2) combination of berm and semi-conical piers with variable diameter in the bed, (3) combination of berm and semi-conical piers with fixed diameter in the bed, and (4) combination of third group with trapezoidal centered slot. Significant decrease (up to 46 percent) in scour depth was observed in berm pier with respect to the cylindrical reference pier. This decrease was also observed in semi-conical pier but it was much lower than the berm pier. In the second group, semi-conical combined pier with the side slope angle of 2 degrees of model piers had the least bed erosion in comparison to other two groups. In this model, the width of the berm is larger and diameter of the pier is lower in bed level. Scour depth is increased in comparison to the berm pier model in this group but it is decreased comparing to the cylindrical reference pier. Very slight decrease has been observed in the scour depth of third group model piers by fixing the pier diameter in the bed depth. Hence, increasing the pier diameter in the bed level has intensified the bed erosion in the semi-conical combined piers. Applying slot in the combined piers of third group was ineffective in increasing the protective role of model piers in the fourth group. Therefore, it can be concluded that the effect of berm model pier in decreasing scour depth was far more than other piers and combining different countermeasures for the bed erosion could not have positive influence on decreasing the scour depth in all the cases. The comparison of maximum scour depth has been conducted in all models in relative to cylindrical reference and berm pier.