عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Scour happening around bridge piers is one of the major reasons to bridge destruction. At present, there are various methods to prevent bridge bases scouring. One method is to set protector boards (collars) around bridge piers. The collar is like that a thin plane surface that surrounding the pier. Many experimental studies were done to illustrate the hydraulic function of collar to protect pier against scour, and also many researchers have examined numerical approachs and developed many packages such as SSIIM, FLUENT, FLOW3D, etc. Collars protect the bed against the vortex system around the bases and it reduces the amount of water scouring. In this study, Lozenge, Square and Circle collars with slight thickness have been used to reduce scouring around bridge bases. In the conducted tests, the diameters of the used collars have been considered twice the base diameter. Various discharges have been considered for these tests and median size of bed grain is considered equal to 1.8 millimeters uniformly (d50=1.8 mm). In order to examine the effect of collar to control scour around the bridge pier in a direct channel, a laboratory model is constructed. The experiments were done in Soil Conservation and Watershed Management Research Institute. They were performed in a flume made of glasses for wall and floor partition with a length of 6 meters, almost 30 cm width and 30 cm height. There was a valve for flow depth control at the end of the flume. In all experiments, to eliminate the effect of flume walls on local scour; pier diameter must not be longer than 10% of flume width according to Chiew and Melville (1987). Considering Raudkivi and Ettema (1983), the standard canal width shall be 6.25 times of pier diameter minimally so the effect of channel wall on pier of local scouring has been eliminated. Therefore, the pier with the width of 21, 30 and 40 mm is used in this study. The collars diameter was fixed twice the pier diameter. The depth of water was constant and equal to three and half times of pier diameters. The time of every test were chosen about one hundred minuets since about 85% of final scouring after 5 hours was occurred in 100 minutes. The experiments have achieved with clear water and discharge was measured by triangular weir adjusted at the end of the flume. The critical velocity of bed material was calculated by Shields diagram. The profile indicator device was used to determine of the bed surface and scour hole dimensions, and then plotted these data by Surfer 11 package.
In general, in the experiments equipped with a collar, it was observed that scouring started due to vortexes activities at the back of the pier. Rising vortexes make grooves beside the base margins so scouring continues by horseshoe vortexes through the development and extension of these groves to pier front and then it penetrates to base underneath and this process digs a scour hole in pier front.
Results showed that the pattern of scouring have directly related to collar’s shape in each pier. Through the use of circle and square collars, the scouring location happened in near the piers, on the other hand, when the lozenge collar was used, scouring happened in a certain place in each pier. By using various collar shapes, the dimensions of the formed hole and hill were different.
The results of using 3 different diameters cylindrical piers and 3 different collar shapes, showed that the scour dimensions directly related to the pier diameter. In such a way that the amount of scouring increases by increasing the pier diameter. In other words, holes and hills are made in a larger region. The results also showed that scour pattern was directly associated with the collar shape. So that the use of circle and square collars lead to occur the scouring location near the pier, however, when the lozenge collar was used the scour hole was formed in a farther distance from the pier. The ratio of the distance of the maximum scouring to the depth of the scour in the lozenge collar, is approximately 52% more than the circle and the square collars. The height of the maximum sedimentation by using the lozenge collar, is 58% less than the circular and square collars.