The effect of simultaneous use of cable and collar in reduction of scour around the group of three piers

This study is presented to investigate the effectiveness of simultaneous use of cable and collar in reduction of local scour depth in groups of two and three piers. The experiments were carried out in a 20m long, 0.60m wide, and 0.60m deep, Plexiglas-sided flume. The working section, in which piers were located, was 8m long with a recess on the bed 0.2m deep and was situated 8m downstream from the entrance of the flume in order to generate fully developed flows. The recess was filled with uniform sediment with the mean particle sizes of 0.71mm and geometric standard deviation of particles was σg=1.135. Cylindrical Perspex pipes with diameter of 60mm were used as pier models. The tests were conducted on a group of three piers in line. For each group, the spacing (S/b where S is the distance between the piers sections and b is the pier diameter) of the piers was varied. The collar with the diameter of 3 times wider than the pier diameter was developed at the bed level around group of three piers. The cable with the diameter equal to 0.15 of the pier diameter with a torsion angle equal to 15° and combination of them around each pier was studied.
In experiment of group piers with collar, two different spacing of S/b=2 and 3 were tested for a group of three piers in line. The experiment with spacing S/b=3 the space between two piers were not completely covered with collar. The scour hole was developed at back of the third pier. Also, two grooves were gradually developed at the down-stream rim of the collar and gradually developed and extended upstream and eventually reached the upstream edge. At this moment, the flow was intensified through the groove, reducing the side slope of the groove and with sediment removal from the grooves, the scour hole extended upstream of the pier and below the collar. By penetration of the flow beneath the front edge of the collar the rate of sediment removal increased. Maximum depth of scour hole is equal in front of the first pier with and without collar. In the first pier, both spaces had the same delay time and maximum scour depth because the space between piers had no effect on efficiency of collar. The results of experiment with cable showed that reduction of scour depth in the second and third pier is more than the first pier, due to sheltering effect of the first pier on the second and third piers. Also, in this case, with increasing the space between the piers the reduction of scour depth was decreased slightly. The simultaneous use of cable and collar showed that reduction of scour depth in upstream of group of three piers with S/b=2 is about 20%, 45% and 53% respectively. Since the strength of wake vortex reduces, reduction of scour depth in upstream of the pier protected with cable and collar is more than the reduction of scour depth of the pier protected with collar and pier protected with cable. In this case, comparison of delay time reveals that using collar caused the delay in scouring mechanism and when the collar and cable combined the delay time is increased. In the first pier, cable and collar cannot cause salient reduction in maximum scour depth due to the reinforcement effects counteract the effect of collar and cable. In the second and the third piers when the collar and cable combined, the delay time and equilibrium time are increased. In these piers, the maximum scour depth is decreased as well. The efficiency of cable and collar to reduce the scour depth in the third pier is greater than the second pier due to increasing the sheltering effect.
The results indicate that the scouring is postponed and reduced in protected pier with cable and collar more than protected pier with collar; therefore cable increases the efficiency of collar. The space between the piers has a considerable effect in reducing the scour depth, so that the scour depth was decreased and the delay time was increased, with increasing the space between the piers. The most reduction of scour depth was observed for group of three piers with S/b=2, in which the scour depth of the first, second and third piers reduced to about 20%, 45% and 53% respectively. In situation that a pier protected with cable and collar with S/b=3, the scouring postponed was maximum. So, use the cable with the collar can greatly reduce the risk of bridge destruction.