Aarabi Mohammad Javad
Experimental Study of Local Scour at Bridge Pier with Raft Footing
Mohammad Reza Chamani
Amir Ahmad Dehghani
Scouring in the river is a phenomena which is the result of interaction of flowing water and sediments which changes the topology of the rivers bed and coastal area. The most common cause for bridge failures reported in the literatures was due to pier scour. Three-dimensional flow filed with sediment transport and mobile channel bed during scour process leads to a complex problem. It is known that pier scour can be studied in either clear-water or live-bed conditions. Most of studies in this topic were carried out empirically. During past decades, several methods were used to control and reduce the scour depth. In addition to structural and geotechnical countermeasures, footing may reduce the scour depth around bridge piers. Raft footings are commonly used in historical bridge; e.g. Sie-o-Se Pol Bridge in Isfahan city in Iran which was built in 1602. It can be also used for low-span bridges where continuous footing are necessary.
In this study, the effect of raft footing on local scour at bridge pier was investigated based on model study under clear-water condition. Experiments were carried out in the Hydraulics Laboratory of the Civil Engineering Department at Isfahan University of Technology. Experiments were performed in a 9 m long, 0.40 m wide, and 0.70 m height flume. The 40-mm-diameter cylindrical pier was made of Teflon. The bed material was uniform sand with median diameter of 0.72 mm and standard deviation of 1.26. Width of raft footing was chosen as one to four times of pier diameter. The top elevation of the raft footing was set at the elevation of bed. The discharge was fixed at 26.1 L/s. The experiments were performed to attain the equilibrium scour depth based on Melville and Chiew (1999) criteria. The topographical mapping at the equilibrium depth are obtained for additional comparisons between the cases of piers with and without raft footing.
It is shown that raft footing alleviate the scour depth. It is shown that scour depth upstream of the raft footing with width equal to pier diameter reduced 52 percent as compared to the scour depth at bridge pier without footing. With width of raft footing reaches two times (or more) of pier width, no scour hole developed upstream of the raft footing. Raft footing also leads to reduced scour depth downstream of the pier. Using raft footing, the time of the equilibrium scour depth was increased as compared to the scour depth at bridge pier without footing. In the case of raft footing with width of pier, the equilibrium time increased 71 percent of the time needed for the equilibrium of scour depth at bridge pier without footing. Scouring downstream of the raft footing created two scour holes at a specific distance from the raft footing. With increasing the width of the raft footing, the location of the equilibrium scour depth downstream of the raft footing migrated to the centerline of the flume. Similar results were seen in previous studies with bed sill located downstream of the pier.