Determine the Best Coverage Pattern of Geotextile for Reduction Scour around Cylindrical Group Piers

Flood currents are considered as a threatening factor which creating local scour along bridge piers. In studying the reasons for the destruction of many bridges, scour resulting from flood currents has been identified as the major reason for the failure. Scour is a natural phenomenon which occurs as a result of erosion in bed and drift riversides by water current. This phenomenon is created locally as a result of mutual effect of water way geometry, bed and current specifications around the pier. As a matter of fact, when a current strikes bridge pier, holes are made around the pier and complex vortex systems are created whose performance causes a hole around piers called a scour hole. The development of this hole around the pier causes under the foundations to get empty leading to their destruction and finally the destruction of the bridge. In order to prevent and decrease scour at the bridge pier, two basic methods have been presented. One of them is the direct method in which the resistance of the bed against tension increases; and the other is the indirect method in which the effect of the destructive forces is reduced by changing the current pattern around the pier. Prediction of depth and the final status of scour hole are the most important cases of hydraulic design of bridges. Geotextiles, which are produced from polypropylene and polyester fibers, form a large group of geosynthetic products. These materials can be used to strengthen river beds adjacent to bridge piers and to control the scour around them concurrently playing the role of riprap and collar. Geotextile spread around bridge piers to prevent the scouring around it. In terms of economics, determine the extent and pattern of suitable geotextile cover is essential.
In this study, coverage area with geotextile around the single pier and a group of two and three piers in the case of clear-water flow over uniform sediment were investigated. The experiments were carried out in a 20m long, 0.60m wide, and 0.60m deep, Plexiglass-sided flume. The piers of Teflon were selected with 5cm diameter and fixed parallel the streamline. The space between the piers was 3 times bigger than pier diameter. The granulated silt of bed materials was chosen so that no ripples are formed in the test area bed. The formation of ripples causes separate small protrusions in the bed; it also causes an increase in current speed compared to the critical speed in the movement threshold of sediments causing some of the upstream sediments of the pier to be washed and carried thus filling the scour hole and causing the scour depth to decrease compared to its real maximum amount. Ripples and non-sticking sediments with mean sized from 0.05 to 0.7 mm develop; hence, the mean diameter of sediments should be over 0.7mm.
The results showed that in group piers the sheltering effect of the first pier decreases 50% of coverage pattern and the reinforcing effect of the last pier increase the coverage pattern about 40%. Sheltering effect in a group of two and three piers let to decreasing scour hole about 50% and 71%, respectively, than the single pier. At the rear pier of a group of three piers, the dimensions of the scour hole were 21% less than a group of two piers because the first and second piers protect the rear pier. With experiment on coverage patterns, it was found that the oval layer in single pier and the semi oval layer in group piers is the best pattern for scour reduction. In this state, after 19 hours, the first signs of scour in downstream are observed whose longitudinal and widthwise progression were about 3cm to the end of the test and its depth, too, reached 0.5cm. Base on the obtained results, with regard to the negligible amount of scour, the semi-oval pattern with more coverage in behind the pier is suggested as the best pattern in geotextile installation. By using geotextile with an appropriate cover, apart from a delay in scouring process, the scour location is transferred to downstream and the scour depth is decreased. In other words, the geotextile layer has a dual performance in decreasing scour. As far as economy and method of application are concerned, the larger layer; and as far as the scour is concerned, the smaller layer would be problematic.