Malihe Keykhee, M Heydarpor, Farhad Mosavi,
Volume 13, Issue 49 (10-2009)
Abstract
Ripraps are placed around bridge piers to prevent scour and secure the piers from failure. Proper riprap cover is essential to be economical. The present study examines using of riprap for reduction of local scour in piers group and the results are compared with data from riprap on a single pier. The models consist of two and three circular-shaped piers in line with the flow, with the diameter of 0.02 m and pier spacing of twice and four times the pier diameter. Four uniform riprap sizes with the diameters of 2.86, 3.67, 4.38 and 5.18 mm were used to cover the piers. The results showed that the effect of wake vortices formed at the downstream side of piers group was decreased as compared with single pier. The reinforcing and sheltering effects caused 31% decrease in front pier and 60% increase in back pier, respectively, for the length of cover riprap. The reinforcing and sheltering effects were decreased by increasing pier spacing, but the riprap pattern was not affected. In triple piers group, scour depth in the second pier was less than the first pier and in the third pier was less than the first and second piers. In double and triple piers group, the sheltering effect reduced the scour depth (46% and 54%, respectively) in the back pier with respect to the single pier. Reduction of dimensions in scour hole of back pier in triple piers group was 67% with respect to double piers group, which is the result of sheltering effect of first and second piers. The best shape for the riprap was semi-oval. The riprap length in double and triple piers group was reduced by 31% and 37.5%, respectively, as compared with the single pier.
M. Majedi Asl, R. Daneshfaraz, S. Valizadeh,
Volume 24, Issue 2 (7-2020)
Abstract
Sand mining from rivers is one of the biggest concerns in the science today. Certain principles and rules for choosing the right place for mining materials and the amount of this mining are missing in the design codes. Therefore, mining of river materials from sites with less potential and near structures has been occasionally seen. In the present study, it has been attempted to reinforce the structure to control the impact of the mining of material, which results in the increased scour by changing the flow pattern around the structure. The experiments were carried out in two simple and armed modes, in sand bed with a grain size of 0.78 mm, with a length of 4.25 meters, inside a canal of 13 meters in length and 1.2 meters in width. The extent of scouring along the longitudinal and transverse directions in different times from the start to the scouring equilibrium was investigated for all substrates under sub-critical flow conditions (range 0.5-0.25). The results showed that the use of a cable-protected method in the upstream pit led to 29.6% reduction in the maximum scour depth at the front and 34% reduction in the back of the pier; also, in the downstream of the pit, it reduced the maximum scour depth by 15% at the front of the pier. Therefore, the cable arrangement used at the piers surface, according to the current research method, resulted in a significant reduction in the depth and extent of scouring in the pier group of the bridge.
