Investigation on Effect of Exit Flow Gradient of Piping Phenomenon in Coastal Slopes under Fluctuations of Water Level in Presence of Cutoff Wall

One of the main reasons of destruction of the coastal slopesand riverbanks is failure of side slopes under the rising water table or sliding of the side slopes under the drawdown of channel water. In this research, effect of variations of water table and channel water level (peak and drawdown conditions of flood, respectively) on the piping of coastal slopes was studied and effect of cutoff wall on the phenomenonwas investigated. Experiments were conducted in a seepage tank having a trapezoidal cross-section with the side slope of 1.5% and 0.58 mm mean diameter of the particles. Cutoff wall was fixed in the relative depths Z/d=0.5, 0.67, 0.83 and 1 (Z is the depth of cutoff wall and d is the depth of channel). Results of peak conditions of flood (rising the water table), under controled experiment showed the relative channel water level was equal to y/d=0.7. In the experiment with cutoff wall Z/d=1 and without occurrance of piping, exit flow gradient was decreased approximately 41 present with respect to the control experiment. Piping was occurred only in the relative channel water level equal to y/d=0.167. Results of drawdown conditions of flood showed, piping phenomenon occured in the all channel water level. In the experiments with cutoff wall, in the cases with high water table, despite of existence of cutoff wall, increasing the exit flow gradient caused piping phenomenon in the all of the experiments because of increasing hydraulic head. Other wise, in the experiment with the relative water table equal to h/d=0.933, piping was occurred in the all depths of cutoff.