Experimental Study On Sudden Siltation Of Open Channel In Silty Coastal Areas Due To Liquefaction

During storm periods, sudden siltation often happened in open channels which were excavated and constructed in silty coastal areas. The siltation sediments were mostly dense and hard, difficult to dredge. It restricted the construction and development of haven channel. As the development of economy and increase of port throughput, the work about analysis of reasons and prevention of sudden siltation was becoming more and more important. In view of the significance in actual engineering, some research works were carried out in this paper.Based on the fact that silty bed is easy to liquefy under wave actions, in-lab flume experiments were carried out with soil taken from the Yellow River Delta. According to the results, this paper pointed out the reasons of sudden siltation which often happened in open channels during storm periods. Beased on the reason analysis of sudden siltation, a series of in-lab flume experiments were designed and carried out with soil taken from the Yellow River delta to test the effects of submerged breakwater on preventing sudden siltation. The prevention effects of submerged breakwaters with different heights were discussed, and a reasonable design elevation was recommended. Following the Geometrical Similarity Principle, an elevation design scheme for submerged breakwater under field conditions was recommended and hydraulic sheeting piles can be employed in its engineering process.Dynamic response of silty seabed under wave cyclic loading was studied though flume experiments. In the experiments, the local weak intensity silt tended to liquefy under wave actions, and the liquefied soil showed oscillation with wave actions. The sliding soil perimeter expanded and deepened gradually to the maximum which is different under different wave actions. The maximum will largen with the increasing of wave height. When the wave height increased to a value, the sliding soil perimeter didn't deepen obviously. But in the horizontal direction, the sliding soil perimeter disintegrated and expanded on a large scale suddenly. With constantly oscillating sliding, the intensity and density of liquefied soil increased and water content reduced. Meanwhile, the escape channel of pore water became unimpeded. Pore water with some clay particle moved up gradually and entered into overlying water. As a result, the liquefied soil coarsened gradually. Finaly, a hyperconcentration sediment layer about 2.0 cm thick was observed near the bottom. The sediment concentration in this layer was 3~4 times higher than the average sediment concentration.Under field conditions, local weak intensity silt at both sides of the channel tended to liquefy and oscillate under intense wave actions. And hyperconcentration sediment layer appeared near the bottom. The liquefied soil and silt in hyperconcentration sediment layer flew into channel by gravity and led to sudden siltation. The liquefied soil in channel densified and coarsened gradually during continuous oscillatory movement under wave actions, and finally formed hard silt layer which was difficult to dredge. This can be the main reason for channel sudden siltation.Based on the consideration that both hyperconcentration sediment layer near the seabed surface and sliding of the liquefied soil caused sudden siltation, a set of in-lab flume experiments were designed and carried out to test the effects of sheet submerged breakwaters. The sheeting submerged breakwater was inserted into bed to prevent the liquefied soil from sliding into the channel, while the upper sides were set some distances above the sediment bed to stop sediments in the high concentration layer. The prevention effects of submerged breakwaters with different heights were discussed. A reasonable design elevation was determined to be 6 cm under experimental condition. The wave could carry sand upwelled when encountering the baffles, and leave the coarse grains near the bottom leap into the simulated channel, so the Anti-arc top is designed to increase the prevention effect. According to the Geometrical Similarity Principle, an elevation design scheme of hydraulic sheeting piles submerged breakwater under field condition was recommended. In allusion to the instability of individual pile dikes,? main reasons were analyzed and some prevention suggestions were proposed against failures of pile dikes.An analytical method of sudden siltation was recommended. Firstly, a discrimination process of silty liquefication was summarized. The possibility of sudden siltation happen can be estimated by distinguishing if silty soil can liquefy. After that, silty soil liquefaction depth should be calculated, and then design reasonable hydraulic sheeting piles submerged breakwater to prevent sudden siltation.