Study On The Mechanism Of Piping In Dike Foundation And The Effect Of Suspended Cut-off Wall

During the 1998 floods, risks caused by dike foundation piping accounted for 52.4% among all the severe risks in the Yangtze River dikes, ranking the first among all types of dangerous situations. Dike foundation piping not only threatens dike safety, but also is the most difficult problems to be treated. Therefore, it is very necessary and practical to study the mechanism of dike foundation piping and the effect of suspended cut-off wall.Physical model tests in a flume were conducted to simulate piping process in three types of different foundations, namely the single, double and triple pervious stratum foundations. The piping processes were recorded that involved the initiation, development and finally dike failure. The critical horizontal average seepage gradients of different dike foundations were measured. The piping characteristics of different dike foundations were analyzed. In addition, model tests were carried out to study the effects of suspended cut-off wall on decreasing the hazard of piping and its effects on increasing the holistic stability of dike foundation in double stratums. From these studies, the following conclusions can be inferred:â‘  Piping will cease by self-healing if the hydraulic head is lower than the critical value. Under this condition, the piping channel can't reach the upstream and its width and depth are limited. As a result, it will not cause dike failure. Otherwise, if the water head exceeds the critical value, the piping channel will propagate persistently to upstream and finally cause dike failure.â‘¡ Critical seepage gradients vary with different dike foundations. For the sand samples in the model tests, the critical average horizontal seepage gradients are 0.278 and 0.214 for single and double stratum foundation respectively. For triple stratum foundation, the critical gradient will decrease along with the decrease of sand layer depth and its minimum value is 0.078 from the tests. From these results, it can be concluded that the horizontal control criteria for dike foundation piping should be related with not only the characteristics of the top sand stratum, but also the foundation structures.â‘¢ Piping mechanisms and the damaged zones vary with different dike foundations. For the single stratum, double stratum and the triple stratum if the top sand stratum is thick enough, piping channel propagates horizontally along the surface of the top sand stratum. The damage zones are just within the piping channel in the top sand layer and are small in depth and width. But for triple stratum foundation, if the top sand layer is relatively thin, piping is vertical. The damage zones are large in width and depth that expand to the sand layer and gravel layer.â‘£ Suspended cut-off wall can effectively increase foundation resistance to piping and it is more efficient when it is situated in land side than in river side. This is an innovative viewpoint compared with traditional concept that adopts seepage control measures in river side and use drainage measures in land side. Therefore it is suggested that suspended cut-off wall can be used for seepage control in future design. In addition, it is recommended that suspended cut-off wall at the land side dike toe or the end of land side berm be taken as an alternative in the selection of possible control...