Research On Piping Development And The Function Mechanism Of Suspended Cut-off Wall

Piping has been regarded as a crucial danger that threatens embankment dams and levees. As the occurrence of piping is unpredictable and the pipin g progression cannot be observed, it is difficult to understand piping’s phenomenon and physicalmechanism scientifically. From the early last century, many researchers had done some works on experiments and theoretical research on piping. However, due to lack of research on erosion mesomechanism, there is still not the numerical work which can simulate the progression of piping erosion and evaluate the seepage control measurements in practice. This dissertation studies the mesomechanical mechanisms of piping erosion based on the cohesionless soil, and the main contents are as follows:Firstly, the numerical model of piping development which contains the mechanism of internal soil erosion and damage of piping tip. This model employs the coupling method of seepage-pipe flow, and considers the seepage force, the particle gravity and the friction among the neighbouring particles in the force analysis of eroded particle. The coupling effect of soil-water is taken into account combine with the particle size distribution, soil porosityand hydraulic conductivity. The direction with maximum erosion volume is considered as the piping progressing direction, and when the soil porosity is beyond the critical value, the soil matric of pipe tip is thought to be damaged. The model employs the erosion rule which is suitable for internal soil erosion to determine the rate of erosion. The soil anisotropy has also been considered in this model. Therefore, the mesomechanism of soil erosion in the tip of piping channel and the transport of eroded soil can be quantitatively expressed and the differences of the piping development under different soil compaction can be demonstrated.A numerical investigation into the functions of suspended cut-off wall is presented, which is utilized through the piping development numerical model. The suspended cut-off wall divides the piping progression into three stages. In the stage of getting over the suspended cut-off wall, due to change of the erosion direction, the critical condition of particle inception is increased, which is the key function of the suspended cut-off wall. Meanwhile, the suspended cut-off wall has changed the seepage field near it, the local hydrodynamic condition of soil erosion was lowered by the weaker seepage field. In addition, if the potential piping exit was not affected by the cut-off wall, the critical condition that triggered piping would not change much. Finally, the factors which can affect the functions of suspended cut-off wall, such as the depth of the wall, the location of the wall and the soil anisotropy, are discussed.The existingempirical formula of suspended cut-off wall has been proved has limitation by the typical levee case in China. The empirical formula(the depth of the wall is 1-1.5 times bigger than the water head) of the suspended cut-off wall is only suitable for the cases which the depth of suspended cut-off wall is deeper than 8m. The particle size distribution, the location of cut-off wall and the compaction of soil are all can influence the effects of suspended cut-off wall.