Constant Gradient Erosion Apparatus for Appraisal of Piping Behavior in Upward Seepage Flow

Seepage direction is crucial for understanding the critical state and development of piping erosion. A stress-controlled apparatus was designed to investigate the piping behavior of cohesionless soil under upward flow condition. The components of the new apparatus included a loading chamber, a vertical and confining loading system, an upstream water supply device, a soil–water separating system, and a water collecting system. The loading chamber provides space for a soil specimen setting and loading. The combination of a vertical and a confining loading system was designed to apply complex stresses to a soil specimen. Under the stresses, the specimen was then eroded by the gradually increasing hydraulic head supplied by the water supply system. The eroded particle and spilling water were collected and detected by the soil–water separating system and the water collecting system. A series of experiments were carried out using the new apparatus. Results demonstrated the repeatability experiments and usefulness of the apparatus. The new apparatus allowed us to investigate the piping behavior under different stress states and hydraulic gradients. With this new apparatus and experiments, we found that lower and high critical hydraulic gradients (CHGs) should be included as the criteria of piping development based on the relationship between the hydraulic gradient and the seepage response. In addition, the stress state on the CHG and the particle erosion rate played important roles in the piping development. The outer pressure on the specimen can retard the development of erosion. In contrast, the hydraulic gradient was found to be positively correlated to the erosion rate. Results also indicated that a specimen would collapse once the amount of eroded small particles exceeds the critical value of 46.5 % of the soil.