When analyzing seepage under a concrete dam, what critical condition at the downstream toe indicates a high risk of piping failure?

The critical condition at the downstream toe indicating a high risk of piping failure is when the exit hydraulic gradient approaches or exceeds the critical hydraulic gradient of the foundation soil. The downstream toe is the specific location where the concrete dam's structure meets the natural ground on the lower water level side, serving as the exit point for water seeping underneath the dam. Seepage refers to the underground flow of water through the permeable soil and rock pores beneath the dam, driven by the pressure difference between the upstream and downstream water levels. The hydraulic gradient ....