Dynamic Analysis of Pore Water Pressure Variation and Stability in Earth Dams During Rapid and Slow Drawdown: Khassa Chai Earth Dam as a Case Study

Abstract

This study uses static and temporal models to investigate the variation of pore water pressure (PWP) along the base of an earthen dam over 30 days. The analysis was performed at four elevations (Y = 10, 20, 30, and 40 m) to capture the pressure distribution throughout the dam body. A detailed finite element mesh was used to ensure high-resolution results, with the pore water pressure variation recorded over ten time intervals, focusing on both fast and slow drawdown conditions. The fast drawdown analysis indicated that pore water pressure dissipated rapidly near the top face, resulting in steep hydraulic gradients, large pressure drops near the dam core, and the formation of negative pressures downstream. In compare, slow drawdown showed a more gradual dissipation of pore water pressure, especially at greater depths, driven by soil permeability. Statistical analysis revealed that the PWP's coefficient of variation (CoV) during slow drawdown increased significantly over time, reaching values ​​of 21.3450 at 5 days, indicating increased variability and potential instability. In rapid drawdown, CoV values ​​peaked at 1.0257 at 2.35 days and then declined steadily, indicating a more transient pressure fluctuation that stabilized over time. Shear strength values ​​increased as pore water pressure dissipated, peaking at 110 m from the top surface, where hydraulic gradients were most pronounced. The factor of safety, which was initially 1.4 under steady-state conditions, decreased to 0.9 during rapid drawdown, but increased to 1.5 after 20–30 days as PWP levels stabilized, indicating a restoration of stability. These results underscore the urgent need for controlled drawdown rates to manage PWP dissipation and ensure dam stability, especially in areas with low-permeability soils.