Understanding the stability of slopes in earthen dams is crucial for ensuring their safety and longevity. Several factors can influence slope stability, but some conditions are inherently more critical than others, especially for specific faces of the dam.

Stability Analysis: Upstream Face Conditions

The upstream face of an earthen dam is particularly susceptible to instability when the reservoir level fluctuates rapidly. Let's examine the impact of each condition:

Sudden Drawdown Impact

Sudden drawdown refers to the rapid lowering of the reservoir water level. This is considered the most adverse condition for the upstream face of an earthen dam. Here's why:

• When the reservoir is full, the saturated soil within the upstream slope is supported by the external water pressure.
• If the reservoir level drops quickly, the external water pressure decreases rapidly. However, the water contained within the soil pores (pore water) cannot drain out as fast due to its seepage path length and permeability.
• This creates excess pore water pressure within the upstream slope. According to the principle of effective stress, the shear strength of the soil is directly related to the effective stress ($\sigma'$), which is the difference between the total stress ($\sigma$) and the pore water pressure ($u$): $\sigma' = \sigma - u$.
• As pore water pressure ($u$) increases due to the trapped water during sudden drawdown, the effective stress ($\sigma'$) decreases.
• A lower effective stress leads to a significant reduction in the soil's shear strength ($\tau$). The shear strength is generally modelled as $\tau = c' + \sigma' \tan \phi'$, where $c'$ is the effective cohesion and $\phi'$ is the angle of internal friction.
• This reduction in shear strength, while the driving forces (like the weight of the soil wedge) remain relatively high, drastically lowers the factor of safety against slope failure. The upstream slope is particularly vulnerable because it is subjected to the full hydrostatic pressure from the retained water before the drawdown occurs.

Steady Seepage Analysis

Steady seepage occurs when water flows through the dam body and foundation under a constant water level in the reservoir. While seepage affects slope stability by increasing pore water pressures and potentially creating seepage forces, it generally:

• Leads to a more gradual reduction in effective stress compared to sudden drawdown.
• Primarily impacts the downstream slope stability, although it does reduce the stability of the upstream slope as well.
• Is usually less critical for the upstream face than the condition of sudden drawdown, as the pore pressures generated are typically lower and more stable.

During Construction Considerations

Stability during construction is vital, especially for the downstream slopes and the partially completed embankment. Factors like placement methods, compaction, and soil properties at different stages are critical. However, this condition relates to the building phase and doesn't represent the same operational hazard to the fully constructed upstream face as sudden reservoir level changes.

Sloughing of Slope Explanation

Sloughing of slope is typically a consequence or a manifestation of slope instability rather than a primary condition that causes the most adverse state. It often involves surface erosion or minor slips, usually resulting from factors like saturation, poor drainage, or wave action, which themselves might be exacerbated by other conditions.

Conclusion on Adverse Conditions

Comparing these conditions, the rapid decrease in external water pressure during sudden drawdown, coupled with the inability of pore water to drain quickly from the upstream face, leads to the most significant reduction in effective stress and shear strength. This makes sudden drawdown the most adverse condition challenging the stability of the upstream face of an earthen dam.