Sea level fluctuation, effective stress

No, the statement is not correct (for a fully submerged sea-bed soil).

Consider soil below sea level at depth $z$ below the bed, with water depth $h_w$ above the bed.

• Total stress: $\sigma = \gamma_w h_w + \gamma_{sat} z$
• Pore pressure: $u = \gamma_w (h_w + z)$

$$\sigma' = \sigma - u = (\gamma_{sat} - \gamma_w) z = \gamma' z$$

The water-depth term $\gamma_w h_w$ appears in both $\sigma$ and $u$ and cancels. So the effective stress depends only on the submerged unit weight and the depth into the soil, not on the sea level. A rise or fall of the sea changes the total stress and the pore pressure by the same amount.

Remarks:

• Even if the sea bed is exposed at low tide, $\sigma'$ stays equal to $\gamma' z$ as long as the soil remains saturated with the water table at the bed surface.
• The effective stress changes only if the pore pressure does not change by the same amount as the total stress, for example if the water table inside the soil falls below the exposed bed surface (drainage), or if the soil is not fully saturated (gas in the pores).
• In normal situations, the strength of the sea-bed soil is not affected by sea-level fluctuation.