Capillarity; water table effect on stress

Capillarity is the rise of water above the water table through the fine voids of the soil, caused by surface tension. The soil voids act like tiny tubes. The height of rise is $h_c = \dfrac{4T\cos\alpha}{\gamma_w d}$, larger for finer soil (silt and fine sand rise more than gravel). Water in the capillary zone is under negative pressure, $u = -\gamma_w z$.

Effect of water table variation

• Rise of water table: the part of the soil that becomes submerged has its unit weight reduced to $\gamma'$ and pore pressure increased, so effective stress at depth decreases. The soil loses strength and bearing capacity.
• Fall of water table (drawdown): pore pressure falls and the soil above becomes moist/dry, so effective stress at depth increases. This increases settlement (e.g. due to pumping).

For depth $z$ below the water table at depth $z_w$: $\sigma' = \gamma z_w + \gamma'(z - z_w)$. Raising the water table reduces $z_w$ and so lowers $\sigma'$.