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Excess vs hydrostatic pore pressure

Short answer

Hydrostatic pore pressure is caused by the weight of the water column above a point and remains constant if the water table is steady. Excess pore pressure is the additional pressure generated by a sudden external load; it dissipates over time as water drains, transferring load to the soil skeleton.

How does excess pore water pressure differ from hydrostatic pore water pressure?

Hydrostatic pore water pressure is the pressure that exists in the pore water because of the depth below the water table, with no flow. Excess pore water pressure is the extra pressure, above the hydrostatic value, created in the pore water by a sudden external load.

PointHydrostatic pore pressureExcess pore pressure
CauseWeight of water column above the pointSudden applied load (e.g. a foundation) on saturated soil
Formulau0=γwzwu_0 = \gamma_w z_wΔu=u−u0\Delta u = u - u_0
Variation with timeConstant while water table is constantDissipates with time as water drains out
Value at t=0t=0γwzw\gamma_w z_wEqual to the stress increase Δσ\Delta\sigma (saturated clay)
Value at t=∞t=\inftyγwzw\gamma_w z_wZero
Effect on effective stressConstant effect, already allowed for in σ′=σ−u0\sigma' = \sigma - u_0Keeps σ′\sigma' low at first; σ′\sigma' rises as Δu\Delta u dissipates

Total pore pressure at any time is u=u0+Δuu = u_0 + \Delta u. Consolidation is the process in which Δu\Delta u dissipates and the load is transferred to the soil skeleton.

✓ Answer checked by Claude Opus 5.5 (high effort)

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