---
title: "Excess vs hydrostatic pore pressure"
subject: "Soil Mechanics"
chapter: 9
key: "semester_4/soil_mechanics:excess-vs-hydrostatic-pore-pressure"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/excess-vs-hydrostatic-pore-pressure
site: BCT Engineering
---

# Excess vs hydrostatic pore pressure

Soil Mechanics · Chapter 9: Compressibility of Soil

- Asked in: 2079 Asoj, 2 marks
- Video: [Concept of Excess Pore Water Pressure-  Introduction to Consolidation of Soils](https://www.youtube.com/watch?v=reto9BpzcBU) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/excess-vs-hydrostatic-pore-pressure
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-9

## Question

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

## Answer

**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.

| Point | Hydrostatic pore pressure | Excess pore pressure |
|---|---|---|
| Cause | Weight of water column above the point | Sudden applied load (e.g. a foundation) on saturated soil |
| Formula | $u_0 = \gamma_w z_w$ | $\Delta u = u - u_0$ |
| Variation with time | Constant while water table is constant | Dissipates with time as water drains out |
| Value at $t=0$ | $\gamma_w z_w$ | Equal to the stress increase $\Delta\sigma$ (saturated clay) |
| Value at $t=\infty$ | $\gamma_w z_w$ | Zero |
| Effect on effective stress | Constant effect, already allowed for in $\sigma' = \sigma - u_0$ | Keeps $\sigma'$ low at first; $\sigma'$ rises as $\Delta u$ dissipates |

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

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Hydrostatic pore pressure is caused by
   A. a sudden applied load
   B. the weight of the water column above the point, u0 = gamma_w zw
   C. soil weight
   D. capillarity
   Answer: B. It exists because of depth below the water table with no flow. (Key point: Hydrostatic)

2. Excess pore pressure is
   A. the pressure below the water table
   B. the extra pressure above hydrostatic created by a sudden external load
   C. always zero
   D. equal to effective stress
   Answer: B. delta u = u - u0, created by an applied load. (Key point: Excess)

3. With time, the excess pore pressure
   A. stays constant
   B. increases
   C. dissipates to zero as water drains
   D. becomes negative
   Answer: C. Consolidation is the dissipation of delta u, transferring load to the skeleton. (Key point: Time variation)

4. At t = 0 in saturated clay the excess pore pressure equals
   A. the hydrostatic pressure
   B. zero
   C. the stress increase delta sigma
   D. the effective stress
   Answer: C. The water takes the full stress increase initially. (Key point: Value at t = 0)

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