Module 8 · Lesson 8.1
Total stress, porewater pressure and effective stress
The one principle that everything else in soil mechanics rests on.
Why this matters
If you remember one idea from soil mechanics, make it this one. The strength of a soil and how much it settles are not controlled by the total load on it, but by the part of that load carried grain-to-grain through the soil skeleton — the effective stress. Change the water pressure without changing the total load and you change the effective stress, and therefore the soil's behaviour. This is why raising a water table can trigger a failure that no new load caused.
What you should already know
- Unit weights (bulk, saturated, submerged) from the phase-relationships module
- The idea of pressure in a fluid increasing with depth
Imagine a horizontal plane at some depth in the ground. Three stresses act on it:
- Total (vertical) stress — the full weight of everything above the plane (soil + water + any surface load), divided by area. It is what a pressure cell placed on the plane would feel.
- Porewater pressure — the pressure in the water filling the voids. Below the water table, in still water, it is hydrostatic: it increases with depth below the water table.
- Effective stress — the difference. It is the stress actually transmitted through the contacts between soil grains — the skeleton stress.
What it calculates: the stress carried by the soil skeleton
- effective (vertical) stress (kPa)
- total (vertical) stress (kPa)
- u
- porewater pressure (kPa)
This assumes
- Saturated soil; the grain-contact area is a negligible fraction of the plane.
In plain terms: The porewater, being a fluid, carries no shear. So it is the effective stress — not the total stress — that mobilises friction between grains and controls strength and compression.
Predict first
A wide raft sits on saturated clay. Heavy rain raises the groundwater table by 2 m, with no change to the loads on the raft. What happens to the effective stress in the clay?
Summary
- Total stress σ = full weight above the plane; porewater pressure u = pressure in the voids; effective stress σ′ = σ − u.
- Effective stress is the grain-to-grain (skeleton) stress; the porewater carries no shear.
- σ′ governs strength and settlement — not σ.
- A higher water table raises u and lowers σ′; pore pressure always subtracts.
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint