Soil Mechanics / Effective stress and stress in the ground
Total stress, porewater pressure and effective stress
The one principle that everything else in soil mechanics rests on.
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Read the lesson register and outstanding review notesWhy 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.