Consolidation and Settlement
The water takes the load first because it cannot escape, and hands it over across decades. Watch the two bands trade while the total never changes.
Skip to the animationLoading a saturated clay raises pore pressure rather than effective stress, because water cannot escape — and settlement arrives over years as that pressure dissipates, at a rate proportional to the square of the drainage path.
The load transfer
- 1The load is applied. Water is incompressible and cannot escape, so it carries almost all of it.
- 2Effective stress is unchanged, so the soil is momentarily no stronger and no stiffer — the undrained condition.
- 3Water begins to drain. Every unit of pore pressure lost becomes a unit of effective stress gained.
- 4The soil compresses as the skeleton takes over. Settlement equals the volume of water expelled.
- 5When u returns to hydrostatic, the skeleton carries everything and the soil is stronger than on the day it was loaded.
The total stress never changed throughout. Consolidation is a drainage problem wearing a compression problem's clothes.
Terzaghi's spring analogy
A spring in a water-filled cylinder with a valve: the spring is the grain skeleton, the water is the pore water, the valve is the permeability. Load the piston and the pressure spikes; open the valve and it bleeds away as the spring takes over.
Permeability changes the rate and not the final settlement. The governing equation is mathematically identical to heat diffusion.
How much
Void ratio against the logarithm of effective stress is a straight line, so each doubling of stress removes the same increment of void ratio. A soft clay settles greatly under the first load increment and progressively resists further loading. The slope is the compression index C_c.
Clay remembers the greatest effective stress it has ever carried — the preconsolidation pressure. Below it the soil is on a stiff recompression branch; above it, settlement increases by a factor of five or ten. Ice sheets, eroded overburden and demolished buildings all leave that memory.
How long
Consolidation time goes as the square of the drainage path, so doubling the layer thickness quadruples the wait. A layer draining from both faces has half the path and a quarter of the time.
Vertical sand drains exploit that directly, cutting the horizontal drainage path from metres to centimetres and turning decades into months. Combined with preloading, it is how soft ground is made buildable.
Secondary compression
After the excess pore pressure has gone, settlement continues at a slower logarithmic rate as the grain structure itself creeps at constant effective stress. It is minor in most inorganic clays and can exceed the primary settlement in peat — which is why building on peat is avoided rather than calculated.
The numbers you will be asked for
- Compression index
C_c = Δe / Δlog σ′
- Primary settlement
S = (C_c H / (1+e₀)) · log(σ′₁/σ′₀)
- Coefficient of consolidation
c_v = k / (m_v γ_w)
- Time factor
T_v = c_v t / H²
H is the drainage path
- 90% consolidation
T_v = 0.848
- Overconsolidation ratio
OCR = σ′_preconsolidation / σ′_present
Watch it work
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