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Settlement

When the effective stress increases, the soil is compressed and the ground sinks. This is called settlement. In clay it happens slowly, because the water takes a long time to be squeezed out of the pores. The oedometer modulus describes how stiff the soil is.

s = \frac{\Delta\sigma'\,H}{M}

settlement of a layer

Symbols

$s$	settlement	m
$\Delta\sigma'$	increase in effective stress	kPa
$H$	layer thickness	m
$M$	oedometer modulus	kPa

Example

4 m of clay with $M = 2000$ kPa gets 50 kPa extra:

$s = 50\cdot 4/2000 = 0.1$ m = 100 mm.

If the groundwater is lowered, the effective stress rises, and buildings can settle even without new load.

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Part of Geotechnical Engineering: Effective stress and settlement.