What is soil and how does it behave?

Soil is a mixture of soil grains, water and air. The air is the space between the soil grains.
When soil is loaded, the space between the soil grains will decrease, the compressibility of the soil will descrease. The soil will become stiffer. The relation between loading and deformation is non-lineair.
When soil is unloaded, the soil will not return to it’s original position. Soil behaves plastic.

The relation between load and deformation in soil is given by Terzaghi’s law.

When soil is loaded, the settlement s is determined as (curve a):

$s=\frac{h}{C} \ln {\frac{\sigma'_0 +\Delta \sigma'}{\sigma'_0}}$

$h$ is the layer thickness [m]
$C$ is the compression index [-]
$C=\alpha \frac{q_c}{\sigma'_0}$

$q_c$ is the cone resistance [kN/m²] $\alpha= 1,5$ (default value soil report, also see EN 1997-1-1 Annex D)
$\sigma'_0$ is the vertical grain stress [kN/m²]
$\Delta \sigma'$ is the change in the the vertical grain stress [kN/m²]

When soil is unloaded, the settlement $s$ is determined as (curve b):

$s=\frac{h}{A} \ln {\frac{\sigma'_0 +\Delta \sigma'}{\sigma'_0}}$

$A$ is the swelling index [-]
$A=\alpha \frac{q_c}{\sigma'_0}$

From the table you see that for sand the A ≈ 10C.
(source ‘Afleiden van de samendrukbaarheid uit in-situ proeven’ – KVIV-TI)