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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):

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


When the soil is loaded again, it will react stiff when the stress \sigma' is less than the prehistorical stress \sigma'_h (curve c).

From the moment the stress \sigma' is higher than the prehistoraical stress \sigma'_h, the soil will react less stiff (curve d).

This is called overconsolidated soil.

The settlement s is the combination of both laws above:

    \[ s=\frac{h}{A}  \ln {\frac{\sigma'_0}{\sigma'_0}}+\frac{h}{C}  \ln {\frac{\sigma'_0  +\Delta \sigma'}{\sigma'_0}} \]

The ratio \frac{\sigma'_0}{\sigma'_0} is called the overconsolidation ratio OCR.

    \[ s=\frac{h}{A}  \ln {OCR}+\frac{h}{C}  \ln {\frac{\sigma'_0  +\Delta \sigma'}{\sigma'_0}} \]



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