# Secondary Compression

Courses > Foundation Analysis and Design > Settlement of Shallow Foundations > Secondary Compression

## Introduction on Secondary Compression :

Secondary compression is a continuation of the volume change that started during primary consolidation, only it usually occurs at a much slower rate. Secondary compression is different from primary consolidation in that it takes place at a constant effective stress, that is, after essentially all the excess pore pressure has dissipated. For general definitions, remarks and types of settlement refer to Settlement types, definitions, and general equation article.

## Concepts and Formulas of Secondary Compression:

Secondary compression index:

$C_{\alpha}=\frac{\Delta e}{\Delta logt}$

This definition is analogous, of course, to the primary compression index Cc.

To determine the magnitude of secondary settlement under the final vertical effective stress

$S_s=\frac{C_{\alpha}H}{1+e_0}log\frac{t}{t_p}$

where t = time of interest; tp = time of end of primary consolidation.

Assumptions about the behavior of fine-grained soils in secondary compression:

• Ca is independent of time (at least during the time span of interest).
• Ca is independent of the thickness of the soil layer.
• Ca is independent of the LIR, as long as some primary consolidation occurs.
• The ratio CJCcis approximately constant for many geo-materials over the normal range of engineering stresses.

What is the approximate values of Ca?

Material Ca/Cc
Granular soils including rockfill 0.01 to 0.03
Shale and mudstone 0.02 to 0.04
Inorganic clays and silts 0.03 to 0.05
Organic clays and silts 0.04 to 0.06
Peat and muskeg 0.05 to 0.07

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