Index of secondary compression

The index of secondary compression is proportional to the logarithm of time and the slope of primary consolidation (it is strongly dependent on the final effective stress in soil):

where:	Cα	-	index of secondary compression
	α	-	deformation of a soil layer
	t1	-	initial time of a period of monitoring (measured from the start of consolidation)
	t2	-	final time of a period of monitoring

Determining the value of index of secondary compression C_α requires either laboratory (e.g. one-dimensional consolidation in oedometer) or in-situ measurements:

Determination of index of secondary compression C_α

Ranges of values of index of secondary compression C_α

sand	0,00003 – 0,00006
silty loess	0,0004
clay	0,01

The ratio between the index of secondary compression C_α and the compression index C_c is approximately constant for most of the normally consolidated clays for loading typical in engineering practice. Its average value is 0,05. 

Variation of natural moisture of soil as a function of the index of secondary compression C_α derived by Mesri appears in figure:

Variation of natural moisture of soil as a function

of the index of secondary compression C_α after Mesri

1	Whangamarino clay
2	Mexico City clay
3	Calcareous organic silt
4	Leda clay
5	Norwegian plastic clay
6	Amorphous and fibous peat
7	Canadian muskeg
8	Organic marine deposits
9	Boston blue clay
10	Chicago blue clay
11	Organic silty clay