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Compression index

It describes variation of the void ratio e as a function of the change of effective stress σef plotted in the logarithmic scale:

Void ratio e versus effective stress σef

It therefore represents a deformation characteristic of overconsolidated soil:

where: Δe - variation of void ratio
Δlogσef - variation of effective stress

Range of compression index Cc (Naval Facilities Engineering Command Soil MechanicsDESIGN MANUAL 7.01)

A typical range of the compression index is from 0,1 to 10. Approximate values for homogeneous sand for the loading range from 95 kPa to 3926 kPa attain the values from 0,05 to 0,06 for loose state and 0,02 to 0,03 for dense state. For silts this value is 0,20. 

For lightly overconsolidated clays and silts tested in USA Louisiana Kaufmann and Shermann (1964) present the following values:

Soil Effective consolidation stress σcef [kPa] Final effective stress in the soil σef  [kPa] Compression index
Cc  [-]
CL soft clay 160 200 0,34
CL hard clay 170 250 0,44
ML silt of low plasticity 230 350 0,16
CH clay of high plasticity 280 350 0,84
CH soft clay with silt layers 340 290 0,52

Prof. Juan M.Pestana-Nascimento (University of California, Berkeley) offers the following typical values of the compression index Cc:

Soil Compression index
Cc [-]
Normal consolidated clays 0,20 – 0,50
Chicago clay with silt (CL) 0,15 - 0,30
Boston blue clay (CL) 0,3 – 0,5
Vickburgs clay - dray falls into lumps (CH) 0,3 – 0,6
Swedish clay (CL – CH) 1 – 3
Canada clay from Leda (CL – CH) 1 – 4
Mexico City clay (MH) 7 – 10
Organic clays (OH) 4 a více
Peats (Pt) 10 – 15
Organic silts and claye silts (ML – MH) 1,5 – 4,0
San Francisco sediments (CL) 0,4 – 1,2
Clay in the old San Francisco Bay 0,7 – 0,9
Bangkok clay (CH) 0,4

In addition, there are empirical expressions available to determine approximate values of Cc for silts, clays and organic soils; their applicability, however, is more or less local:

Soil Equations Reference
Transformed clays Skempton 1944
Clays Nishida 1956
Brazilian clays
Sao Paulo clays
Cozzolino 1961
New York clays Terzaghi a Peck 1948
Clays of low plasticity Sowers 1970
Taipei clays and silts
Moh a kol. 1989
Clays
Pestana 1994

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