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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 load 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)

0.30 - 0.50

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 and more

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