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Soil Cohesion

Cohesion

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Cohesion of Soil


Cohesive soils are clay type soils. Cohesion is the force that holds together molecules or like particles within a soil.

Cohesion, c, is usually determined in the laboratory from the Direct Shear Test.
Unconfined Compressive Strength, Suc, can be determined in the laboratory using the Triaxial Test or the Unconfined Compressive Strength Test.
There are also correlations for Suc with shear strength as estimated from the field using Vane Shear Tests.

 

 

c = Suc/2

               Where:

               c = cohesion,  kN/m2 (lb/ft2), and
               Suc = unconfined compressive strength,  kN/m2 (lb/ft2).

 

Guide for Consistency of Fine-Grained Soil, NAVFAC 7.02

SPT Penetration (blows/ foot)

Estimated Consistency

Suc (tons/ft2)

<2

Very Soft

<0.25

2 - 4

Soft

0.25 - 0.50

4 - 8

Medium

0.50 - 1.0

8 - 15

Stiff

1.0 - 2.0

15 - 30

Very Stiff

2.0 - 4.0

>30

Hard

>4

 

Empirical Values for Consistency of Cohesive Soil, (from Foundation Analysis, Bowels)

SPT Penetration (blows/ foot)

Estimated Consistency

Suc (kips/ft2)

0 - 2

Very Soft

0 - 0.5

2 - 4

Soft

0.5 - 1.0

4 - 8

Medium

1.0 - 2.0

8 - 16

Stiff

2.0 - 4.0

16 - 32

Very Stiff

4.0 - 8.0

>32

Hard

>8

 

Typical Strength Characteristics (from Lindeburg, Civil Engineering Reference Manual for the PE Exam, 8th ed.)

USCS Soil Group

c, as compacted (lb/ft2)

c, saturated (lb/ft2)

GW

0

0

GP

0

0

GM

-

-

GC

-

-

SW

-

-

SP

-

-

SM

1050

420

SM-SC

1050

300

SC

1550

230

ML

1400

190

ML-CL

1350

460

CL

1800

270

OL

-

-

MH

1500

420

CH

2150

230

 

Other soil and soil related properties are listed below:


Angle of Internal Friction
Bearing Capacity Factors
Cohesion
External Friction Angle
Factor of Safety
Lateral Earth Pressure Coefficients
Modulus of Vertical Subgrade Reaction
Soil Unit Weights
Young's Modulus or modulus of elasticity

 

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