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Cohesion of Soil
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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.
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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
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SPT Penetration
(blows/ foot)
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Estimated Consistency |
Suc (tons/ft2) |
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<2
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Very Soft
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<0.25
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2 - 4
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Soft
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0.25 - 0.50
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4 - 8
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Medium
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0.50 - 1.0
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8 - 15
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Stiff
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1.0 - 2.0
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15 - 30
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Very Stiff
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2.0 - 4.0
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>30
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Hard
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>4
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Empirical Values for Consistency of Cohesive Soil, (from Foundation Analysis, Bowels)
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SPT Penetration
(blows/ foot)
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Estimated Consistency |
Suc (kips/ft2) |
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0 - 2
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Very Soft
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0 - 0.5
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2 - 4
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Soft
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0.5 - 1.0
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4 - 8
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Medium
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1.0 - 2.0
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8 - 16
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Stiff
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2.0 - 4.0
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16 - 32
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Very Stiff
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4.0 - 8.0
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>32
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Hard
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>8
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Typical Strength Characteristics (from Lindeburg, Civil Engineering Reference Manual for the PE
Exam, 8th ed.)
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USCS Soil Group
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c, as compacted
(lb/ft2) |
c, saturated (lb/ft2) |
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GW
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0
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0
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GP
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0
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0
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GM
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-
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-
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GC
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-
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-
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SW
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-
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-
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SP
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-
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-
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SM
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1050
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420
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SM-SC
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1050
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300
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SC
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1550
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230
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ML
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1400
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190
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ML-CL
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1350
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460
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CL
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1800
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270
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OL
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-
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-
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MH
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1500
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420
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CH
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2150
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230
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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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