# Friction Angle of Soils + Typical Values

Courses > Soil Mechanics > Physical Properties of Soil > Friction Angle of Soils + Typical Values

### Introduction

Soil friction angle is a shear strength parameter of soils. Its definition is derived from the Mohr-Coulomb failure criterion and is used to describe the friction shear resistance of soils together with the normal effective stress.

In the stress plane of Shear stress-effective normal stress, the soil friction angle is the angle of inclination with respect to the horizontal axis of the Mohr-Coulomb shear resistance line.

### Concepts and Formulas

#### Difference between Cohesive and Granular soil

"Granular soil" means gravel, sand, or silt (coarse-grained soil) with little or no clay content. Granular soil has no cohesive strength. Some moist granular soils exhibit apparent cohesion. Granular soil cannot be molded when moist and crumbles easily when dry.

"Cohesive soil" means clay (fine-grained soil), or soil with a high clay content, which has cohesive strength. Cohesive soil does not crumble, can be excavated with vertical sideslopes, and is plastic when moist. Cohesive soil is hard to break up when dry, and exhibits significant cohesion when submerged. Cohesive soils include clayey silt, sandy clay, silty clay, clay and organic clay.

#### Typical values of soil friction angle for different soils according to USCS

 Description USCS Soil friction angle [°] Reference min max Specific value Well graded gravel, sandy gravel, with little or no fines GW 33 40 [1],[2], Poorly graded gravel, sandy gravel, with little or no fines GP 32 44 [1], Sandy gravels - Loose (GW, GP) 35 [3 cited in 6] Sandy gravels - Dense (GW, GP) 50 [3 cited in 6] Silty gravels, silty sandy gravels GM 30 40 [1], Clayey gravels, clayey sandy gravels GC 28 35 [1], Well graded sands, gravelly sands, with little or no fines SW 33 43 [1], Well-graded clean sand, gravelly sands - Compacted SW - - 38 [3 cited in 6] Well-graded sand, angular grains - Loose (SW) 33 [3 cited in 6] Well-graded sand, angular grains - Dense (SW) 45 [3 cited in 6] Poorly graded sands, gravelly sands, with little or no fines SP 30 39 [1], [2], Poorly-garded clean sand - Compacted SP - - 37 [3 cited in 6] Uniform sand, round grains - Loose (SP) 27 [3 cited in 6] Uniform sand, round grains - Dense (SP) 34 [3 cited in 6] Sand SW, SP 37 38 [7], Loose sand (SW, SP) 29 30 [5 cited in 6] Medium sand (SW, SP) 30 36 [5 cited in 6] Dense sand (SW, SP) 36 41 [5 cited in 6] Silty sands SM 32 35 [1], Silty clays, sand-silt mix - Compacted SM - - 34 [3 cited in 6] Silty sand - Loose SM 27 33 [3 cited in 6] Silty sand - Dense SM 30 34 [3 cited in 6] Clayey sands SC 30 40 [1], Calyey sands, sandy-clay mix - compacted SC 31 [3 cited in 6] Loamy sand, sandy clay Loam SM, SC 31 34 [7], Inorganic silts, silty or clayey fine sands, with slight plasticity ML 27 41 [1], Inorganic silt - Loose ML 27 30 [3 cited in 6] Inorganic silt - Dense ML 30 35 [3 cited in 6] Inorganic clays, silty clays, sandy clays of low plasticity CL 27 35 [1], Clays of low plasticity - compacted CL 28 [3 cited in 6] Organic silts and organic silty clays of low plasticity OL 22 32 [1], Inorganic silts of high plasticity MH 23 33 [1], Clayey silts - compacted MH 25 [3 cited in 6] Silts and clayey silts - compacted ML 32 [3 cited in 6] Inorganic clays of high plasticity CH 17 31 [1], Clays of high plasticity - compacted CH 19 [3 cited in 6] Organic clays of high plasticity OH 17 35 [1], Loam ML, OL, MH, OH 28 32 [7], Silt Loam ML, OL, MH, OH 25 32 [7], Clay Loam, Silty Clay Loam ML, OL, CL, MH, OH, CH 18 32 [7], Silty clay OL, CL, OH, CH 18 32 [7], Clay CL, CH, OH, OL 18 28 [7], Peat and other highly organic soils Pt 0 10 [2],

References:

1. Swiss Standard SN 670 010b, Characteristic Coefficients of soils, Association of Swiss Road and Traffic Engineers
2. JON W. KOLOSKI, SIGMUND D. SCHWARZ, and DONALD W. TUBBS, Geotechnical Properties of Geologic Materials, Engineering Geology in Washington, Volume 1, Washington Division of Geology and Earth Resources Bulletin 78, 1989, Link
3. Carter, M. and Bentley, S. (1991). Correlations of soil properties. Penetech Press Publishers, London.
4. Meyerhof, G. (1956). Penetration tests and bearing capacity of cohesionless soils. J Soils Mechanics and Foundation Division ASCE, 82(SM1).
5. Peck, R., Hanson,W., and Thornburn, T. (1974). Foundation Engineering Handbook. Wiley, London.
6. Obrzud R. & Truty, A.THE HARDENING SOIL MODEL - A PRACTICAL GUIDEBOOK Z Soil.PC 100701 report, revised 31.01.2012
7. Minnesota Department of Transportation, Pavement Design, 2007

#### Correlation between SPT-N value, friction angle, and relative density

Correlation between SPT-N value and friction angle and Relative density (Meyerhoff 1956)

SPT N3
[Blows/0.3 m - 1 ft]

Soi packing

Relative Density [%]

Friction angle
[°]

< 4

Very loose

< 20

< 30

4 -10

Loose

20 - 40

30 - 35

10 - 30

Compact

40 - 60

35 - 40

30 - 50

Dense

60 - 80

40 - 45

> 50

Very Dense

> 80

> 45

#### Which test gives a better estimation of friction angle?

Usually, the economics of the project dictates the type of test you would use for determination of the friction angle. Nonetheless, the best test to determine the friction angle of soil is the one that is more analogous to the problem at hand. For example, if you are to determine bearing capacity of a square footing, triaxial test is the best one.

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