Friction Angle of Soils + Typical Values

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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.

 

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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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• Cohesion Intercept of Soils + Typical Values
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