Shear Strength of Granular Soils
17.4 Shear Strength of Granular Soils
Granular soil is a frictional material. The friction angle ( φ ′) is affected by the grain size distribution and dry density. In general, φ ′ increases as the dry density increases and as the soil becomes more well graded,
TABLE 17.1
Summary of Common Shear Strength Tests Additional
Information Direct
Test Type Applicability
Advantages
Disadvantages
a. ASTM D3080* shear
a. Effective strength
a. Simple and
a. Only for drained
b. U.S. Army, 1970 grained and fine-
parameters for coarse-
inexpensive
conditions
b. Failure plane forced to c. Saada and grained soils
b. Thin sample allows for
rapid drainage of fine-
occur at joint in box Townsend, 1981
grained soils
d. Head, 1982 distribution of stress and strain
c. Nonuniform
d. No stress-strain data Triaxial
a. ASTM D2850* strength parameters
a. Effective and total
a. Easy to control
a. Apparatus more
b. U.S. Army, 1970 for coarse-grained and
drainage
complicated than
c. Donaghe et al., fine-grained soils
b. Useful stress-strain
other types of tests
b. Drained tests on fine- 1988 b. Compared to direct
data
grained soils must be d. Head, 1982 shear tests, triaxial
c. Can consolidate
sheared very slowly e. Head, 1986 tests are preferred for
sample hydrostatically
or to in situ K o state of
fine-grained soils
stress
d. Can simulate various loading conditions
Direct a. Most common
a. Bjerrum and simple
o a. K consolidation
a. Nonuniform
distribution of stress Landva, 1966 shear
application is
b. Gives reasonable
undrained shear
b. Saada and strength of fine-
values of undrained
and strain
Townsend, 1981 grained soils
shear strength for
design use
Unconfined a. Undrained shear
a. Not applicable to soils a. ASTM D2166* strength of 100%
a. Very rapid and
b. U.S. Army, 1970 saturated samples of
inexpensive
with fissures, silt
seams, varves, other c. Head, 1982 homogenous,
defects, or less than unfissured clay
100% saturation
b. Not suitable as the b. Sample disturbance only basis for design
not systematially
on critical projects
accounted for
Lab vane Same as for unconfined
Same as for unconfined Head, 1982 test
Same as for unconfined
test
test
* Designation for American Society of Testing and Materials test procedure. Kovacs [1981] and Carter and Bentley [1991]. The friction angle also increases as the angularity of the
soil grains increases and as the surface roughness of the particles increases. Wet sands tend to have a φ′ that is 1˚ or 2˚ lower than for dry sands [Holtz and Kovacs, 1981]. The intermediate principal stress
( 2 ) also affects φ′. In triaxial tests σ 2 is equal to either the major principal stress or minor principal stress ( σ 1 or σ 3 , respectively); however, most field problems occur under plane strain conditions where σ 3 ≤σ 2 ≤σ 1 . It has been found that φ′ for plane strain conditions (φ′ ps ) is higher than for triaxial conditions ( φ′ tx ) [Ladd et al., 1977]. Lade and Lee [1976] recommend the following equation for estimation of φ′ ps :
(17.4b) In practice φ′ for granular soils is determined using correlations with results from SPT, CPT, and other
φ′ ps = φ′ tx
( φ′ tx ≤ 34°)
in situ tests, as discussed in Chapter 15 , or laboratory tests on samples compacted to the same density as the in situ soil. Appropriate laboratory tests are drained direct shear and CD triaxial tests. CU triaxial tests with pore pressure measurements are sometimes used for granular soils with appreciable fines. The method used to prepare the remolded sample and the direction of shearing relative to the direction of
45 ANGLE OF INTERNAL FRICTION
VS DENSITY (FOR COARSE GRAINED SOILS)
RELA TIVE DENSITY 100% 75%
GW MATERIAL f ’ (DEGREES)
30 IN THIS
f’ OBTAINED FROM EFFECTIVE STRESS 0 APPROXIMATE CORRELATION FAILURE ENVELOPES
IS FOR COHESIONLESS 25
MATERIALS WITHOUT PLASTIC FINES
ANGLE OF INTERNAL FRICTION 20
DRY UNIT WEIGHT (g D ), PCF
VOID RATIO, e
POROSITY, n
(G = 2.68)
FIGURE 17.2 Correlation of friction angle of granular soils with soil classification and relative density. (Source: U.S. Navy. 1986. Soil Mechanics, Design Manual 7.1, p. 7.1-149. Naval Facilities Engineering Command, Alexandria, VA.)
et al., 1977]. The c ′ of granular soils is zero except for lightly cemented soils which can have an appreciable
c ′ [Clough et al., 1981; Head, 1982].