Shear strength, compressibility and permeability of embankment materials and soil foundations - Geotechnical Engineering of Dams - page 267

Table 6.4. Factors affecting the shear strength of rockfill.

Variable Description of effect on shear strength Importance of variable References

Confining Mohr-Coulomb shear strength envelope Major - see Douglas (2003),

pressure is curved, with lower friction angles at Figures 6.36, 6.37 Indraratna et al. (1993,

high confining stress 1998), Charles and

Watts (1980), Marsal
(1973)

Density or Shear strength increases with increased Medium to minor Douglas (2003), Marsal

void ratio density, lower void ratio. The effect is (1973), Chiu (1994),

greatest at low confining stress, with Nakayama et al. (1982)

little effect at high confining stress

Unconfined Shear strength increases with increased Minor to medium Douglas (2003),

compressive rock strength below UCS 100 MPa but Anagnosti and Popovic

strength of little effect for stronger rock. Little (1982)

the rock effect at low confining stress, medium

effect at high confining stress

Uniformity Uniformly graded rockfill exhibits Minor Saras and Popovic

coefficient more curvature in the Mohr Coulomb (1985), Chiu (1994),

strength envelope than well graded. Douglas (2003)

No clear effect on strength (Douglas)

Maximum Most accept that shear strength Minor Chiu (1994), Charles

particle size increases with particle size (Chiu, et al. (1980), Anagnosti

Douglas), but some claim no effect and Popovic (1981)

(Charles et al.), or the opposite effect
(Anagnosti and Popovic)

Finer Shear strength reduced significantly Medium USBR (1966), Douglas

particles with more than about 35-50% silty or (2003)

content clayey sand passing 2 mm size (USBR).

Rockfill with 20% fines passing

0.075 mm had lower strength (Douglas)

Particle Angular particles have higher strength Minor Douglas (2003), Sarac

angularity than sub angular and rounded particles and Popovic (1985),

at low confining pressures, little Bertacchi and Berlotti

difference at high confining pressures (1970)

Tangent to circle
through origin

secant to failure
envelope

τ

φ sec

φ t

Tangent to failure
envelope

Failure
envelope

Assumes φ φ t

and c 0

σ n2 σ n

τ 0 τ env τ t τ env τ t τ 0

τ t τ env τ 0

Figure 6.36. Methods for representing the shear strength of rockfill (Douglas 2003).