Granular Materials in Civil Engineering: Recent Advances in the Physics of Their Mechanical Behavior and Applications to Engineering Works - Multiscale Geomechanics: From Soil to Engineering Projects - page 58

− a rockfill made of fine grained argillite produced by quarry blasting, having
very angular particles of comparatively low strength, used for the construction of the
Pyramid Dam in California;

− a rockfill made of crushed basalt extracted from a quarry, with angular sound
particles;
− a coarse alluvium, predominantly made of sound unweathered rounded gravels
and cobbles of fine-grained amphibolite, used for the construction of the Oroville
Dam in California.

These three groups of materials, although strongly different, display the same
qualitative trend concerning grain size influence, suggesting the existence of a fairly
general scale effect rule.

An independent validation of this approach can be made by using the data
gathered by Charles and Watts (see Figure 3.10). Their data was obtained on
materials with maximum grain sizes of D Max = 38 mm, whereas the results
previously obtained by Marsal and Marachi concerned coarser materials ( D Max = 150
to 200 mm). A correction of Charles and Watts' curves by the method presented
above on the confining stress corresponds here to a simple shift of their results
towards the left of the diagram, as confining stresses in abscissae are displayed in
logarithmic scale. Figure 3.12 presents the curves A , C and B2 from Charles and
Watts' [CHA 80] corrected by an horizontal similarity factor equal to
mm 38

. The transformed curves A' and C' in the diagram are now in far

150

mm

better agreement with the results obtained by Marsal and Marachi than the original
curves A and C . For the slate, B2, with low-strength particles, the transformed curve
is now located below the usual range for rockfill materials, which is more in
agreement with the poor quality of this material, which was why it was selected.

Thus, the scale effect in Barton's data and the apparent distortion in Charles and
Watts' diagram appear to be due to this physical similarity rule resulting from
fracture mechanics laws governing grain breakage. The adjusted data compilation in
Figure 3.12 also suggests a typical shear strength envelope for coarse granular
materials corresponding to the dotted line plotted in the central part of the
experimental data range. This central line, corresponding to the shear strength
envelope range of granular materials with grain size D Max = 150 mm, can be fitted by
a power law:

τ σ ≈⋅ 4

0,77

[3.27]

; σ in kPa

and

n

n