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 45

Other experimental data plotted in the same diagram have shown the same linear
relation with a slope − function of apparent friction − essentially being dependent on
the mineralogy and shape of the grains [FRO 79].
Figure 3.4. Experimental validation of energy dissipation relation based on friction
In Figure 3.4b, we show the results of triaxial compression tests on crushed
basaltic reduced rockfills published by Charles and Watts [CHA 80]. The granular
material is well graded, with grain sizes varying from sand to gravel with maximum
grain size equal to 38 mm. The samples, of diameter equal to 230 mm, were tested at
the same initial density and confining pressures, varying from 27 to 700 kPa. The
diagram shows the evolution of the friction angle at peak with the dilatancy rate.
From the experimental points, we can fit a curve corresponding to equation [3.17],
which gives a value of the apparent friction equal to 44° with a dispersion equal to
plus or minus 5%.
In these tests, the effect of grain breakage reduces the dilatancy rate at peak
when the confining stress increases. Despite the phenomenon of grain breakage, the
experimental values remain close to the ones predicted by the equation of energy
dissipation by friction, even if the results obtained at peak are more dispersed than
those deduced from a fitting over the whole test data.
If we plot these results in the Mohr plane (see Figure 3.5) we can observe the
effect of dilatancy on shear strength by comparing two circles at the same
confinement. The decrease in dilatancy at elevated stresses due to grain breakage