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 52

specific to these materials. We have to underline that these results are obtained by
using a simplified view of the internal actions at contact points:
− the kinematic screws of the local relative contact displacements are reduced to
simple sliding, neglecting rolling and spinning motions;
− the static screws of the local contact forces are reduced to resultant contact
forces, neglecting rolling and spinning torques.
This simplified representation of the contact actions, associated with the concept
of internal actions, appear therefore to be very efficient, at least when usual civil
engineering granular materials are concerned.
As for practical consequences, we can connect the results obtained in the
analysis of strain localization and shear bandings, the failure criterion corresponding
to Coulomb's criterion, and the tendency of the movement to converge towards the
kinematics of plane strain. All these characteristics are the basis of classical studies
of stability of slopes and bearing capacity of foundations by 2D limit analysis. This
represents an essential part of the possible applications of this theoretical approach.
Some other practical properties of granular media will be discussed later on.
3.2. Influence of grain breakage on the behavior of granular materials
Grain breakage in granular materials during loading was underlined long ago by
studies of grain size distributions before and after testing, see for example Marsal's
work [MAR 73]. However, the analysis of the influence of this phenomenon on
granular materials' mechanical behavior is more recent, as the results presented by
Bolton and Mc Dowell [BOL 98] show, giving the impact on the non-reversible
compressibility. The original approach presented here consists in analyzing the
effect of local breakage on the maximum shear strength of the granular assembly
and the induced scale effect. The latter can be explicitly expressed by a “scale effect
relation” acting on the expression of the intrinsic failure criterion. This result leads
to practical applications in the stability analysis and the design of engineering works
[FRO 09], as shown later on in sections 3.3.2 and 3.3.3.
3.2.1. Introduction to the grain breakage phenomenon
3.2.1.1. Elementary grain breakage
The basic description of elementary grain failures when internal forces evolve in
granular media may be summarized as follows (see Figure 3.9a):
− as contact forces increase, maximum strength is reached in some of the grains
that fail;