Strona 2_redak - Instytut Agrofizyki im. Bohdana Dobrzańskiego ...

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grain was poured into the mould along the axis of symmetry. With increasing angle of
sample inclination β, the angle of internal friction φ decreased significantly.
According to the Coulomb yield criterion, the slip plane is inclined at the
angle α = ±(π/4+φ/2) to the plane of the higher main stress. Figure 3.6 presents the
theoretical slip planes and the plane of free surface of grain sample, inclined to one of
them at angle γ 1 . Figure 3.6a refers to the triaxial compression test at sample
inclination angles β: 10 o , 20 o and 30 o , while figure 3.6b refers to angles β: 0 o , –10 o and
–20 o . While in the case of an isotropic medium there are many possible orientations
of the slip plane, in the case of an anisotropic matrerial the slip plane is determined by
the direction of the lowest strength.
a) b)
c)
Fig. 3.6. Sample of granular material in triaxial compression test: a), b) non-spherical grains,
c) spherical grains
In a material composed of spherical granules there may occur anisotropy of
concentration of normal directions to the points of grain contact. In a hexagonal
structure, the normal directions to the points of contact are mutually inclined at 60 o ,