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

60
and the upper boundary was 18.5 mm above the corrugation peaks. The dilation in the
boundary layer resulting from shearing during discharge gives rise to an overpressure.
The overpressure is independent of silo scale causing a decrease in the relative
overpressure with increased silo size.
The shear zone in granular material has recently become the object of wider
interest of researchers. The determination of the thickness of the shear zone is
important for the estimation of forces transferred from the granular material to the
structure. The thickness of the shear zones depends on the wall roughness, the grain
diameter, the specimen size and the boundary value problem considered [160]. The
relation between shear band thickness and grain size has profound implications for
investigations of progressive failures within granular solids. According to direct
experimental observations of Roscoe [143], the width of shear bands is about
10 times the average grain diameter. Investigations concerning the shear band formation
are mainly based on computer simulations or theoretical modelling. The thickness
of fully developed shear band was found to be approximately 16 times the mean grain
diameter. Only a few researchers investigated experimentally the formation of the
shear band in bulk of grain [109, 122, 177].
Triaxial compression tests [64] were performed to obtain information on the
displacement distribution of particles inside the shear band. The sample was 30
cm high and 15 cm in diameter. The volume of the sample was divided into 30
cylindrical regions using two different colours of seeds: stained and not stained
mustard seeds. Each region was 3 cm high and 3 cm thick. In the vertical
direction the sample was divided into three cylindrical coaxial regions by
inserting two cylindrical moulds of diameters of 3 cm and 9 cm into the sample
mould. Layers of seed of 3 cm in height were poured into each of the three
cylindrical regions of the sample. Ten layers of stained and not stained seed were
poured into each column of the sample. Vertical cross-section of the sample of
triaxial compression at ε 1 = 0.17 with the deformed meshes indicated is shown in
figure 7.8. Orientation angle of the shear zone α and its thickness were also
indicated in figure 7.8. The shear zone was oriented at an angle of:
π ϕ
(7-4)
α = +
4 2
with the horizontal axis (direction of the minor principal stress σ 3
) as predicted by
the Mohr-Coulomb theory where φ is the angle of internal friction.
Average value of the angle of internal friction for stained and not stained
seed was 26 o . Distributions of displacement across the shear band, at axial strain
ε 1 of 0.1 and 0.17, are shown in figure 7.9 and 7.10. Vectors connecting the line of
original and deformed mesh represent the distribution of displacement across the