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

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poured into the box without vibration and the reference stress of 100 kPa was applied.
A top plate was rotated backwards and forwards three times through an angle of ten
degrees to consolidate the sample. Following consolidation, the sample was sheared
under normal stress equal to reference stress, at the rate of 10.8 mm min -1 . Second
sub-test was performed for consolidation reference stress of 100 kPa and test load
value of normal stress of 50 kPa. Three replications were performed. Triaxial compression
tests with a sample 15 cm in diameter and 30 cm high were conducted on
wheat to compare results with the results of direct shear test. Comparison of angles of
internal friction of wheat obtained in direct shear test and those obtained in triaxial
compression test is shown in figure 7.3.
Mean values for five levels of moisture content taken as one sample were not
significantly different except the values for 10% of m.c. To obtain such an agreement
of the results of two testing methods, the procedures had to be modified. In the case of
direct shear test the shear deformation was extended up to 0.10 of sample diameter
instead of 0.05 as recommended by Eurocode 1. In the case of triaxial compression
test the sample was compacted by tapping during filling, up to a density equal to that
of the sample in direct shear test. No clear explanation may be given for the
discrepancy of results of direct shear test and triaxial compression test at 10% of grain
moisture content. The probable reason is difference in the mechanism of deformation
in the two tests combined with distinctly lower (than at higher levels of moisture
content) grain-on-grain coefficient of friction.
Angle of interm al friction ϕ (deg)
36
32
28
24
20
9 10 11 12 13 14 15 16 17 18 19 20
Moisture content (%)
Fig. 7.3. Mean values and 95% confidence intervals of the angle of internal friction of wheat
determined with the direct shear test and the triaxial compression test