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

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Fig. 9.2. Apparatus for measuring wall friction and typical shear-displacement curves [50]
The procedure is as follows:
1. The reference stress 1 r should be taken as the largest horizontal silo pressure p h .
2. The sample should be poured into the test cell, without vibration or other
compacting forces, and the consolidation stress 1 r applied. A top plate should be
rotated clockwise and anticlockwise about the vertical axis several times through
an angle of at least 10 degrees to consolidate the sample.
3. After filling the cell and before shearing, the cell should be rotated and lifted
slightly of the test surface, so that only friction between the particles and surface
is measured.
4. Shearing of the sample should be carried out at a constant rate of approximately
0.04 mm s -1 .
5. The residual friction force F r (see figure 9.2), attained at large deformations,
should be used in the calculation of the coefficient of wall friction for action
calculations.
6. The sample value of the coefficient of wall friction for action calculations
should be determined as:
Fr
= , (9.3)
N
where:
F r is the final or residual value of the shear force (figure 9.2)
N is the applied vertical load on the cell.
While testing for this study five levels of normal pressure 1 r were applied of: 20,
30, 40, 50 and 60 kPa, shear velocity was set at 0.35 mm s -1 .
The angle of wall friction for the evaluation of flow assessment have been
determined following the tilting table method (fig. 9.1 a). The sample of grain
confined with the square frame (20x20 cm) was loaded in the range of 0.5-2.5 kPa
and tilting angle at initiation of the sample sliding was registered.