Effect of Compaction on Strength and Arching of Cohesive Material in Storage Bins

Arching in storage bins for cohesive materials is a common problem in handling bulk solids. The formation of arches in cohesive bulk solids is influenced by many factors, including material properties and storage conditions. An experimental study was carried out to determine the effect of compaction on arching in storage bins for wheat flour with different moisture contents. A model bin 475 mm in height and 600 mm x 375 mm in cross-section was designed and fabricated to conduct tests. A unique feature of this model bin test system was an adjustable hopper which allowed for discharge opening to be increased during test without disturbing the material in the hopper. This feature made it possible to quantify arch spans in the hopper without disturbing the stored material. Wheat flour at two different moisture contents of 8.6% and 14.2% was prepared as the test material. A universal testing machine was used to apply pressure to compact the stored material in the model bin. Compaction pressure was measured in the hopper by using an en masse pressure measuring system. Direct shear tests were performed to determine the angle of internal friction and cohesion of wheat flour subjected to various compaction pressures. The unconfined yield strength was then calculated from the measured values of internal friction and cohesion.;The variation in moisture content of wheat flour had noticeable effect on the arching span. Specifically, arching span increased as the moisture content increased. The required hopper opening for arching-free flow for 14.2% MC was 42% greater than that for 8.6% MC (122 mm vs. 86 mm).;It was observed that the arching span increased with compaction pressure when the compaction pressure was low. Increase in compaction pressure from 0.2 to 5 kPa led to a 64% increase in required hopper opening for arching-free flow for flour at 8.6% MC, and 49% at 14.2% MC. However, compaction pressure had little effect on arch formation after it reached 5 kPa.;It was observed that the internal friction angles were about the same for the wheat flour at two moisture contents (37.1° vs. 37.5°), but cohesion for 14.2% MC was 72% higher than that for 8.6% MC (1.21 vs. 2.08 kPa). The unconfined yield strength increased from 2.46 kPa to 4.22 kPa, or by 72% as the moisture content of wheat flour increased from 8.6% to 14.2%.