Tanzania Journal o Agriculture Science 1998, Vol. 1(2): pp 187-194
The angles of repose and internal friction are important parameters which determine the flow
characteristics of agricultural granular materials, such as flour, maize, beans, wheat, sorghum
and rice. The angles have a big influence on the design offlow and storage structures of
agricultural materials such as hoppers, silos, bunkers and bins because they determine
whether the flow will be smooth or not, and whether the bursting forces in relation to vertical
forces will be great or not. An apparatus for determination of the angle of repose was specifically
designed for the purpose. The a'ngles of repose for three different grains, namely maize,
sorghulll and rice at four moisture content levels, namely 10, 15, 20 and 25%wb were determined
using the designed apparatus. A tri-axial compression machine was used to determine
the angles of internal friction for the same grains and same moisture contents. The data obtained
were fed into SAS statistical software for step-wise regression analysis. A model of the
form <t> r = 22.35 + 0.013<1> 2; + 0.019[MC] 2 where <l>r = the angle of repose, <I> 1 = angle of
intemlilfriction and MC = percent moisture content on wet basis was established, and used
to predict the angleofreposefor the tested grains ~ith high accuracy (R2 = 0.97). It was
concluded that the angle of repose was consistently higher than the angle of internal friction
for all the granular materials tested, and the angle of internal friction was highest for maize,
followed by rice and finally sorghum.