From NOEL COCHRANE
The article entitled ‘The self-organising sand pile’ (15 June) seems
to contain at least four implicit presumptions including dry sand, the grains
are of moderate size, the grains are rough and lumpy in shape and affected
by normal gravitational forces. In that form most engineers would agree
with an angle of passive or critical repose of 33 to 35 degrees. This represents
our angle of internal friction.
Larger lumpy particles such as rock rubble will have an angle of repose
of 40 to 45 degrees.
Very fine dry particles may have an angle of repose of 15 to 20 degrees
but with some it will be almost zero. Ball bearings would be likely to give
0 degrees, unless rusty.
If the particles are not lumpy but naturally rounded, as in piles of
dry grain such as wheat, the critical angle of repose will be more like
25 degrees.
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Very fine, dry particles such as clays are affected by forces regarded
as cohesion and a friction angle is normally not attributed to them. Great
changes in criticality are also brought about by the presence of moisture,
free water, and water at excessive pore pressure.
The sand modelling described could be described as challenging but I
am not surprised that IBM found natural sands play dirty, because they very
often are dirty.
Noel Cochrane Crawley, West Sussex
