Social engineering

Social engineering

For desert dunes it’s 34°,
but 44 for sand dunes in the rain;
and 45 for sulphur, dynamite, 
asbestos, rubble, ash, quicklime or bones;
a range from 32 to 43
for mining spoil, and 38° for snow.

At steeper angles, slopes begin to slide 
as shape and gravity and time combine 
with unforeseen to turn a trickle into 
slump, collapse or avalanche. So mind,
for all you touch or near, the need to know—
and not exceed—their angles of repose.

————

Note: The angle of repose is the steepest angle relative to the horizontal plane at which granular material can be piled without slumping. At this angle, the material on the slope is on the verge of sliding. Any steeper, and it will collapse. The angle of repose differs between materials, for example it is 25-30° for gravel, 38° for snow, 34° for dry sand and 44° for wet sand.

“When loose particles are tipped from above, as they are in a crane-tip or a child’s sandcastle, they come to rest in a regular conical shape. The angle at the base of the cone (the angle of repose) varies with the composition of the material. In the case of the material forming the tip at Aberfan this angle was about 35°-37° to the horizontal. When the particles are dry or only slightly moist they do not stick together, as even children building sand-castles soon learn. If the material is wet — but not too wet — the particles will stick together and may stand up at an angle of repose greater than that taken up by dry material; if the material is very wet the angle of repose will be reduced, and perhaps greatly reduced. This is because the space between the particles is filled with water. This water is under pressure which varies with the height of the heap and the volume of water within it. The effect of the presence of this water is to reduce the dead weight of the heap downwards and this also reduces the resistance to lateral movement. Water is almost incompressible and if it cannot escape from the heap it acts much as a hydraulic jack in tending to lift the heap upwards. When the heap is on a deep slope the effect of gravity is to pull the heap not only downwards but also sideways. If the material starts to move on a slope it will continue downwards with less friction than normal, becoming almost fluid. The effect of adding very fine grained material (such as “tailings”) to a heap is to permit the material, when dry, to stand at an angle of repose greater than normal. But if additional water is added, whether from above by rainfall or from below by a spring or watercourse, there will be a collapse, just as a sandcastle collapses when a child empties its bucket of water on it.”

Welsh Office, 1967: Report of the Tribunal appointed to inquire into the Disaster at Aberfan on October 21^st, 1966.