The angle of repose or the critical angle of repose, of a granular material is the steepest
angle of descent or dip relative to the horizontal plane to which a material can be piled without
slumping.
At this angle, the material on the slope face is on the verge of sliding.
The angle of repose can range from 0° to 90°.
Smooth, rounded sand grains cannot be piled as steeply as can rough, interlocking sands.
If a small amount of water is able to bridge the gaps between particles, electrostatic
attraction of the water to mineral surfaces will increase soil strength.
When bulk granular materials are poured onto a horizontal surface, a conical pile will
form.
The internal angle between the surface of the pile and the horizontal surface is known
as the angle of repose and is related to the density, surface area and shapes of the particles,
and the coefficient of friction of the material.
However, a 2011 study shows that the angle of repose is also gravity-dependent.
Material with a low angle of repose forms flatter piles than material with a high angle
of repose.
The term has a related usage in mechanics, where it refers to the maximum angle at which
an object can rest on an inclined plane without sliding down.
This angle is equal to the arctangent of the coefficient of static friction μs between
the surfaces.
Applications of theory
The angle of repose is sometimes used in the design of equipment for the processing of
particulate solids.
For example, it may be used to design an appropriate hopper or silo to store the material, or to
size a conveyor belt for transporting the material.
It can also be used in determining whether or not a slope will likely collapse; the talus
slope is derived from angle of repose and represents the steepest slope a pile of granular
material will take.
This angle of repose is also crucial in correctly calculating stability in vessels.
It is also commonly used by mountaineers as a factor in analysing avalanche danger in
mountainous areas.
Measurement There are numerous methods for measuring angle
of repose and each produces slightly different results.
Results are also sensitive to the exact methodology of the experimenter.
As a result, data from different labs are not always comparable.
One method is the triaxial shear test, another is the direct shear test.
If the coefficient of static friction is known of a material, then a good approximation of
the angle of repose can be made with the following function.
This function is somewhat accurate for piles where individual objects in the pile are minuscule
and piled in random order.
where, μs is the coefficient of static friction, and θ is the angle of repose.
Exploitation by antlion and wormlion larvae
The larvae of the antlions and the unrelated wormlions Vermileonidae trap small insects
such as ants by digging conical pits in loose sand, such that the slope of the walls is
effectively at the critical angle of repose for the sand.
They achieve this by flinging the loose sand out of the pit and permitting the sand to
settle at its critical angle of repose as it falls back.
Thus, when a small insect, commonly an ant, blunders into the pit, its weight causes the
sand to collapse below it, drawing the victim toward the center where the predator that
dug the pit lies in wait under a thin layer of loose sand.
The larva assists this process by vigorously flicking sand out from the center of the pit
when it detects a disturbance.
This undermines the pit walls and causes them to collapse toward the center.
The sand that the larva flings also pelts the prey with so much loose, rolling material
as to prevent it from getting any foothold on the easier slopes that the initial collapse
of the slope has presented.
The combined effect is to bring the prey down to within grasp of the larva, which then can
inject venom and digestive fluids.
Methods in determining the angle of repose Tilting box method
This method is appropriate for fine-grained, non-cohesive materials, with individual particle
size less than 10 mm.
The material is placed within a box with a transparent side to observe the granular test
material.
It should initially be level and parallel to the base of the box.
The box is slowly tilted at a rate of approximately 0.3 degrees/second.
Tilting is stopped when the material begins to slide in bulk, and the angle of the tilt
is measured.
Fixed funnel method The material is poured through a funnel to
form a cone.
The tip of the funnel should be held close to the growing cone and slowly raised as the
pile grows, to minimize the impact of falling particles.
Stop pouring the material when the pile reaches a predetermined height or the base a predetermined
width.
Rather than attempt to measure the angle of the resulting cone directly, divide the height
by half the width of the base of the cone.
The inverse tangent of this ratio is the angle of repose.
Revolving cylinder method The material is placed within a cylinder with
at least one transparent face.
The cylinder is rotated at a fixed speed and the observer watches the material moving within
the rotating cylinder.
The effect is similar to watching clothes tumble over one another in a slowly rotating
clothes dryer.
The granular material will assume a certain angle as it flows within the rotating cylinder.
This method is recommended for obtaining the dynamic angle of repose, and may vary from
the static angle of repose measured by other methods.
When describing the angle of repose for a substance, always specify the method used.
Angle of repose of various materials Here is a list of various materials and their
angle of repose.
All measurements are approximated and not exact.
See also The angle of repose plays a part in several
topics of technology and science, including: Aeolian processes
Avalanche Barchan
Bulk cargo Concrete slump test
Mass wasting Oceanic trench
Retaining wall Sand volcano
Scree Slope stability
References