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Concept
Dilatancy (granular material)
Summary
In soil mechanics, dilatancy is the volume change observed in granular materials when they are subjected to shear deformations. This effect was first described scientifically by Osborne Reynolds in 1885/1886 and is also known as Reynolds dilatancy. It was brought into the field of geotechnical engineering by Peter Walter Rowe.
Unlike most other solid materials, the tendency of a compacted dense granular material is to dilate (expand in volume) as it is sheared. This occurs because the grains in a compacted state are interlocking and therefore do not have the freedom to move around one another. When stressed, a lever motion occurs between neighboring grains, which produces a bulk expansion of the material. On the other hand, when a granular material starts in a very loose state it may continuously compact instead of dilating under shear. A sample of a material is called dilative if its volume increases with increasing shear and contractive if the volume decreases with increasing shear.
Dilatancy is a common feature of soils and sands. Its effect can be seen when the wet sand around the foot of a person walking on beach appears to dry up. The deformation caused by the foot expands the sand under it and the water in the sand moves to fill the new space between the grains.
The phenomenon of dilatancy can be observed in a drained simple shear test on a sample of dense sand. In the initial stage of deformation, the volumetric strain decreases as the shear strain increases. But as the stress approaches its peak value, the volumetric strain starts to increase. After some more shear, the soil sample has a larger volume than when the test was started.
The amount of dilation depends strongly on the initial density of the soil. In general, the denser the soil, the greater the amount of volume expansion under shear. It has also been observed that the angle of internal friction decreases as the effective normal stress is decreased.
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