A USGA study suggests that choosing the right bunker sand, particle shape is just as critical as the particle size distribution and is characterized by the degree of angularity and sphericity.
When choosing the right bunker sand, particle shape is just as critical as the particle size distribution and is characterized by the degree of angularity and sphericity, according to a USGA study.
Sphericity is an indication of shape. Round particles have high sphericity and oblong particles have lower sphericity. Angular sands resemble a cut piece of glass with sharp, jagged edges, while a well-rounded sand is smooth and looks more like a potato.
The image above shows the six classifications used to describe angularity and the three classifications that describe sphericity. Selecting sands that range from very angular to subangular and have low to medium sphericity will likely produce desirable bunker playing conditions. Well-rounded, rounded and high-sphericity sands lead to soft conditions and buried lies.
A study comparing glass particles to bunker sands revealed that glass-derived sand had statistically greater angle of repose and ball penetration resistance when compared to four conventional bunker sands of comparable particle size (Owen et al, 2005). The glass-derived material was more stable because 34% and 58% of its particles were in the angular and very angular shape classifications, respectively, compared to only 21% and 0% of particles in the same categories for the conventional sands.
The glass material also had lower sphericity. The USGA is not suggesting you use recycled glass for bunker sand, simply highlighting how these results emphasize the importance of sand particle shape for firm bunker
Another shape characteristic is particle roughness; however, this is not quantified in a lab. Crushed materials will have higher surface roughness and the combination of rough and angular particles will have greater propensity to pack and produce firmer conditions. However, when splashed from bunkers onto putting greens, approaches and collars, turf damage may occur from the grinding action that comes when foot traffic and routine maintenance pass over the splashed sand.
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