It's all about the sand

Several factors need to be considered when filling bunkers.

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January 3, 2006

Sand bunkers remain one of the most important concerns for golf course superintendents, and like greens conditions, they’re controversial. Most of the issue is with golfers, who tend to view sand as too soft, hard, wet, dry or inconsistent. The amount of sand in a bunker also can be an issue. Golfers need to be reminded sand bunkers are a hazard, but at the same time, bunkers need to be a fair hazard.

The major issues with bunkers are sand quality and playability, and obtaining quality bunker sand, which isn’t easy.

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Some bunker designs can be a real hazard.

For many years, selecting bunker sand involved calling the local sand pit. Superintendents were often told, “We have mason sand, brick sand and concrete sand. I think what you want is our mason sand. That stuff would work great for you.” Mason sand might or might not be desirable sand for bunkers, but in the current golf climate, the days of calling the local sand pit are over. There are few places in the United States that have natural sand deposits that meet specifications for great bunker sand. Most premium bunker sand is manufactured in only a few locations throughout the country. These manufactured sands, along with a few rare natural deposits, make the process of finding great bunker sand difficult and expensive.

What makes great bunker sand? That’s a complex question. The U.S. Golf Association considers a list of eight factors when selecting bunker sand: particle size, particle shape, crusting potential, chemical reaction and hardness, infiltration rate, color, penetrometer value and playability. Depending on location and climate, how these factors are ranked vary slightly.

However, there seems to be a common denominator of great bunker sand, and it might be the biggest factor – the fried egg test, or in technical testing terminology, the penetrometer value, which measures the energy required to bury a ball in sand. This value shows the ability of sand to resist the golf ball from burying, or in more scientific terms, its resistance to compression. However, the penetrometer device is questionable, and some think a better device is needed. A chief limitation is that a penetrometer doesn’t factor in ball spin, which has major input on the resulting lie in a bunker. The two biggest factors affecting the penetrometer value are the particle size and shape of the sand.

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A big rain can change a bunker's playability instantly.

Particle size
It’s recommended the majority of the particle sizing, about 75 percent or more, fall in the medium-coarse range (0.25 mm to 1.0 mm). The additional 25 percent or less should fall in the medium-fine to very fine range (0.25 mm to 0.05 mm).

One factor that influences particle size when selecting bunker sand is the makeup of the root zone. Sand is blasted onto green surfaces from adjacent bunkers frequently. Therefore, sand can create problems if the particle size is significantly smaller than the root-zone makeup. So it’s important to select a sand that also will integrate with the root-zone material and not cause any layering-type problems.

Particle shape
Once sand is found with the correct particle sizing, the job is only partially finished. Particle shape is the next characteristic that influences a great sand and might be the most important of all. This also is the one characteristic that influences the penetrometer value the greatest.

One term frequently used when comparing sand shapes is sphericity. Sand with a high degree of sphericity is one that’s round or almost round. Sand that has a low degree of sphericity is one that’s elongated or flatter. The most desired shape for bunker sand is a particle shape that’s angular. Therefore, it possesses many sharp and well-defined edges and has low sphericity. Sand that’s smooth and has high sphericity isn’t well suited for bunkers.

The difference between these two sand types is the particle shape of the angular sand. It compacts well because of the sharp angular edges and elongated shapes. Round sands can’t compact. Therefore, highly angular sand with low sphericity will obtain the best penetrometer reading and offer the best resistance to compression from the golf ball. This translates into a low tendency for a ball to bury in the sand, which minimizes the dreaded fried egg lie.

Penetrometer value
The penetrometer has been the test of choice for determining the potential for a ball to bury. As mentioned above, ball rotation (spin) is a factor not taken into consideration. There are other factors that can determine the ball’s lie in a bunker that testing doesn’t consider: shot trajectory, ball angle entry and incoming ball velocity. A ball can enter a bunker at a bad angle when it’s 90 degrees to the sand slope with a high trajectory. This angle offers the least reaction between the ball and the sand, and results in the greatest possibility of a fried egg lie.

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Impeccable bunkers are expected on a daily basis at many clubs.

The speed of the ball (velocity) when it hits the sand is another factor. Many shots, with a high ball speed when entering the face of the bunker, are destined to be buried even with the good bunker sand. When considering these factors, it’s understandable why, in recent years, there has been talk of determining better methods to evaluate bunker sands.

Some of the original penetrometer values published by Thomas Turf Services (reference the table at the top of page 48) also are being questioned. Many superintendents agree values should be higher. Using the table, a sand with a value of 2.4 kg/cm2 or greater is rated as a low tendency to bury. However, clubs are reporting significant problems with sand of this penetrometer value. Many believe the magic number should be near or greater than 3.0 kg/cm2 to have sand perform as needed.

Crusting potential
Crusting is the formation of a thin, crust-like layer on the surface of the sand. This layer usually ranges from 1/16 inch to 3/8 inch. Crusting is a direct relationship to the purity (cleanness) of the sand. The crusting potential is directly proportional to the amount of silt and clay in the sand. The higher the percentage of silt and clay, the higher the crusting potential.

Crusting occurs when the bunker surface receives moisture from rain or overhead irrigation. With the sophistication of sand production facilities, the cleaning (washing) process all but eliminates any crusting problems when purchased from those facilities.

Chemical reaction
Chemical reaction and hardness will determine the makeup and stability of sand. Some sand, such as calcareous sand, is prone to physical and chemical weathering. This will cause long-term problems from the breakdown of the sand and a build-up of fine particles. These fine particles will cause firmer sand that will have decreased infiltration rates. Fortunately, the makeup of most sand is quartz, which is silicon dioxide (SiO2) and resists chemical and physical breakdown.

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Particle size and shape are just two things to think about when choosing bunker sand. 

Infiltration rate
Infiltration rate (hydraulic conductivity) is a straightforward characteristic and is usually high in most sand. This is different than root zones for greens, which normally have an organic component blended with sand that lowers the infiltration rate significantly. Bunker sand should have an initial minimum infiltration rate in the range of 20 to 30 inches per hour. If a sand meets the criteria for particle sizing and has high purity (cleanness), then meeting the infiltration rate usually isn’t a problem.

Color
The color of bunker sand is subjective. Most golfers like the look of white bunker sand contrasting with green surrounds. However, white sands can cause problems on bright, sunny days, creating significant glare. Slightly off-white (light tan) might be a better choice. If local sand meets all the performance criteria for great bunker sand, color probably shouldn’t be an issue.

Playability
The playability of a bunker sand will differ from golfer to golfer and is another subjective component of bunker sand selection. Unfortunately, not all golfers like the same bunker sand. Tour professionals and most low-handicapped players prefer firm sand, which allows spin to be produced on the ball. A higher-handicapped player, who can’t develop the clubhead speed needed to get through a firm sand bunker shot, prefers softer sand.

Comments from players often reflect the type of sand used. If you have firm sand, disgruntled players will insist there isn’t enough sand in the bunker. With softer sand, players might insist there’s too much sand in the bunker. In both cases the sand depth may be identical.

One of the earliest methods by which a golfing membership evaluated different bunker sand was the construction of a test bunker. After sand selection was narrowed to three or four similar types of sand, many clubs would divide their practice bunker into sections and fill each section with a different sand. This would give the membership a chance to test the playability and offer feedback.

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Proper sampling procedures will ensure correct lab tests.

Another testing ground that’s become popular is redoing a worst-case-scenario bunker. This offers a live playing situation a practice test bunker can’t.
Factors such as the fried egg lie and buried lie can be evaluated under true playing conditions. Maintenance practices and environmental conditions also can be evaluated. This might cause a small problem with the test bunker playing completely different than other bunkers on the course. However, good communication with the membership makes it easier.

Lab testing
The most important part of selecting sand is to have a complete bunker sand analysis performed by an accredited laboratory. Characteristics tested for are: particle size, particle shape, infiltration rate, penetrometer value, crusting potential, color and purity. Some labs also test for chemical hardness.

When considering sand for testing, superintendents should request a minimum of a truckload (15 to 20 yards) to evaluate. There should be a good representative sample of the pile taken immediately after dumping. The preferred testing method should be a 2-inch PVC pipe inserted into the pile in numerous locations. The sand in the pipe should then be combined in a bucket. A one-gallon size sample from the bucket should be used for laboratory testing, which ranges from $100 to $200 per sample. This is a minor cost compared to the overall cost of bunker sand for a golf course.

Additionally, factors such as surface and internal drainage, design and maintenance practices influence great bunkers. With an eye on new testing procedures and the ability to manufacture high-quality bunker sand, hopefully the future of bunkers will be less controversial. GCN

Kevin J. Ross, CGCS, is director of golf course management at Country Club of the Rockies, Vail, Colo., and president of Ross Golf Agronomy. He can be reached at kjross@vail.net.