2. Particle shape

Sand particle shape is just as critical as the particle size distribution and is characterized by the degree of angularity and sphericity. 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. Figure 1 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 (these factors will be discussed below) 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. We’re 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.

3. Uniformity coefficient

The uniformity coefficient (Cu) can be useful for predicting the relative firmness of sand by identifying if the material is narrowly or widely graded. A narrowly graded sand contains many particles of similar size and will have a relatively low Cu. Such material is more likely to produce buried lies and is more prone to erosion. Conversely, a widely graded sand is one that has particles of many different sizes and will generally produce firmer conditions due to the greater tendency for the sand to pack together.  As a guideline, the Cu should range between 2.0-5.0. In a study by Bigelow and Hardebeck (2004), no meaningful relationship was detected between the Cu and the modified-penetrometer readings used for measuring firmness. However, Crum et. al. (2003) found a strong correlation between the Cu and bearing capacity – a measurement of material strength – in their study “Agronomic and Engineering Properties of USGA Putting Greens.” The researchers used a modified California Bearing Ratio testing device to quantify the firmness and stability of sand rootzone mixtures for putting greens. When the Cu increased from 1.8 to 3.0, bearing capacity doubled. While this test is not used to evaluate bunker sands, it demonstrates the value of the Cu for predicting material stability in this study.  

While the Cu is important for predicting bunker sand firmness, it must be considered in conjunction with the other factors discussed in this article. For example, a high-Cu sand that is rounded may still result in soft conditions and buried lies, while an angular sand with a relatively low Cu may result in acceptable firmness. If the sand exhibits a low Cu value – i.e., less than 2 – it should be very angular to angular if the goal is to avoid a high propensity for buried lies.

4. Infiltration rate

The physical soil testing laboratory community has suggested that the minimum infiltration rate for bunker sand is 20 inches per hour, which was based on the 20-24 inch per hour infiltration rate guideline in the 1993 USGA Recommendations for a Method of Putting Green Construction. However, in my experience testing bunker sands on golf courses, I’ve seen that infiltration rates often decrease significantly – e.g., from 22.8 to only 7.5 inches per hour – in just two to three years! Silt and clay from wind-blown dust, organic debris from clippings, leaves from trees or shrubs, algae growth and soil contamination all can decrease infiltration rates. Bunker sands that initially drain from 30-100 inches per hour should work well, depending on the climate and site conditions.

5. Penetrometer test

The penetrometer test is often perceived as the best predictor of bunker sand firmness, but this test is only marginally useful for indicating firmness and aversion to buried lies in the field. However, it is helpful for comparing the relative firmness of different sands. This test utilizes a handheld penetrometer with a golf ball on the end. The golf ball is pushed halfway down into a small amount of oven-dried, loosened sand in a container with rigid sides. The force used to push the golf ball down to its hemisphere is recorded in kilograms per square centimeter. The higher the force, the firmer the sand. The average value from a series of five drops produces the final penetrometer reading. Brown and Thomas (1986) determined that sands with penetrometer readings less than 1.8 kilograms per square centimeter were more prone to the dreaded buried lies, while sands with penetrometer readings greater than 2.4 kilograms per square centimeter generally had few instances of buried lies. However, let the buyer beware. This test is highly variable as labs use different containers to hold sand for this procedure, and the procedure itself may vary with each penetrometer operator. It is not recommended to compare values between labs, and it is not recommended to place great emphasis on this test. Rather, focus your attention on the particle size distribution, Cu, sand shape and infiltration rate when selecting bunker sands.

Bunker sands that exhibit regular crusting require more-frequent raking to break up the crust and provide an acceptable playing surface. Bunkers that set-up are rarer but are more likely to occur with highly calcareous sands such as those containing coral. An extreme case of set-up was observed at a course in Hawaii with sand containing very high levels of calcium carbonate. The set-up was so severe that it was difficult to break up with a hammer!

8. Acid reaction

The acid reaction test involves applying several droplets of a strong acid, such as hydrochloric acid, on the bunker sand and observing the reaction. A highly calcareous sand will have a strong effervescence reaction with bubbling off carbon dioxide. Such material is more likely to set-up in the bunker and may degrade due to chemical weathering, causing a reduction in infiltration rate. While silica sand is generally preferred over calcareous sands due to its resistance to chemical weathering, many golf courses have been using calcareous sands successfully in bunkers for many years. Therefore, a high acid reaction should not disqualify a sand for consideration in bunkers.  

9.  Color

Laboratories assess sand color using the Munsell color charts. Although the color of the bunker sand is a characteristic that deserves attention, it is strongly recommended to focus the selection process on the eight sand characteristics described above. In general, the whiter the sand, the greater the potential to see green algae forming on bunker faces and the easier it is to notice any deterioration in color. Therefore, whiter sands generally have a shorter useful life. Depending on expectations for aesthetics, a brilliant white sand may only last three to five years.

Soil moisture release curve

The SMRC and moisture column tests are used to identify the moisture content of the sand at varying depths. These tests are becoming increasingly useful for course managers to decide the best sand depth for varying sand types, site conditions and liner options. A soil physical testing laboratories guideline for interpreting the moisture column or SMRC test is to determine the depth at which the volumetric water content is equal to or less than 15% after 24 to 48 hours of drying. Sam Ferro, president of Turf and Soil Diagnostics soil testing laboratory, said, “Bunker sand should have a moisture content less than 15% at 1 inch below the surface. Higher moisture content is likely too wet and may lead to problems such as algae growth. However, while lower moisture content is preferred, some water-holding is desirable because it contributes to sand firmness.” For many bunker sands, the depth at which moisture levels dry to 15% at 1 inch below the surface will range from 5-10 inches in instances where the sand is placed on top of a liner and there is no access to free drainage – in this case free drainage is defined as a continuous column of sand extending to the drain pipe.

There are several methods to determine a SMRC. In the most common method, a small cylinder is filled with sand and then water is applied to reach saturation. The sand is then allowed to drain for up to 48 hours to reach equilibrium. Tension in the form of suction is then applied to represent the sand depth in 2-inch increments, drawing water out of the sand. To better understand this test, visualize the height of a water column 2 inches deep compared to one that is 8 inches deep. The 8-inch-deep column will produce more weight or pressure – i.e., head – than the 2-inch-deep column. The tension applied in the small cylinder filled with sand in this test simulates the weight of the water at different sand depths. The greater the tension, the greater the sand depth. Increasing tension is applied to reach representative depths of up to 10-12 inches and the moisture content is measured corresponding to each 2-inch depth increment.

Moisture column tests

The moisture column test is like the SMRC, yet simpler. In the moisture column test, a clear plastic column is filled with sand and placed on top of filter fabric, or a sample of a bunker liner material such as polymer-treated gravel or porous concrete. The sand is saturated and allowed to dry for 48 hours. The moisture content is measured at the interface of the filter fabric or liner and the sand, then at 4 inches, 6, 7 and 8 inches from the bottom. Moisture is recorded every two hours. In Figure 2, Bunker Sand A is placed in a column 8 inches deep over a gravel material treated with polymer. After 24 hours have elapsed, the sand is still quite wet at the 4- and 6-inch depths, with moisture content of 21.6% and 18.8%, respectively. At 7 inches the sand still has slightly higher water content than desirable at 15.9%. After 48 hours of drying, Sand A still has a high moisture content at the 6-inch depth. Based on this test, the depth for Sand A should be 7-8 inches given the choice of liner.

The moisture column test for Sand B (Figure 3) shows that after 24 hours, the sand has dried to a more ideal moisture of 14% at 7 inches from the bottom of the column and after 48 hours the 6-inch depth is optimal at 15.5%. Sand B can be installed at about 6 inches of sand given its lower moisture retention and better drainage characteristics. 

The SMRC and moisture column tests assume the sand is saturated throughout the depth of the column, which may not occur that often in arid regions, and these tests do not account for evaporative loss or factor additional water added to the system from irrigation, rainfall or surface runoff. However, these tests do represent the perched water table created by the lining material placed over the top of drain lines. While the liners are composed of different materials, Mr. Ferro notes that, “Anecdotally, the liner material doesn’t appear to be the most significant factor affecting bunker sand water retention.” In other words, the height of the perched water table is not significantly different among the liners. In addition, most underlying soils – other than sites with a deep sand profile – will also increase moisture content in the sand. One way to avoid issues with a perched water table in bunker sand is by placing the drain lines on top of the bunker lining material and filling the entire area with bunker sand. 

This design provides a free-draining sand column from the surface to the drain line, omitting the perched water table and reducing the moisture content in the sand. As a result, this design also reduces the sand depth required to achieve good playability. Field tests using a handheld moisture meter have confirmed that the volumetric water content in the top 2 inches of the sand in bunkers constructed with free drainage is significantly less than in bunkers where drainpipe is hidden underneath the liner.

The results from these tests can be useful to plan for the best sand depth at your facility, and to match sand choices with liner options. As courses pursue firmer sands with greater angularity and wider particle size distribution, the suggested depth may continue to increase, up to 8-10 inches in some cases depending on the sand and bunker design. With seemingly ever-increasing bunker sand costs and the expense of trucking the material to the course, planning for 8 inches of sand rather than 4 or 5 inches can result in a significant cost increase.

Practical sand selection guidelines for bunkers

The following two sections will provide suggestions for bunker sand selection when faced with different field scenarios and will address frequently asked questions.

Common Scenarios

1. Selecting bunker sand for a wet climate

There is a paradox between selecting bunker sands that have a high enough infiltration rate to handle heavy rain events yet remain firm enough to encourage inbound golf balls to bounce from the impact point. In climates where heavy rain occurs frequently,  using a sand that has a Cu in the lower range – i.e., a value from 2-3 – and contains coarse sand in the higher end of the desirable range – 10%-15% between 1 and 2 mm – should perform well. Additionally, the silt and clay must be kept to a minimum, ideally less than 2%, and the very fine sand content – between 0.05 and 0.1 mm and measured on the No. 270 sieve – must be less than 5%.

2. Sand selection for steep, flashed bunker faces

Sands that are widely graded – i.e., with a high Cu – angular and rough will have greater potential to remain on steep bunker faces. The angle of repose for a given sand provides a general guideline for the “do-not-exceed” slope for the bunker faces. The range of angle of repose for bunker sands is narrow, from as low as 29 degrees to as high as about 35 degrees. There is limited research relating erosion potential to the angle of repose, but the study “Erosion Potential of Various Golf Course Bunker Sands” revealed that sands possessing the highest angle of repose – 33.1 and 34.9 degrees – resulted in less erosion from steep slopes when compared to sand with an angle of repose of 30.3 degrees. Thus, a good guideline is to avoid building bunker faces that exceed a slope of 33 degrees. In addition to selecting a sand with a relatively high angle of repose, courses can use bunker liners to discourage erosion from steep slopes. Furthermore, smoothing bunker faces with the rounded side of a bunker rake is one common technique courses will use to create firm bunker faces and discourage sand from eroding down steep slopes. 

3. Sand selection for wind-swept sites

Wind erosion can result in significant sand loss from bunkers, which increases costs due to sand replacement and the labor necessary to install the sand. The sand particles in the finer fraction – i.e., less than 0.25 mm in diameter – are more prone to wind erosion. While K.W. Brown and J.C. Thomas suggest 78%-100% of sand particles should be sized between 0.1 and 1 mm in diameter, at windy sites more than 80% should be between 0.25 and 1 mm and 10%-20% should fall in the coarser fraction from 1-2 mm in diameter. An option recently employed at the Sheep Ranch at Bandon Dunes Golf Resort, Oregon, was to omit all sand-filled bunkers. The land, formerly used as a wind farm, is so windy that all the windmills fell! The Coore and Crenshaw design team recognized that keeping sand in the bunkers would be a serious issue and smartly built the course with no bunkers to avoid the maintenance headache that constant erosion would have created.

4. Sand selection for installation on top of a liner

Although many bunker liners are permeable, the distinct change in texture between sand and liner creates a perched water table. Anecdotal evidence suggests that there may not be a great difference in the height of the perched water table among the liners when drainage pipe is located beneath the liner. The ideal sand depth for a given sand and liner combination can be determined by using the SMRC or moisture column test. Courses should determine the depth at which the sand dries to approximately 15% moisture content at 1 inch beneath the surface after 24 to 48 hours of drying. The trend to pursue firmer bunker sand with finer particles and more widely graded particles increases the height of the perched water table and may require depths of 8 inches or more to achieve the desirable surface moisture content.

5. Sand selection with playability in mind

Subjective characteristics of the bunker sand selection process include the color of the sand and playability. Let the golfers be a part of the selection process, but only provide bunker sand materials for consideration that have been vetted for the desirable physical characteristics described in this article. Consider using several sands in separate trial bunkers for golfers to evaluate. This also will allow the golf course superintendent to monitor sand performance characteristics such as the ability of the sand to remain on the bunker face, crusting potential and responses to raking patterns and various raking implements.

Frequently asked questions

1. Why does water movement in bunker sand slow over time?

Think of the sand in a bunker as a large filter. Bunker sand captures silt, clay, grass clippings, algae growth, and leaves, just to name a few common contaminants. There is also soil contamination from edging operations and bunker washouts.

While bunker liners help reduce contamination from the subsoil, they cannot prevent contamination carried by wind or surface water. Additionally, porous aggregate liners used in the market result in a perched water table at the interface of the sand and the liner when drainage pipe is located underneath the liner. At typical sand depths of 4-6 inches on the bunker floor, the sand is often chronically wet with frequent rainfall or irrigation. This condition leads to algae growth and even black layer. There is anecdotal evidence from testing aging bunker sands that algae growth can significantly decrease bunker sand performance.

In a three-year span at one course in Arizona, the infiltration rate of the bunker sand decreased from 24 inches per hour to an average of 12 inches per hour. The organic matter in this sand increased from 0.3% to nearly 1%, which likely can be attributed to algae growth. The silt and clay content increased marginally during the same period from 3.2% to 3.9%. Fine and very fine sand content also increased slightly. These finer particles resulted in an increase in the Cu from 3.5 to 4.1. This decline in bunker sand performance in such a short time is not uncommon, especially in bunkers that are chronically wet and experience algae formation. 

2. What is the recommended sand depth for bunkers?

The recommended sand depth will depend on the sand physical characteristics, the underlying material – i.e., liner or soil type – and, on a more practical level, availability and cost. At minimum, sand depth should be 4 inches on bunker floors to prevent golfers from striking a liner or underlying soil while playing a shot. If the bunkers were constructed with drainage pipe underneath the liner, use the moisture column test to help determine the appropriate sand depth.

One way to prevent issues with a perched water table in the bunker sand is to install the drainage pipe on top of the liners. This design allows for free drainage because there is a continuous column of sand from the surface to the pipes. This type of design has been used successfully and is becoming more commonplace. An alternative method is to construct a square or rectangular open area bordered by durable plastic or wood at the lowest point in the bunker, where the drains intersect with the exit drainpipe. This area is filled with gravel to an elevation approximately 4 inches beneath the top of the liner. The remaining void is filled with bunker sand. One can also fill the entire open area with bunker sand rather than use gravel.

3. What are some strategies to improve playability in bunkers?

New sand added to bunkers is often soft at first but will firm within a few months. Courses can expedite this process by compacting the sand with a mechanical bunker rake or vibratory plate compactor, or by using the rounded side of the bunker rake to smooth the sand and create a firm surface. On the other hand, excessively firm bunkers are often contaminated with organic debris, algae and fine material such as silt, clay and fine sand. These bunkers will likely need to be raked with deep tines to rough up the sand and soften it. With severely contaminated sand, the deep tines may be needed several times per week, and eventually the sand will need to be replaced. A short-term solution is to add 1-2 inches of new sand. While this strategy can usually be expected to improve cosmetics and playability, the benefits typically only last one or two seasons.

4. How do we increase the longevity of our sand?

While course managers cannot thwart the accumulation of wind-blown silt, clay and very fine sand, they can routinely rake and blow organic debris out of bunkers to limit sand contamination. Courses can also monitor sand contamination in bunkers by using a jar test and comparing the results from year to year. A bunker liner may also help prolong the life of bunker sand by reducing contamination from soil or rocks underneath the liner. Grading around bunkers should discourage surface water, and the debris it carries, from entering bunkers. In bunkers where the sand remains drier and where there is free drainage from the top of the sand to the drain lines, anecdotal testing has revealed significantly less organic matter contamination and longer duration of desirable drainage and playing characteristics.

USGA agronomists have encountered many instances where bunker playability is underperforming due to poor sand selection. Do not make the same mistakes that others have. Utilize laboratory testing and the guidelines and perspectives provided in this article to help guide your bunker sand selection process.

Brian Whitlark is an agronomist in the West Region

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References:

Bigelow, C., and G. Hardebeck. 2004. Characterization of sand materials for golf course bunker use. Purdue.edu.

Brown, K. W., and J. C. Thomas. 1986.  Bunker sand selection. Golf Course Management. 54(7): 64-70.

Crum. J.R., T.F. Wolff, and J.N. Rogers III. 2003. Agronomic and engineering properties of USGA putting greens. USGA Turfgrass and Environmental Research Online. 2 (15). August 1. 1-11.

Moore, J.F. 1998. How to select the best sand for your bunkers. USGA Green Section Record. January/February. 36(1): 9-12.

Nemitz, J., C. Bigelow, A. Moeller, G. Hardebeck, K. Walker, and D. Smith. 2007. Erosion potential of various golf course bunker sands. Abstracts: 2007 International Annual Meetings (ASA/CSSA/SSSA).

Owen, A.G., A.R. Woollacott, and S.W. Baker. 2005. An evaluation of recycled glass-derived sand for use in golf course bunkers. International Turfgrass Society Research Journal. 10(Part 2): 1138-1143

United States Golf Association. 2018. USGA recommendations for a method of putting green construction. USGA Green Section Record. February 16. 56(4): 1-16.

United States Golf Association. 2019. Monitoring sand contamination in bunkers. USGA Green Section Record. November 1. 57(21).