Investigating the Options in the New USGA Green Construction Recommendations
By T. Powell Gaines
Figure 1. Three models of USGA constructed greens. Cylinder #1 contains 12" root zone mix above a 2" intermediate layer of 1-2mm sand above a 4" layer of 3/8-1/4" gravel. Cylinder #2 contains a 12" root zone mix above a 2" intermediate layer of 1-4mm sand/gravel above a 4" layer of 3/8-1/4" gravel. Cylinder #3 contains a 12" root zone mix above a 1/4-1/8" gravel with no intermediate layer.
When the USGA introduced the new version of its green construction recommendations (4) in March 1993, there were several changes made that expanded the types and sizes of materials that can be used in the USGA green profile, including the gravel bed, the intermediate layer, and the root zone mix. The new recommendations also offered an option, based on specific criteria, that allows for a change in the makeup of the profile itself. The nature of these changes is such that many golf courses can use lesscostly materials or construction methods compared to the previous USGA recommendations.
Since 1960, a feature of the USGA method of golf green construction (3) has been the use of a 2inch intermediate layer of 1-2mm sand above a 4inch layer of 3/8 to ¼ inch pea gravel. The rationale was that the gravel should be not more than 5 to 7 times larger than the intermediate layer sand to prevent migration of particles from one layer to the next. In the latest version of the USGA recommendations for a method of putting green construction (4), the size range of the intermediate layer has been increased to 1-4mm (90% minimum), making it easier to find an appropriate material at a reasonable cost. The gravel is still 3/8 to 1/4 inch (65% minimum). In addition, the USGA now is offering an alternative to this method by omitting the intermediate layer and using only a 4inch layer of 1/4 to 1/8inch gravel below the 12inch root zone mix, provided the particle size of the gravel and root zone sand pass three criteria.
The USGA method for determining the physical properties of a greensmix does not include putting all three components (greensmix, intermediate layer, and gravel) together in layers when testing, but to test the physical properties of the greensmix in 2" x 3" cylinders separately from the intermediate layer and gravel.
We thought it would be interesting to construct threelayered models of the USGA methods of golf green construction (the former 12mm intermediate layer method and the two new options) and to compare water permeability rates of the three models with those of the greensmix determined separately under USGA protocol.
Therefore, the purpose of this study was to compare the water permeability rate of a root zone mix using the USGA method (3) with those of three constructed models to see if the values were comparable, and to determine if the two new changes to the USGA methods of golf green construction work as well as the previous method (1-2mm intermediate layer sand) with no movement of particles from the root zone mix into the larger 1-4mm intermediate layer or into the 1/4 to 1/8inch gravel when the intermediate layer is omitted.
Materials and Methods
Three models of USGAbuilt greens were constructed in 20" x 4" plexiglass cylinders (Figure 1). The cylinders had a linch (3/16" i.d.) drain tube 1/2" from the bottom and a linch (3/16" i.d.) water inlet tube 1 1/2" from the top.
Cylinder #1 represented the previous USGA method of golf green construction (3), which contained a 12" root zone mix above the 2" intermediate layer of 12mm sand, which was above a 4" layer of 3/8 to 1/4inch pea gravel. Cylinder #2 represented the newly recommended USGA method (4) and contained a 12" root zone mix above a 2" intermediate layer of 14mm sand/gravel, which was above a 4" layer of 3/8 to 1/4inch pea gravel. Cylinder #3 represented the optional USGA method in which the intermediate layer is not required, with a 12" root zone mix above a 4" layer of gravel that met all criteria. Only the root zone mix in Cylinders #2 and #3 was the same (Table 1), and the two intermediate layers and the two pea gravels in Cylinders #1 and #2 were different (Table 2), as was the 1/4 to 1/8inch gravel (Table 3).
The physical properties of the toot zone mixes (Table 1) were determined by the USGA method (3), and water permeability rates of the three models of USGA profile greens in Cylinders #1, #2, and #3 were determined by Darcy's Law (2). The root zone mixes in the three cylinders were compacted manually by tamping the damp root zone mix with a round piece of wood that snugly fit the cylinder until the volume ceased to be reduced by compaction. The samples were allowed to remain overnight under a continuous hydraulic head of approximately I inch before being timed the next day. The standard deviation (+/-) of the water permeability rate was determined by the variance in the mean of eight consecutive timings. The Darcy Equation allows for the determination of hydraulic conductivity (water permeability rate) regardless of the size of the cylinder, which is how the water permeability rate of the three models in the cylinders can be compared to the water permeability rate of the greensmix in the 2" x 3" cylinders using the USGA method of determining the physical properties of a greensmix (3).
Results and Discussion
The physical properties and particle size analysis of the three greensmixes are shown in Table 1. The greensmix in Cylinders #2 and #3 was the same. Both greensmixes meet all USGA physical property parameters and particle size recommendations for a putting green root zone mixture (greensmix).
The particle size analysis of the two intermediate layers and the two pea gravels in Cylinders #1 and #2 are shown in Table 2. The intermediate layer in Cylinder #1 had 78% particles within 12mm and 21% particles with 23mm. This intermediate layer represents the previous USGA recommendations of a 12mm intermediate layer for golf green construction. The pea gravel in Cylinder #1 had 81.6% particles within the USGA recommended range of 3/8 – 1/4" (65% recommended minimum) and 17.5% particles between 1/4" and 1/8". The intermediate layer and pea gravel in Cylinder #1 are compatible with each other and meet USGA recommendations for golf green construction.
The intermediate layer in Cylinder #2 had 99% particles within the new USGA recommended range of 14mm with 90% particles between 2mm and 4mm (Table 2). Actually, this intermediate layer material is a grit rather than a very coarse sand and would not have passed the previous USGA recommendation of 12mm. This intermediate layer material represents the new USGA recommendation of a 14mm intermediate layer for golf green construction. The pea gravel in Cylinder #2 had 91.4% particles within the USGA recommended range of 3/8" 1/4" and 8.4% particles between 1/4" and 1/8". The intermediate layer material and pea gravel in Cylinder #2 are compatible with each other and meet USGA recommendations for golf green construction.
The root zone mix and the ¼ - 1/8" gravel in Cylinder #3 meet the three new USGA criteria for when an intermediate layer is not required (Table 3). This sand and gravel passed the bridging, permeability, and gravel uniformity factors as shown in Table 3; therefore, an intermediate layer is not required with this sand and gravel.
Water Permeability Rates
For greensmix #1, water permeability rates were 14.5 +/- 0.4 in/hr as determined by the USGA procedures to simulate a compacted golf green (3), and 14.6 +/- 0.6 in/hr for Cylinder #1 when determined by the Darcy Equation (2) following overnight saturation of an approximate linch hydraulic head (Table 1). These values represent eight consecutive 10minute timings and are practically the same. The standard deviations of +/-0.4 and +/-0.6, respectively, show approximately the same precision in these two methods of measuring water permeability rates. For greensmix #2 and #3, water permeability rates were 16.8 +/- 0.5 in/hr as determined by the USGA procedure (3), compared to 16.0 +/- 0.5 in/hr for Cylinder #2 and 16.7 +/- 0.4 in/hr for Cylinder #3. These values show very close agreement and equal precision between water permeability rates as determined by the USGA procedure (3) and from cylinders using the Darcy Equation (2). Using a 1-4mm intermediate layer and 3/8-1/4" gravel in Cylinder #2 did not cause a significant difference in water permeability rates compared to omitting the intermediate layer and using the 12" greensmix above a 4" layer of ¼-1/8" gravel in Cylinder #3.
RootZone Particle Migration
The picture of the three models shown in Figure 1 was taken after the three cylinders had been treated with an approximate 1" hydraulic head above the greensmix for one week. The results showed no movement of root zone particles into the intermediate layer in either Cylinder #1, which had the smaller 12mm intermediate layer, Cylinder #2, which had the larger 14mm intermediate layer, or Cylinder #3, which had the rootzone mix directly above the l/41/8" gravel with no intermediate layer. These results showed that the two new methods of USGA golf green construction using a larger 14mm intermediate layer (Cylinder #2) and a root zone mix above a ¼-1/8" gravel with no intermediate layer (Cylinder #3) did not cause any root zone particle migration as was also the case for the previous USGA method of golf green construction using a 12mm intermediate layer (Cylinder #1).
This study shows that water permeability rate values obtained from three different models of USGA constructed golf greens were approximately the same as values obtained by the USGA method (3), and that the precision of these values as determined by the standard deviation in the variance of the mean of eight consecutive timings was the same for each method of measurement. These findings show that the USGA method of determining water permeability rates of a greensmix using 2" x 3" cylinders gives values that are not statistically different from values obtained by constructing models of the entire 18" putting green profile. Furthermore, no root zone particle migration occurred in any of the tree models after maintaining a1" hydraulic head for one week. These results suggest that the two new USGA methods of golf green construction should work just as well as the previous USGA method of golf green construction, while allowing for potentially less costly materials or construction methods to be used.