The USGA putting green specifications have been revised, after careful research by Dr. Norm Hummell and the USGA. One of the changes relates to one of the most argumentative issues about USGA greens in general. This issue centers around the intermediate sand layer (choker layer), which acts as an intermediate barrier between the rootzone and gravel drainage layer. The whole idea of a USGA green is to provide the turf with a large and deep enough "pot" to regulate moisture holding capacity, drainage and infiltration... all at the same time.

The changes and relative sizes of particles (of both the sand and the gravel) affect the moisture holding capacity and drainage, as a function of pore space. The changes in pore space at the interface layers determines largely the properties the rootzone sand has.

These relationships have been now been defined for construction of new greens both WITH or WITHOUT the intermediate (choker) layer.

1. The intermediate level should be uniformally installed to a thickness of between 2 - 4 inches.

2. Make sure the surface of each layer conforms to the contours of the proposed finish grade. In otherwords, don't change the depth of materials to make them "level", if the playing surface of the green itself will not be level.

When both the intermediate layer and gravel are used, the guidelines are as follows for recommended particle size(s).

Material Description
Gravel:
(using intermediate layer) Particles are not more than 10% greater than 1/2" (10 mm)

At least 65% of the particles should be between 1/4" (6 mm) and 3/8" (9 mm)

Not more than 10% of the particles less than 2 mm

Intermediate Layer: At least 90% of the particles between 1 mm and 4 mm

There are about 25 mm in one inch.

The following deals with the selection of gravel, when the choker layer is not used. Note that a specific relationship between the rootzone sand and the gravel must exist for proper performance when the choker is not used.

Factor Recommendation
Bridging factor * D15 (gravel) 5 x D85 (rootzone)

Permeability factor * D15 (gravel) 5 x D15 (rootzone)

Uniformity * D₉₀ (gravel) / D₁₅ (gravel) 2.5

* No gravel particles greater than
12mm

* Not more than 10% of gravel less
than 2mm

* Not more than 5% of gravel less than 1mm

The engineering criteria for these values are explained below and are specified within these ranges for proper "bridging" between the rootzone and the gravel layer. Bridging allows proper transition of sand particles with the gravel, so that the green maintains proper water movement and other critical benefits of USGA greens.

The criteria as outlined rely on 15% of the sand rootzone materials to bridge (mesh) with the smallest 15% of the gravel particles. When this occurs, smaller voids are produced and they prevent migration of the rootzone sand into the gravel. This is necessary for adequate water movement from the rootzone into the gravel.

The D₈₅ (rootzone) is defined as the particle diameter below which 85% of the soil particles (by weight) are smaller than the rest of the rootzone particles. The D₁₅ (gravel) is defined as the particle diameter below which 15% of the gravel particles (by weight) are smaller than the rest of the gravel.

* For bridging to occur, the D₁₅ (gravel) must be less than or equal five times the D₈₅ (rootzone).

* To maintain water flow across the rootzone gravel interface, the D₁₅ (gravel) shall be greater than or equal to five times the D₈₅ (rootzone).

* The gravel shall have a uniformity coefficient (gravel D₉₀ / gravel D₁₅) of less than or equal to 2.5.

* In addition, all gravel must pass through a 1/2" (12mm sieve screen) and not more than 10% can pass through a No. 10 (2mm sieve screen). Still yet, not more than 5% of the gravel can pass through a No. 18 (1mm sieve).

The architect and the construction superintendent must work closely with the soils testing laboratory for selecting an acceptable gravel, when no choker layer is used. Two suggested options include:

1. Send samples of different gravels to the lab when submitting samples of components for the rootzone mix. General guidelines are to look for gravels in the 2mm to 6mm range. The lab can first determine the best rootzone mix and then test the gravel samples to see if they meet the guidelines below. Or,

2. Submit samples of the components for the rootzone mix and ask the laboratory to provide a description (based on the rootzone tests) of the particle size distribution required for the gravels. The lab will do this by using the three equations included in the table.

Use the description to find one or more appropriate gravel materials, Then, submit the gravel materials to confirm the proper size.

The strict adherence to the criteria is imperative. If these guidelines are not followed, failure of the greens may result. Remember, the choker layer was used as a transition between the rootzone sand and the gravel. Construction without the choker means that specific tolerances between the sand and gravel must be observed.