David M. Kopec, Extension Turfgrass Specialist
Last months issue provided information regarding the new suggested guidelines for gravel and choker particle sizes for USGA greens. The particle sizes in that article are for cases when the choker layer (intermediate layer) is used. The choker layer is installed between the gravel and the rootzone mixture.
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.
SIZE RECOMMENDATION FOR GRAVEL
WHEN THE INTERMEDIATE (CHOKER) LAYER IS NOT USED
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
* Not more than 10% of gravel less
* 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 be observed.
Next month, the revised particle sizes and physical properties of the rootzone mixture will be discussed.
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona. The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.