Product Manual

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B. Selection of Gravel When the Intermediate Layer Is Not Used
If an appropriate gravel can be identified (see Table 2), the intermediate layer need not be included in the
construction of the green. In some instances, this can save a considerable amount of time and money.
Selection of this gravel is based on the particle size distribution of the root zone material. The architect
and/or construction superintendent must work closely with the soil testing laboratory in selecting the
appropriate gravel. Either of the following two methods may be used:
Send samples of different gravel materials to the lab when submitting samples of components for the root
zone mix. As a general guideline, look for gravel in the 2 mm to 9.5 mm range. The lab first will determine
the best root zone mix, and then will test the gravel samples to determine if any meet the guidelines
outlined below.
Submit samples of the components for the root zone mix, and ask the laboratory to provide a description,
based on the root zone mix tests, of the particle size distribution required of the gravel. Use the
description to locate one or more appropriate gravel materials, and submit them to the laboratory for
confirmation.
Gravel meeting the criteria below will not require the intermediate layer. It is not necessary to understand
the details of these recommendations; the key is to work closely with the soil testing laboratory in
selecting the gravel. Strict adherence to these criteria is imperative; failure to follow these
guidelines could result in greens failure.
The criteria are based on engineering principles which rely on the largest 15% of the root zone particles
"bridging" with the smallest 15% of the gravel particles. Smaller voids are produced, and they prevent
migration of root zone particles into the gravel yet maintain adequate permeability. The D85 (root zone) is
defined as the particle diameter below which 85% of the soil particles (by weight) are smaller. The D15
(gravel) is defined as the particle diameter below which 15% of the gravel particles (by weight) are
smaller.

For bridging to occur, the D15 (gravel) must be less than or equal to eight times the D85
(root zone).

To maintain adequate permeability across the root zone/gravel interface, the D15 (gravel)
shall be greater than or equal to five times the D15 (root zone).

The gravel shall have a uniformity coefficient (Gravel D90/Gravel D15) of less than or equal
to 3.0.
Furthermore, any gravel selected shall have 100% passing a 1/2" (12 mm) sieve and not more
than 10% passing a No. 10 (2 mm) sieve, including not more than 5% passing a No. 18 (1 mm)
sieve.
Step 4 - The Root Zone Mixture
The sand used in a USGA root zone mix shall be selected so that the particle size distribution of
the final root zone mixture is as described in Table 3.