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Gmax Testing of Natural Turf


While synthetic surfaces are relatively uniform and consistent, natural turf surfaces are generally not. Soil conditions can vary widely within a field, as can moisture levels, the amount of turf cover, and many other factors. As a result, it is much more difficult to accurately test and characterize the shock-absorbing properties of natural turf fields.

Standards for testing
Natural turf fields can be tested in accordance with ASTM Standard F355-A and Specification F1936. For the most part, the test procedures will be identical to those outlined on the page that discusses "Testing Artificial Turf."

Most often, however, natural turf fields are tested in accordance with ASTM Standard F1702. This Standard:
  • Specifies the use of a smaller, lighter, portable test device (commonly referred to as a "Clegg tester" or a "Clegg hammer").
  • Accomodates the variability of natural turf surfaces by specifying a more flexible procedure for selecting "test units" - the points on the field where Gmax readings will be taken.
  • Allows for evaluating an unspecified number of test units - recognizing that the actual number will be dictated by the need to assess a large enough sample to accurately characterize the condition of the field, while limiting the size of the sample to minimize the costs of the testing.
Test Procedures
In consultation with each client, Sports Turf Solutions develops a testing plan to characterize the condition of the field, at an acceptable level of accuracy. The key element of the testing plan is the number of test units selected for evaluation - the size of the test sample. The number of test units needs to be large enough to provide reasonable assurance that the test results are representative of the field as a whole. At the same time, the size of the sample directly impacts the cost of the test, so the sample needs to be limited in order to keep costs within reason.

After the sample size is determined, the location of each test unit is plotted on the field. This is accomplished by superimposing a grid of equally-spaced horizontal and vertical lines on a diagram of the field. On a typical football field, the grid would cover an area 320 feet long and 180 feet wide. Each grid line is assigned a number, so that each intersection of a horizontal and vertical line can be identified by a unique number-pair. (Using the football field as an example, if the grid lines were at 5-yard intervals, there would be a total of 325 potential test units on the field.) At this point, we create a randomly generated list of unique number-pairs. Each number-pair equates to a specific test unit. Using the list, the test units are plotted on the diagram, and then flagged on the actual field.

After the test units are sited on the field, they are evaluated to insure that there is a representative sample of each observable surface condition relative to usage and wear. If not, additional test units are selected to provide adequate replication. Generally, a minimum of three test units is desired for each condition observed on the field.

Finally, using the Clegg tester, four g-max readings are taken within a one-square-meter area around the point that identifies the test unit. The tester is moved after each reading, so that no two readings are taken on exactly the same spot. The four readings are averaged to determine the g-max value of the test unit.

In addition to its g-max value,  each test unit is described relative to its apparent level of wear, the amount of soil compaction, the moisture content of the soil, the amount of vegetative cover, the amount of thatch (if any), and other relevant variables.

The results of the testing and observations are summarized in a comprehensive report.

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