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
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
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.
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
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
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