and then trying to walk on them. Being rounded and of similar size leads to an unstable footing. If the marbles were broken (angular) and of different sizes they would fit , or lock, together and produce a more stable surface. Sands complying with USGA specifications do not necessarily produce a stable surface.
Sand particle shape has also been included and discussed in the USGA specifications primarily because of the stability, or lack of, associated with shape. Rounded particles are the least stable and very angular particles are the most stable because of the differences in friction resistance associated with those shapes. Although, in
topdressing applications angular particles can be extremely abrasive to the turfgrass and
physiological stress. turfgrass manager is
Rounded particles are less abrasive but are to obtain the sand material with the optimum
also less stable. characteristics.
cause considerable the struggles of the
Products have been developed and added to soil in an attempt to increase stability (Adams et. aI, 1989; Beard et. aI, 1988; Gibbs, 1990). Meshed products and many kinds of fibers have been added with what I think is limited effectiveness. Root systems (Reid et. aI, 1982) of the turfgrass plant (if there is one) behave in this way and act somewhat like reinforcing rod in concrete to increase strength. In fact, many types of fibers have been added to concrete to increase the strength with some success.
The objective of this research was to characterize commonly used sands and mixtures using routine soil mechanics procedures. Conventional laboratory testing was used to measure frictional resistance and geotechnical modeling was used to predict behavior of the sands in this study.
METHODS AND MATERIALS
Four sands were selected based on their use and particle-size distributions. The sands varied from 2NS, a sand mixture specified by the Michigan Department of Transportation, to TDS 2150, a sand quarried from the dune deposits along Lake Michigan near Grand Haven, Michigan. The characterization data for these sands are
included in Appendix A. To predict bearing capacity of these sand materials a computer algorithm ofTerzaghi's
equation was used. The variables used to study the changes in bearing capacity were angle of internal friction, inclination, and water content (matric potential).
Comparing the sieve results in Appendix A with the USGA recommendations indicates the 2NS and Mortar sands do not fit the recommendations. Both sands do not have the required 60 % in the Medium and Coarse sand fractions while the 2NS also has more than the maximum of 10% above 1.0 mm in size. These two sands are
too well-graded (wide distribution) to fit the USGA specifications. The other two sands selected, Michigan #40 and TDS 2150, fit within the USGA specifications. are uniformly graded and similar in particle-size distribution.
From an engineering perspective, the 2NS and the mortar sand are generally found to be the better materials when considering them for use in construction. This because as the particle-sizes that make up a sand become more widely distributed the sand becomes more stable. Greater stability results from the smaller sized particles filling the voids of the larger sized particles. The problem that arises when considering well-graded sands for putting greens or athletic fields is the relationship between gradation and porosity. As the sand becomes more well-graded it becomes less porous reducing the hydraulic conductivity and slowing drainage.
Michigan #40 and TDS 2150 sands theoretically do not have the problem of low porosity because they are poorly-graded samples that fall within the USGA specifications. But, these sands have lower strength and less stability because of the uniform distributions.
The difficulty in the design of the sand mixture arises because strength is inversely related to porosity and the greater the porosity, generally the better the environment for turf grass growth. The success of the turfgrass depends upon its ability to flourish in the sand mixture, while at the same time the stability of the sand mixture depends, in part, on a well established rootzone system ..
We have begun to quantifY the relationships between sand-size distribution, angularity, pore-size distribution, soil strength and compressibility. It is already known, at least qualitatively, the wider the distribution of sand, the greater the stability. With further research we hope to quantifY these relationships in the form of improved material specifications to give turf grass managers better tools to make sand selections.