|
Agronomic and Engineering Properties of USGA Putting Greens
By James R. Crum, Thomas F. Wolff, And John N. Rogers III
Through practical field experiences and research, the USGA putting green recommendations have been revised a number of times, but at no time were they drastically changed. They remain the standard by which most golf course architects design and golf course construction companies build their highest-quality golf course putting greens.
Rootzone specifications for USGA putting greens require greater than 92% sand, not more than than 5% silt (0.05-0.002 mm), and not more than 3% clay (<0.002 mm) in the soil. Additionally, there must be a minimum of 60% coarse and medium sand, not more than 10% fine gravel and very coarse sand, not more than 20% fine sand, and not more than 5% very fine sand. The resultant rootzone is dominated by macropores (large pores that are air-filled at field capacity) that drain rapidly and maintain large amounts of aeration porosity important for turfgrass growth.
Within the engineering literature, high-sand-content soils are considered cohesionless materials, or materials that do not stick together. High-sand-content soils must rely on particle-to-particle touching and the friction produced to create a stable surface required for the game of golf. As we traveled the United States and visited many golf courses, it became apparent that all USGA specification putting greens do not behave alike. Some have strong, stable surfaces and others have surfaces that are difficult to manage because when a load was applied (i.e., foot or equipment traffic), deformation occurred. With these observations in mind, we initiated research to apply the principles of soils engineering to the issue of ensuring stability of sands used in golf course putting greens.
The specific objectives of the research were to: (1) develop six experimental sands that varied in particle size and gradation to represent a range of USGA specifications, (2) determine bearing capacity of the six experimental sands and relate their strength to size and gradation characteristics of the sands, and (3) determine bearing capacity of established putting greens and relate their strength to sand characteristics.
Michigan State University researchers adapted a California Bearing Ratio (CBR) testing device to measure in the field a putting green's soil strength against failure under compression. The CBR device was mounted to a tractor, and a plunger was pushed into the ground with a jack. A load cell with digital readout measured the force on the plunger or the pressure required to deform the putting green surface for each 0.01-inch displacement.
|
|
Laboratory testing focused on the effect of particle size, expressed as median grain size (D50) and gradation, expressed as coefficient of uniformity (CU) on bearing capacity.
Sieve analysis of coarse-grained material (sand, in this case) is generally expressed in two ways: percent retained and percent passing (or percent finer). The United States Department of Agriculture (USDA) has defined five classes of sands based on their particle size: very coarse sand (VCoS, 1.0-2.0 mm diameter), coarse sand (CoS, 0.5-1.0 mm diameter), medium sand (MS, 0.25-0.5 mm diameter), fine sand (FS, 0.10-0.25 mm diameter), and very fine sand (VFS, 0.05-0.10 mm diameter). The USGA has modified these classes slightly in the FS class range (0.15-0.25 mm diameter) and the VFS class range (0.05-0.15 mm diameter).
The percent retained in each class is calculated as the proportion of the total weight of the sample that is of the given size class. The percent passing is calculated as the proportion of the total sample weight that is finer, or passes a particular size (e.g., the percent passing a 2 mm sieve). Often, both ways of expression are presented as tables and as graphical representation of the data. The graphical representation of the percent retained data appear as histograms and of the percent passing as semi-log line graphs. The usefulness of the semi-log graphical presentations comes in the quantification and calculation of coefficients. For example, to determine the median grain size we would determine the D50, or the diameter (D) at which 50% is larger and 50% is smaller. The coefficient of uniformity, Cu, is calculated as D60/D10, or a way to express the shape of the finest 60% of the percent passing curve. The larger the Cu, the greater the range in particle size and the more well-graded the sand.
Six gradations of sand were prepared for each of three different D50 sizes termed fine (FG), medium (MG), and coarse (CG). For each of these sands, two gradations were prepared: a very uniform gradation with a low Cu (LCU) and a more well-graded one with a higher Cu (HCU). In order to ensure consistency, these six sands were produced in the laboratory. These sands were made from a commonly available construction sand (MDOT 2NS) that had a wide range of particle sizes. To prepare the laboratory gradations, the 2NS sand was divided into a number of very narrow gradations by sieving. These were then recombined to achieve the desired gradations for testing. All six of these test sands were designed to meet the USGA guidelines for golf putting greens.
A soil's strength against failure under surface compressive load is termed its bearing capacity. This was directly tested in the lab by developing a modified California Bearing Ratio (CBR) testing device. This device has a circular plunger with a cross-sectional area of three square inches, which is forced into a sample volume of sand placed in a mold using a load frame. A load cell above the plunger displays the force pushing down on the soil sample. The depth the plunger has penetrated into the soil is measured with a dial gauge. Dividing the force by the piston area gives the applied pressure. The bearing capacity, or ultimate pressure that the soil can withstand before it fails corresponds to the peak of the curve and the soil's bearing capacity.
A field CBR device was adapted from the original California Bearing Ratio testing device. The CBR device can be pinned to the three-point hitch or clamped to the loading bucket of most tractors. The device has a plunger that is pushed into the ground with a jack. A load cell with digital readout measures the force on the plunger. This force is recorded for a set of corresponding vertical displacements of the plunger into the ground, measured by a dial gauge clipped to the plunger arm and measuring movement relative to a reference beam.
The force measured by the load cell is divided by the area of the load piston to obtain the pressure on the surface of the putting green. This calculation is performed for every increment of vertical displacement. Force is recorded at every 0.01 inch of displacement for consistency. The pressure at each 0.01-inch displacement is plotted versus the vertical displacement (Figure 1). The initial part of the curve, labeled A, represents the pressure causing initial deformation of the surface layer. It is obvious from the graph that the surface offers little resistance to deformation. The portion of the graph labeled B shows that increasing stresses are developed as the underlying sand-based rootzone deforms under the surface layer. The underlying sand requires significantly greater stresses to produce additional deformation. At point C the sand fails and further displacement occurs with less stress (area D).
For the six sands used in this experiment, each falls within USGA specification guidelines but represents the extremes allowable for median size and distribution. Our expectation was that the sands with poor gradation (low Cu) would have lower ultimate bearing capacities than the sands with more well-graded distributions of sand sizes. Sands with poor gradation do not have the internal frictional forces required to make them strong. Sands that are well graded have frictional forces produced by smaller particles fitting within the voids of larger particles.
The bearing capacity tests show the benefits of sands with a high Cu. The laboratory bearing results show the well-graded sands (FGHCU, IGHCU, and CGHCU) were capable of withstanding an ultimate pressure greater than those sustained by the uniform sands. For example, the fine-grained high-Cu sand has an ultimate bearing capacity of approximately 44 pounds per square inch (psi), as compared to an ultimate bearing capacity of approximately 23 psi for the coarse-graded low-Cu sand. The higher-Cu sands have nearly double the bearing capacity as the low-Cu sands. It should be reiterated that although these sands display such a wide variety between their ultimate bearing capacities, they all fall within USGA gradation specifications and would be considered acceptable sands for golf putting green construction.
The testing conditions in the lab were somewhat different from those in the field. In the lab, there was no layer of turf, organic matter, and roots incorporated in the surface of the soil. Also, in the lab, the sand is contained in a rigid mold that will not allow lateral deformation or strain of the sand. This leads to a well-defined peak stress at failure and a non-ambiguous bearing capacity. In the field, the surface layer applies a tensile confinement that allows significant deformation to occur at increasingly greater pressures on the sand without producing a well-defined peak stress at failure. Also, in the field, the sand-based rootzone can strain or deform somewhat laterally, similarly reducing the tendency to exhibit a peak.
The sand-based rootzone does not reach a distinct failure point because of the tensile confinement applied by the surface layer filled with organic matter and roots. Also, the rootzone material has the freedom to deform laterally and redistribute the pressure to the adjacent soil. Although the field and lab tests are not exactly equivalent, it is noted that the lab results tend to act as upper and lower limits, bracketing the field results.
It is also shown that the slope of the pressure-displacement curves, or rate at which the pressure increases with increasing displacement, is highest for the confined lab bearing test and lowest for the field bearing test. The high rate of increase in pressure due to increasing displacement for the confined lab test occurs because the sand is confined from both lateral deformation (due to the rigid mold) and vertical deformation (due to the applied surcharge). The rootzone material is allowed to deform laterally, thus leading to its lower rate of increase in pressure due to increasing displacement.
Figures 2 and 3 display bearing capacity curves for newly (two- to four-months-old) established golf putting greens constructed of 100% sand (IGHCU) and of a mixture of IGHCUsand and 10% sandy loam textured topsoil. Within each figure the bearing capacity curves of the six experimental sands are also displayed for comparison.
In general, the 100% sand bearing capacity curves in the field and in the laboratory compare well, but in the field there is no lateral confinement and no distinct maximum bearing capacity as under laboratory conditions. With the addition of soil, a rootzone mixture with a higher Cu is produced. In fact, our results show a much higher bearing capacity than was seen from the six experimental sands. Qualitatively, it is easy to see and feel that the sand:soil rootzone is firmer and stronger than the 100% sand rootzone. Our data suggest the latter has a bearing capacity on the order of twice as high as the 100% sand rootzone.
To date we have not done enough field testing on a wide enough range of rootzones to develop bearing capacity criteria for what might be "too soft" or "too hard" for golf putting greens. We plan to continue this work and collect field bearing capacities from a wider variety of soils and putting greens to begin characterizing the strength properties of sandy rootzones.
Particle gradation greatly influences the engineering properties of high-sand-content soils. In our studies, increasing the Cu of intermediate grade sands from 1.8 to 3.0 approximately doubled (from 22 to 42 psi) the laboratory bearing capacity. Increasing the Cu in the fine and coarse grain sizes of sands also dramatically increased the bearing capacity of the sands.
Appreciation is expressed to the United States Golf Association and the Michigan Turfgrass Foundation for their generous support of this research.
Note: This article first appeared in the USGA's Turfgrass and Environmental Research Online (http://usgatero.msu.edu). The specific URLfor the article is http://usgatero.msu.edu/v02/n15.pdf.
James R. Crum, Ph.D., Professor of Turfgrass Soil Management; Thomas F. Wolff, Ph.D., Associate Professor and Associate Dean, College of Engineering; John N. Rogers III, Ph.D., Professor of Turfgrass Management, Michigan State University, East Lansing, Mich.
|
|
|
|
