AGCSATECH UPDATE AGCSATECH UPDATE
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Varying specifications or omitting vital components without a full understanding of the ramifications, places the whole construction process at risk of failure
It is important that consideration be given to designing a drainage system that can cope with major rainfall events. Photo: Geofabrics Australia
Often we see particle size analysis conducted using a dry sieve method. This does not give a true result as clay and silt particles can stick to the larger particles and do not register in the results. The percentage of clay and silt particles contained in a sample can have the greatest bearing on the performance of a sand profile. Results often vary by as much as 10-15 per cent in the clay and silt fractions when comparing the same sample under wet vs dry sieve analysis. A wet sieve analysis should always be used to determine the particle size distribution of a sand sample that is being considered for use in sports turf construction. If an initial sample conforms to specification, the next step is to arrange stockpiling of material at the quarry for your project. Dependent on the size of the project, sand is generally stockpiled in 500 tonne lots. In large projects a detailed quality control programme at this stage is essential. Individual stockpiles should be numbered and samples collected for testing. If stockpiles are tested and meet the set specification they can then be approved for delivery to the site. This process should begin long before construction begins to ensure adequate time is allowed for thorough testing. Even for a small construction project such as a single golf green, this process should also be followed. A thorough quality control process should be mandatory. Both the USGA and conventional specifications are based on a set of criteria that ensures predictable performance of the rootzone. Moving outside of these specifications and the confidence intervals set by the USGA, places the construction at risk of poor performance or even failure. There are a range of factors that ultimately influence the physical performance of a sand including; l
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Particle size distribution: Particle size distribution and even particle shape of a sand will affect the bulk density, hydraulic conductivity, aeration porosity and volumetric water content of a sand profile. Sands with the highest percentage of particles in the medium sand range (0.25 to 0.5mm) are most desirable. Sands with a broader distribution of coarse sand and very fine sand may have a greater tendency
AUSTRALIAN TURFGRASS MANAGEMENT 19.3
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to compact, with the finer particles able to fill the voids between the larger particles reducing total porosity. This will in turn have a negative effect on the bulk density, hydraulic conductivity, aeration porosity and volumetric water content. Bulk density: Bulk density could be described as a measure of a soil’s tendency to compact – the higher the bulk density reading, generally the lower the total porosity. This can also have an influence on a soil’s hydraulic conductivity. Total porosity: Total porosity is the sum of the total pore spaces within a sand profile made up by aeration porosity (large pore spaces) and volumetric water (small pore spaces). The particle size distribution generally influences both aeration porosity and volumetric water. A sample that has a higher percentage of coarse and very coarse sand will tend to have higher aeration porosity and lower volumetric water content. This is due to the greater percentage of large pore spaces. On the other hand, a sample with a higher percentage of fine and very fine sand will tend to have higher volumetric water content and lower aeration porosity due to the higher percentage of small pore spaces. Hydraulic conductivity: Hydraulic conductivity is a measure of the volume of water (mm/hr) that can pass through a saturated soil profile. There are several methods that can be used to assess hydraulic conductivity, sometimes offering differing results dependent on the method of compaction used. The preferred method to gain uniform compaction of a sample is that where the soil and entire container is dropped vertically onto a flat surface from a measured height. Saturated hydraulic conductivity is usually assessed after 16 and 32 drops with both usually taken into account to gauge the level of compaction and change in bulk density.
CONCLUSION There are multiple components that make up a perched water table specification, all of which can have a bearing and need to be taken into account to provide a successful outcome. Varying specifications or omitting vital components without a full understanding of the ramifications places the whole construction process at risk of failure. While the initial cost of construction may seem expensive, if constructed and maintained properly a perched water table can last for many decades and may only need re-surfacing rather than a complete reconstruction. This may help to make construction costs more palatable, when compared to the cost and hassle of a complete reconstruction due to the selection of inferior materials or poor quality control. In part two of this article we will examine how a perched water table works, moisture release curves, organic amendments and compare several sand test results in order to better understand sands used for sports turf construction.