The basic functions of the soils are to hold enough water for the grass/plants to survive in the heat, and to dispose of any excess water during the wet. It is very important to choose the correct soils at the time of construction, or reconstruction. Are your turf fields too dry/wet, patchy or droughty?

Ground Science staff have been trained by Dr Bent Jackobson and utilise

technology.

How we eventually make the decision about which soil is more suitable over another is not a simple process, particularly when one element is not within a desired range. So we need to investigate the scenario does one test, one specific characteristic make a soil unsuitable. What we first need to understand is:

• What influence does each physical characteristic play in the final wicket product.
• Does the variability in the wicket preparation play more of a role in the wickets performance than the physical characteristic itself as a measured item.
• How accurately does the laboratory test reflect the insitu performance of the soil.
• What are the repeatability levels of the current test methods

Clay content: is probably one of the most critical components of a wicket soil. Clay is a particle less than 2um in size. Due to the small nature of a clay particle a clay soil holds large amounts of water (the surface area of a set volume of clay particles far exceed that of an equivalent volume of sand particles). There are many types of clay minerals with the most common including Kaolinite. Illite and Smectite Montmorillonite. Ideally wicket soils should contain more Montmorillinite and illite. The clay content provides strength to a wicket.

The most common test used to measure clay content is by mixing the soil into water (generally approximately 50grams of soil diluted into 1 litre of water). A hydrometer is placed into the soil water mix. Due to the small nature of the clay particles they remain suspended for longer periods than the heavier silt and sand particles. The hydrometer bulb is held in suspension by the particles suspended in the liquid. As the heavier particles start to fall out of suspension and settle on the bottom of the cylinder the hydrometer start to sink lower and lower into the liquid. Details calculations are performed to determine to particle size and quantity to provide a particle size distribution plot.

During the preparation of the particle size analysis by hydrometer method, the clay soil must be thoroughly dispersed. If the soil is not dispersed appropriately to produce individual particles, the particles adhering to one another act as a larger particle falling out of suspension sooner and the test will indicate higher silts contents. It must also be noted that black soils are generally black due to the high organic content. Organics are typically lighter particles and once dispersed in the hydrometer test may influence the result. It is therefore necessary to remove the organics in a pretreatment process using hydrogen peroxide which affectedly burns off the organics.

In any laboratory test, repeatability and uniformity are the key elements in achieving a true result. The initial process of preparing a wicket soil for analysis in current test methods allows the practioner a large variation in the consistency of the mix which in my view has a substantial influence on the result. The problems begin when a wicket soil may arrive in a laboratory in three possible conditions:

• Moist/wet clods
• Dry lumpy clods
• Dry manufactured crumbles (<5mm)

Tests such as cracking pattern, shrinkage and crushing strengths rely on the wicket soil to be in a paste condition. The consistency of the paste needs to be made into two different stiffness’s to perform these tests. The cracking pattern and shrinkage cores require a paste that is able to be smeared into an evaporating dish or smeared into a 10mm diameter cylinder tube and extracted to form a core, ie a paste of a similar consistency to peanut butter. The crushing strength requires a stiffer paste. The soil needs to be at a moisture condition that allows spherical balls to be rolled on a glass plate without being too wet where the material sticks to the surface or too dry where the ball crumbles before a ball is formed. The rolled balls need to be between 5mm and 20mm diameter and not have any flat spots, surface cracks or imperfections. The difficult element in this preparation is, understanding that different clay types and clay contents will govern the amount of water required to create these pastes. It may vary considerably with soil types.

It is obvious that the moisture content at which these two pastes are formed, will influence the outcome of the test. When we look at a model of soil we have three components, air, water and solids. When we create a paste, the amount of water and the size of the pore spaces are the two elements that are going to change once the specimen dries. In a shrinkage test this is easy to appreciate as we are measuring the reduction in size due to the moisture loss. If a paste is made wetter, there is more water to loose and potentially a greater measure of shrinkage.
In the crushing strength test the moisture allows us to achieve a compact ball of soil and the optimum moisture will allow us to achieve the maximum compaction level.

Cracking pattern

The consistency of the paste used in the cracking pattern and shrinkage test is somewhat wetter than that of the crushing strength. Again what we would ideally like to be able to achieve is a simulation of what happens in the field. What we need to remember is that a wicket table is a body of soil generally 300mm thick. The current method of measuring a soils cracking pattern is to wet up the soil into a toothpaste like consistency and then smearing it into a 100mm diameter porcelain drying dish. The material is then left to air dry and crack at room temperature.

Shrinkage

The shrinkage test paste is probably the most critical in achieving a repeatable result between operators within a company and within laboratories the world over. As previously discussed it is the moisture loss that provides the shrinkage to the specimen. We must understand that the shrinkage cores test is not the same as the Linear Shrinkage test and the two produce very different results. It is apparent that the soil preparation method to produce a paste for the shrinkage test must be refined and tightened to limit the possible variations that currently occur. Measuring soils shrinkage from an unknown variable moisture condition to a dry state is not favorable, which is what occurs in the current method.

Summary

The process in selecting a cricket wicket soil or determining whether a soil meets a project specification must be done with a holistic understanding of the importance of each assessed element and whether any unfavourable element within the suite of the tests is going to substantially affect the overall playability of the final product. If we were to review cricket materials world wide from major stadiums, we would find that not many if any would meet all the criteria commonly found in Australian specifications. Refer to table 1 below.

Table 1 – Results of some international cricket soils tested in 2007