Aussie Thumper Impact Severity Instrument

 

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John R Grealy & Associates
27 Bowden Grove
Oaklands Park
South Australia 5046

Phone: 08 8296-2279
Mobile: 0407 430 661
info@aussiethumper.com.au

The Science

The Science of the Aussie Thumper™ - test and record surface hardness
Technical overview / discussion: H. K. Schulze B.Tech (Elec.) Notary Public; Patent Attorney.

There are various current techniques for measuring soil effects. Unfortunately tests which are most appropriate for one purpose such as road subgrades are not obviously appropriate for other purposes such as heaviness or speed of a racing track.

The recently established test which is known as the AT Standard uses the well respected dropped hammer approach except that it uses the extent of momentary maximum relative movement of an impacting head under a rapidly applied Standard impact force as an indication of track status.

The AT Standard relies on a maximum extent of relative displacement from a Standard impact replicating a hoof striking and being withdrawn from the turf.

The work required which is in effect the effort that a horse or object needs to push in and then pull out of the turf once impacted can be reflected directly in the level of momentary maximum movement. A pure water mass will provide most displacement and a hard surface eg concrete surface may provide close to zero displacement.

The amount of effort required which can be equated to the energy or work required to insert a hoof to obtain a footing and then extract the hoof will directly correlate with a maximum level of insertion potentially occurring and as such is a measure of the quality most of interest to the racing or other similar applications.

A well known alternate test is the Clegg test which is a highly respected for a number of applications. It is based on the California Bearing Ratio (CBR) which was developed for road foundations in the USA. The test can be performed on laboratory-prepared samples or in the field. The CBR test is described in ASTM Standards D1883-05 and D4429, and AASHTO T193.

The CBR rating was developed for measuring the load-bearing capacity of soils used for building roads. The harder the surface, the higher the CBR rating. A CBR of 3 equates to tilled farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of 10. High quality crushed rock has a CBR of 100.

The CBR is intended to provide information regarding a longer term static load bearing capacity of soil from a single relatively simple test. It was not designed to test the dynamic effect and work requirements imposed by a horses hoof when striking and sinking in turf.

This requires information regarding the softness of the soil but also the reactiveness of the soil, the stickiness of the soil, the viscosity of the soil and the porosity of the soil where under sudden impact liquid may be squeezed through very small interstices. It can be thought of as a sponge which if impacted on quickly may still be quite solid while if impacted slowly it will easily give.

This is the basis for an AT Standard test which has been designed specifically where dynamic effects rather than static action have to be catered for . This AT Standard test is an Active Transition test which uses the same initiation as the CBR test but measures maximum relative movement achieved from a rapid impact force.

The AT measurement uses a falling weight as an impact force which is consistent with the other well established systems but provides a permanently stored dynamic maximum displacement measure from a single impact from close to a simulated hoof shape of 125mm dia. The dynamic force of the weight hitting a lowermost target pushes the base directly into the turf being tested and measures the maximum movement momentarily achieved. This is designed to simulate most closely the conditions of a hoof and its effects on the turf and the results have been correlated with actual conditions that are recognized by experts in the field. A rebound deadener is incorporated to minimize rebound clutter.

The actual reading is a result of the actual force being applied to a head-weight and the attached parts including the swinging arm scriber. Any result in these circumstances is relative and each unit is indexed to indicate relative results from actual ground conditions well recognized and established by experts. The range of measurements from numerous tests which have been conducted over many racetracks and parks compare best for the range of actual turf conditions encountered in practice.

The AT standard allows anyone to apply the test by simply resting the testing equipment on the ground to be tested, it uses a separate ground support by which the amount of dynamic movement can be referenced, then standardizes the alignment and release needed for repeatable results.

The AT test should NOT be compared with either the Penetrometer System or the Clegg System both of which have been designed for different purposes and as has been explained are either static tests or if dynamic have a penetration approach rather than an impact approach. Another device that is used and again is highly respected is the Penetrometer.

Different types of Penetrometers have been developed to measure soil penetrability (Bengough et al. 2001; Lowery and Morrison 2002) that operate on static or dynamic principles. The static Penetrometer is pushed into the soil at a constant rate, while the dynamic Penetrometer is driven into the soil by repeated hammer-blows (used mainly in civil engineering). Herrick and Jones (2002) described a dynamic Penetrometer for use in soil science, enabling cheap, repeatable soil strength assessments in the field. It consists of a metal rod with a conical tip at one end, an anvil or strike plate around the rod and a sliding hammer with a fixed mass at the other end (Figure 1). The cone is pushed into the soil by successive blows of the sliding hammer against the anvil. The strike of the hammer applies an amount of kinetic energy determined by the work required to raise the mass of the (frictionless) hammer through a distance influenced solely by gravity (Herrick and Jones 2002).

This again is very respected but has been taken from use in many widely varying applications and may not be most appropriate for turf testing for the following reasons:

  1. The test itself requires multiple impacts for a single output reading. Eg. once a sponge has been squeezed it will take some time to sop up the liquid again which means that these “time constants” in soil which is to say how long before it takes to return to full saturation etc are not as such catered for.
  2. The impact is directed through a pointed cone which penetrates the soil. This is not consistent with the shape of a horses hoof and is acting in penetrating and pushing aside and not dynamically hitting the soil from above with a flatish surface. Resistance will be primarily from sliding friction on the side of the cone. It is suggested that frictional resistance of the soil with a selected surface material of the side of the cone may be a significant factor in any reading. However this is hardly a characteristic that will be of direct interest in relation to the track reaction to a hoof.
  3. The reading which is manually obtained is a deceleration measurement but there will be potentially a number of different deceleration rates throughout a penetration process. Eg. a car when slowing down will slow down quicker if the brake is applied harder. If the pointed cone strikes different types of soil as it passes through different layers which of these readings will be taken as the most relevant? What this means is that the system requires either an average of all of the deceleration rates that will occur and for this to be meaningful, this has to be considered over a given time period presumably for each detected impact or perhaps a small middle period of this?.

Alternatively the figure is a maximum deceleration rate but then this needs to be referenced to the actual depth of the probe at the time so that some relevance can be given to the reading in the context of the soil depth being assessed.

The quaintly named “Aussie Thumper” is an instrument that is more complex than a Penetrometer but which uses the Active Transition Standard technique to provide a repeatable and consistent Standard AT reference which is considered to be at least as useful as any of the other techniques and from field testing comparable studies matches actual ground conditions with a greater degree of reliability in fact.