I have been working in the geotechnical industry for eight years. In that time, I have known numerous people who have sustained serious crush injures to their hands and fingers from using a Conventional Scala Penetrometer. After a recent injury at my current company, I thought there must be a safer way to carry out Scala Penetrometer testing and I began to research safer alternatives. I found out that overseas people have been using a device called a Safety DCP, or Safety Scala Penetrometer. This article introduces the Safety Scala to New Zealand.
Scala Penetrometer Use in New Zealand
The Scala Penetrometer which is also known as a Dynamic Cone Penetrometer (DCP) is a common geotechnical site investigation tool used in New Zealand. The device works by dropping a hammer on an anvil and driving a rod into the ground, measuring the penetrative resistance to derive the soil density. The Scala Penetrometer is used in the geotechnical industry for determining bearing capacity and ‘good ground’ as defined in New Zealand standard NZS 3604:2011. The Scala Penetrometer is also used in the Civil Engineering for subgrade verification, California Bearing Ratio (CBR) estimation and the monitoring of engineered fills. Scala Penetrometer is a popular tool because it is a quick, cheap and a relatively easy tool for gathering geotechnical information.
The New Zealand standard for the Scala Penetrometer is defined in ‘NZS 4402 Test 6.5.2:1988, Determination of the Penetration Resistance of a soil’ and is the same as the equivalent Australian Standard. The standard defines the required weight of the hammer (9.0 kg ± 0.1 kg), drop height (510 ± 5 mm) and shaft and cone dimensions (figure 1). The basic design has been unchanged for at least 30 years.
Figure 1: New Zealand Scala Penetrometer Standard – NZS 4402 Test 6.5.2:1998
Scala Penetrometer Injuries
The Scala Penetrometer has been associated with many injuries to operators, in particular crush injuries to fingers and hands being trapped between the falling hammer and anvil. An official Information Act (OIA) Request to WorkSafe indicated that since September 2018, WorkSafe has been notified of 8 serious injuries caused by a conventional Scala Penetrometer (figure 2). The true number of injuries is likely to be a lot higher given not every accident would be reported.
Figure 2: WorkSafe New Zealand Official Information Act Request – Notified Scala Penetrometer Injures
The consequences of these injuries are often severe, with physical disfiguration as well as mental distress. An accident involving a Scala Penetrometer often leads to extended time off work, and even longer time out of the field where the total cost to a company likely far exceeds the cost for new equipment.
The causes of these accidents are often contributed to inadequate training, inexperienced operators, fatigue, complacency and rapid testing, however the root cause is the exposed pinch point between the hammer and the anvil (figure 3). Particularly as the operators’ hands need to be in close proximity to this hazard zone.
Figure 3: Exposed pinch point on conventional Scala Penetrometers.
The Safety Scala differs from the conventional Scala Penetrometer in that the anvil is fully enclosed by the hammer, meaning the risk of finger injuries is eliminated. The Safety Scala is precision made to have the same drop height (510 mm) and weight (9.0 kg) as the conventional standard as such complies with NZS 4402 Test 6.5.2:1998. The Safety Scala has long handles on the side to make it easy to use and reduces the amount the operator has to bend over during operation, as such reducing the risk of back injuries.
Figure 4: Safety Scala, showing components
Testing and Verification:
Testing was undertaken to compare the results of the Safety Scala to a conventional Scala Penetrometer in August 2022. The aim was to find out if there was any measurable difference in the two types of equipment. The testing was completed within Christchurch’s redzone near New Brighton and in close proximity to a borehole available on the NZGD that indicated the presence of medium dense SAND. A hand auger borehole was also completed at the testing which confirmed the underlying soil conditions to consist of sandy topsoil, reworked SAND and natural clean SAND of the Christchurch formation. Ten tests to a target depth of 1.9 m were completed with each device within a 2 m square.
The tests indicated the presence of variable soil within the top 0.8 m bgl, likely reworked soil before becoming more consistent within the natural clean SAND. The results from each test were then averaged to compare the results (figure 5).
Figure 5: Summary of test results completed in Red Zone. Average of all tests completed.
The results indicated similar results for each test method, although it became obvious when looking at the raw data that there were large variations in the density of the ground within relatively short horizontal distances, even within the natural soil.
From the Red Zone trial, it was concluded that the variations in the soil density over small horizontal distances was more significant than any differences in the type of Scala Penetrometer.
In addition to this investigation, field trials in Australia with a similar device completed by S.J. Clarke also concluded that the variation in ground conditions, even over horizontal distances of only 0.3 m, was more significant than any differences in the efficiencies of the various test types.
The Safety Scala has been developed to reduce finger crush injuries and is now available in New Zealand. The device also significantly reduces back strain and serious injures due to its long handles and ergonomic design. The Safety Scala has been trialled in the South Island and has proven to be robust and popular with users.
The idea of the Safety Scala is not new, they already are very common in Australia, where they have the same standard as us in New Zealand. Testing in both Australia and New Zealand has shown that any differences between the Safety Scala and Conventional Scala was insignificant in comparison to variations in ground conditions over a short distance.
– New Zealand Standards, NZS 4402 Test 6.5.2:1988, Determination of the Penetration Resistance of a soil’
– S.J. Clarke, 2019, Development of a Safety Dynamic Cone Penetrometer.