Field of Play development projects relies on a solid foundation if they are to last and for SPORTENG, quality control through the construction phase includes a proof roll test of the subgrade prior to installing the overlying pavement layers to check how well they have been compacted. For this blog post, we asked Patrick Beazley, one of our civil engineers, a few questions to help explain this simple but important test.
So what exactly is a proof roll and why do we do it?
A proof roll is an assessment of the subgrade layer prior to installing overlying pavement layers. The subgrade proof roll is a visual assessment to determine if it will enable the construction of the subsequent pavement layers.
There are a couple of options; one being a smooth-drum roller which is probably the most common one that we see. Another option is a fully laden truck or water truck with a single axle with a lot of load on the wheels.
Proof rolling with a water truck full of water
You walk alongside the roller or truck as it travels over the finished subgrade to see if there is any visible deformation. It is not a very technical test and is a visual assessment by the engineer who decides if the test has been passed and that the subgrade will enable the construction of the pavement layers.
Proof roll inspection SPORTENG conducted at the Waverley netball courts (Glen Waverley, Victoria).
When proof rolling, we use a specific weighted piece of equipment (8T single axle truck) to slowly drive up and down over the prepared surface, checking for any deflection of the surface. This will indicate whether there are weak areas in the pavement base, that may fail during the design life of the pavement. It ensures a solid foundation of your sports field and is an essential step of the quality control through the construction phase.
What are you looking for while watching the proof roll?
We are looking to see how much deflection is in the subgrade during the proof roll. The desired outcome is a subgrade that shows no signs of deflection on the surface. Minimal surface compaction of the crusted top layer is also acceptable.
What we don’t want to see if a subgrade that heaves under the applied load. This typically indicates there is a high level of moisture or high organic content in the subgrade.
What about indentation of the surface or tyre imprints? Are they to be expected and are they considered normal?
Sometimes with the smooth drum roller in particular, you do see a little crushing of the top layer of the subgrade, particularly if the layer is quite sandy or heavy clay. So we do expect to see breaking away at the immediate surface, but that's generally okay because that's not really a failure of the entire subgrade, just localised at the surface.
If the subgrade does fail the proof roll test, what then?
It depends on the cause and extent of failure as to what the instruction may entail. For example, on a recent project in Brisbane, the subgrade material was quite sandy and in a small area it was wet, and that area failed. In that case, providing the areas surrounding it, which is the same material didn't fail, we considered that it only failed because it was wet. We advised the contractor to let that area dry out and then re-test it. To accelerate the process, the contractor might rip it up a little bit and then re compact.
In other cases, where there have been quite big rock sizes in the crushed rock and not enough smaller rocks that aid with interlocking. If the material is suitable the contractor could blend with finer material to assist with interlocking.
For clay subgrades that fail-proof rolls often the approach is to stabilise the layer with cement/lime stabilisation or in more drastic cases over excavate the material and replace it with suitable material.
What if there are some really large soft spots? How comprehensive can the works get?
Sometimes there can be large areas of failure. These often need to be excavated and the material replaced, often to a depth of 300 millimetres or more. If possible, we try to encourage the re-use of existing material to reduce costs, but this is not always possible. The contractor can do this by excavating the material and cultivating it once exposed to dry it out and then replacing it back where it came from, compacted fully. This does take time and if there is insufficient time, new material would need to be brought in. When we excavate the failed area, we excavate beyond the failed area and tie it into the surrounding good ground.
Anything else you want to add?
A Contractor should be ready for the inspection when the engineer arrives to carry out the proof roll. For example, the subgrade should be flat and have been rolled already so that the contractor is confident it will pass. The subgrade should also have passed the required compaction tests. They also need to ensure no other equipment is in the way and we have full access to the subgrade with the proof rolling machinery and all we need to do it witness the test.
Finally, it’s important to note that this is a boring test! In an ideal world, the surface would have been prepared, the contractor already rolled the subgrade to the required compaction and the proof roll test would not result in any failed areas.
Rectifying subgrade failure can require extensive works!
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