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Measures of Compaction
Compaction is a topic taught on the Concrete Pipe Installer course delivered by NZWETA with the support of concrete pipe manufacturers Hynds, Humes, and the Concrete Pipe Association of Australasia.
Dry Density Ratio
The most commonly understood measure of compaction in the construction industry is the ‘Dry Density Ratio’. This is also known as ‘Relative Density’ ‘Compaction Ratio’ or simply ‘compaction’.
Most people who work in the road sector will be familiar with one of these terms. Others may not be as used to using them.
Dry Density Ratio or Compaction is measured by a number, expressed as a percentage, which usually lies in the range 85% to 105%.
What does this number mean? What exactly does this percentage measure?
It is simply the ratio of the Dry Density of the material measured in the field to the Dry Density of the same material subjected to a standard test in the soils laboratory.
Dry Density Ratio =
| Dry Density (field)
Dry Density (lab)
| X 100%
|
So a dry density ratio less than 100% means that the lab test gave a higher density and greater than 100% means a higher density was achieved in the field.
Density Index
Another measure of compaction with which you are probably not familiar is the Density Index. This is only used for non-cohesive materials without a fine fraction. Pure sands are the most common example.
The density index is the density of the compacted granular fill material divided by the maximum possible density the fill material could have if it was totally compacted. Density index is given as a percentage. So, a DI of 50% means that the density is 50% of the maximum available for that material.
Another way to think about the density index is to imagine a block cut out of the compacted material. If the density index is 70%, then that means that 30% of the block is voids (and 70% is the material).
You will rarely (if ever) encounter Density Index, but if you do, the following table will give you an approximate comparison so you can ‘get a feel’ for the compactive effort required.
Measure |
Approximate |
Equivalent |
Value |
Dry Density Ratio |
85% |
90% |
95% |
Density Index |
50% |
60% |
70% |
The Right Way to Compact Fill Materials
As you saw in the experiment, compaction can make a big difference to the load a pipe can bear.
There are various ways of achieving compaction. You can use:
- a crowbar
- mechanical compaction
- flooding
Crowbar
If you are using a crowbar, make sure you work over the entire area that requires compaction without missing sections.
Mechanical compaction
Mechanical compaction refers to compaction using equipment such as a wacker.
When you are compacting the haunch and side zones the plate on the wacker needs to be narrow enough to fit between the pipe and the side of the trench. If the space is too narrow you may need to use a crowbar instead.
If you are using mechanical compaction make sure you do enough passes of the plate to achieve the compaction required by the specifications for the job.
It is also extremely important that the correct equipment is selected to ensure that excessive loads are not placed on the pipeline during construction.
Flooding
Flooding can be convenient in some cases but it is not very efficient.
Flooding alone usually only achieves a density index of about 30%, which is not enough for most installations. For clean, non-cohesive soils the density index can be improved by using a concrete immersion vibrator in combination with flooding.
If you use this method of compaction you must be fairly certain that the flooding will not cause the pipe to float.
General points on compaction
To be effective compaction must be done the right way. Whichever method you use (crowbar, mechanical or flooding) you must observe the points below:
- Haunch zone compaction is critical. There must be no voids in this zone.
- All compaction should comply with the specification for the job.
- Bring up the fill on both sides of the pipe together in layers of about 150 mm. Compact as required as you go.
- Top down compaction after you have finished backfilling is no substitute for doing it the right way. Top down compaction only puts extra load on the pipe.
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