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 Understanding Compaction Curves
The graph shown here illustrates the affect water has on soil compaction at the time that that compaction is carried out.
In the example (left), the results of this Moisture/Density Relationship (or compaction test), would be 2.01 t/m³ (MLDD) at 11.0% (OMC).
One soil sample is split into four sub-samples of equal size. These sub-samples are compacted at different moisture contents using a uniformed test procedure (in this case AS1289 5.1.1). The only variables are the moisture contents at the time of compaction, which are usually kept close together at regular increments. (eg. 1.5% each).
Looking at the example above, the curve starts with a compaction point at 8.9% moisture giving a Dry Density* of 1.948 t/m³. The compaction second point at 10.1% has a dry density of 1.996 t/m³. Here we can see that as the moisture of the sample at the time of compaction has under the same compactive effort achieved a greater compaction result.
More water is added to the third compaction point prior to being compacted. It's moisture result was 11.4% with a dry density of 2.006 t/m³. Looking at the graph, we can see that the sub-sample although still more dense than the second point, has just gone beyond the peak. The moisture content is now just above the Optimum Moisture Content and may appear to move around under compactive effort.
Finally the fourth point is compacted at an even higher moisture content (13.0%) culminating in a lower dry density result (1.964 t/m³). AT this stage, the sample is wet and moving about considerably under compactive effort.
* = in order to compare the soil densities of test points, all moisture is removed to give a Dry Density. Comparing Wet Densities with different moisture contents to each other is like comparing apples to oranges.
Density Ratio & Moisture Content
The most common need for MDRs, is to determine whether or not a Field Density Test has passed or failed its required specification. In this example, we will say that the Field Compaction is required to pass 95% of the 100% as determined by laboratory compaction using AS1289 5.1.1 (Standard Compaction).
The graph above shows the MLDD (Maximum Laboratory Dry Density) as being 100% which it always will be on all compactions or MDRs. If you draw a line to represent where 95% (the required compaction specification) would measure up to, where the line intersects the curve we can learn what would be the maximum and minimum moisture range that we need to achieve the desired compaction. (See 'A' & 'B' above)
In this case the minimum would be 10.4% and the maximum 11.7%. Anything outside this range cannot be compacted under AS1289 5.1.1 and meet the 95% requirement.
Although in the field with modern machines (eg. vibrating pad foot rollers), it is possible to achieve the required compaction with less moisture than specified at point 'A'.
Keeping this principle in mind during the construction of any earthworks gives all those involved in the project, a distinct advantage in achieving good field compaction.
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