How much is a cubic metre?
On the face of it, the question ‘how much is a cubic metre’ is pretty simple: it’s the volume occupied by a cube where all sides are 1m in length. However, for those of us working in bulk earthworks, it’s slightly more complicated.
Three stages of earthmoving
Earthmoving generally involves three processes:
Digging
Hauling
Placing
At each of these stages, the material has a different density. That’s why we like to be careful with how we name the volume it occupies.
What’s the difference between your Bs, Ls and Cs?
A lot actually. Prior to hauling, material is in its natural state. It’s sitting in a bank waiting to be moved and is defined in Bank Cubic Metres (BCM). Remember, that could be in a hill that has never been touched, or it could be in a stockpile. The idea is that it’s in its original form.
As soon as we dig or even disturb any material, we introduce voids – ie it swells – and those voids fill with air. The voids could be the size of a pin head in sand or a basketball in hard rock. At this stage, we refer to the material in Loose Cubic Metres (LCM).
Finally, we place and compact the material, minimising air voids. Here we have Compacted Cubic Metres (CCM).
Precision drives productivity
Knowing the difference is far from semantics. It can influence decisions such as plant selection at the extreme and, at the simplest end, does influence productivity rates.
On a small job, it might not make a notable difference. However, once you start dealing with 100,000 cubic metres, or even millions, using the right measure rapidly adds up. We typically find that loose gravely sands and clays are denser in their CCM state than their BCM state. The reverse is true for hard rock, because once rock is shattered, it cannot be put back together. These differences can be large – easily above 5%. If your job is only making a 10% margin, the impact of not considering the expected changes during forecasting can be catastrophic.
To identify the best possible measure, there are plenty of guides available. We like the guides provided in the CAT handbook. However, to get a more specific idea for soft material, we prefer to send soil testers out to a site to take in-situ readings. They then knock out a Maximum Dry Density (MDD) test in the lab. If you divide the in-situ measure by the MDD, you’ve got the difference between BCM and CCM. You can also get a rough estimate of LCM by simply filling a known tub size with material and weighing it.
Granular materials nearly always densify as we take them from BCM to CCM – ie we need more than 1 BCM to make 1 CCM. When we have no better data to go by, we typically let 1 BCM equal 95% of 1 CCM if we are dealing with sands and gravels. For stiff clays, we’d say the difference is closer to 98%. Hard rock can be more difficult because you generally don’t do compacting tests on rockfill. As a starting point, we allow a 10% swell for rock that is blasted and 5% for rock that is hard to rip. In other words, the density of blasted rock in its CCM state is 10% less than when it is in its BCM state.
When we’re brought in as independent reviewers, we often see that either no measure has been applied, or the wrong one has been used for the material type and stage in question. Particularly at those higher volumes, it’s worth being this specific so the rest of your production calculations line up.
