Doing too good a job – yes, a blog about compaction
I know your mum always told you to do your best, but she was wrong. Sometimes you have to do just enough for it to be ‘a better job’. Let me explain.
When it comes to earthmoving, there are some fundamental rules: you can’t make money with a dozer in reverse, low fuel burns are a reason to worry and, most importantly, the key to making money is to minimise the energy you use. What’s the easiest way to minimise energy use when constructing earthworks? Do less work. Seems simple, but if it really was that simple, why do so many operators, foremen and engineers get excited when they see high density results on general fill? To me that says they are doing more work than required!
MDDs aren’t a race to the top
Many specifications call for results such as ‘the dry density of the embankment must be 95% MDD’. That’s 95% of the Maximum Dry Density (MDD) that can be achieved in a lab context when a controlled amount of energy is imparted on a sample. So, if a test shows you that your field result achieved 98% MDD, it means you’ve delivered 3% greater density than required. This seems like a small number but, remember, to achieve that result you had to move 3% more material than required. If you’re getting paid by the compacted cubic meter, that’s a problem, because you effectively wasted the time and resources to achieve that extra 3%! When many civil jobs are priced with less than 10% margin, that ‘small’ 3% represents a 30% decrease in potential profits! If you’re a contractor, overshooting your MDD isn’t smart for business. However, if you’re the client, this is just as relevant. Exceeding the specified MDD not only slows your contractor down, it might mean you’re paying more than you need to if your rate for cut-to-fill is greater than the rate for cut-to-spoil!
Easy fixes
Our recommendation for bulk earthworks jobs is to pay close attention to your results to ensure you’re achieving an embankment that is dense enough to pass, but not so dense that you’re burning resources. The best way to keep an eye on this is to keep a plot on the wall showing density results over time. You’ll soon see if you’re consistently making your mum proud by doing too good a job and losing money along the way.
If you’re the client, ask for the monthly report to include a plot of densities verses dates and consider challenging your contractor to minimise excess work as a way of increasing output.
