Higher densities, better physical properties, all civil engineering graduates know this. The specification for highway works has a method specification for general soil compaction and an end of product density for special fills. Now special fills that require density measurements are generally those behind structures, with limited space and therefore limited choice of compaction plant, you can begin to understand this, especially as the structure will not move. Where do you feel serious bumps on motorways? generally at structures. So is density measurement working? The difficulty I approach, is returning to measuring densities on general embankment fill, the method compaction (admitted as conservative in the DMRB) seems to have served us well, where is the evidence contrary to this? the specification remains the same! and is emulated in other sectors (railways) and countries. Where are we returning to densities? well Trunk roads in Scotland, but it appears to be a fact gathering exercise at the moment, there is no end product performance. We the contractors measure densities and report against no % requirement. If this is the thin end of the wedge when density measurement is routine, and Scotland blazes the comeback of the measured soil density, then history has been lost, and productivity will be lost, waiting for someone to make a decision. Never an easy situation where boxes are not easily ticked. This is not the time to talk of testing errors and sand replacement densities, a UK, university taught method, almost symbolised as a method to calibrate other methods. I would just say , how many other countries use this method? No one would dispute maximum density in soil, without producing pore pressures, provides the best properties, but measuring it layer by layer in road construction and comparing it to laboratory testing is retrograde. It also raises safety issues where testing techs have to mingle with earthmoving machines. Or as I have been told when raising this issue, stop the earthworks till the testing is complete!! Try tying up a sub-contract where testing techs stop the sub-contractor desperate to work in our fluky weather!! Perhaps the industry is not providing / evolving experienced engineers, (perhaps because historical evidence is not taught?) such that they understand by heel and eye what good compaction is. We live in a tick box environment, we don't need a soil density box that we had thirty years ago.

3 Responses to “Measuring Soil Densities”

  1. Ian January 10, 2009 at 6:18 pm

    Just reviewing a new Transport Scotland tender, soil density measurement is still there for method compaction. Lets say for a moderate muck shift 200,000M3 say 400,000 Tonnes, this is convenient maths for the sampling rate is 1 per 400 Tonnes. A 1000 measurements say at half an hour per measurement at £25 an hour for a tech is £12500, minimum. Be interesting how the CDM regs view this dangerous mixing of techs and plant. The CDM supervisor (as I understand it) has to look at the safety of the design, this is part of the design, it’s the specification. Now if we have to stop one earthworks team for safe sampling, this 500 hours for 200,000M3 will cost in excess of £200,000. (6 ADT, 1 dozer and roller, 1 backactor.) OK the dozer and roller will not stop, but its serious money for what control? Just fact finding? a spec typo? wake up TS you are endangering lives.

  2. October 30, 2009 at 1:21 pm

    Ian, how’s about the more operative control of bound materials? what I mean is the portable falling weight deflectometer. If the volume of the soil to be spread’n’pressed is big enough and the moisture and mixture composition relatively even – why not to try to percuss down the metal disk placed upon the surface?

    • Ian October 31, 2009 at 5:26 pm

      well Yorick, this is happening. Portable FWDs are being carried out, but as more or less fact finding at the moment. It will come to end product testing and not by density but by stiffness. It would pay unbound subbase manufactures to conduct some research so they are prepared for the future that they can sell subbase by it’s performance.

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