Relative density vs MDD

[soilnwavesco]

Hi everyone,

I'm currently working as a site engineer on a drainage project in New Zealand. In the embedment zone of a pipe, the civil specifications call for a relative density (Dr) above 60% in some instances, and a compaction ratio (MDD,OMC) (standard compaction) above 95% in others.

I'm interested in your opinions on using relative density versus compaction ratio for these specifications. What are the advantages and disadvantages of each method? Under what circumstances would you prefer one over the other? Are there any specific criteria or guidelines that influence this decision?

Thanks for your insights.

Cheers,

Jhon

 

[EireChch]

I worked in NZ also and remember seeing Dr as a spec. I thought it was unusual. I have only ever seen if spec'd for ground improvement since.

Dr is actually very hard to measure. Since you are in NZ you will probably be trying to correlate a scala penetrometer to Dr. Is that the case? how will you confirm Dr?

Regardless, any method to correlate Dr is somewhat crude. Using MDD instead is far more accurate (in terms of geotechnical accuracy) to measure and can be easily checked by sand replacement and nuclear tests

 

[soilnwavesco]

Hi Eire,

To confirm the Relative Density (Dr) or Density Index (Id) as it's referred to here, we conducted tests to determine the minimum and maximum dry density of the material. We then used a Nuclear Density Meter (NDM) to test the compacted material (coarse) in-situ, allowing us to obtain the in-situ density and calculate the Dr.

 

[geotechguy1]

They like specing relative density in NZ for whatever reason although I frequently get the impression that most engineers writing the specs don't know what relative density or MDD are (or you get some muppet from a company that rhymes with Bad Engineering Consulting Assholes or Hopeless insisting everything be compacted to 95% of the vibrating hammer MDD).

If you can convince them of this, there is a very rough published correlation that RC = 80 + 0.2*Dr so you would have ~92% MDD as a target.

 

[soilnwavesco]

Hi Geotechguy1,

I think the concepts might differ, but I'd appreciate your opinion. I believe that unbound materials with no plasticity should be tested using Relative Density (RD) to target a minimum voids ratio. Depending on the type of project, such as road construction, Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) should be selected based on the foundation criteria. However, I'd like to understand this better, as in my home country, we typically use MDD for all applications.

Thanks,

 

[geotechguy1]

At the end, you are still either going to test using an NDM, or use a Clegg or a Scala correlation, right?

The point isn't to reduce the void ratio for the sake of reducing the void ratio, the point is to achieve certain characteristics of strength, stiffness, permeability, and/or (perhaps most importantly and forgotten) - achieve uniformity of compaction so you have relatively uniform support. Having the target be relative density or some percentage of one of the MDD tests or a scala or clegg target is ultimately just a proxy for those properties in all cases.