Significance of "one-point" proctor in the field?

[blueridge]

Can anyone explain the procedure and significane of running a one-point proctor in the field and adjusting the optimum weight? I ask this question because we have a project with brown silty clay with a standard proctor of 99pcf with field density tests consistently over 100% standard proctor. The contractor is using a dual wheeled sheepsfoot roller and is making numerous passes.

 

[Focht3]

Sounds like the on-site soils are getting more compactive energy causing the field dry densities to be higher than the ASTM D 698 (Standard "Proctor&quot lab optimum dry density. Or your material has changed.

Running single points in the field is done to check for changes in the material. If they plot "on" the lab moisture-density curve then your existing Proctor is fine. If they're "significantly" off, you need a new Proctor curve.

 

[Rjeffery]

There are also a 'family' of curves and a method for correcting the MDD and OMC for oversize fraction. Virginia DOT uses that procedure as does COE. Check on the corps web site for the procedure. But as focht3 indicates, a one point proctor will only put you in the ball park (in the pitch).

 

[SUGARCANE]

I think focht3 comments have answered the query in this instance.

For some uses, one point compactions are particularly useful as a quick means of evaluating the workmanship of engineered fills. In particlular on variable materials (eg made grounds/ash etc).

When there is confidence that the material is at a suitable moisture content, insitu BULK densities (Nuke or Cores) can be related to one point Bulk densities to give % relative compaction. Although this doesn't give % Proctor, its a damn site quicker and cheaper than taking bulks back to the lab, determining optimum moisture/density and then reporting results which by this time may have failed!

It does have its limitations and care should be excercised when materials are significantly wetter than optimum as % relative compaction should always be high given such materials are easier to compact.

 

[VAD]

One should also check the moisture contet of the compaction tests that give the higher than standard Proctor densities and compare this wth the standard Procture optimum moisture. Generally, the higher compaction density results are associated with heavier field compaction effort. Hence you would be in the realm of results more comparable to the results of a modified Proctor compaction test.

The only thing that one should look out for despite that a high compaction is achieved is how is this higher compacted material likely to behave. If the moisture content is about 5% lower than the standard Proctor optimum then I would accept the compaction as suitable as today's compactors tend to give larger density results at times. I would also and consider the moisture if about 5% lower to be around the optimum moisture of the modified test. However, if the the moisture is less than 2 to 3% of the the modified compaction moisture and the density is higher than the Standard Proctor then I would rework the ground and add moisture. The same situation would be applicable if the density was below the standard Proctor density and moisture lower than optimum.

For the higher compacted soils at moisture less than their corresponding optimum, there is a tendency for them to lose strength much more rapidly than a soil that is compacted at a lower density but with moisture at or slightly greater than optimum. If I am constructing a roadbed, I would prefer to have my soil in a state where the strength does not deteriorate with in service moisture movement.

It is common practice for Contractors to achieve high densities on clay type soils with low moistures and argue when they have been told to add moisture as the moisture requirement is not satisfied. Equally as well many Engineers, are puzzled and they often pass such jobs as high compated density should equate with a superior material.

Having said the above it is equally necessary to understand the soil that you are working with i.e. all dogs of the same colour do not have the same names. Soils that appear to the eye as clay etc do not all behave the same. If one takes moulded samples of a low to intermediate plasticity clay made at different moistures and immerse them in water one can radily determine what their propensity for water absorption is like. The same can be done for materials that appear similar to study the difference in behaviour.

Some of this is learned in the field by diligent observation together with examining closely the material structure by getting one's "hands dirty" rather than asking the tech to see the density sheets with numbers.

The above is my opinion which may be in disagreement with the experience of others and is based on trying at all times to ensure that the Tech in the field needs guidance as he is often the one that has to make the call. The Engineer knows after and unfortunately this is where the Engineer should be but the practice today is such.

There should be a pre-requisite that all new Techs and Engineers spend some time in the field not running Nukes but sand cone and volutester. With the sand cone digging the holes leaves a lot of memory in the brain as pain equals gain. This forces one to get intimate with the soil and results attained. There is also the need to conduct some more practical resesrch on compaction which we have taken for granted, but on which so much of our design relies on at times.

[Cheers]

 

[BigH]

I always like the rubber balloon and Ontario's Field Relative compaction where you bang the material recovered from the hole into a mould you carry out with you. One test per hour - approximately! We had a good thread before about "Contractors Don't Like to Add Water . . .". If some haven't seen it before, maybe it would interest them in companion to this thread.

 

[Focht3]

On the experience front,

I spent a summer in Saudi Arabia doing field densities using rubber balloon equipment. Soil was sabkha - "gravelly sand" made up of broken seashell. After two months of this, I learned how to prepare a hole so that I could go an entire day without having to replace a balloon. One test per hour is about the right pace for this type of equipment.

I learned an awful lot that summer -

 

[blueridge]

Thanks for your responses. A new proctor was run on the soil that looked the same as the existing proctor. The soil that came from deeper cuts weighed about 10% more but looked almost identical.

P.S. - Your numerous responses to such a detailed question has prompted me to make a donation to this website to keep this thing running. Thanks again.