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Clay compaction method

Clay compaction method

Clay compaction method

some groundworkers that we are working with have been tasked with filling an old basement with reworked clay from our site. Their specification requires 175mm layers with 4 passes of a their weight of vibratory roller. They however are doing 350mm layers with 8 passes.

My question is, is there a direct correlation with thickness/passes, or shoud they be doing more passes?


RE: Clay compaction method

Clay is difficult to compact unless it is slightly wet of optimum moisture.  It requires thin lifts, particularly with light compaction such as I presume you are using.  In some cases it even needs to be compacted with a special type of compactor such as a "sheepsfoot" roller.  Not likely in your confined area though.

Your compaction criteria should be to a specified density under some laboratory moisture-density relationship comparative.  A common requirement is 95% of the standard Proctor density for US applications.

RE: Clay compaction method

The bigger issue with your original post is the layer thickness.  Just doubling the number of passes for a thicker layer, may or may not work.

Allowable layer thickness is dependent on the size/weight of the roller.

Additionally, Ron makes a good point that generally a density or even strength requirement is specified.

Mike Lambert

RE: Clay compaction method

In my opinion, 14 inch lifts with hand operated equipment is probably unacceptable. Stick to the 8 inch lifts and verify compaction with testing and monitoring of moisture and density.

amount of compaction and acceptability is also related to your end use and performance requirements for the site which you have not indicated.  

RE: Clay compaction method

Thanks for you replies.

The basement in question is that of a former boiler room on a large hospital site being developed for housing. it is being compacted with a 6300kg vibrating sheepsfoor roller with a giving a linear load of 30kg/cm.

End use is set to be residential, however the developer is set to pile all their plots across the entire site due to the variable nature of the top 1-2m of clay being of reworked nature.

So end loading is not going to be huge on it, was just a simple broad question of if you double layers thickness, would doubling the passes of roller give the same result. I reality they are doing more than just double the number of passes. Would this roughly give the same level of compaction?

RE: Clay compaction method

"simple broad question of if you double layers thickness, would doubling the passes of roller give the same result."
Probably not.
For hand or very small equipment, I usually request even thinner lifts.  Actually, if the contractor can get a decent sequencing of his operations, thinner lifts with pre-moisture conditioned fill soil is usually very quick to accomplish, especially when the results are well above the borderline requirements.  
It is faster to do something better,-- once -- than having to rework areas due to failing density tests.

RE: Clay compaction method

In my experience, if the right compaction equipment is used for the type of material being compacted, if the material is near its optimum moisture, is not segregated (not a concern with clay) and lift thickness is around 6" to 8", compaction is achieved in about 4 passes.  If it takes more than six passes, one or more of the criteria above needs adjustment.  Too many passes with a vibratory roller will loosen the material rather than compact it.  This is one of those cases where more is not necessarily better.

To determine the correct number of passes, use a nuclear densometer to check the density of the material after every pass and make a simple graph of the density rise.  At a given number of passes, the density curve will drop ("break"), much like you see in a moisture/density curve of a proctor plot.  If additional passes are made after the break point, the density will increase with one pass and then decrease back down again on the next pass.  Once you find the break point, you have all the compaction you're going to get with that roller regardless of the number of passes made.  If the density meets specifications at that point, everyone goes home happy.  If not, one or more changes must be made to the criteria items above, usually the moisture content.

One other possibility is that you never find the break point in the compaction curve.  In this case, the density goes up with each additional pass, but in decreasing increments until eventually no further densification occurs.  If this happens, most likely the lift thickness is too thick and it is very unlikely that the required density has been achieved.

An exception to all of the above is when a modern roller is used that utilizes "intelligent compaction" technology.  These rollers have the ability to measure the stiffness of the material directly using accelerometers and reduce energy output as they near what would otherwise be the break point.  By reducing energy output, they can continue to increase densification with additional passes.

RE: Clay compaction method

When a contractor proposed an alternate approach to achieving compaction, I'll query about their method to see what it is, if they've used it before, and did they achieve the degree of compaction?  Finally, was there any testing performed to confirm the compaction had been achieved.

If they want to use thick lifts (+8 inch loose with heavy equipment, +4 inch with manually operated equipment), I'll tell them it is their option to try it and demonstrate the effort will work.  This means 1) watching them perform the work and then 2) testing at variable elevations in the compacted lift to confirm they've achieved the desired result.  This lets me develop a reasonable expectation of production time, as well as verify what operation they will use (this is useful in determining if they've really used the method which has been tested for).

And, I'll let them know if it doesn't provide the result required in the specs, they'll need to redo it.


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