90% Soil Compaction with Tracks of a Tractor

[ocgeo]

A site with sandy clay to clayey sand, trace gravel, has obtained 90 percent relative compaction up to almost 20 feet in depth, with the tracks of a 25 ton tractor, using enough passes, a loaded front bucket, and 8 inch thick loose lifts. It can be done, more to come if interested (track width, length, # of passes).

Has anyone else had luck with compacting with this type of compaction method?

After all, we are the observation and testing firm and do not direct; means and methods is up to the contractor.

 

[oldestguy]

You probably could get by with fewer passes. That's "relative" density, right? If so that's darn good. I'm usually satisfied at 70% relative density.

 

[ocgeo]

Actually it is 90 percent relative compaction based on the modified proctor, we are getting about 120 to 125 lb/ft3 dry density, so I would estimate the relative density about 70 to 80 percent, but does have some fines.

 

[oldestguy]

Still good. Remember we build on natural soil where the natural density seldom is over that 90% number,if that were to compare to compacted sample. 82% is common for silty clay in nature around my area.

 

[fattdad]

I'm not surprised. Were you using Standard or Modified proctor values for reference density?

90%RC is about 50% relative density.

Was the lab work based on performance for 90% RC, Standard proctor?

f-d

ípapß gordo ainÆt no madre flaca!

 

[ocgeo]

We only use the modified here in CA.

It is a 25 ton tractor, and has a 3.5 yd loaded bucket, so the tracks contact pressure is about 20 psi (about 3000 psf), very low.

Normal pneumatic tires exert the pressure of the tire, about 35 to 100 psi.

A sheepsfoot knob exerts about 300 to 1000 psi.

Favorable items of the tracks:

They do have 'knobs' so you will get increased pressure at those points, but only to a shallow depth

The bucket is loaded in the front, so the pressure may be more towards the bucket (greater pressure, however the engine is in the rear, so may be balanced

The key to our success: very thin lifts and increase the number of passes

​

 

[fattdad]

Just to add a few further thoughts: What is the line of optimums for the use of rubber tired compaction? This is an industry-wide issue. When we do a Proctor (i.e., either modified or standard), we obtain a reference moisture content that we call, "Optimum water content." That's actually only the optimum water content for proctor compaction. IT'S NOT THE OPTIMUM WATER CONTENT FOR A SHEEPSFOOT, RUBBER TIRE, VIBRATORY, ETC; HOWEVER!

It's funny to me how we never recognize that laboratory optimum may have no direct relationship to the field compaction method. Somewhere I have content on this, but it's not a click away.

f-d

ípapß gordo ainÆt no madre flaca!

 

[ocgeo]

Agree, I also saw a thread about getting compaction above 100%, which of course is achievable, the reason the Modified Proctor evolved from the Standard Proctor, as the equipment became more capable of better compaction. It was amazing to see an aggregate base turn to what almost looked like glass, while installing rammed aggregate piers; I never tested the compaction but I would imagine it was above 110%.

 

[aeoliantexan]

In my experience, compaction can be achieved with the tracks if the lifts are thin, water content favorable, and sufficient passes are applied. If they placed 20 feet of material in one lift and compacted it from the surface, it cannot be dense below a couple of feet at most. The stresses are too low at depth. How were the density tests run? if a drill rig was used and Shelby tubes pushed, the samples may have been compacted by friction between the soil and the inner wall of the tube.

 

[BigH]

fattdad - further to your point - nor any relation to specifications (say dams) where they want OMC to OMC+25 - but never really identify if it is the lab Proctor or compactor OMC.

 

[fattdad]

haha! Yes, BigH! It's very interesting to me, the separation of first principals from the whole compaction realm! I'll even ask, "What's the optimum moisture content for 95% compaction? It's clearly not the same as the OMC for 100% compaction. We know this, 'cause we know that the OMC for Modified is lower than the OMC for Standard. Therefore, the OMC for 95% compaction would be greater than the OMC for 100%!

Our practice makes it out like OMC is a standard - some vertical line on the moisture-density curve. It's not!

There's a ZAV and there's a LOO (line of optimums). Has the whole industry forgotten this!?!

Rant over!

f-d

ípapß gordo ainÆt no madre flaca!

 

[geotechengineer1]

I've had several projects involving 95% SPMDD specs (had to argue to change the spec from 'maximum dry density' to 'standard proctor maximum dry density' because the specifier believes the standard proctor is the maximum dry density, period.), where compaction was completed with fully loaded ultra class haul trucks. Guess how many tests were 100-105% of SPMDD, dry of optimum? Then some 'expert' from a different jurisdiction comes along and insists we have to moisture condition the soil to the 'optimum moisture content'. When we can do 1m loose or perhaps more with no moisture conditioning.