Sidewalk Subgrade

[durtaholic]

Hello All... New to the site and appreciate all of the valuable information. So today I was sent to a job to check subgrade for sidewalk area. It was an island area with curb line placed already.

Anyways , the sub-grade material was a plastic clay with an optimum moisture of 15 per lab max. I burned a moisture on site and showed 6 %. The contractor was doing the old
" flood the surface and run the vibra-plate over the top " dig down 2 inches and dry as can be. The material probed pretty firm but I failed the test due to the effort , or lack there of, and the bone dry clay material . They are really limited as far as equipment to get in there and scarify and process. Only thing I can think off at this point is a rototiller ? Might be able to get a backhoe crammed in there and rip it a bit but will be tight .

So is it nit-picking to have them scarify and re-compact a firm sub-grade because the moisture is not there ? The high clay is what makes me a little nervous ...

Thanks again for the great site..

 

[Ron]

Good call!

Use a roto-tiller and mix a little bit of hydrated lime (spread it about 1/4" deep over the surface before mixing) in as it is being mixed. Mix 6 to 8 inches deep and increase the moisture content by spraying, not flooding. Take moisture content to about 16-17 percent and start compaction with "jumping jack", not plate compactor. "Walk" the compactor out of the ruts it will produce while the moisture content is decreasing. You'll see this happen.

If you get it too wet and it won't seem to dry, remix with portland cement in the same quantity and re-compact. It will certainly set up then (you will have a soil-cement base).

Plastic clays with low moisture will "bridge", giving a false sense of stability. If left uncompacted and then covered with concrete, moisture can then permeate at its own pace. When it gets high enough, some settlement will then take place and you'll have an uneven sidewalk.

 

[moe333]

It seems there is a potential for swelling of the clay if it gets wet in the future. Swelling could cause differential heave of the sidewalk. You may need to treat the soil to a greater depth to mitigate potential heave....if this is an issue for this project.

 

[fattdad]

If it is natural soil and below optimum, who cares. If it was compacted structural fill and placed under a CQA program, prior to drying out, who cares?

It's rediculous to believe that "optimum" moisture content should be controlled in native soils or in fill that's already been placed. If the sidewalk subgrade is undisturbed and either natural soil or previously placed fill, lay in your subbase and pour.

If there is no documentation of either, then yes, moisture condition and compact (or undercut and install suitable subgrade materials that you can more easily compact).

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

what does the spec say?

our local standard specs require testing and approval of subgrade, regardless of if it was natural, in-situ soil or compacted fill. Under pavement and sidewalks, 90% of standard proctor to a depth of 12 inches is required. Regardless of moisture content, it has to meet the density requirement. but flood and whack without scarification is a waste of time and effort. If it doesn't meet requirements, they must do a better job.

 

[hokie66]

Agree with fattdad. Sounds like nit-picking to me.

 

[durtaholic]

Thanks for the responses.. Contractor chose to excavate about 8" of material ( soil ) and compacted aggregate base as a replacement. Worked out fine .

 

[itzobi]

I fail contractors on this all the time. The fact of the matter is this.

They bid the job, and have agreed to do so in accordance with the contract, plans and specifications.

If the specifications call for 6" of scarification and to bring to proctor optimum moisture and compaction, then they must do whatever they can to achieve this, tight area or not.

Unless the engineer will approve otherwise, I would just fail them repeatedly until they get it right, at least process the full depth as required by the specifications. Unless an engineer is willing to allow it of course, I would reduce their payment for the prep if that was the case.

My specifications for SG prep read that the OM content must be achieved only "at the time of compaction" not at the time of test. Granted the moisture shouldn't evaporate that quickly, but given the fact that nuclear gauges will only read the upper 3" of soil, it is highly plausible. So unless you were there moments after compaction to test it, you need to factor this in.