Compaction and ASTM D-698 Standard Proctor 3

[ben1212]

Ladies and Gentlemen,

I have a basic understanding of ASTM D-698 and how the test is performed and how the numbers are calculated. At my firm we usually have notes similar to "The fill shall be compacted to at least 95 percent of the ASTM 698 maximum dry density at a moisture content ranging between -2 and +3 percent of optimum moisture content." which are taken directly from the recommendations of the geotechnical report.

My question is, how do these numbers relate to the compaction of soil below a grade bearing/ slab-on-grade type foundation? Particularly when you have deep fill, lets say 6 to 12 feet on a hillside. If the compaction were at least 100 percent or 101, or whatever, how does that relate in real terms to the compaction of the fill and more importantly to the potential for settlement of the fill?

If the compaction is 95% does that mean that the fill could settle 5% of its depth if exposed to water that would likely consolidate the fill and compact it more? Which leads me to another question, if the fill is compacted to 95% of the dry density, does that number change if the fill is exposed to rain before the concrete is placed?

I look forward to the discussion!

 

[shin25]

When a site which is 6 to 8 ft deep is filled with borrowed soild, the contractor cannot fill the entire depth with one lift. He will be required to fill with multiple lifts and every lift has to attain the maximum dry density as out lined in the specification.

When it rains (if the site is not flooded), the rain water theoretically will run off the site, so above the water table, the soil will not remain staturated. So, theoretically, rain should not change the soil density. Now, if the fill extends below the water table then you must need to consider the pour water pressure from the water table and the same way the effect of dewatering.

Now, usually, when the soil attains its maximum dry density, the soil structure is fully compacted and the long term settlement due soil perticle restructuring is less. But, the immediate or elastic settlement needs to be considered as usual.

 

[hokie66]

You cannot depend on mechanical compaction to completely eliminate long term settlement in a deep fill. Some of the consolidation is time dependent no matter how much effort is expended in compaction. This is the reason that relieving slabs should always be used adjacent to basement walls, bridge abutments, etc. There are plenty of examples where this time dependent settlement has been ignored with expensive consequences.

 

[msucog]

large area fills are particularly susceptible to fill induced settlement over a relatively short period immediately after the fill is placed. the "short period" is different depending on the geology and soil structure of the residual soils under the fill and fill thickness as well as a few other parameters. i've seen 12"+ of fill induced settlement under 35' of fill occurring within about 3-4 months after placement. the residual soils has 10+ bpf material but due to the geology, it was prone to significant settlement. some soils experience long term settlement depending on the soil type and drainage conditions.

the building loads may induce additional settlement. from my experience in my geology, settlement of the fill itself is typically not a problem if it's compacted to at least 95% std...it's usually the stuff under the fill or poorly compacted fill that causes the headaches. large loads may do some funky stuff to the fill too.

as a comparison, residual soil might only be 75-85% of the standard Proctor max dry density in its natural state. once it's disturbed, the residual stresses/strength that built up over thousands/millions of years are destroyed so then it has to be beat back together. that is where fill gets its strength.

well compacted fill should not take additional water (except maybe the upper few inches). poorly compacted fill will take on water.

also to note, "on a hillside" has its own implications. retain the services of a geotech.