Is anything less than 95% standard proctor acceptable?
Is anything less than 95% standard proctor acceptable?
(OP)
For example, using a sandy/gravel fill with a maximum dry density of 129.2 pcf the contractor was only able to achieve 91% compaction. This house pad is between 2-4' and will support a slab on grade post-tensioned foundation. Any thoughts on if the compaction is adequate and will not settle over time?





RE: Is anything less than 95% standard proctor acceptable?
RE: Is anything less than 95% standard proctor acceptable?
RE: Is anything less than 95% standard proctor acceptable?
RE: Is anything less than 95% standard proctor acceptable?
Had another test done by another geotech company. Turns out the first company was using a dry density number that was too high. The pad was above 95% in 2 locations, and above 98% in the other 2 locations.
RE: Is anything less than 95% standard proctor acceptable?
RE: Is anything less than 95% standard proctor acceptable?
Gosh. I didn't get any bites. Was hoping to stir up some conversation.
RE: Is anything less than 95% standard proctor acceptable?
RE: Is anything less than 95% standard proctor acceptable?
The original testing company tested the material before it was delivered then used that value to compare. The new testing company tested proctor value on site.
RE: Is anything less than 95% standard proctor acceptable?
Most times when slabs are designed, it is "always" with a layer of crushed or gravel beneath. I've even seen one recently (to my consternation) that the site had a granular base, then the poly vapour barrier, then a 50 mm lean concrete mud mat, then placing the BRC and a 100 mm slab. I can't say I've seen a mud mat between the slab and granular before . . .
RE: Is anything less than 95% standard proctor acceptable?
RE: Is anything less than 95% standard proctor acceptable?
To me there is a difference between natural soil at a given density and fill at a given density. Addmittedly that difference is much less with a well graded sand and gravel such as being discussed here, but there is still a difference.
Part of the difference is that natural deposits are assumed to varry uniformally between borings. Now we all know that is not always the case, but it is still almost always one of our base assumptions. Fills on the other hand can and do vary considerably in short distance, unless the contractor does a good job of control. That is where density testing comes in, consistently good tests generally mean that the contractor has good control.
As for 95% modified versus other compaction requirements, nothing special in my mind. The higher you spec the better job the contractor has to do to meet the requirement. Most of my work is done in silts and clays, we typically use 95% modified under structures and slopes and 85 to 90 for landscape areas. For base rock we typically use 98% standard (our base is usually limestone and we found that the material tended to breakdown some under modified energy).
Mike Lambert
RE: Is anything less than 95% standard proctor acceptable?
RE: Is anything less than 95% standard proctor acceptable?
Then, I can't recall ever checking out an excessive settlement job that can be blamed on say 88% or some very far off test result.. Usually it is more in the category of no compaction or excessive lift thickness leaving loose zones. Building on cinders or, a sideways slip into loose stuff.
So, at 95% we know it works. Some scientific reasoning there.
I do know of an excessive swelling job or two when 95 percent was actually achieved on an expansive clay. The learning curve was expensive then.
In a few jobs with residual clay soils for K-Mart parking lots I have asked for an unconfined compression of at least a given figure, say 2.0 T/sf, with a minimum percent of Modfied Proctor at 83% (because acceptable undisturbed ground was 83%.) Mr. Freezing ground then takes over regardless the next winter.
RE: Is anything less than 95% standard proctor acceptable?
when i wrote earthwork specs, i wondered what kind of feedback i would get from contractors if i specified 103% standard compaction for the stone.
one item to point out for the non-soil-testing types. There is a precision statement to the proctor testing. You can certainly get 2 different numbers on the same soil and both are valid tests. I'm always skeptical when i hear about soils being double-checked by others. Fill soils are rarely homogeneous and technician locations are dubious.
RE: Is anything less than 95% standard proctor acceptable?
Also, testing can have some built in errors. A funny example was a time I was on a proposed landfill site and an opposing party hired a nearby very experienced professor to counter what I might find. We had a test pit dug. I mentioned the soil there was "structured" (a soil scientist term). He hadn't heard of that term. Probably also never heard of "peds", as in pedology. He had along his lab tech to do some field density tests. His main device was a quart jar fitted with a small cone and filled with white sand. That goes to tell what sort of soils engineering education comes from his office and the degree of precision his students were taught..
I looked forward to having him on the other side in court some day, but it didn't happen.
RE: Is anything less than 95% standard proctor acceptable?
For clayey fills, the big difference appears when the completed fill is subjected to water infiltration. The lower the percent compaction, the more voids available to fill with water. Assume a silty clay with a maximum Standard Proctor density of 105 pcf and optimum water content of 20%. Compacted at optimum,the consistency will probably be very stiff, with an unconfined compressive strength between 4 and 8 kips per square foot. At 98% compaction, it can absorb enough water to have a water content of about 23%, 3% above optimum. It would probably be stiff (2 to 4 ksf). At 95% compaction, the potential saturated water content is about optimum + 5%, and it would be firm (1 to 2 ksf). At 90% compaction, the potential saturated water content would be about optimum + 8%, and the soil would be soft, unsuitable for footing support and likely to compress significantly under its own weight.
If you can keep water out of the fill, The degree of compaction is much less critical than if the fill may get wet in the future.
RE: Is anything less than 95% standard proctor acceptable?
While relative density might perhaps be a better predictor of performance, it is difficult to find a lab with the equipment to properly run the test and its repeatability with various soil types is questionable. The moisture-density relationship (Proctor) coupled with appropriate in-place density testing is a valid and repeatable test series that can be readily applied and interpreted. One problem is that most contractors don't know much about it and do not use it to advantage.
But the question of what percent is appropriate remains. For repetitive loading such as pavements, 95% of the standard Proctor is woefully deficient, particularly in sandy soils or clayey/silty sands above optimum moisture. One must also keep in mind that compaction does not equal stability. You can achieve compaction in soils that have little or no load-induced stability. Don't mix up the two!
Back to the void ratio. Can the void ratio change in soils that were only compacted to 90% of the standard Proctor? Of course, but the change has to be induced by something. The most common "somethings" are changes in groundwater levels (if the level rises into the 90% compacted soil and then recedes, the void ratio will generally change, resulting in compaction greater than 90%....which results in settlement. The quantification of the settlement is site specific and depends on several variables) and vibration. The vibration can come from adjacent traffic, equipment within the building or even thunder. The goal of selecting an appropriate compaction level to specify is to mitigate such future settlement.
Also keep in mind that the two Proctor methods, standard and modified, were developed based on available equipment to cause compaction. In the early days of the use of moisture-density relationships, the equipment was not particularly heavy and certainly not sophisticated. Vibratory compaction was not used. For those considerations the "standard" Proctor was correlatably useful, particularly in clayey sands, clays and silts. As equipment became more sophisticated and variations in material blends were used, it was recognized that one could easily achieve over 100% compaction in some soils with the better equipment. That led to the development of the modified Proctor to better exemplify the compactive effort that could be put into the soil with better equipment.