Standard Proctor Density

[PEFLWI]

We are constructing a large mat foundation (75'x90') to support several large pieces of equipment. The geotechnical engineer is recommending a compaction of 95% using the Standard Proctor Test. The contractor is saying that 95% is very difficult and expensive to achieve. I have specified 95% using either the Standard Proctor or Modified Proctor Tests for many years and I have never encountered this objection from a contractor. Has anyone had this problem? The geotechnical engineer suggested that the contractor may be confusing relative density with compaction. Thoughts?

 

[GeoPaveTraffic]

95 percent standard should never be a problem to obtain. Contractor is one of the following: cheap, inexperienced, lazy, or confused.

Mike Lambert

 

[GeoEnvGuy]

Concur with geopavetraffic my only other comment is 95% spmdd seems low to me for a foundation I typically see 100% spmdd or modified Proctor specified.

 

[oldestguy]

Interesting subject. How's come we can support a floating slab foundation, as in this case, on soil with standard penetration test result at say 30 blows? The relative density of say 80 percent (not even 95% standard Proctor) and we are happy with it. Being ultra conservative only makes the job cost more, with no obvious benefit. Contractor obviously confused.

 

[Ron]

I notice that you are in Florida. You are dealing with coastal plains soils. First of all, a standard Proctor is rarely specified in Florida except for embankment soils. Most structural fill and subgrades are compacted to a Modified Proctor. In most areas of Florida you can get 95% of a standard Proctor by very minimal compactive effort. Your contractor doesn't want to do any compaction because it costs him more money. He's not likely confused about relative density because he has likely never heard of it since it isn't used much as a standard in Florida.

I've been practicing in Florida for 44 years and the only time I run into this is when a contractor is too cheap to follow the specs. Hold him to it. I would consider it a minimal standard to meet. If I had specified it, I would have required at least 95% of a Modified Proctor.

 

[cvg]

inspected a project the other day. told the contractor he needed to test the compaction and make sure it met 95%. contractor said, we didnt know we had to do that. "we do commercial construction and we usually dont do that, so what if it settles a little bit" my guess is your contractor did the same thing, bid the job without any geotech testing. that will cost him money and he underbid the job

 

[r13]

Is contractor perform testing the standard practice?

 

[cvg]

just depends
for this project, geotech certification of all earthwork was required to be submitted to the owner. pro tip: read the notes on the plans

 

[r13]

We usually suggest the owner to hire their own lab to avoid conflict of interest. Just not positive on residential project though.

 

[cvg]

yep, I agree, that is good practice. but we didn't design this, just doing the inspections for the AHJD

 

[darthsoilsguy2]

is it fill? or cut? to get to foundation bearing elevation. it would be reasonable for a contractor to think they were not responsible for compaction work/testing on cut. there are specs out there which direct the contractor to get in the cut and compact the bearing surface... but it really only helps to have uniform surface to cast against as any settlement squeezed out is super minimal and is in the layer that consolidates during the construction cycle prior to finishes anyway.

 

[BigH]

Under foundations, I've always required modified Proctor, say 97% but prefer 100% on heavier structures (Ontario Granular A). Had an argument one time with a younger engineer who wanted to specify 100% standard Proctor. I refused to budge. Why? Standard Proctors are just that "standard" as in Ho-Hum. When Modified is specified, the Contractor (maybe not the OP's one) knows it is important and he puts in the effort.

 

[JedClampett]

I'd hold the line on this one, not so much for your project, but the ones in the future. I do a lot of work in Florida and the contractors get away with murder. And it's not totally their fault, they're just doing what they've always done.
If it's easy to get the 95%, then the contractor has no complaint. And if it's hard, then it was really needed.
Didn't Hammurabi say that a good foundation makes for a good project? No? Well he should of.

 

[oldestguy]

Per retired13, don't get side tracked with a meaningless question.

 

[r13]

I am curiously awaiting the answer to the question posted by one of the geotechnical practitioner. I think the answer is quite beneficial to the non-practitioners as well, when facing different situations. As usual, you didn't disappoint me by sentimental comment instead of offering knowledge of your practice.

 

[Ron]

@retired13....that was not the question...it was a point raised by darthsoilsguy2 as a valid point; however, the question was why would a contractor balk at achieving compaction when such a low standard was specified.

Native soil or fill, I would still require compaction to at least 95 percent of the Modified Proctor. There is a premise by some who feel that native soils do not need additional compaction. I'm not one of them. That applies to overconsolidated soils but not so much in coastal plains sands. I have tested native soils many times and found them to be significantly less than appropriate compaction. Particularly true in coastal plains soils where near-surface density can vary significantly.

The contractor is just trying to get away without having to meet a minimal requirement. This is absurd.