Standard or Modified Proctor Density? 7

[BSVBD]

I consider it necessary to provide General Structural Notes that are comprehensive to the skills and trades having to comply with them.

Within our General Earthwork Notes on our construction documents, for various purposes, our firm specifies that soil must, "... be mechanically compacted to 98% modified proctor density."

Question #1: Why "Modified" rather than "Standard" proctor density?

Question #2: Could the 98% (or 95%), be comparable to a percentage of the allowable soil bearing capacity?

Through the research i have done, if it is difficult for me to understand what this proctor is a percentage of, how can i expect the excavators or others to comply?

Not forsaking terms of liability, i would like to express specifications in terms of adequate simplicity to avoid potential liability.

Question #3: From what i understand, this Modified Proctor Density is a laboratory test. The ultimate objective i am trying to obtain for our structure is that of the allowable soil bearing capacity. What can be done in the field to avoid counter-productive and expensive laboratory testing?

I appreciate your input.

 

[Once20036]

Here's how I understand it:
Both the standard proctor test and modified proctor are means of checking the compaction.
I believe that before WWII there was only the proctor test. Around the time of the war, there were larger and larger airplanes imposing more and more loads on the runways, and they needed a stricter method of measuring the compaction. So, 95% proctor is less dense than 95% modified proctor.

Essentially, it's a laboratory test. The compact a soil sample at varying moisture contents and determine the most it could be compacted. Then you tell the field guys that they need to achieve some portion of that maximum value. They may need to add or subtract moisture from the soil in order to make that possible, but it will help to mitigate the potential for settlement.

I don't believe the is any direct relationship between bearing capacity and proctor tests.
I don't believe test can be administered without the laboratory testing, and without a technician on site comparing the compacted soils to the laboratory results. I don't agree that these tests are counter-productive.

On a small residential job where you don't have a geotechnical engineer, I`m not sure how compaction is typically addressed, but I feel like this is the question that you're getting at.

 

[BSVBD]

Once20036, thank you for the history and details. Through research, i've discovered much of what you've shared and more.

You are on track, in that, the majority of structures our firm designs for, are small commercial, light industrial.

90% of the jobs we design for do NOT have a geotech on board. We use the assumed 2,000 PSF soil bearing capacity for this area.

For larger structures where geotechnical engineers are the norm, i agree that the tests are likely the norm and necessary. For the smaller projects that i typically work on, the typical general contractors we deal with, most of whom we deem as competent and responsible (since they have a reputation to uphold), will not consult with a geotech, and, thus, in these majority of instances, would consider any laboratory testing as counter-productive unless questionable soils are encountered.

Even on larger projects, where a geotech engineer is used, the compaction tests seem like another practically avoidable step unless we encounter questionable soils. For the past two decades that i have been with thsi firm, since we have NEVER encountered a proctor test, that i am aware of, i am not seeing an urgency or a necessity if the allowable soil bearing capacity is obtained.

 

[mike20793]

I'd like to hear the geotechnical engineers chime in on this one; good question! I recently had a report that called for vibratory densification and required modified proctor for compaction testing, so I was on the phone with the geotechnical engineer quite a bit making sure I had my specs and notes correct. I asked why the modified proctor and not the standard proctor and he said soils typically use the standard proctor and sands use the modified proctor. He didn't really get into why, but this was the first time I saw a report require the modified proctor.

 

[Ron]

BSVBD..... It would probably be a good idea for you to take a basic soil mechanics and soils lab course if given in your area. It should have been covered in your engineering curriculum; however, that is not always the case.

The "Proctor" test is more properly called a moisture-density relationship test. It is a laboratory test that was developed by R.R. Proctor in the 1930's. It is used as the basis of comparison to an in-place (field) density test to determine the percent compaction. It is common for building construction to specify 95% of the modified Proctor as a compaction standard. For pavements, 98% of the modified Proctor is usually required.

The original test was the "standard" Proctor. It uses a 5.5 lb rammer falling 12" for 25 times in each of 3 layers within a standard mold in the lab. The modified Proctor uses a 10 lb rammer falling 18", also in a standard mold. Get a copy of each standard (ASTM D698 for standard Proctor and ASTM D1557 for modified Proctor) and read it. Read corresponding in-place density test standards as well.

One piece of specifying advice....don't specify a standard you don't understand! It helps protect your liability to understand what you specify.

Compaction, since it is usually related to surface or very near surface conditions, has little to do with bearing capacity of soils. There are some tests that will give you an idea of bearing capacity such as plate load tests.

 

[JAE]

I once read that the Standard Proctor was established to best mimic the standard types of compaction equipment used in construction at the time of its development.

Years later there was an observance that compaction equipment had gotten better, heavier, etc. and the Modified Proctor was developed to better mimic those types of equipment.

However, the Modified was found to be difficult to achieve in numerous conditions so a lot of today's specifications rely on the older "standard" Proctor.



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[cvg]

since we have NEVER encountered a proctor test, that i am aware of, i am not seeing an urgency or a necessity if the allowable soil bearing capacity is obtained.

A very high percentage of construction defects are the result of earthwork problems including: expansive soils; landslides; settlement; earth movement; improper compaction; inadequate grading; and drainage.

compaction increases the in-situ density which reduces settlement and increases stability. Done properly it requires adding water and mechanical effort. a good contractor will have a feel for how much water and how much effort this takes. it may be possible to achieve good compaction without testing. However, what happens if they run out of water or the compactor breaks down? What happens if they haul in some crappy fill dirt that won't easily compact?

Cost is not really a big issue, a proctor can be run for about $100 and a field density with nuke gauge for about $65. Is it really worth saving a couple hundred dollars?

 

[JAE]

Sometimes you need to remove water (i.e. dry out the soil) - depends on which side of the proctor density curve you are on.


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[Ron]

JAE...you are correct in both statements. Equipment improvements necessitated the modification to the standard Proctor. It is not uncommon with current equipment to exceed 100% compaction with the standard Proctor and it can also occur with the modified Proctor.

 

[Ron]

Attached is a short description of how a Proctor curve is used in construction. A similar version of this appears in the textbook "Principles and Practices of Commercial Construction", 9th Edition, Pearson Publishing.

 

[BUGGAR]

"Fill soils shall be moistened or dried and compacted to at least 95 percent of the maximum dry density determined in accordance with ASTM D(lab test method). The density of soils following compaction shall be determined in accordance with ASTM D(sand cone or nuke meter)."

 

[shaneelliss]

Some of this has already beeen addressed, but I will just add my understanding to the conversation as well.

Question 1: Modern day compaction equipment can compact soils to the modified proctor density values with reasonable effort. Modified proctor densities are generally called out these days because of this.

Question 2: It is important to remember that in a lot of soils the allowable soil bearing capacity is generally determined based on settlement estimates under load. It isn't that the soils will collapse completely swallowing the structure if the bearing capacity is exceeded, it is just that it will settle more than is considered acceptable (usually about 1"-2" long term from the reports I have seen). They put the soil under load and see how it compresses, then they create a curve from this that shows expected settlement versus applied bearing pressure, then they determine what they think is acceptable settlement and find out what the allowable bearing pressure is at that settlement based on the curve.

If the existing soils have an allowable bearing capacity based on acceptable settlement, and then you add fill on top of that and don't compact that fill properly, you will get settlement in the uncompacted/lightly compacted fill plus settlement in the underlying soil, and these added together probably exceed what is considered acceptable settlements. Usually worse is when you add fill, but the contractor drives over one area more than other areas, effectively compacting some areas more than others, and then you get differential settlement under the building footprint because some areas are compacted more than others. Differential settlement is the real bad guy in most structures with soil problems. If the left half sinks 2" but the right half only settles 1/2", you get broken foundations, cracked walls, overstressed beams, etc.

Long story short, I have never seen a direct correlation between allowable bearing capacity in the fill versus compacted density. The goal in proper compaction of fill is just uniformity in density and minimum settlement.

Question 3: If the fill soils are fairly typical in your area, without a lot of variation in grain size and composition, or if the contractor always uses the same type of structural fill from the same source, a careful contractor will have a good feel for how much water and effort they need to put into compacting the fill to get a decent compaction. But they will probably never get the feel of it if they have never had to do tests to show them what proper moisture and compaction looks like. And if compaction isn't specified, some contractors will not feel any motivation to get the understanding they need to do the job right.

I have seen once or twice where engineering specifications call out a specific soil type and gradation for fill and then describe a compaction pattern to follow, such as "three passes in each direction with a double drum compactor over properly moistened soil deposited in 8" lifts." I don't like to do that because it puts more liability on me, it is less accurate, and soils tests are cheap.

 

[jike]

I was fortunate to have worked on projects in various parts of the country. Some areas used Modified Proctor and others used the Standard Proctor. I relied on the local soils engineer to direct me in this regard. I inserted the appropriate method in the general notes / specs depending on what they used in that area of the country.

 

[BSVBD]

This certainly provides for better understanding and knowledgeable specifying.

Thank you all for your input.

 

[cvg]

modified proctor is appropriate for some soils and for some projects. standard is acceptable for others. both are used as appropriate. Contrary to common belief, more compaction is not always better.

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[Okiryu]

Check the in-situ water contents of the soils that you are planning to compact and compare they with your proctor data. You will have an idea if soils will need to be conditioned to get a certain degree of compaction. For example, you will note that clays at their in-situ conditions may be difficult to compact at 90 plus percent of proctor since they have high natural water contents...

 

[oldestguy]

BSBVD:

It is difficult for me to understand why you have a specification showing some degree of compaction related to the Proctor test when you don't seem to know why. It also is difficult for me to see you calling laboratory testing counter productive, when in connection with field testing of compaction (I trust you do that) you then have better assurance your support will be there. If you don't do field testing, then who does it, the earthwork guy? That's like the fox guarding the hen house. I'd like to be a competitor of yours if you routinely use 2,000 psf as bearing cap'y for foundations. By doing these field and laboratory tests you ought to be able to use 4,000 psf or more in some cases, reducing the cost of foundations. No point in having a spec that you don't know how to enforce, because it may bite you some day.

One other item no one mentioned is using high compaction on some clay soils (especially expansive ones) you open yourself to problems later when those soils expand to the density they "like to sit at". A foundation or floor raising up can really get peop0le more bothered than settlement in some cases.

In short, it would seem that you would be farther ahead cost-wise and risk-wise by bringing a geotech on board and using them to assist in setting bearing pressures to make the most economical use of that ground or compacted fill. As you apparently are working now, I think you are more at risk for legal problems than doing this. Many times I have been out on a job where a compaction spec is used and have found it necessary to justify tests not passing, when the soils are modified some during lab work such that the requirement for the earthwork guy is impossible to meet, practically and not really needed anyhow. My PE written document gets the job going, yet I have taken some of the risk knowing all is OK.

 

[BSVBD]

oldestguy - Thank you for your helpful and sincere comments.

It is, likewise, difficult for me to understand why the firm that i work for has a specification showing some degree of compaction related to the Proctor test when neither myself nor my supervisor has a thorough, thus acceptable, understanding of it. This is why i am posting to improve the specifications of the firm i am employed under.

oldestguy, you are further correct, in that i did refer to laboratory testing as counter-productive. Please forgive me. My statement was simply a reflection of the mentality, that a "bread-n-butter" contractor client has for the typical small-scale building foundation we frequently design for. Somehow, i need to satisfy our clients, in a professional manner, WITHOUT compromising structural integrity. Collectively, we all understand this.

Again, please forgive me... i'm dealing with many, occasional practices that i don't necessarily agree with, care for nor approve of. Considering all aspects of the construction process, from schematic design to occupancy permit, job-specific magnitude and incidentally, this does not necessarily mean any of the aforementioned are absolutely wrong.

As you have stated, as you "...have found it necessary to justify tests not passing...", i also need to justify with my supervisor and our clients, as well, why i need to (frequently) "go-against-the-grain" of their desires, especially their claim of "we've been doing this for years", for the sake of structural safety.

I am asking questions on this forum because, i frequently challenge specifications within my firm, as well as practices and desires of our clients, that "we have been doing for years", that i feel either need to change or that we need to further understand.

Thank you, oldestguy, and all others, for this professional assistance that you continue to provide!

 

[oldestguy]

BSBVD: I appreciate your problems there. Sometimes the boss is wrong, but he still is the boss. Short of modify he spec for small jobs consider asking if a geotech might be asked just to visit the job as it starts or maybe after excavated to roughly evaluate it as to whether there is any need for further investigation. I've done this many a time, using a rod probe to rough estimate the support capabilities, especially for light loads. It amounts to a short time on the job and maybe a letter confirming verbal job statements. No fancy test on the site or in the lab.

 

[XR250]

Why not just have the owner hire a Geotech to oversee this aspect? Why take on that liability?